(jU^S^^-M STATE OF ILLINOIS WILLIAM G. STRATTON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION VERA M. BINKS, Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief URBANA REPORT OF INVESTIGATIONS— NO. 162 GEOLOGIC ASPECTS OF RADIO WAVE TRANSMISSION BY M. WILLIAM PULLEN PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1953 LIBRARY nr.T ft 1QM ILLINOIS STATE GEOLOGICAL SURVEY 3 3051 00005 8440 STATE OF ILLINOIS WILLIAM G. STRATTON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION VERA M. BINKS, Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON. Chief URBANA REPORT OF INVESTIGATIONS— NO. 162 GEOLOGIC ASPECTS OF RADIO WAVE TRANSMISSION BY M. WILLIAM PULLEN PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1953 MANUSCRIPT COMPLETED JANUARY 1952 ORGANIZATION STATE OF ILLINOIS HON. WILLIAM G. STRATTON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION HON. VERA M. BINKS, Director BOARD OF NATURAL RESOURCES AND CONSERVATION HON. VERA M. BINKS, Chairman W. H. NEWHOUSE, Ph.D., Geology ROGER ADAMS, Ph.D., D.Sc, Chemistry LOUIS R. HOWSON, C.E., Engineering A. E. EMERSON, Ph.D., Biology LEWIS H. TIFFANY, Ph.D., Pd.D., Forestry GEORGE D. STODDARD, Ph.D., Litt.D., LL.D., L.H.D., President of the University of Illinois DELYTE W. MORRIS, Ph.D., President of Southern Illinois University GEOLOGICAL SURVEY DIVISION M. M. LEIGHTON, Ph.D., Chief (67358— 2M— 3-53) srs7 STATE GEOLOGICAL SURVEY DIVISION Natural Resources Building, Urbana M. M. LEIGHTON, Ph.D., Chief Enid Townley, M.S., Geologist and Assistant to the Chief Veld a A. Millard, Junior Assistant to the Chief Helen E. McMorris, Secretary to the Chief RESEARCH (Not including part-time personnel) GEOLOGICAL RESOURCES Arthur Bevan, Ph.D., D.Sc, Principal Geologist Frances H. Alsterlund, A.B., Research Assistant Coal Arthur Bevan, Ph.D., D.Sc, Acting Head G. H. Cady, Ph.D., Senior Geologist and Head, Emeritus Ernest P. Du Bois, Ph.D., Geologist George M. Wilson, M.S., Geologist Robert M. Kosanke, Ph.D., Geologist Raymond Siever, Ph.D., Associate Geologist Jack A. Simon, M.S., Associate Geologist John A. Harrison, M.S., Assistant Geologist Margaret A. Parker, M.S. Assistant Geologist Frederick E. Williams, M.S.,, Assistant Geologist Kenneth E. Clegg, M.S., Research Assistant Oil and Gas A. H. Bell, Ph.D., Geologist and Head Lester L. Whiting, B.A., Associate Geologist Virginia Kline, Ph.D., Associate Geologist Wayne F. Meents, Assistant Geologist Kathryn C. Irving, Technical Assistant Eugene Frund, B.A., Research Assistant Petroleum Engineering Paul A. Witherspoon, M.S., Petroleum Engineer and Head Frederick Squires, A.B., B.S., Petroleum Engineer, Emeritus Industrial Minerals J. E. Lamar, B.S., Geologist and Head Donald L. Graf, Ph.D., Associate Geologist James C. Bradbury, A.M., Assistant Geologist Robert J. Cronk, M.S., Assistant Geologist Raymond S. Shrode, M.S., Assistant Geologist Clay Resources and Clay Mineral Technology Ralph E. Grim, Ph.D., Consulting Clay Mineralogist W. Arthur White, M.S., Associate Geologist Herbert D. Glass, Ph.D., Associate Geologist Edward C. Jonas, M.S., Assistant Geologist Groundwater Geology and Geophysical Exploration Frank C. Foley, Ph.D., Geologist and Head Merlyn B. Buhle, M.S., Associate Geologist Theodore R. Walker, Ph.D., Assistant Geologist (on leave) Richard F. Fisher, M.S., Assistant Geologist (on leave) Robert D. Knodle, M.S., Assistant Geologist John W. Foster, M.S., Assistant Geologist James E. Hackett, M.S., Assistant Geologist Margaret J. Castle, Assistant Geologic Draftsman (on leave) Lidia Selkregg, D.N.S., Research Assistant Geophysical Research M. William Pullen, Ph.D., Geologist and Head Robert B. Johnson, M.S., Assistant Geologist Bennie Ellis, Assistant (on leave) Engineering Geology and Topographic Mapping George E. Ekblaw, Ph.D., Geologist and Head William C. Smith, M.A., Assistant Geologist Stratigraphy and Areal Geology H. B. Willman, Ph.D., Geologist and Head J. S. Templeton, Ph.D., Geologist David H. Swann, Ph.D., Geologist Elwood Atherton, Ph.D., Associate Geologist Charles W. Collinson, Ph.D., Assistant Geologist Donald B. Saxby, M.S., Assistant Geologist T. C. Buschbach, M.S., Assistant Geologist Howard R. Schwalb, B.S., Research Assistant Charles C. Engel, Technical Assistant Joseph F. Howard, Assistant Topographic Mapping in Cooperation with the United States Geological Survey. February 18, 1953. GEOCHEMISTRY Frank H. Reed, Ph.D., Chief Chemist Grace C. Johnson, B.S., Research Assistant Coal Chemistry G. R. Yohe, Ph.D., Chemist and Head Francis Scheidt, B.S., Research Assistant Rudolph L. Pedrotti, M.S., Research Assistant Physical Chemistry J. S. Machin, Ph.D., Chemist and Head Tin Boo Yee, M.S., M.A., Assistant Chemist Frances H. Staplin, M.A., Research Assistant Fluorine Chemistry G. C. Finger, Ph.D., Chemist and Head Robert E. Oesterling, B.A., Assistant Chemist Joseph E. Dunbar, M.S., Special Research Assistant Melvin J. Gortatowski, M.S., Special Research Assist- ant Chemical Engineering H. W. Jackman, M.S.E., Chemical Engineer and Head P. W. Henline, M.S., Chemical Engineer R. J. Helfinstine, M.S., Mechanical Engineer B. J. Greenwood, B.S., Mechanical Engineer James C. McCullough, Research Associate (on leave) Walter E. Cooper, Technical Assistant Raymond H. Pellum, Technical Assistant (on leave) Edward A. Schaede, Technical Assistant X-Ray W. F. Bradley, Ph.D. Chemist and Head Spectrochemistry (formerly Physics) Kenneth B. Thomson, Ph.D., Physicist R. J. Piersol, Ph.D., Physicist, Emeritus Juanita Witters. M.S., Assistant Physicist Analytical Chemistry O. W. Rees, Ph.D., Chemist and Head L. D. McVicker, B.S., Chemist Emile D. Pierron, M.S., Associate Chemist Francis A. Coolican, B.S., Assistant Chemist Charles T. Allbright, B.S., Research Assistant Robert L. Eissler, B.S., Research Assistant William J. Armon, B.S., Research Assistant Evelyn Veazey, B.S., Research Assistant Raymond Isaac, B.S., Research Assistant Eugene Lange, Technical Assistant George R. James, Technical Assistant Lois Jean Ruffner, Technical Assistant MINERAL ECONOMICS W. H. Voskuil, Ph.D., Mineral Economist W. L. Busch, A.B., Assistant Mineral Economist Ethel M. King, Research Assistant EDUCATIONAL EXTENSION Gilbert O. Raasch, Ph.D., Geologist in Charge Margaret H. Bargh, B.S., Research Assistant Louis Unfer, Jr., M.A., Research Assistant RESEARCH AFFILIATES IN GEOLOGY T Harlan Bretz, Ph.D., University of Chicago Stanley E. Harris, Jr., Ph.D., Southern Illinois Uni- versity C. Leland Horberg, Ph.D., University of Chicago Heinz A. Lowenstam, Ph.D., California Institute of Technology Paul E. Potter, Ph.D., Assistant Geologist, Stale Geo- logical Survey William E. Powers, Ph.D., Northwestern University Paul R. Shaffer, Ph.D., University of Illinois Harold R. Wanless, Ph.D., University of Illinois J. Marvin Weller, Ph.D., University of Chicago CONSULTANTS Geology, George W. White, Ph.D., University of Illinois Ralph E. Grim, Ph.D., University of Illinois L. E. Workman, M.S., Former Head, Subsurface Division Ceramics, Ralph K. Hursh, B.S., University of Illinois Mechanical Engineering, Seichi Konzo, M.S., University of Illinois Editing, Dorothy E. Rose, B.S. GENERAL ADMINISTRATION (Not including part-time personnel) Library Anne E. Kovanda, B.S., B.L.S., Librarian Ruby D. Frison, Technical Assistant Frances Beer, B.A., Technical Assistant Mineral Resource Records Vivian Gordon, Head Gretchen B. Bauerle, Technical Assistant Shirley R. Cole, B.S., Technical Assistant Judy Ann Day, Technical Assistant Mary L. Bahe, B.A., Technical Assistant Lorna J. Elliott, Technical Assistant Publications Jane V. Olson, B.A., Associate Technical Editor Barbara Zeiders, B.S., Assistant Editor Meredith M. Calkins, Geologic Draftsman Gordon W. Johnson, B.S., Assistant Geologic Draftsman Technical Records Berenice Reed, Supervisory Technical Assistant Marilyn DeLand, B.S., Technical Assistant General Scientific Information Irene H. Benson, Technical Assistant Lois Winiarski, Technical Assistant February 18. 1953 Other Technical Services Leslie D. Vaughan, Research Associate Beulah M. Unfer, Technical Assistant A. W. Gotstein, Research Associate Glenn G. Poor, Research Associate Gilbert L. Tinberg, Technical Assistant Wayne W. Nofftz, Supervisory Technical Assistant Donovon M. Watkins, Technical Assistant Financial Records Velda A. Millard, In Charge Leona B. Kenward, Clerk-Typist III Virginia C. Sanderson, B.S., Clerk-Typist II Joann M. Dunbar, Clerk-Typist I Irma E. Toppe, Clerk-Typist I Clerical Services Mary Cecil, Clerk-Stenographer III Mary M. Sullivan, Clerk-Stenographer III Ethel M. Henwood, B.A., Clerk-Stenographer II Lyi.a Nofftz, Clerk-Stenographer II Kathryn M. Retallick, A.B., Clerk-Stenographer II Phyllis J. Barham, Clerk-Stenographer I Lillian Weakley, Clerk-Stenographer I Shirley Colvin, Clerk-Stenographer I Barbara Jolloff, Clerk-Stenographer I Mary J. de Haan, Messenger-Clerk I Automotive Service Glenn G. Poor, In Charge Robert O. Ellis, Automotive Mechanic Everette Edwards, Automotive Mechanic David B. Cooley, Automotive Mechanic s Helper CONTENTS Page Chapter 1. — Introduction 9 Radio waves and geology 9 Objectives 10 Scope 11 Acknowledgments 11 Chapter 2. — Theoretical concepts of propagation 12 Radio waves 12 Influence of earth materials 12 Ground-wave at broadcast frequencies 13 Wave-guide transmission and propagation 13 Geologic wave-guides 13 Chapter 3. — Previous work 15 Chapter 4. — Equipment for radio field intensity measurements 19 Previous work 19 Present work 19 Instrumentation for reconnaissance investigation 19 Instrumentation for detailed investigation 20 Instrumentation in mobile operation 22 Instrumentation and investigation in the laboratory 22 Chapter 5. — Radio field intensity measurement 24 Field intensity and loop orientation 24 Field intensity records 24 Ground-wave versus sky-wave signals 24 Constancy of ground-wave intensity 26 Modulation effect 27 Outline of field procedure 27 Chapter 6. — Effects of cultural and natural features 28 Wires and steel bridge 28 Description of Area I 28 Wire fences 32 Description of Area II 32 Description of Area III 32 Overhead wires 33 Grounded electric service poles 33 Topography 34 Topographic shadow effect 34 Shadow effect from woods 37 Description of Area IV 37 Streams 38 Buried pipes 38 Road materials 38 Lakes and ponds 38 Chapter 7. — Effects of meteorological conditions 39 Chapter 8. — Effects of geologic features 41 Faulting 41 The Shawneetown fault in Illinois 41 Signal from broadcast station WILL 42 Signal from broadcast station KWK 42 Signal from broadcast station WJPF 43 Signal from radio range station AF 43 The Shawneetown fault in Kentucky 44 [5] Page Inman East fault 46 Cryptovolcanic structure near Kentland, Indiana 47 Geologic setting 47 Field hazards 49 Field intensity measurements 49 Field intensity contour maps 49 Signal intensity versus magnetic intensity 52 Signal intensity behavior 52 Depth to bedrock 52 Dome structure with suspected igneous origin 53 Geologic setting 53 Field hazards 54 Field intensity measurements 55 Ore bodies 55 Geologic setting 55 Ore deposits 56 Areas of working and abandoned mines 56 Prospective ore-bearing areas 56 Areas of newly discovered ore bodies 57 Underground mined-out areas 59 Geologic setting 60 Truax-Traer coal mines 60 Re-examination of B. & VV. mine 61 Soils 62 Soil influence on signal strength 63 Bedrock valleys and depth to bedrock 64 Nonglaciated areas 64 Glaciated areas 65 Chapter 9. — Summary and conclusions 67 Appendix A. — FCC ground conductivity map of the U.S 71 Appendix B. — Glossary of radio terms 72 ILLUSTRATIONS Figure Page 1. Field intensity contour map near Homewood, Illinois 20 2. Circuit of field intensity meter 21 3. Wooden- bodied station wagon with radio field intensity measuring equipment 21 4. Operator in working position 22 5. Signal intensity record 25 6. Signal intensity record 25 7. Four-hour record of WLW illustrating sky-wave 26 8. Record of WGN illustrating ground-wave 26 9. Location of broadcast transmitters — distance from area of traverse in area I 29 10. Natural and cultural features along line of traverse 29 11. Signal intensity curves of signals arriving from the northeast 30 12. Signal intensity curves of signals arriving from the southeast 30 13. Signal intensity curves of signals arriving from the southwest 31 14. Signal intensity curves of signals arriving from the northwest and southeast 31 15. Natural and cultural features along line of traverse in area II 32 [6] Page 16. Natural and cultural features along line of traverse in area III 33 17. Signal intensity curve near Harrisburg, Illinois 34 18. Topographic map of Shawneetown Hills area 35 19. Signal intensity curve illustrating shadow effect of interposed hills 36 20. Signal intensity curve run across Shawneetown Hills 37 21. Natural and cultural features along line of traverse in area IV 38 22. Map of traverse across Shawneetown fault in Gallatin County, Illinois 42 23. Signal intensity curve recorded across the Shawneetown fault (Sta. WILL) 43 24. Signal intensity curve recorded across the Shawneetown fault (Sta. KWK) 43 25. Signal intensity curve recorded across the Shawneetown fault (Sta. WJPF) 44 26. Signal intensity curve recorded across the Shawneetown fault (Radio Range Sta. AF) .... 44 27. Topographic map of the Shawneetown area 45 28. Field intensity curve recorded across the Shawneetown fault in Ohio River bottoms in Kentucky . 46 29. Field intensity contour map near Kentland, Indiana (Sta. WIND) 48 30. Field intensity contour map of the Kentland, Indiana, area (Sta. WAAF) 50 31. Topographic map of the Hicks dome area, Hardin County, Illinois 51 32. Geologic map and cross section of the center of Hicks dome, Hardin County, Illinois 53 33. Signal intensity curve recorded across Hicks dome 54 34. Signal intensity curve recorded over a rich iron deposit in the Galena, Illinois, area 57 35. Signal intensity curve recorded over the Kittoe ore body (Sta. WMAQ) 58 36. Signal intensity curve recorded over the Kittoe ore body (Radio Range Sta. CHI) 59 37. Map of traverse across mined-out area, Gallatin County, Illinois 59 38. Signal intensity curve recorded across mined-out area, Gallatin County, Illinois 61 39. Map of traverse between Clinton and Springfield, Illinois 62 40. Signal intensity curve recorded across various soil types near Kenny, DeWitt County, Illinois . . 63 41. FCC map of ground conductivity in the United States 69 [7] Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/geologicaspectso162pull GEOLOGIC ASPECTS OF RADIO WAVE TRANSMISSION M. WILLIAM PULLEN CHAPTER 1 — INTRODUCTION RADIO WAVES AND GEOLOGY THE POPULAR AND SCIENTIFIC appeal of electronics has stimulated the use of radio methods of exploration in the search for natural resources in the earth's crust. Some of these methods seem to be imprac- tical, but others show promise of success. It appears, from previous work and this study, that radio waves penetrate bedrock and other earth materials, and their depth of penetration depends on the power and fre- quency of the waves and on the effective conductivity of the earth materials. Effective conductivity of earth materials to radio waves is controlled by the average value of conductivity and dielectric constant for the distance below the surface of the earth at which there are ground currents of appreciable amplitude. 1 Effective conduc- tivity (ground conductivity) of earth mate- rials to radio waves is determined by the resistance, 2 dielectric constant, 3 and magnetic permeability 4 of earth materials. 5 Dielectric hysteresis 6 (attenuation by a dielectric me- dium) may also be a factor in the attenuation of radio fields. Radio wave propagation is a highly com- plex process dependent on many variables. One of the most important variables is the conductivity of earth materials, which changes from place to place within relatively short distances. This conductivity largely determines the strength of a radio field as 1 Terman, F. E., Radio engineer's handbook: New York, McGraw-Hill, 1st ed., p. 708, 1943. 2 Idem., p. 674. 3 Standards of good engineering practice concerning stand- ard broadcast stations 550-1600 kc: Federal Communica- tions Commission, Washington, D.C., U. S. Govt. Printing Office, pp. 33-34, 1940. 4 Smith, Woodrow, Antenna manual : Santa Barbara, Edi- tors and Engineers, Ltd., p. 149, 1948. 5 See Glossary of Radio Terms : Appendix B. 6 Skilling, H. H., Fundamentals of electric waves: New York, John Wiley, 2nd ed., p. 149, 1948. measured in air at a distance from a trans- mitter. 7 It is the influence of earth materials on radio field intensity which is of interest to the geologist. The present investigation is concerned with the relationship between radio fields and earth materials. If geologic features such as folding, faulting, and abrupt lithologic changes present electrical discontinuities (changes in effective conductivity and dielec- tric constant) which influence the behavior of radio fields measured in air at the earth's surface, recognition of such behavior through field strength measurements would provide a means of mapping these geologic features. If significant field strength anomalies are found in areas of known geologic features, and can be correlated with them, it would appear that unknown geologic features might be interpreted from measurements of field strength. Because radio waves at broadcast frequen- cies (550-1600 kc.) are readily available and are known from previous work to be influ- enced by earth materials, they were employed throughout most of the investigation. They are propagated primarily by ground-wave and by sky-wave. Ground-wave intensity at a given distance from a transmitter, at these frequencies, is relatively constant and prac- tically all daytime propagation is possible only by this means. Sky-wave fluctuates in intensity almost continuously. Sky-wave propagation, at broadcast frequencies, is op- erative only at night, such propagation during daytime being theoretically impos- sible. 8 Therefore, field intensity measure- ments were restricted to daytime ground- wave signals for the present study. 7 Terman, op. cit., pp. 708-709. 8 Electronics Engineers of the Westinghouse Electric Cor- poration, Industrial electronics reference book: New York, John Wiley, p. 337, 1948. [9] 10 RADIO WAVE TRANSMISSION Attenuation measurements on diamond drill cores were made in the laboratory, but laboratory conditions are so different from those of the field that the measurements were possibly only indicative of electromag- netic conductivities of rock cores. Early measurements in the field made with prim- itive equipment suggested that certain radio fields were influenced by specific geologic features; later more elaborate instrumenta- tion indicated in much greater detail the influence of natural and cultural features as well as geologic conditions. Many influ- encing features other than geologic were therefore investigated, and it was found possible to recognize and to separate in some instances the influence of geologic features on radio field intensity. OBJECTIVES Striking similarity between a ground con- ductivity map of the United States (fig. 41 ) , published by the Federal Communications Commission in 1938, 9 and the U. S. Geolog- ical Survey geologic map of the United States 10 was the impetus for this investiga- tion. A search for the explanation of this similarity led to the study of irregular and unpredictable radio reception. The major objectives were to collect data on field intensity of transmitted radio waves, and to determine what influence geologic conditions and earth materials have on field strength. To attain these objectives, it was necessary to develop suitable instrumenta- tion and field techniques. Instruments were needed that would be compact and portable, yet rugged enough to withstand field opera- tion, and that would give reliable continuous measurements. Field techniques had to be developed that would permit rapid and reli- able field intensity measurements. Field intensity anomalies caused by specific geo- logic features needed to be examined to ascertain if there were optimum frequencies, powers, orientations, and distances that would provide the strongest or most readily identifiable signal anomaly. 9 FCC, Standards of good engineering practice, op. cit., pp. 33-34. 10 Geologic map of the United States: U. S. Geol. Survey, 1932. Before the influence of earth materials on radio field strength could be determined, it was necessary to be able to recognize other factors that affect field strength. Therefore, a secondary objective of the work became the recognition of the other factors as hazards. The following features and factors were tentatively considered potential field hazards : A. Cultural 1. Wire fences 2. Electric power, telephone, and other transmission lines 3. Pipe lines 4. Bridges 5. Buildings and towers 6. Road materials (concrete, black- top, gravel, and dirt) 7. Stability of power output at the radio transmitter B. Natural 1. Trees and other vegetation 2. Bodies of water — lakes, ponds, and streams 3. Topography C. Meteorological 1. Barometric pressure 2. Wind velocity and direction 3. Sunlight and cloudiness 4. Temperature 5. Humidity 6. Magnetic storms 7. Precipitation 8. Natural electromagnetic phenom- ena The following types of geologic conditions were selected for investigation : 1. Bedrock with faulting 2. Bedrock with folding 3. Cryptovolcanic structure 4. Variation in soil types 5. Variation in bedrock lithology 6. Variation in depth to uniform bedrock 7. Buried glacial drift-filled valleys 8. Ore deposits This investigation was experimental, from a geological point of view. It is hoped that the data will contribute to a better practical understanding, and to the eventual theoret- ical understanding, of the phenomena in- volved in anomalous radio reception. INTRODUCTION Jl SCOPE Measuring and recording equipment was tested experimentally to achieve satisfactory instrumentation. Various methods of analysis and presentation of the combined electro- magnetic and geologic field data were tried before a satisfactory system was worked out. Several hundred miles of traverses were selected along which radio field intensities were measured and recorded. The traverses were run mainly in Illinois, but to clarify the picture, some were run in Kentucky, Indiana, and Wisconsin. A laboratory investigation of transmission of radio signals through, or along, diamond drill cores of different lithologies was made in an attempt to determine the transmission or attenuation behavior of the cores. In the field, various man-made and natural features were examined for their in- fluence on radio field intensity. The value and character of their influence depends on their orientation with respect to the direction of signal arrival and the point of measure- ment, and on the frequency, power, and dis- tance to the radio station being monitored. With empirical data on the value and char- acter of signal anomalies caused by these features, it became possible to investigate and evaluate the influence of geology on field in- tensity. Traverses could then be run meas- uring radio field intensities across areas of known geology, both in the presence and the absence of obvious natural and man- made field hazards. The present work sets forth experimental data that illustrate many of the factors and features affecting radio field intensity at broadcast frequencies. The report evaluates the use of field intensity measurements as an aid in geologic exploration and offers a new concept of the methods of transmission of radio waves through earth materials. ACKNOWLEDGMENTS This work was part of a research investi- gation program of the Division of Ground- water Geology and Geophysical Exploration of the Illinois Geological Survey. The writer gratefully acknowledges the active interest and support of the investigation by M. M. Leighton, Chief of the Survey. For encouragement to undertake the work, the writer is indebted to Carl A. Bays, formerly Geologist and Engineer and Head of the Division of Groundwater Geology and Geophysical Exploration of the Illinois Geological Survey; Ernest P. Du Bois, Geologist in the Coal Division, Illinois Survey; Stewart Folk, former Associate Geologist in the Oil and Gas Division of the Illinois Survey; to Harold R. Wanless, Professor of Geology at the University of Illinois; to A. James Ebel, former Assistant Professor of Electrical Engineering at the University of Illinois; and to R. D. Car- michael, Dean Emeritus of the Graduate School of the University of Illinois. For advice and guidance, the writer is especially indebted to Carl A. Bays and H. R. Wanless, J. R. Sommers, Robert Floyd, Stanley Snow, and Robert Mann, all elec- trical engineers with Carl A. Bays and As- sociates, Inc., aided with advice on instru- ment operation and maintenance and in interpretation of signal intensity anomalies caused by or related to factors other than geologic conditions. Appreciation is also extended to M. B. Buhle, R. D. Knodle, Jack Wolf, and Ben Ellis, of the Illinois Geological Survey, who drove many hundreds of miles on field intensity traverses and assisted in instru- ment maintenance and field operations. CHAPTER 2 — THEORETICAL CONCEPTS OF PROPAGATION RADIO WAVES Radio waves (electromagnetic waves) are subject to the same laws as light waves in regard to reflection, refraction, diffrac- tion, polarization, interference, and speed of propagation. A radio wave transmitted from a nondirectional antenna at approximately 186,000 miles per second spreads out and travels in all directions. There appear to be two boundaries for this spreading wave, the surface of the earth and some ionized layers that are about 30 to 250 miles above the earth's surface. The behavior of the wave depends upon its frequency. If the frequency is higher than approximately 30 mc, the wave may pass through the ionosphere and travel into space beyond, while in the vicinity of the antenna at the earth's surface, much of it will be rapidly absorbed and attenuated by earth materials. If the frequency of the wave is lower than approximately 30 mc, it may be reflected earthward by the ionosphere, while in the vicinity of the antenna it will follow the earth's surface for some distance before becoming absorbed or attenuated by earth materials. The part of the wave that follows the earth's surface is called surface or ground- wave. The remainder of the wave is called the sky-wave or space-wave. According to Brainerd et al.: 1 The ground wave is usually further subdivided into a direct wave, a wave reflected from the ground (of importance when the receiving an- tenna is well above the ground) and a surface or guided wave. The ground wave is usually re- fracted in passing through the lower atmosphere, and this combined with the guiding effect which exists (the earth may act as a wave guide some- what as one wire of a transmission line does) tends to cause the ground wave to follow the curvature of the earth when the frequency is not too great. But the ground wave often suffers se- vere attenuation, so that it cannot account for long-distance transmission except at relatively- low frequencies. 1 Brainerd, J. G., ed., Ultra-high-frequency techniques: New York, D. Van Nostrand, pp. 436-437, 1946. According to Laport: 2 In free space devoid of all substance, includ- ing air or gases, an electromagnetic wave is propagated without any dissipation of its energy. The inverse relationship between field strength and distance is due to the expansion of the wave in three dimensions and the distribution of radi- ant energy over a larger and larger volume of space, so that the power flow follows the inverse- squares law with respect to distance. However, ground-wave signal strength measured in microvolts per meter at a dis- tance from the antenna commonly differs from that calculated from the inverse pro- portion relationship. This is because the earth's surface, or ground, is not a perfect conductor but has resistance, or a finite con- ductivity, so that some of the field strength is absorbed or attenuated. INFLUENCE OF EARTH MATERIALS The influence of earth materials on radio wave propagation and reception has been recognized for many years. Theories have been formulated to account for the kind and amount of influence by earth materials, and field observations have been conducted to test these theories. Sommerfeld 3 considered ground-wave propagation and arrived at an empirical equation which expresses signal intensity as a function of power, distance, frequency, and earth conductivity. Byrne, 4 and later Higgy and Shipley, 5 made radio transmission surveys in Ohio and found that their measured field strengths agreed reason- ably well with field strengths predicted from Sommerfeld's equation. 2 Laport, Edmund A., Radio antenna engineering: New York, McGraw-Hill, p. 9, 1952. 3 Sommerfeld, A., The propagation of waves in wireless telegraphy: Annual of Physik, vol. 4, no. 28, p. 665, March 1909. 4 Byrne, J. F., Radio transmission characteristics of Ohio at broadcast frequencies: Ohio State Univ. Eng. Expt. Sta. Bull. 71, July 1932. 5 Higgy, R. C, and Shipley, E. D., Radio transmission survey of Ohio: Ohio State Univ. Eng. Expt. Sta. Bull. 92, May 1936. [12] PROPAGATION 13 GROUND-WAVE AT BROADCAST FREQUENCIES Broadcast frequencies range from 550 to 1600 kc. Sky-wave reception from broad- cast stations is not usually possible in the daytime, and as all measurements were taken in the daytime, they were necessarily meas- urements of the ground-wave. The normal range of ground-wave reception is from approximately 50 miles, at the higher fre- quencies, to more than 400 miles at lower frequencies. Terman 6 cites the work of Howe, who, assuming an average value for ground con- ductivity, concludes that radio waves pene- trate the earth's surface at least 20 feet at 10 mc and 45 feet at 1 mc. Therefore, ground-wave signals at broadcast frequencies and lower were used in the present investi- gation. WAVE-GUIDE TRANSMISSION AND PROPAGATION In addition to the ground-wave theory, another method of propagation, not yet gen- erally recognized as being particularly ap- plicable at broadcast frequencies, is by wave-guide. This theory is a generally ac- cepted explanation for transmission through hollow metal tubes at microwave frequen- cies. Wave-guide transmission may be thought of as transmission of electromagnetic waves in a dielectric medium bounded by one or more conducting planes. 7 Wave- guides, manufactured for use in ultra-high frequency transmission, are usually rectan- gular or circular in section. Propagation can also take place in the Z direction (trans- verse) between two roughly parallel planes having finite conductivity. The wave lengths of an electromagnetic field that can be transmitted through a wave-guide are limited by the physical di- mensions of the wave-guide itself. When 6 Terman, F. E., Radio engineers handbook: New York, McGraw-Hill, 1st ed., p. 698, 1943. 7 Sarbacher, R. I., and Edson, W. A., Hyper and ultra- high frequency engineering: New York, John Wiley, 1947. Skilling, H. H., Fundamentals of electric waves: New York, John Wiley, 2nd ed., 1948. Brainerd, op. cit., pp. 455-494. Ramo, Simon, and Whinnery, J. R., Fields and waves in modern radio: New York, John Wiley, pp. 292-295, 1947. wave lengths exceed the cut-off frequency dimension of the wave-guide, the waves are not transmitted along the guide. When the wave lengths are smaller than the cut-off frequency dimension of the wave-guide, the waves may be transmitted by one of several possible modes. Although much is known about the behavior and mechanics of guided waves at higher frequencies, there are not enough data to permit a description of the behavior of guided waves at all frequencies. Ramo and Whinnery 8 state: For any given set of planes with arbitrary fixed spacing, there should be some frequencies and some angles of reflection for which boundary conditions could be satisfied by a wave having a component of propagation in the Z direction. The wave-guide theory is applicable not only at very high frequencies but also at lower frequencies. At frequencies below a few hundred kilocycles the ionosphere can act as a good reflector of radio waves. 9 Since at these frequencies the earth is also a good reflector, one can consider the surface of the earth and the ionosphere as boundary conductors of a large parallel-plane wave- guide having an air dielectric. Transmission of low-frequency waves over large distances (thousands of miles) is possible by this mode of propagation. If the earth is considered as the floor of a wave-guide, the variable elec- trical conductivity of the floor will cause some energy to be attenuated, thereby caus- ing a change in field intensity. Theoretically, as the floor of the guide becomes lower in conductivity or as the frequency decreases, the wave penetrates deeper into the floor. GEOLOGIC WAVE-GUIDES Wave-guide propagation in bedrock strata may take place under certain geologic con- ditions. A wave-guiding system may be thought of as a dielectric region between two parallel conducting planes. In rocks, a dry, poorly conducting rock stratum may be considered as the more-or-less dielectric re- gion. If the rock strata above and below this dielectric region are porous and saturated with electrolyte, or have low electrical re- 8 Ramo and Whinnery, op cit., p. 294. 9 Jordan, E. C, Electromagnetic waves and radiating sys- tems : New York, Prentice-Hall, p. 662, 1950. 14 RADIO WAVE TRANSMISSION sistivities, as shales and clays, they could be considered as parallel conducting planes. Theoretically, in a wave-guide with the parallel planes perfect conductors and with an air dielectric, a uniform plane wave should propagate between the planes in a Z direction with a phase velocity equal to the velocity of light and with no attenuation. 10 In a geologic wave-guide, with poorly con- ducting strata as the dielectric and good 10 Ramo and Whinnery, op. cit. 292. conducting strata as the roughly parallel planes, similar wave propagation may be possible, but the waves would be subject to much attenuation (from losses in the con- ductors and dielectric), and the wave veloc- ities would be lower. Regardless of the concept of propagation, whether by guided wave or the so-called ground-wave, it is known that signal inten- sity is affected measurably by the earth ma- terials along the signal path. CHAPTER 3 — PREVIOUS WORK There is only a limited amount of litera- ture that deals directly with the relationship between transmitted radio fields and geo- logic conditions. However, there is consider- able collateral literature, dealing more or less indirectly with the subject, in the fields of radio, electronics, communication and propagation engineering, and physics. Bailey et al. 1 experimented with radio wave propagation using a frequency of 60 kc. Horizontal antenna lengths ranged from 14,000 to 17,000 feet. The effects of differ- ent earth materials beneath the antennas on directional characteristics were recognized. To the authors, variations in reception (and propagation) characteristics, at least in part, correlated with geologic formations, which they illustrated with cross sections of the rocks beneath their antennas. Eve et al. 2 attempted to demonstrate penetration of rocks by radio waves in the Mount Royal tunnel. Because the tunnel was open at both ends and traversed by rail- road tracks, their results were not considered conclusive. However, their data suggested that penetration is a function of frequency and that the higher frequencies were attenu- ated more than the lower frequencies. Sig- nals at broadcast frequencies and lower were detectable throughout the entire tunnel. Eve et al. 3 experimented with radio wave penetration of rocks in Mammoth Cave, Kentucky. This site was selected as the test- ing place because of its miles of underground passageways and rooms which contained no railroad tracks, wires, or other metallic con- ductors. Using various types of receiving an- tennas, signals from the surface at broadcast frequencies and lower were detected 150- 1 Bailey, A., Dean, S. W., and Wintringham, W. T., The receiving system for long-wave transatlantic radio telephony: Proc. Inst. Radio Eng., vol. 16, no. 12, pp. 1645-1705, De- cember 1928. 2 Eve, A. S., Steel, W. A., Olive, G. W., McEwan, A. R., and Thompson, J. H., Reception experiments in Mount Royal tunnel: Proc. Inst. Radio Eng., vol. 17, no. 2, pp. 347-376, February 1929. 3 Eve, A. S., Keys, D. A., and Lee, F. W., The penetra- tion of rock by electromagnetic waves and audio frequencies : Proc. Inst. Radio Eng., vol. 17, no. 11, pp. 2072-2074, No- vember 1929. Also, U. S. Bureau of Mines Tech. Paper 434, pp. 37-40, 1928. 350 feet below the surface. The overburden is composed of limestone and sandstone. Using an audio frequency of 500 cycles, signals were detected through 900 feet of rocks, suggesting again the increase in depth of penetration with decrease in fre- quency. Volker Fritsch 4 has written extensively on the influence of underground geology on transmitted radio fields. He has described and illustrated numerous geologic conditions that improve signal reception and others that weaken or prevent it entirely. He dem- onstrated that radio signals at various fre- quencies can be detected in tunnels and mines, and cites experiments by Lowy, who detected 700 meter signals at a depth of 1000 meters. Underground in mines at Kot- terbach, using frequencies of 300 meters (1000 kc) or greater, Fritsch correlated signal strength values with fractures, dip and strike of formations, and ore bodies. In a coal mine at Grunbach, situated in a synclinal structure, Fritsch found recep- tion very poor. In a mine at Ostrau (Mo- ravia), reception from surface stations was possible at depths of 400-500 meters because of the presence of a good geologic conductor which dips steeply (or vertically). Frac- tures, ore bodies, or other geologic struc- tures (conductors) favor reception if they 4 These articles by Volker Fritsch were translated and ab- stracted by Professor Ernst Cloos from inaccessible German papers for the Geological Society of America: Eineges iiber die Grundlagen der Funkmutung: Montan. Rundschau. Jahrg. 26, no. 4, pp. 1-6, 1934. Beitrage zur Radiogeologie: Beitr. angew. Geophysik., Bd. 5, H. 3, pp. 315-364, 1935. Beitrage zu den Beziehungen swischen Ausbreitung Hertz'scher Wellen und geologischer Beschaffenheit des Untergrundes (Funkgeologie). Grundlagen und Anwendung der Kapazitatsmethode: Beitr. angew. Geophysik. Bd. 5, H. 4, pp. 375-379, 1936; Bd. 6, H. 1, pp. 100-119, 1936. Beitrage zur Funkgeologie, III. Einiges iiber die Ausbrei- tung Hertz'scher Felder in Gebirgen : Beitr. angew. Geo- physik., Bd. 6, H. 3, pp. 277-306, 1937. Beitrage zur Funkgeologie, IV. Darstellung der Eingen- schaften geologischer Leiter: Beitr. angew. Geophysik., Bd. 6, H. 4, pp. 407-412, 1937. Beitrage zur Funkgeologie, VII. Einiges iiber die Ausbrei- tung elektromagnetischer Wellen in Bergwerkschacten und Stollen: Beitr. angew. Geophysik., Bd. 7, H. 4, pp. 449-461, 1939. Die funkgeologische JJntersuchung des Zinnobervorkom- mens von Schonbach bei Eger (Sudetenland) : Neues Jahrb. f. Geol. B., Vol. 84, H. 1, pp. 90-116, 1940. Messverfahren der Funkmutung: Munich, R. Oldenbourg, 1943. [15] 16 RADIO WAVE TRANSMISSION connect the receiver with the surface. Fritsch believes that changes in field inten- sities can be predicted over an area of known geologic conditions and, also, that observed changes in field intensities may lead to the discovery of unknown geologic conditions. Felegy and Coggeshall, 5 investigating the applicability of radio for emergency mine communications, successfully transmitted and received radio signals to and from the surface through intervening rocks. Ampli- tude-modulated radio transmission with voice modulation was used at frequencies from 33 to 220 kc running from 2-3 watts transmitter output power. Continuous two- way communication via rock strata (400 feet of sandstone, 150 feet of conglomerate, 30 feet of slate and clay, and a thin layer of surface soil) was maintained at the Re- liance Colliery, Mt. Carmel, Pennsylvania, at distances up to 1050 feet, and intermittent communication was possible up to 2040 feet. The above authors refer to the work of Wadley, 6 who claims to have transmitted signals through 5000-6000 feet of quartzite, using 500-foot linear antennas that touched nothing but air, both underground and at the surface. He used code signals from a 10 watt transmitter on frequencies between 100 and 300 kc. Ernst Cloos 7 published what is probably the first geological report in this country which recognizes definite geologic influence on behavior of field strength. Using crude equipment, he was able to map faults and steeply dipping contacts between different kinds of rocks in the Baltimore area by an audible decrease in signal strength or com- plete absence of signal near or over these features. Best results were obtained when using a 250-watt broadcast station on a fre- quency of 600 kc. He concluded that in an area of known geologic conditions, with recognition of intensity disturbances caused by overhead wires, railroad tracks, road 5 Felegy, E. W., and Coggeshall, E. J., Applicability of radio to emergency mine communications: U. S. Bureau of Mines Rept. Inv. 4294, May 1948. 6 Wadley, T. L. (Underground communication by radio in gold mines on the Witwatersrand). Suid-Afrikaanse Weten- skaplike En Nywerheidnavorsingsraad : Telekommunikasies Navorsinglaboratorium, Johannesburg, South Africa, T.R.L. 3, Nov. 1946. 7 Cloos, Ernst, Auto-radio — an aid in geologic mapping: Am. Jour. Sci., ser. 5, vol. 28, pp. 255-268. 1934. cuts, and the like, if the remaining inten- sity anomalies could be repeated over a peri- od of days, months, and years, they could be definitely correlated with the geologic con- ditions. i Spieker 8 recognized a strong correlation between a radio transmission map of Ohio 9 and the geologic map. The radio trans- mission investigation was made to determine the most economical and efficient communi- cations system that could be set up for use by the Ohio State Highway Patrol. On the radio transmission map the state was divided into zones classified as to effectiveness of transmission. Spieker observed that the area of best transmission was underlain generally by Ordovician, Silurian, and Devonian lime- stones; the second best area by Devonian and Mississippian shales; the third by Penn- sylvanian and Permian rocks of varied li- thology but with considerable sandstone ; the fourth and poorest area by thick Pleistocene deposits. From these observations Spieker con- cludes: 10 The generalization is obvious that radio trans- mission is affected by the texture of the rock im- mediately beneath the surface ; tight, solid rock affords the best conditions and loose, open-tex- tured materials the worst. This is supported by the fact that the values fall off notably as exist- ing river channels are crossed, due perhaps in part to the topographic deflection, but probably also to the alluvium in the valleys. Barret 11 was granted a United States patent wherein he claims the ability to make use of electromagnetic waves for acquiring useful subsurface geologic information. He describes suitable apparatus and techniques for determining the location and character of hidden geologic faults, for locating and defining buried masses such as salt domes and igneous plugs, and for locating and de- fining electrical discontinuities in buried strata. s Spieker, F. M., Radio transmission and geology: Bull. Am. Assoc. Petr. Geol., vol. 20, no. 8, pp. 1123-1124, August 1936. 9 Higgy, R. C, and Shipley, E. D., Radio transmission survey of Ohio: Ohio State Univ. Eng. Expt. Sta. Bull. 92, May 1936. 10 Spieker, op. cit., p 1124. 11 Barret, W. M., Electrical apparatus and method for geo- logical studies: U. S. Patent 2,172,688, 1939. PREVIOUS WORK 17 More recently, Barret 12 conducted a dem- onstration before a group of geophysicists, geologists, and other technical men to prove that radio waves may be transmitted to depth in the earth. The site was at the Morton Salt Company's Kleer mine at Grand Saline, Texas, where signals were received underground on a frequency of 1602 kc from a transmitter on the surface 1200 feet away. Electric and telephone lines were cut and grounded at the top and bot- tom of the shaft, and pipes and the like were also grounded. The receiver was lo- cated in an abandoned part of the mine which was free from metal and separated from the shaft by 1800 feet of circuitous tunnels. Code signals from the portable transmitter at the surface apparently trav- eled through some 700 feet of sedimentary rocks before they were picked up by the receiver. Howell 13 conducted field intensity investi- gations in faulted areas of California and New Jersey. He found that a decrease in intensity occurred above some faults in addi- tion to a possible change in the direction of the field. He observed, like Cloos, that rel- atively weak electromagnetic fields seem to be more strongly influenced by geologic con- ditions than strong fields. Blackburn 14 investigated field intensity variations in areas of known geologic con- ditions and concluded that field variations reflect geologic conditions. He claims to have used his "radiographic" method in commer- cial work in the United States and Canada. He ran continuous traverses and recorded field measurements on a graphic recorder of the Esterline-Angus type. Kerwin, 15 at Massachusetts Institute of Technology, reviewed the literature and concluded that geologic mapping based on observation of field intensity variations should be practical. Supported by a grant 12 Barret, W. M., Salt mine test proves earth penetration by radio waves: World Petroleum, vol. 20, no. 3, pp. 62-63, March 1949. 13 Howell, B. F., Jr., Some effects of geologic structure on radio reception: Geophysics, vol. 8, no. 2, pp. 165-176, April 1943. 14 Blackburn, M. S., Radiographic method of geophysical exploration: World Oil, vol. 126, no. 11, August 11, 1947. 15 Kerwin, Larkin, Use of the broadcast band in geologic mapping: Jour. Applied Physics, vol. 18, no. 4, pp. 407-413, April 1947. from the Geological Society of America, he designed suitable field equipment and con- ducted several successful preliminary investi- gations of known geologic situations. He made continuous surveys and recorded the measurements graphically. He found that field intensity decreased over a basic dike with an electrical resistivity lower than that of the surrounding conglomerate, but in- creased over a dike with a resistivity higher than that of the surrounding rocks. Mcllwain and Wheeler 16 presented a paper at the technical session of the 1948 National Convention of the Institute of Radio Engineers which is available only in abstract form. A theoretical and experimental study of the propagation of radio waves through ground has resolved certain inconsistencies in prior work. Tests covered depths to several hundred feet and frequencies from 0.6 to 1.000 mc. As expected, dry ground is better than wet. At lower frequencies, ground behaves as a homogeneous, poorly con- ducting medium ; at the higher, the rate of at- tenuation increases much more rapidly, indicat- ing pockets of moisture separated by dry ground. A special technique has been used to test the horizontal propagation through substrata, which is especially useful to detect and trace dry layers, sandwiched between wet layers. The results show the limitations of radio waves for deep geophys- ical prospecting, though they may be useful for related exploration. Haycock, Madsen, and Hurst 17 investi- gated propagation characteristics of electro- magnetic waves in earth and through rocks, to evaluate the possibility of using radar methods and techniques to determine geo- logic discontinuities within the earth. Velocity, attenuation, and frequency of electromagnetic waves in earth materials were measured experimentally in the field. From standing wave measurements, the wave length and velocity of propagation in the earth materials used were calculated to be about one-tenth of that in the air. At- tenuation measurements made with trans- mission lines and antennas buried in soil indicated 7.5 db per 100 feet at 350 kc, 11.7 db per 100 feet at 600 kc, and about 62 db per 100 feet at 5 mc. The authors success- 10 Mcllwain, Knox, and Wheeler, H. A., The propagation of radio waves through the ground: Proc. Inst. Radio Eng., vol. 36, no. 3, p. 377, March 1948. 17 Haycock, O. C, Madsen, E. C, and Hurst, S. R., Propagation of electromagnetic waves in earth : Geophysics, vol. 14, no. 2, pp. 162-171, April 1949. 18 RADIO WAVE TRANSMISSION fully demonstrated penetration of 400 feet of overburden by radio waves in mine-tunnel tests; frequencies between 300 and 1000 kc are apparently best suited for such through- the-earth propagation. The authors conclude that, because of the apparent short propagation distances possible in earth materials as compared with the far greater distances possible in radar work, and because of directional antenna limitations at frequencies between 300 and 1000 kc, radar techniques for location of under- ground discontinuities appear to be inade- quate. It seems apparent, from a review of previous work, that there is some relation- ship between observed variation in field in- tensity and surface and subsurface geologic conditions. There is, however, a wide di- vergence of opinion as to the exact nature of the relationship and of the mechanics and phenomena involved. And there has been no systematic investigation described, and no extensive treatment of the subject, from the geological point of view. CHAPTER 4 — EQUIPMENT FOR RADIO FIELD INTENSITY MEASUREMENTS PREVIOUS WORK Instrumentation for measurement of sig- nal intensity progressed from the simple scheme of Cloos, 1 who used a 1933 Majestic automobile radio and loud speaker, to the more elaborate equipment of Kerwin, 2 who used a radio direction finder with a shielded loop antenna. Kerwin measured signal strength in the intermediate frequency (I.F.) stage of his receiver with a Vomax vacuum-tube volt-meter and recorded it on an Esterline-Angus continuous recording milliammeter. Felegy and Coggeshall 3 used conventional 6-tube amplitude modulation superhetero- dyne receivers with a frequency coverage from 80-175 kc in one band. Their trans- mitters had two stages (oscillator and ampli- fier), were amplitude-modulated, and had a power output of 2-4 watts, depending upon the impedance match obtained between the transmitters and the radiating material. Howell 4 operated at broadcast frequencies using a portable direction finder with loop antennas for determining direction of signal arrival, and a portable field strength meter (a tuned radio-frequency receiver with a nondirectional antenna) for determining variations in field intensity. Blackburn 5 used a small Hallicrafter com- munications receiver at broadcast frequen- cies and recorded signal strength continuous- ly with an Esterline-Angus recorder actuat- ed by a speedometer cable drive. 1 Cloos, Ernst, Auto-radio — an aid in geologic mapping: Am. Jour. Sci., ser. 5, vol. 28, pp. 255-268, 1934. 2 Kerwin, Larkin, Use of the broadcast band in geologic mapping: Jour. Applied Physics, vol. 18, no. 4, pp. 407-413 April 1947. 3 Felegy, E. W., and Coggeshall, E. J., Applicability of radio to emergency mine communications: U. S. Bureau of Mines Rept. Inv. 4294, May 1948. 4 Howell, B. F., Jr., Some effects of geologic structure on radio reception: Geophysics, vol. 8, no. 2, pp. 165-176, April 1943. 6 Blackburn, M. S., Radiographic method of geophysical exploration: World Oil, vol. 126, no. 11, August 11, 1947. PRESENT WORK Several systems of measuring signal strength were used early in the present work. These included equipment loaned by the University of Illinois broadcast station, WILL, and field intensity meters construct- ed in the Illinois Geological Survey labora- tory. Instrumentation for Reconnaissance Investigation The field intensity contour map (fig. 1) was made from measurements taken with an Illinois Survey laboratory-constructed field intensity meter designed for use in conjunc- tion with automobile and battery-portable radio receivers (fig. 2). The Dixmoor water well (see fig. 1), owned by the village of Homewood, Illinois, produces water from porous Niagaran reef rock at depths from 92-104 feet and a crevice zone from 195-201 feet, as indicated by a geophysical log of the hole. The village needed a larger water sup- ply and started a test hole approximately one mile west of the Dixmoor well hoping to encounter water-bearing reef rock. The field intensity traverse was run as the well was being drilled, to ascertain if such a sur- vey, in advance of drilling, might not indi- cate the areal extent of the reef. Intensity values in the vicinity of the Dixmoor well are between 20 and 30 microamperes. It is possible that the area with values of 40 microamperes and less indicates part of the areal extent of the reef because reef rock was encountered in the test hole. The areas to the south and west have vastly different intensity values, perhaps indicative of strata other than reef rock. The entire area was resurveyed two weeks after the initial sur- vey, using different equipment, but of the same type, and signal values were essentially duplicated. [19] 20 RADIO WAVE TRANSMISSION WJOB 1230 kc 250 w,l5mi. T. 36 N. R. 13, 14 E. ILLINOIS STATE GEOLOGICAL SURVEY FIG. . — Field intensity contour map near Homewood, Illinois (based on spot readings with auto- radio intensity meter). The validity of intensity measurements made with this type of equipment is condi- tioned by spot readings, field hazards, the personal element in tuning in a signal for maximum intensity, reading and recording the value, and the height of the receiving antenna above the earth's surface. However, in spite of the opportunity for error, inten- sity measurements with this equipment showed anomalies where they might be ex- pected, many of which were supported later through rechecks employing more elaborate instruments. Instrumentation for Detailed Investigation The chief component of the ultimate in- strumentation used in the present work is the commercial field intensity meter, type 308-B, built by the Radio Corporation of America. This instrument is shock-mounted on aircraft-type Lord mounts on a small table fastened to the wooden floor of a wooden-bodied station wagon (figs. 3 and 4). The 308-B is a compact, fairly rugged precision instrument, easy to operate, and it covers a frequency range from 120 kc to 18 mc in six bands using three separate rotat- able shielded loop antennas. The power supply for the 308-B meter is an RCA-type 93-A vibrator unit with a nonspillable 6-volt storage battery, and a shielded cable for carrying voltage to the meter. This unit is fastened to the floor be- neath the table. An auxiliary 6-volt storage battery is connected in parallel with that of the 93-A battery to prolong battery life and permit longer intervals of operation. The recorder, an Esterline-Angus model A.W. with a 10-milliampere movement, is similarly shock-mounted on a small table fastened to the floor of the vehicle (fig. 4). A glass pen traces the field intensity record on a paper chart driven past it at a constant speed (one of several speeds available from a spring-drive mechanism). However, on traverses, the chart is actuated by a Clark recorder drive, model 102-A (fig. 4). The complete Clark recorder equipment includes a recorder drive, a speedometer tee for tying- in to the car speedometer, and intercon- necting flexible drive cables. With this ar- rangement, the vertical scale of the chart is directly proportional to the mileage of the traverse as registered on the car speedometer. In addition to the signal intensity recording pen of the recorder, there are two side-mark- ing chronograph pens. EQUIPMENT 21 Xl 1*1 <• COIL & TO PLATE OF LAST IF. TUBE INED— L „ 2 MEG. TO RCVR. IF FREQ. * ILLINOIS STATE GEOLOGICAL SURVEY r ♦ 1.4V. _45V. *45V Fig. 2. — Circuit of field intensity meter designed for use with automobile and battery portable radios. Fig. 3. — Wooden-bodied station wagon with permanently mounted radio field intensity measuring equipment. 22 RADIO WAVE TRANSMISSION p IG . 4_Operator in working position. The 308-B field intensity meter (center) ; Esterline-Angus recorder and Clark speedometer drive (left). Instrumentation for reliable continuous measurement of field intensity, automatic- ally recorded, is essential in undertaking a comprehensive investigation of the influence of earth materials on radio wave transmis- sion. These conditions are adequately met with the RCA 308-B field intensity meter, the Esterline-Angus recorder, and the Clark speedometer-actuated recorder drive. Instrumentation in Mobile Operation The instruments were permanently mounted in an International station wagon with a wooden body. This vehicle was chosen on the basis of tests in which measure- ments were taken from inside the vehicle, from the tailgate of the vehicle, and from 10 to 50 feet away from the vehicle. The measurements indicated differences insuffi- cient to warrant their being taken from outside the vehicle. Dewitt and Omberg 6 6 Dewitt, J. H., Jr., and Omberg, A. C, The relation of the carrying car to the accuracy of portable field intensity measuring equipment: Proc. Inst. Radio Eng., vol. 27, no. 1, pp. 1-4, January 1939. investigated the accuracy of portable meas- uring equipment and found that radio fre- quency fields are distorted in the vicinity of metal-bodied cars due to a secondary field resulting from eddy currents. They found that the wooden-bodied station wagon is almost completely free from field distortion. Instrumentation and Investigation in the Laboratory There has been comparatively little lab- oratory investigation on the behavior of elec- tromagnetic waves in rocks. This may be due in part to the apparent lack of economic application and to the difficulty of simulat- ing field conditions. Wheeler 7 made some laboratory investigations on the dielectric constant and the A.C. and D.C. conduc- tivity of oil sands. His instrumentation con- sisted of a radio frequency generator, a radio frequency bridge, and a communications- type receiver. Frequencies employed ranged from 1 to 30 mc. Although his results were 7 Wheeler, R. T., The dielectric properties of oil sands : Petr. Engineer, vol. 19, no. 9, pp. 141-154, June 1948. EQUIPMENT 23 far from complete, he concluded that the dielectric constant decreases rapidly with increase in frequency above 1 mc, and salt water in a sand is the chief factor in rais- ing its dielectric constant. At the Illinois Geological Survey labora- tory, diamond drill cores were used in radio wave attenuation tests at frequencies rang- ing from 100 kc to 18 mc. Radio signals were transmitted through, or along, dia- mond drill cores of different lithologies to investigate the attenuation of signals trans- mitted through rocks. The rock cores were used as transmis- sion lines connecting a signal generator with a field intensity measuring receiver. Curves were drawn for the transmission ability of each core for frequencies between 100 kc and 18 mc. For control, transmission curves were drawn for air path and for direct coaxial connection between transmitter and receiver for the same range of frequencies. The results indicated that for certain fre- quencies all cores tested behaved as trans- mission lines yielding field intensities be- tween those obtained with direct connection and with air path. Admittedly, laboratory conditions differ considerably from those in the field ; thus the measurements, while possibly indicative of electromagnetic trans- mission in rocks, were not conclusive. CHAPTER 5 — RADIO FIELD INTENSITY MEASUREMENT FIELD INTENSITY AND LOOP ORIENTATION Signal intensities were measured in the field. Relative field intensity was measured, rather than actual field intensity, in micro- volts per meter. The chief interest is in sig- nificant changes of intensity ; actual intensity values were rarely measured because they require considerable additional instrument calibration and manipulation which slows the speed of the surveys. The shielded loop antenna of the 308-B meter is bidirectional. There are two places, 180 degrees apart through the complete 360 degrees of rotation, where signal in- tensity is at its maximum ; also two places, 180 degrees apart, where signal intensity is at a null, at a minimum, or absent. Most of the several hundred miles of recorded traverses were run keeping the loop man- ually oriented in the direction of maxi- mum signal intensity. This is not difficult to accomplish on straight roads following section or fractional section lines, but re- quires more attention on winding roads, especially in hilly terrain where a reference point on the horizon is difficult to maintain. Maximum signal intensity is indicated by the highest reading obtainable on the D.C. milliammeter of the 308-B meter. As the loop is rotated away from maximum signal orientation, there is a decrease in the D.C. milliampere readings. FIELD INTENSITY RECORDS Field intensity values, indicated by the D.C. milliammeter of the 308-B, were auto- matically recorded on the paper chart of the Esterline-Angus graphic recorder (fig. 4). Signal intensity is recorded on the chart as the chart is driven past the main recording pen (figs. 5 and 6). In addition to the main recording pen, there are two chronograph or marking pens on the Esterline-Angus. They are located near the right and left margins of the chart, are 6-volt D.C. solenoid-actuated, and manually controlled from switches under the front edge of the recorder table (fig. 4). The pens trace vertical lines parallel to the chart margins, but, when actuated, they mark a short line normal to the vertical trace. The pen at the right was used to ink marks to corre- spond with map reference points along a traverse. In figure 5, these marks, labeled distance mark, bear the same number (map reference station number) as numbered points one-half mile apart on a geographic base map. The pen at the left was used to ink marks to correspond with field strength anomalies caused by a readily identifiable field hazard, such as an overhead wire, a railroad track, or a bridge. The dashed lines connect points on the curve (signal strength anomalies) with their respective solenoid pen marks. When the marking pens are not used, the map reference points are indicated on the chart by the main recording pen in the form of an arc (fig. 6), by momentarily turning the selector switch of the 308-B to a calibrate position. The readily identifi- able signal strength anomalies are indicated on the chart with an X or a check mark, and labeled stream, bridge, oh for over- head wire, etc. (fig. 6). The telephone and electric wire conditions are noted on the left margin of the chart. This method of indicating identifiable signal strength anomalies, wire conditions, and map ref- erence points on the chart is faster and possibly more accurate than the solenoid marking-pen method, and is now employed wherever roads are not too rough. GROUND-WAVE VERSUS SKY-WAVE SIGNALS The Federal Communications Commission specified that in the broadcast band (550 to 1600 kc), primary service area means the area in which the ground-wave is not sub- [24] MEASUREMENT 25 S,GN *L INTENSITY INCRE ILLINOIS STATE OEOLOOICAL SURVEY *& Fig. 5. — Signal intensity record (taken from a chart recorded in the field) showing signal in- tensity curve, distance marks, map reference station numbers, and readily identifiable signal strength anomalies. ject to objectionable interference or objec- tionable fading. 1 Westinghouse engineers 2 describe ground-wave as follows: In the broadcast band, there is virtually no sky-wave propagation in the daytime. Therefore, primary coverage is determined by power, fre- quency, and ground constants. At night the ionosphere contributes to sky-wave propagation. If the radiating system radiates an appreciable amount of energy at high angles measured from the ground, the sky-wave is reflected by the ionosphere into a region having appreciable ground-wave coverage. There it adds with its characteristic varying and uncer- tain phase position to the ground-wave produc- ing fading. The sky-wave produces secondary coverage beyond the fading region, but it is relatively unreliable. Ground-wave propagation is good at the low- 1 F. C. C. Rules and Regulations, Rules governing stand- ard broadcast stations: Section 3.11, Rules in force as of March 1, 1940: U. S. Gcrvt. Printing Office, Washington, D.C.. 1940. 2 Electronics Engineers of the Westinghouse Electric Cor- poration, Industrial electronics reference book: New York, John Wiley, p. 337, 1948. *'•"««■ WTEHS.TY .HC****** ILLINOIS STATE SEOLOSICAL SURVEY Fig. 6. — Signal intensity record (taken from a chart recorded in the field) showing signal in- tensity curve, distance, map reference station numbers, readily identifiable signal strength anomalies, and wire conditions, (oh stands for overhead wire, x indicates its position on the curve.) frequency end of the broadcast band but dete- riorates near the high-frequency end of the band. . . . During the day coverage is completely by ground-wave. At night a region of ground-wave coverage is surrounded by a fading zone or at least a zone of inadequate signal. The fading zone is surrounded by a ring of sky-wave coverage. Signal intensity records of sky-wave prop- agation illustrate the characteristic fading and variance of signal level. An illustra- tive record (fig. 7) was made with the 308-B equipment at a fixed location, be- tween 9 p.m. and 1 a.m. local time. Signal intensity records were made in the daytime of ground-wave propagation with the 308-B equipment at a fixed location (fig. 8 is illus- trative of ground-wave field strength). 26 RADIO WAVE TRANSMISSION ILLINOIS STATE GEOLOGICAL SURVEY Fic. 7. — Four-hour record of WLW (Cincinnati, Ohio) 700 kc, 50,000 watts — illustrating sky- wave signal intensity fluctuation, recorded at a fixed location in Urbana, Illinois, 200 miles from Cincinnati. These curves illustrate the remarkably con- stant signal intensity usually encountered in ground-wave propagation within the pri- mary coverage area. CONSTANCY OF GROUND-WAVE INTENSITY The validity of the premise that influenc- ing features cause varying signal intensity depends upon the assumption that the signal being measured is fluctuating very little in strength at its site of emanation. Federal Communications rules and regulations speci- fy that each station shall be operated at all times as near to the authorized power as practicable. The operating power tolerance may be permitted to vary from 5 percent above to 10 percent below the authorized power for short periods. 3 The preceding 3 Standards of good engineering practice concerning stand- ard broadcast stations 550-1600 kc. : Federal Communi- cations Commission, U. S. Govt. Printing Office, Washington, D.C., p. 51, 1940. Fig. 8.— Record of WGN, 720 kc, 50,000 watts- illustrating ground-wave signal intensity, re- corded at a fixed location in Urbana, Illinois, 130 miles from Chicago. assumption was verified by many signal in- tensity measurements. Signal intensity was recorded, from a fixed location, for time intervals ranging from 5 minutes to 12 hours. The ground-wave intensity curves show slight changes during short intervals and somewhat larger changes over longer intervals. The variations in signal intensity as transmitted, for the stations monitored, are negligible in comparison with variations caused by influencing factors in the field. Thus it appears that an electromagnetic field radiating from a modern broadcasting station is sufficiently constant in intensity, in the ground-wave area, that significant MEASUREMENT 27 changes can be attributed to factors other than fluctuation at the transmitter. This was borne out by repeated runs over the same traverse, the field chart record being essentially duplicated on runs made weeks, months, and years apart. If the signal strength varied significantly at the transmit- ter, measured intensity curves over a trav- erse at various time intervals would not be reproducible. Modulation Effect With some types of field intensity meters, signal intensity fluctuates with the modu- lation of the carrier signal. Modulation varies with the type of program being trans- mitted. A symphony orchestra or singer running the gamut of tone, pitch, and vol- ume causes more irregular modulation levels than do the more quiet programs. The modulation effect is commonly most evi- dent when monitoring a station at a very short distance. In such instances a signal with little modulation effect should be selected. The modulation effect appeared occasion- ally using the reconnaissance-type field in- tensity meter (fig. 2) in conjunction with battery-portable and automobile radios. Kerwin describes modulation effect in con- nection with field intensity surveys in Massachusetts. 4 One of the disadvantages of using the broad- cast band with the present equipment here be- came apparent. The modulating voltage had some effect on the instantaneous readings, al- though not the average. Therefore readings extending over some period of time were neces- sary to obtain accurate values. The type of program being broadcast also has an effect as the modulation of musical programs is much smoother than those of the "soap-opera" type, and gave records which were much easier to interpret. Fortunately, the RCA 308-B field in- tensity meter is essentially free from the modulation effect. All recorded signal in- tensity curves made with the 308-B can be 4 Kerwin, Larkin, Use of the broadcast band in geologic mapping: Jour. Applied Physics, vol. 18, no. 4, pp. 407-413, April 1947. examined and interpreted without consid- eration of a modulation effect. OUTLINE OF FIELD PROCEDURE The procedure developed for field in- tensity surveying is as follows: — 1. Select an area for running traverse. 2. Locate area on geographic base map. 3. Assign map reference station num- bers at one quarter mile, one half mile, or other selected intervals. 4. Move measuring equipment to the area for a daytime (ground-wave) traverse. 5. Choose a standard broadcast station the primary service area of which in- cludes the area to be surveyed, and with preferred direction orientation to the area. 6. Select a broadcast signal that shows no modulation effect. 7. Calibrate the signal being measured so that the needle of the signal in- tensity meter points to the middle of the scale. This helps keep the needle on scale with signal intensity increases and decreases. 8. Connect (Esterline-Angus) recording meter to the field intensity meter (308-B). 9. Connect speedometer drive (Clark) to the recording meter. This pro- vides a vertical scale in miles, or frac- tions thereof, for the chart. 10. Start running the traverse and keep the shielded loop antenna oriented for maximum signal intensity at all times. 11. Keep running field notes on the chart, noting streams, bridges, wires, etc. 12. Mark points on the signal intensity curve that correspond to map refer- ence stations. This facilitates geo- graphic orientation of the chart. 13. Run the same traverse several times using different broadcast stations with different frequencies, powers, dis- tances, and directions of signal arrival. CHAPTER 6 — EFFECTS OF CULTURAL AND NATURAL FEATURES The following are selected examples of features, primarily other than geologic, which affect signal intensity. They are spe- cific features, representative of many ob- served in three years of field intensity sur- veying. WIRES AND STEEL BRIDGE Wires, because of their widespread dis- tribution in Illinois, are the single greatest hazard influencing radio field strength. From hundreds of miles of observation, it has been found that the closer the wires are to the ground, the smaller their influ- ence upon signal strength. Fences influence measurements of signal intensity less than telephone or electric wires strung on poles above the road. This condition may per- haps best be interpreted by the height rela- tionship of the wires to the shielded loop antenna of the 308-B field intensity meter. The loop antenna in operating position is six feet above the ground; if the height of the wires is the same or greater than that of the loop, their effect on field intensity may be large, but if below the height of the loop, their effect is usually smaller. The kind and amount of effect wires have on field intensity depend upon their orienta- tion with respect to the direction of arrival of the signal and to the geographic point of measurement. Three miles south of Gibson City, Illinois (Area I), a series of traverses was run between stations 61a and 62, to determine the effects of telephone wires and a steel bridge on broadcast signal intensity, with signals varying in power, frequency, direc- tion of arrival, and distance traveled (figs. 9 and 10). The power of the broadcast transmitters ranges from 250 to 50,000 watts, the frequencies 550 to 1580 kc, the distance 21 to 160 miles, and directions of arrival are widely separated (fig. 9). The 1980-foot traverses were run from south to north on a two-lane concrete highway (Illi- nois 47) which runs across an essentially flat plain ; both electric and telephone wires parallel the highway, which crosses a steel bridge (fig. 10). Description of Area I The topography is essentially that of a flat ground moraine. Approximately 200 feet of glacial drift overlies several hundred feet of Devonian and Silurian limestone and dolomite. Because the bedrock is just west of the local crest of the LaSalle anti- cline it probably dips slightly to the west and south. Because of the more or less uni- form geologic structure of the area investi- gated and the readily apparent field hazards (wires and bridge), recorded signal strength anomalies may be attributed chiefly to those field hazards rather than to any geologic component. In all the recorded curves the effect of the steel bridge greatly reduces or eliminates the signals (figs. 11-14). Signals arriving from the south decrease in intensity be- neath the overhead telephone wire (fig. 10), but increase immediately in front (south) of the bridge (figs. 12 and 13). The steel bridge appears to act as a transmitting ele- ment and reradiates the signal in the direc- tion of travel. This may be the source of the intensity peaks north of the bridge (figs. 12 and 13). A greater number of peaks occur at high frequencies than at low fre- quencies. Station KSD at 550 kc (fig. 13A) shows one intensity peak north of the bridge ; WDZ at 1050 kc (fig. 12B) shows three peaks; WKID at 1580 kc (fig. 12D) shows four intensity peaks. Signals measured arriving from the north increase in strength at the north end of the bridge and beneath the overhead telephone wire (fig. 11). The intensity increase un- der the telephone wire may be caused by re- radiation from the metal bridge. [28] CULTURAL AND NATURAL FEATURES 29 lOOmiles ILLINOIS STATE SEOL05ICAL SURVEY Fig. 9. — Location of broadcast transmitters — dis- tance from area of traverse (Area I, 3 miles south of Gibson City, Illinois) and directions of signal arrival. Signals arriving from the west do not increase in intensity at either the north or south end of the bridge. They decrease un- der the telephone wire and near a steel guy wire on an REA electric pole west of the road (fig. 14 A, C). The curves of signals arriving from the west are smoother and have fewer peaks north and south of the bridge than the curves of signals arriving from the north or south. The bridge struc- ture is probably reradiating signals arriving from the west along the direction in which they are traveling (east) so that they are not in evidence north or south of the bridge along the line of traverse. One signal from the southeast (fig. 14D) has enough southerly component to make the curve resemble the other signals from the south. However, it increases slightly under the telephone wire, thus be- having more like the signals from the north. Summary. — Interpretation of signal in- tensity curves recorded between stations 61a and 62 suggests the following conclusions : — Signals arriving from a southerly direc- tion show : 1. Reduced intensity under the tele- phone wire crossing the road. 2. Increased intensity just south of the metal bridge structure. 3. Intensity greatly reduced or absent on the bridge. 4. One, two, three, or four intensity peaks north of the bridge, depending upon frequency. The lower the fre- quency the fewer the peaks, and the higher the frequency the greater the number of peaks. Signals arriving from a northerly direc- tion show: 1. Increased intensity just north of the metal bridge structure. 2. Increased intensity under the tele- phone wire crossing the road. 3. Intensity greatly reduced or absent on the bridge. Signals arriving from a westerly direc- tion show: 1. Intensity greatly reduced or absent on the bridge. 2. Reduced intensity under the telephone wire crossing the road. 3. Reduced intensity near a steel guy wire on an REA electric pole west of the road. In addition to the influence of wires and the steel bridge on signal intensities, the influence of other cultural and natural fea- tures was investigated in other areas. Their effects on radio fields were carefully ana- lyzed in a manner similar to that described in the foregoing section. The following sections summarize the effects of these other features but detailed analyses are omitted. The features described are supported by observations of similar features along many hundreds of miles of traverses. Rood 2 strond electric R.E.A. line Steel bridge over a stream Telephone wire crosses the road overhead 8 strand telephone line Map reference station number ILLINOIS STATE 6E0L00ICAL SURVEY Fig. 10. — Natural and cultural features along line of traverse (Area I, 3 miles south of Gibson City, Illinois). 30 RADIO WAVE TRANSMISSION ILLINOIS STATE SECL06ICAL SURVEY Fig. 11.— Signal intensity curves, recorded along traverse (Area I, 3 miles south of Gibson City, Illi- nois), of signals arriving from the northeast. ILLINOIS STATE GEOLOGICAL SURVEY Fig. 12. — Signal intensity curves, recorded along traverse (Area I, 3 miles south of Gibson City, Illi- nois), of signals arriving from the southeast. CULTURAL AND NATURAL FEATURES 31 ILLINOIS STATE GEOLOGICAL SURVEY Fig. 13. — Signal intensity curves, recorded along traverse (Area I, 3 miles south of Gibson City, Illi- nois), of signals arriving from the southwest. ILLINOIS STATE 6E0L06ICAL SURVEY Fig. 14. — Signal intensity curves, recorded along traverse (Area I, 3 miles south of Gibson City, Illi- nois), of signals arriving from the northwest and southeast. 32 RADIO WAVE TRANSMISSION WIRE FENCES Description of Area II In Area II, five miles west of Gibson City, Illinois (fig. 15), the topographic re- lief is approximately 80 feet. Signals from seven broadcast stations were measured while traverses were run across the area. Signal arrival directions were roughly north, east, south, and west. Signal frequencies ranged from 580 to 1580 kc, distances from 24 to 160 miles, and powers from 250 to 50,000 watts. The line of traverse (4000 feet) is on the south slope of the Normal recessional moraine. 1 Thickness of the glacial drift ranges from 260 to about 320 feet, depend- ing on surface elevation. The underlying Pennsylvanian bedrock surface is of low relief and lies at approximately 540 feet above sea level. 2 The beds are Lower McLeansboro in age, and probably of uni- form sedimentary cyclic lithology. Here they lie about six miles west of the local crest of the LaSalle anticline and probably dip gently westward. The seven runs along this traverse were all made from south to north along a sec- ondary gravel road. A wire fence paralleled the road on the west side for the entire length of the traverse (fig. 15). At the north end of the traverse (on the west side) a farmyard was completely enclosed by a wire fence. A wire fence ran a short distance north from the south end of the traverse (on the east side of the road), and from the north end of the traverse a wire fence ran a short distance to the south. These fences on the east were joined at right angles by several east-west running wire fences. The fences were of two- and three-strand barbed wire, on both wooden and metal poles, and one fence was of coarsely woven rectangular wire net. The effects (if any) of the fence parallel- ing the traverse on the west are unknown because it was a constant factor throughout 1 Leighton, M. M., Ekblaw, G. E., and Horberg, Leland, Physiographic divisions of Illinois: Jour. Geol., vol. 56, no. 1, fig. 4, p. 22, January 1948; reprinted as Illinois Geol. Survey Rept. Inv. 129, 1948. 2 Horberg, Leland, Bedrock topography of Illinois: Illinois Geol. Survey Bull. 73, pi. 2, 1950. Map reference station number Road Iron windmill tower Wire fence 2 strand electric RE. A. line ILLINOIS STATE GEOLOGICAL SURVEY Fig. 15. — Natural and cultural features along line of traverse (Area II, 5 miles west of Gib- son City, Illinois). the entire traverse. A slight decrease in intensity was observed at the north end of the traverse at the fenced-in farmyard. The decrease may have been caused by the additional fence or by a metal windmill tower at the east side of the road. The effects of the fences on the east are slight. A fairly consistent, but small, de- crease in intensity was recorded near a fence on the east where another fence (oriented east-west) met it at right angles. It is pos- sible that these decreases may have resulted from reradiation by the fences in some out- of-phase relationship, or that the wire fence configuration was such that it absorbed or otherwise attenuated part of the radiated field. Description of Area III In Area III, five miles southwest of Champaign-Urbana, Illinois (fig. 16), a one-mile traverse was run across 45 feet of topographic relief. Signals from 11 broad- cast stations were measured while running the traverse across the area. Signal arrival directions were roughly north, east, south, and west. Signal frequencies ranged from 580 to 1580 kc, distances from 4 to 140 miles, and powers from 250 to 50,000 watts. The line of traverse followed a secondary gravel road across the ground moraine about four miles west of the Champaign reces- CULTURAL AND NATURAL FEATURES 33 sional moraine. 3 Drift thickness ranges from approximately 230 to 280 feet depend- ing upon ground elevation. The underlying bedrock surface is fairly uniform and lies approximately 475 to 500 feet above sea level. 4 The beds are Tradewater in age and probably of uniform sedimentary cyclic lithology. Here they lie about two miles west of the local crest of the LaSalle anti- cline and probably dip gently to the west. The eleven runs along this traverse were made from west to east along a secondary gravel road. Twelve different fence sys- tems were encountered along the traverse. No one fence ran the entire length of the traverse. There were short east-west fences parallel to the traverse ; some north-south fences joined the east-west fences; some north-south fences ended at the line of trav- erse without meeting or joining any of those oriented east-west. The fences were two-, three-, and four-strand barbed wire on both metal and wooden poles. Wire fence effects on field intensity were either slight or nonexistent for the eleven traverse runs. The slight effects were not consistent for the various signals measured. For all practical purposes of radio field in- tensity investigation, except in rare and spe- cial instances, effects of wire fence can gen- erally be ignored in Illinois. 3 Leighton, Ekblaw, and Horberg, op. cit. 4 Horberg, Leland, op. cit., pi. 2. OVERHEAD WIRES The effects of an overhead telephone wire were reported in the discussion of Area I and the effects of both telephone and elec- tric wires were observed in field measure- ments in Area III (fig. 16). These wires in Area III crossed the line of traverse at right angles at both the east and west ends. The field strengths of the eleven signals measured decreased beneath the north-south electric and telephone wires. Signals arriv- ing from the north and south underwent the greatest attenuation; signals from the east and west the least. GROUNDED ELECTRIC SERVICE POLES A special type of signal intensity anomaly is illustrated by figure 17. The traverse was run from west to east, starting 2Vi miles east of Harrisburg, Illinois, along State Highway 13. REA electric service wires on poles parallel the road along the west half of the traverse; along the east half there were no wires of any kind. The series of peaks (intensity increase and decrease) which make up the curve for the west half is caused by the grounding of every other or every third REA pole. Grounding of certain poles is a common practice. The maximum intensity peaks occur opposite the ungrounded poles, the minimum peaks op- posite the grounded poles. Map reference station number 703 Wire fence 2 strand telephone wire -Road \2 strand electri REA. line Topographic profile Vertical exaggeration 16.4 ILLINOIS STATE GEOLOGICAL SURVEY Scale 1/2 I mile Fig. 16. — Natural and cultural features along line of traverse (Area III, 5 miles southwest of Champaign-Urbana, Illinois). 34 RADIO WAVE TRANSMISSION TOPOGRAPHY In trying to determine the effect of topography on signal intensity it is difficult to eliminate the effects of geologic condi- tions involved in the control of the topogra- phy. In drift-covered areas, topographic variation may correspond to variation in drift thickness above the bedrock, or the relief may be the result of bedrock topogra- phy; thus signal intensity anomalies could be the result of drift thickness and/or topographic variation, or changes in sub- surface geologic conditions. In areas of moderate relief, with little or no glacial drift cover, it is difficult to lay out a traverse that will not cross lithologic boundaries in the bedrock. Areas of great relief, with or without glacial drift cover, are commonly associated with numerous lithologic changes in the bedrock. Areas of small relief hardly offer fair tests of topographic influence on radio signal in- tensity. Thus, the problem of completely isolating a signal anomaly due solely to topographic influence is most difficult. Area II (fig. 15), which has about 80 feet of topographic relief, was selected to study the possible effect of topographic re- lief on signal intensity because the only known change in geologic conditions is the 80-foot variation in thickness of drift cover. In all the recorded curves there is a slight increase in signal strength atop a small hill (fig. 15, point 1 along the traverse), which may be attributed to the topographic high and/or to the increase in drift thick- ness over the essentially flat bedrock sur- face. This slight increase in signal intensity is insignificant compared to the relief of the curves recorded along this 3960-foot traverse. Area III (fig. 16) has about 45 feet of topographic relief, caused by variation in drift thickness. As in Area II, the bedrock relief is probably small. Eleven runs were made across the traverse using different sta- tions, powers, frequencies, distances, and directions of signal arrival, and at no place was there even a small signal anomaly which appeared related in any way to the 45 feet of topographic relief. ILLINOIS STATE 6E0L0GICAL SURVEY Fig. 17. — Signal intensity curve recorded along traverse near Harrisburg, Illinois. High ampli- tude of curve in west half of traverse is caused by grounded electric service (REA) poles. Maxi- mum intensity peaks occur opposite ungrounded poles, minimum peaks opposite grounded poles. Topographic Shadow Effect The only significant signal anomaly caused by topography in Illinois can be called a topographic shadow effect. This shadow effect, resulting from a barrier to signal propagation, is well illustrated in figures 18, 19, and 20. The map (fig. 18) shows the Shawneetown Hills and station points along the traverses. Signal intensity was recorded continuously as a traverse was run from south to north (stations 600-601-602-603) around the east side of the hills. A control traverse (fig. 20) was run from north to south over the hills (fig. 18, stations 264-263-614-615). The signal measured, on both traverses, was CULTURAL AND NATURAL FEATURES 35 R- 9 E RJQ E, SCALE 3 MILES 3 Fig. 18. — Topographic map of Shawneetown Hills area, Gallatin County, Illinois. Traverses were run along roads between the numbered points (map reference station numbers). 36 RADIO WAVE TRANSMISSION from WEBQ, Harrisburg, Illinois, 22 to 23 miles to the west. Signal intensity on the traverse around the hills (fig. 19) shows a decided de- crease between station 601 and 602. Here the Shawneetown Hills come directly be- tween the antenna of WEBQ and the measuring equipment along the traverse. North and south of the hills (out from behind them) signal intensity increases strongly. In comparing the curves of the two trav- erses it should be remembered that the first was run from south to north and the second from north to south. Signal intensity along the control traverse is slightly higher north and south of the hills than it is on the hills. The intensity decrease on the hills may be due to either the geology, which differs from that of the Ohio River flood plain, or perhaps to attenuation by woods and vegetation on the hills. The signifi- cant feature of the curve is the ap- parent absence of any signal anomaly due to topographic influence. If topography exerted any great influence one might expect a strong signal increase on top of the hill, which rises about 200 feet above the sur- rounding Ohio River flood plain. Summary Hundreds of miles of recorded field in- tensity measurements at broadcast frequen- cies in Illinois have shown that small topo- graphic relief (20 to 60 feet) has little or no influence on signal strength. In areas of greater relief (60 to 400 feet), except for the topographic shadow effect, signal anoma- lies resulting from topographic influence appear, in general, to be insignificant. Where they are significant, it is difficult to assign the cause of the anomaly solely to topographic influence because in such areas geologic conditions sometimes change rapidly within short distances and may be the chief cause of an anomaly. Concerning topographic influence, Ker- win concludes: 5 5 Kerwin, Larkin, Use of the broadcast band in geologic mapping: Jour. Applied Physics, vol. 18, no. 4, p. 413, April 1947. ILLINOIS STATE 6EOLGICAL SURVEY Fig. 19. — Signal intensity curve recorded in the Shawneetown Hills area, Gallatin County, Illi- nois. Intensity decrease between stations 601 and 602 caused by a shadow effect of the hills. Arrow above "WEBQ" indicates direction of signal arrival. It was also observed that topography appeared to have relatively little effect. In driving over stretches of country in Arlington where the geol- ogy was uniform, the field remained essentially constant except for manhole effects, etc., whether the receiver was passing over rolling hills, pro- ceeding through cuttings, or on level ground. A notable exception was observed on top of Pine Hill, a prominence beside the Medford Dike and very close to the transmitting antenna of the broadcasting station being recorded. At the top of the hill the signal was extremely strong, but it weakened considerably as the bulk of the mountain was interposed sharply between the receiver and transmitter. At broadcast frequencies and lower, the topographic effect on signal intensity in Illinois is not appreciable. Apparently, topo- graphic effect increases with increase in frequency until at very high frequencies (30 to 300 mc), ultra-high frequencies (300 CULTURAL AND NATURAL FEATURES 37 to 3000 mc), and super-high frequencies (3000 to 30,000 mc) even a small knoll may produce a dead spot (area of no field strength) behind it. 6 SHADOW EFFECT FROM WOODS Description of Area IV In Area IV, four miles west of Monti- cello, Illinois, a series of traverses was run between stations 346 and 347 (fig. 21) to investigate the influence of trees on signal intensity. The 10 broadcast transmitters ranged in power from 250 to 50,000 watts, in frequency from 550 to 1580 kc, in dis- tance from 17 to 140 miles, and the signals had widely separated directions of arrival. The 3960-foot traverse was run from south to north on a secondary gravel road. The bedrock immediately underlying the trav- erse area is McLeansboro or Carbondale of the Pennsylvanian system, and the area is about 15 miles west of the axis of the LaSalle anticline. Here the bedrock is part of the south slope of a large buried valley, the Mahomet, which is considered pre- glacial. 7 The glacial drift thickens from about 300 feet (sta. 346 at the south) to approximately 400 feet (sta. 347 at the north), as the traverse crosses the south wall toward the center of the valley. The area investigated is covered by ground mo- raine, with 10 feet of topographic relief along the 3960-foot traverse, and lies about five miles west of the Cerro Gordo reces- sional moraine. 8 Interpretation of the ten signal intensity curves recorded along this traverse show or suggest the following conclusions: — Signals measured after they have come through the woods show: 1. Little or no attenuation for frequen- cies lower than 1000 kc. 2. Progressively greater attenuation for signals higher than 1000 kc. 3. Signals measured near the fringe edge ILLINOIS STATE GEOLOGICAL SURVEY 6 Morecroft, J. H., Principles of radio communication: New York, John Wiley, 3rd ed., pp. 372-380, 1944. 7 Horberg, Leland, A major buried valley in east-central Illinois and its regional relationships: Jour. Geol., vol. S3, no. 5, 1945; reprinted as Illinois Geol. Survey Rept. Inv. 106, p. 353, 1945. 8 Leighton, Ekblaw, and Horberg, op cit., p. 22. Fig. 20. — Signal intensity curve recorded in the Shawneetown Hills area, Gallatin County, Illi- nois. Slight decrease in field intensity between stations 263 and 614 occurs on the hills. of their primary coverage area are not a good test for attenuation by woods, as part of the arriving sig- nal may be propagated by components other than ground-wave. Signals measured before they entered the woods show : 1. Decreased intensity along the margin of the woods, possibly the result of reflection or absorption at the boundary of the woods. Signals measured arriving from all direc- tions show: 1. Decreased intensity at margin of woods. 2. Individual trees along the roadside (fig. 21) cause no noticeable influ- ence upon signal intensity. 38 RADIO WAVE TRANSMISSION The influence of the geologic component is uncertain and probably not significant. If bedrock and moraine conditions in the preglacial valley exert any influence on sig- nal intensity it cannot be recognized as such in the recorded curves. STREAMS In Area III the line along which signal intensities were measured crosses a small stream nearly at right angles (fig. 16). Field intensity decrease over the stream was recorded for signals arriving from north- south directions (Chicago and St. Louis), 120 and 140 miles distant. Field intensity increase over the stream was recorded for all other signals (arriving from east-west directions), 4 to 80 miles distant. Signal arrival with respect to stream orientation and possibly distance appear to govern the type and magnitude of intensity variation over the stream rather than frequency or power of the signals. Collateral observations indicate that stream-caused signal anomalies (either in- crease or decrease) are usually sharp but of relatively small magnitude. BURIED PIPES The influence of buried pipelines on sig- nal intensity is not established. Along the traverses run in Illinois numerous pipelines were crossed, but in each instance the signal intensity anomalies caused by nearby elec- tric, telephone, or transmission wires, over- rode those of the pipelines. If signal in- tensity measurement across a pipeline could be made in the absence of overhead wires and other hazards, a small to medium anomaly might be expected. The anomaly would be either an intensity increase or de- crease depending upon orientation of the long axis of the pipeline in the radio field being measured. Map reference station number Road ^Single strand telephone wire *" ^^Wire fence » » » j. t\ K »<*> Woods ILLINOIS STATE 6EOLO6ICAL SUBVEY Scale 1/2 Fig. 21. — Natural and cultural features along line of traverse (Area IV, 4 miles west of Monticello, Illinois). ROAD MATERIALS Signal intensity is apparently not influ- enced by dirt, gravel, or tar roads. Numer- ous traverses along roads changing from dirt to gravel to tar showed little if any variation in signal intensity. Crossing a reinforced concrete highway would prob- ably cause an intensity anomaly, but where such roads were crossed, associated wires seemed to override completely any anomaly caused by the differences in material sur- facing the highways. LAKES AND PONDS The influence of lakes and ponds on signal intensity is not certain. Because of scarcity of lakes and ponds in the areas in- vestigated, and the lack of good field condi- tions for measuring signal intensity around them, it was difficult to ascertain their in- fluence. Bodies of water can be expected to influence signal intensity in varying de- gree, depending upon their size, shape, chemical composition, and orientation with respect to direction of arrival of the signal. CHAPTER 7 EFFECTS OF METEOROLOGICAL CONDITIONS Meteorological conditions appear to exert only a small influence on radio waves at broadcast frequencies, in primary ground- wave coverage areas, over short periods (fig. 8). However, they are known to in- fluence radio waves of low frequency (long waves) over long time intervals, and waves of high frequency (short waves) over small time intervals. There are seasonal varia- tions on long-wave signal intensity, the greatest signal strength usually being ob- served during the winter months. 1 Morecroft, 2 describing a two-year field strength record of a long-wave transatlantic signal, reports no evident correlation be- tween magnetic storms and field intensity. He does report an evident correlation be- tween the number of sunspots and average field intensity, with field strength greater during solar activity. Short waves have small ground-wave cov- erage areas. Their chief value in communi- cations is their ability to travel great dis- tances via sky-wave propagation paths. Short-wave signals are influenced greatly by cycles of solar activity, and may become severely attenuated during magnetic storms. Gracely, 3 in his work on variations of ground-wave signal intensity at standard broadcast frequencies, attempted to correlate variations with temperature, precipitation, humidity, atmospheric pressure, dew point, and vapor pressure. He concludes there is closer and more continuous correlation with temperature than with any other meteoro- logic feature. Most of Gracely's signal intensity meas- urements were spot readings, on six differ- ent paths, made at 1 :30 p.m., E.S.T., daily or every fourth day, for periods of nine months to four years. He discovered that 1 Morecroft, J. H., Principles of radio communication: New York, John Wiley. 3rd ed., p. 381, 1944. 2 Idem., pp. 383-385. 3 Gracely, F. R. ? Temperature variations of ground-wave signal intensity at standard broadcast frequencies: Proc. Inst. Radio Eng., vol. 37, no. 4, pp. 360-363, April 1949. intensity variation was greater over long paths (up to 558 miles) than over short paths (down to 76 miles). Precipitation along the paths investigated frequently cor- related with marked increases in signal in- tensity which remained for several days fol- lowing periods of heavy rainfall. However, there were other periods of rainfall when no such signal intensity increase was ob- served. This led to investigation of ground moisture, along the various paths, taking into account the rates at which the ground gains moisture from precipitation and loses it by runoff and evaporation. The applica- tion of this anatysis led only to another par- tial correlation. A few local coincidences between signal intensity and humidity were observed but also permitted only partial correlation. In attempting to correlate signal intensity variation with temperature, the effects of vegetation along the paths, and effects of the gradient of the index of refraction of the lower troposphere were considered. Al- though some correlations between vegetation and signal intensity may be possible, vege- tation alone is not responsible for the varia- tions recorded. Similarly, it is difficult to explain the regular intensity-versus-temper- ature observations in terms of a lower trop- ospheric reflected wave component which is varying irregularly in height of reflection and length of path. For ground-wave signal intensity at standard broadcast frequencies the follow- ing generalizations, according to Gracely, appear to be established with reasonable cer- tainty: signal intensities decrease at higher temperatures; this decrease becomes greater with distance. In field experimentation Kerwin 4 em- ployed a stationary monitoring field intensity meter and obtained continuous field intensity 4 Kerwin, Larkin, Use of the broadcast band in geologic mapping: Jour. Applied Physics, vol. 18, no. 4, pp. 409- 413, April 1947. [39] 40 RADIO WAVE TRANSMISSION records on an Esterline-Angus recording milliammeter. He found that in general, the fieldswere stronger during rainy weather, and less affected by geology. He concluded that for periods of three to four hours, with constant weather and daylight conditions, the field remained steady. Figure 8 is a typical curve recording variation in field intensity of ground-wave at a fixed location. These curves some- times exhibit no more variation over a period of eight hours than they do for one hour. However, eight-hour curves occa- sionally exhibit minor variations, usually in the form of slow, gradual shifts in in- tensity. Most of the traverses for the present work were run in periods five minutes to half an hour long. The longest traverses, run at speeds up to 45 miles per hour, took about one hour. Thus it appears that, with reasonably constant weather conditions for short periods of time, signal intensity is probably not significantly influenced by temperature or other meteorologic features. One exception to the preceding general- ization is the instantaneous intensity change caused by natural electromagnetic phenome- na. These phenomena cause momentary bursts of high signal intensity. A com- mon example is the burst of noise (static) which accompanies a lightning flash during a thunderstorm. This type of interference is common at broadcast frequencies, is especially evident at low frequencies (100 to 400 kc), but is less noticeable at high fre- quencies. An intensity curve recorded from sta- tion WILL (580 kc) (fig. 23) fails to show static bursts (summertime fair- weather static) because they were not large enough to deflect the recording pen, al- though they were heard while monitoring the signal. A curve from radio range sta- tion AF (317 kc) (fig. 26), run along the same traverse 15 minutes later, shows the effect of almost continuous static bursts. Such intensity anomalies are not likely to be confused with those caused by field haz- ards or geologic conditions because the static- induced anomalies are characteristically in- stantaneous bursts of increased intensity. They are also readily identifiable when monitoring the signals with headphones. Repeated runs over selected traverses, days, weeks, months, and years apart, under reasonably constant but different meteoro- logical conditions, did not reveal any no- ticeable variations from the general char- acter of the relative field intensity curves, as measured with the RCA 308-B field in- tensity meter. CHAPTER 8 — EFFECTS OF GEOLOGIC FEATURES The experimentally obtained data on the influence of cultural, natural, and meteoro- logical features on signal intensities pro- vide a starting point for investigation of geologic influence. The problems of ascer- taining geologic effects on signal intensities are, first, separating them from effects of field hazards and, second, making reason- able correlations with geology. Obviously, the effects of all field hazards have not been completely investigated, but it now appears possible to recognize and correlate some geologic effects. Radio engineers have long held the con- cept that radio waves penetrate only shal- lowly into the earth's surface. 1 One of the methods of calculating the constants of the earth along a path consists of taking sam- ples of the earth and measuring their con- ductivities and dielectric constants. 2 It would appear, as the result of tests with cave and mine communications, that radio waves do penetrate and are transmitted through some bedrock. Thus, it seems logical to postulate that abrupt changes in physical and structural characteristics of the rock strata may cause observable intensity and perhaps other changes in the radio field. If the electromagnetic field is partly in the air and partly in the ground, a change in the part of the field below the surface may be reflected in the air over the earth's surface and be measurable. The following field examples have been selected from many hundreds of miles of traverse as representative of the effects of a variety of geologic situations upon field intensity. These examples demonstrate not only the geologic effects, but also limitations of the method. FAULTING Stratigraphic and lithologic discontinuity of the type usually presented by faulting 1 Terman, F. E., Radio engineers handbook, New York, McGraw-Hill, 1st ed., p. 698, 1943. 2 Idem., p. 709. might be expected to cause signal intensity anomalies. To appraise these effects, field hazards, if present, must be recognized and their influence accounted for. Signal intensity was recorded across faults in Gallatin, White, Hardin, Pope, Johnson, Union, Jackson, Williamson, and Franklin counties, Illinois. Signal intensity associated with some of these faults was apparently unaffected, with some it may have been influenced slightly, while with others there appeared to be a strong influence. An in- tensity anomaly may be associated with a fault when lithologic discontinuity also of- fers electrical, chemical, or magnetic discon- tinuity to the field. Where fault structures were traversed, field intensity anomalies were most often associated with lithologic discontinuity. This is illustrated by the Shawneetown fault in Illinois (figs. 22-25), where signal intensity is commonly higher on the side of the fault nearest the trans- mitter. Shawneetown Fault in Illinois A major fault zone, known as the Shaw- neetown in Illinois, trends from east to west across part of Illinois and Kentucky. This fault in southern Illinois was selected to test geologic influence on radio field in- tensity. According to Butts, 3 there is as much as 2,300 feet of displacement along the north side of Gold Hill (sees. 27 and 28, T. 9 S., R. 8 E.), Gallatin County, where Pennsylvanian beds of the Trade- water group to the north are in contact with Mississippian Ste. Genevieve lime- stone to the south. At Horseshoe Gap, three miles southwest of Equality, the relative displacement along the fault is not less than 3500 feet. Numerous traverses, using signals from different stations, were run across the Shawneetown fault. Figure 22 shows the 3 Butts, Charles, Geology and mineral resources of the Equality-Shawneetown area: Illinois Geol. Survey Bull. 47, pp. 58-59, 1925. [41] 42 RADIO WAVE TRANSMISSION PENNSYLVANIAN Mc Leonsboro Carbondale Tradewater ILLINOIS STATE GEOLOGICAL SURVEY Fig. 22. — Route of traverse (along Illinois Highway 1) across the Shawneetown fault in Gallatin County, showing topographic contours, potential field hazards, and radio broadcast station data. route of one of the traverses. Station 208 is at the junction of Illinois highways 13 and 1, about three miles east of Equality. Running south, station 210 is at the junc- tion of Route 1 and the L & N railroad ; station 212, just north of the Saline River; station 215, about two miles northwest of Gibsonia. The contour lines crossing the route along which the traverse was run (Illinois Highway 1) are topographic. The potential field hazards, electric and tele- phone wires, railroad tracks, streams, and bridges, are indicated. The geologic sec- tion has been generalized from Butts. The arrows beside the radio station data indi- cate the directions of signal arrival. SIGNAL FROM BROADCAST STATION WILL The signal from WILL, Urbana, 111., 170 miles to the north (fig. 23), shows a fairly constant intensity level north of the fault, except for the influence of electric and telephone wires and the metal bridge approximately at the fault trace. South of potential field hazards, electric and tele- phone wires is still evident, but the general signal intensity drops off. The WILL transmitting antenna is on ground under- lain by Pennsylvanian strata, and the 170- mile signal path from Urbana to the area of traverse is underlain by Pennsylvanian rocks. Crossing the fault, the radio field (above, along, and possibly carried by Penn- sylvanian strata and glacial drift) encoun- ters a geologic discontinuity as Pennsylvanian strata abut against Mississippian beds. The exact effect of the discontinuity on signal intensity cannot be differentiated from the influence of the metal bridge. However, the lower signal intensity level south of the fault is interpreted as influenced by the fault. SIGNAL FROM BROADCAST STATION KWK The signal from KWK, St. Louis, Mo., 125 miles northwest (fig. 24), shows (ex- cept for the influence of cultural features) fairly constant intensity levels in this area. The signal level north of the fault is no- ticeably higher than the level south of the fault. The influence of the fault on signal intensity is confused with the influence of GEOLOGIC FEATURES 43 ILLINOIS STATE 6E0L0GICAL SURVEY Fig. 23. — Signal intensity curve of WILL re- corded across the Shawneetown fault, Gallatin County, Illinois. Intensity decrease is evident south of the fault (see fig. 22). the metal bridge, which is situated close to the fault trace. However, the lower signal intensity south of the fault suggests influence by the fault. SIGNAL FROM BROADCAST STATION WJPF The signal from WJPF, Herrin, 111., 37 miles west and slightly north of the trav- erse (fig. 25), also shows fairly constant intensity levels, except for the influence of cultural features. The WJPF transmitter is northwest of the fault, on terrain under- lain by Pennsylvanian strata. The signal level north of the fault is considerably higher than that to the south. The decrease in intensity at the bridge masks the influ- ence of the fault, but the fault may cause lower intensity to the south. SIGNAL FROM RADIO RANGE STATION AF The signal from radio range station AF, Advance, Mo., 102 miles southwest, is trans- mitted at 317 kc. The numerous sharp in- tensity peaks (fig. 26, to the right) repre- sent bursts of static and noise. An almost continuous crackling, popping, and rushing noise was heard in the headphones while monitoring the signal along the traverse. This is characteristic of reception of low frequencies with low signal intensities in summer weather. Signal level decreases at the bridge and immediately to the south, and also in the vicinity of the railroad tracks. Influence on signal intensity by the fault is not readily apparent. The slight decrease of in- ILLINOIS STATE GEOLOGICAL SURVEY Fig. 24. — Signal intensity curve of KWK re- corded across the Shawneetown fault, Gallatin County, Illinois. Intensity decrease is evident south of the fault (see fig. 22). 44 RADIO WAVE TRANSMISSION ILLINOIS STATE GEOLOGICAL SURVEY Fig. 25. — Signal intensity curve of WJPF re- corded across the Shawneetown fault, Gallatin County, Illinois. Intensity decrease is evident south of the fault (see fig. 22). tensity immediately south of the bridge is the only obvious anomaly on the curve that cannot be solely accounted for by either cultural or meteorological effects. If the fault trace was not close to the bridge, the decrease could possibly be assigned to the bridge influence alone, but under the cir- cumstances the anomaly is assigned to both bridge and fault influence. It is possible that a signal at low frequency (317 kc) with this orientation is not as strongly in- fluenced by this fault as signals at higher frequencies. Shawneetown Fault in Kentucky Before investigating the Shawneetown fault in Illinois, it was considered likely that this major structure would cause a ma- jor variation in signal strength. Preliminary field investigation across the Shawneetown fault, with reconnaissance-type equipment, and spot readings 330 feet apart, revealed large signal anomalies. Subsequent continu- ous traverses across the fault show that spot readings, unless removed from field hazards, can be misleading. The comparatively small signal anoma- lies, recorded on continuous traverses across the fault, may be due to a general signal attenuation by field hazards. Electric and telephone wires parallel the road and pass overhead ; railroad tracks, a stream, and a river cross the traverse, and the traverse itself winds over terrain with about 80 feet of topographic relief. Perhaps the lack of ILLINOIS STATE GEOLOGICAL SURVEY Fig. 26. — Signal intensity curve recorded across the Shawneetown fault, Gallatin County, Illinois. The intensity decrease immediately south of the bridge is the only obvious anomaly that may be due in part to the influence of the fault. GEOLOGIC FEATURES 45 u (/) 46 RADIO WAVE TRANSMISSION large signal strength anomalies across the fault should be no more surprising than the small anomalies that were recorded in the midst of field hazards. The anoma- lies (figs. 22-26), though smaller in mag- nitude than might be expected, appear to be the results of fault influence. It was desirable to further substantiate the Shawneetown fault influence on signal strength by crossing it, if possible, where there were no associated field hazards. Ac- cordingly a location was chosen on a level alluvial plain along the Ohio River bot- toms in Kentucky (fig. 27). The plain was about car-roof height in corn at the time of the traverse. Thus, the factors of soil, topography, and vegetation were uniform. Figure 28 shows the curve re- ILLINOIS STATE GEOLOGICAL SURVEY Fig. 28. — Field intensity curve recorded along traverse across the Shawneetown fault, on Ohio River bottoms in Kentucky. corded along this traverse. The signal was from WEBQ, Harrisburg, 111., 23 miles west and slightly north. The transmitting antenna is north of the fault on terrain un- derlain by lower McLeansboro beds of the Pennsylvanian system. The curve shows an intensity decrease near the fault, and general field intensity is higher north of the fault. In the entire length of the traverse (more than a mile) there is only the one major anomaly (fig. 28). The signal path from WEBQ partly parallels or coincides with the strike of the fault. If a signal from the north or south were used (with the signal path at a high angle to the strike of the fault), the effect of the geologic discontinuity on that signal might be greater and the signal strength anomaly stronger. This traverse (fig. 28) is interpreted as unquestionably demon- strating geologic influence on signal in- tensity. Inman East Fault Traverses across the Inman East fault, Gallatin County, 111. (fig. 18), yielded the common type of signal anomaly with in- tensity higher on the side of the fault closer to the transmitter. They also yielded a special type of anomaly which may result from some of the electromagnetic field being transmittted by limestone beds. The Inman East fault trends northeast- southwest across the Ohio River bottoms in Gallatin County (fig. 18). The strati- graphic throw ranges from approximately 200 to 400 feet. The downthrown strata are on the east side of the fault. Intersec- tion of the major plane by drill holes in the Inman East oil pool (sees. 11, 14, T. 8 S., R. 10 E.) indicates a dip of about 60 degrees to the southeast. 4 In the area of the traverses, alluvial and outwash glacial debris is approximately 100 to 150 feet thick; the underlying lower McLeansboro rocks (Pennsylvanian) present a surface of gentle relief; the near-surface beds in- volved in faulting range from No. 6 coal bed to 50 feet above the West Franklin limestone (250 feet above No. 6 coal bed). 4 Pullen, M. W., Subsurface geology of Gallatin County north of the Shawneetown fault: In Illinois Geol. Sur- vey Rept. Inv. 148, 1951. GEOLOGIC FEATURES 47 Field intensity using the signal from WJPF (250 watts, 1340 kc), 46 miles west of the fault trace, was lower east of the fault than west. Field intensity from WGBF (5000 watts, 1280 kc), 30 miles northeast of the fault, was lower on the west than on the east side, but the intensity immediately east of the fault was con- siderably higher than elsewhere on the east side within a few miles of the fault. Two explanations of the anomaly are sug- gested : first, intensity is increased by re- flection of part of the field at the struc- tural discontinuity or by excitation of the structure ; second, intensity is reinforced at the fault by that part of the field which is transmitted from the east to the fault plane by the West Franklin limestone. The first explanation is compatible with radio theory. If the second explanation is valid it may operate alone or in combination with the first. The transmitting antenna of WGBF, at Evansville, Ind., is on or close to the outcrop of the West Franklin limestone. The lime- stone, with some faulting, extends south- west and abuts along the Inman East fault plane at depths of 150 to 300 feet on the east (downthrow) side. In Schlum- berger electric logs the West Franklin lime- stone has high apparent resistivity as com- pared to strata above and below the lime- stone. Because of the thickness of the lime- stone in relation to the electrode configura- tion, it is probable that there is even greater resistivity contrast in true in-place resistiv- ities. Electric logs of some holes that inter- sect the fault indicate that the material along the fault plane itself has higher electrical resistivity than adjacent shales, siltstones, and sandstones. In several wells, the surface pipe is seated and cemented in the West Franklin lime- stone (Carter Oil Co., E. H. Busick C-87, sec. 11, T. 8 S., R. 10 E.), thus directly connecting the limestone strata with the surface of the ground. The limestone offers the emitted radio field a comparatively high resistance path. Kerwin 5 demonstrated high field intensity 3 Kerwin, Larkin, Use of the broadcast band in geologic mapping: Jour. Applied Physics, vol. 18, no. 4, p. 412, April 1947. associated with rocks of high electrical resistivity and low intensity with rocks of low resistivity. The increase in intensity close to the Inman East fault, as com- pared to intensity measured in the air path away from the fault, might be the result of bridging between the West Franklin lime- stone (carrying part of the field in the ground from Evansville) and the surface either by the cemented metal surface pipe in the drill holes or by high resistance material along the fault plane, or both. If the intensity anomalies across the In- man East fault are valid expressions of geo- logic influence on field intensity, then bed- rock structure influence is making itself felt through 100 to 150 feet of alluvium and glacial outwash. CRYPTOVOLCANIC STRUCTURE NEAR KENTLAND, INDIANA Geologic Setting The Kentland cryptovolcanic structure lies between the towns of Kentland and Goodland, Newton County, in north- western Indiana. Here, disturbed Ordovi- cian rocks cropped out (before quarry op- erations) in a flat glacial plain. The largest of these quarries (McCray) lies about 200 feet south of U. S. Highway 24 in sec. 25, T.27N.,R.9W. (fig. 30). The disturbed Ordovician rocks have been described by Shrock and Malott; 6 the history and evolu- tion of geologic thinking has been sum- marized and the stratigraphy and structure have been described by Shrock; 7 and the paleontology of the rocks has been discussed by Shrock and Raasch. 8 Field intensity surveys were run over this area to ascertain if disturbed Ordovi- cian rocks (apparently pushed up through younger overlying strata) had any measur- able influence on signal intensity. The topography of the area is that of a level glacial drift plain. The drift thick- 6 Shrock, R. R., and Malott, C. A., The Kentland area of disturbed Ordovician rocks in northwestern Indiana: Jour. Geol., vol. 41, no. 4, pp. 337-370, 1933. 7 Shrock, R. R., Stratigraphy and structure of the area of disturbed Ordovician rocks near Kentland, Indiana: Am. Midland Nat., vol. 18, no. 4, pp. 471-531, 1937. 8 Shrock, R. R.. and Raasch, G. O., Paleontology of the disturbed Ordovician rocks near Kentland, Indiana : Am. Midland Nat., vol. 18, no. 4, pp. 532-607, 1937. 48 RADIO WAVE TRANSMISSION R. 9 W. R. 8 W. FIELD INTENSITY CONTOUR MAP NEAR KENTLAND, IND. WIND Chicago 147 GEOGRAPHIC ORIENTATION POINTS ALONG TRAVERSES 2.5-7 RELATIVE FIELD INTENSITY VALUES 27 ILLINOIS STATE GEOLOGICAL SURVEY Fig. 29. — Field intensity contour map near Kentland, Indiana (Sta. WIND) ens in all directions away from the quar- ries in the cryptovolcanic rocks. Drill- hole data indicate a maximum thickness of 160 feet locally. The geological map of Indiana (1932) 9 shows the quarry area surrounded by De- vonian (New Albany) strata, with Missis- sippian (Osage) immediately east and south. On the basis of fauna found in the quarries, Raasch and Bays 10 consider most of the exposed disturbed Ordovician strata Black River and Trenton in age. Shrock 11 considers the disturbed Ordovi- cian rocks an inlier surrounded by Silurian, Devonian, Mississippian, and possibly Penn- sylvanian strata. He believes that most or all of the disturbed Ordovician strata ex- posed in the present quarries have been uplifted at least 1500 feet. The uplift 9 Logan, W. N., Geological map of Indiana: Indiana Conserv. Coram., Div. Geol. Pub. 112, 1932. 10 Raasch, G. O., and Bays, C. A., personal communica- tion, 1949. "Shrock, op. cit., p. 517. caused faulting, fracturing, shattering, and brecciation of the strata. The Ordovician away from the quarries assumes its normal regional attitude. Logan reported a well at Kentland 12 about three miles west of the quarry area that penetrated 100 feet of glacial drift, 145 feet of Devonian, 305 feet of Silurian, and 570 feet of Ordovician strata. If the present exposed upstanding mass of dis- turbed Ordovician rocks is the result of a post-Silurian or Devonian diastrophism, then it was forced up through the overlying Silurian or Silurian-Devonian strata. If the diastrophic incident was post-Ordovi- cian-pre-Silurian, it is possible that part of the disturbed Ordovician mass may have been covered by younger sediments. Since the geologic relations at the margins of the disturbed mass are unknown, the age of the diastrophic event cannot be ascertained. 12 Logan, W. N., Economic geology of Indiana: Handbook of Indiana Geology, pt. 5, p. 950, 1922. GEOLOGIC FEATURES 49 Regardless of whether Silurian beds were directly involved in the cryptovolcanic ac- tion or were later deposited around or partly over the Ordovician mass, they are probably in direct contact with the upthrown Ordo- vician mass, or at least not far from it. Field Hazards Field hazards in the area are numerous. Both telephone and electric wires are paral- lel to and cross the roads. A railroad track runs east and west through the middle of the area (figs. 29, 30). These field hazards cause diverse intensity anomalies, for the most part recognizable. Field Intensity Measurements The field intensity curves from radio sta- tion WIND, 560 kc, 5000 watts, 55 miles northwest (transmitter at Gary, Ind.), are nearly constant and relatively flat. In gen- eral, field intensity decreases sharply be- neath overhead wires and increases near and at the cryptovolcanic structure. The map, based on the intensity curves (fig. 29), shows a closed intensity high around the cryptovolcanic structure. The transmitting site of station WIND is on terrain under- lain by Devonian and Silurian rocks. Additional traverses were run over the area using station WDAN, Danville, 111., 1490 kc, 250 watts, 45 miles south and slightly west of Kentland. Intensity does not change over the cryptovolcanic structure. The transmitter site at Danville is under- lain by Carbondale strata of Pennsylvanian age. The signal from station WDZ, Tuscola, 111. (now relocated at Decatur), 1050 kc, 1000 watts, 80 miles, 35 degrees west of south, from Kentland, showed a small intensity increase over the cryptovolcanic structure. The WDZ transmitter site is underlain by the crest of the LaSalle anti- cline, and Devonian-Silurian dolomites are the first bedrock encountered (at depths from 30 to 50 feet). There may be some geologic relationship which causes the sig- nal intensity from WDZ to increase over the cryptovolcanic structure while that of station WDAN remains constant. The frequencies of the two stations are close, the transmitting powers 1000 and 250 watts, but the stronger station is 80 miles away and the weaker only 45 miles. Signal in- tensity from station WDAN, on Pennsyl- vanian strata, is not influenced by the Ordo- vician strata exposed around Kentland. Some of the sedimentary layers between the Pennsylvanian and Ordovician have low electrical resistance, such as shale, and as comparatively good electrical conductors they may act as shields and prevent the radio fields from reaching the Ordovician, Silurian, or Devonian strata below. Again, two explanations of the anomaly are suggested : first, intensity increases at the structure by reflection or excitation; second, intensity is reinforced at the struc- ture by that part of the field which may be transmitted by the Devonian-Silurian rocks from Tuscola to Kentland where these beds are in contact with the disturbed Ordovician rocks. Field Intensity Contour Maps A regional survey of the area was made using the signal from station WAAF, Chi- cago, 111. Radio station WAAF operates on a frequency of 950 kc, with a power of 1000 watts, and is 75 miles, 15 degrees west of north, from the town of Kentland. The field intensity map (contours repre- sent measured intensity values) gives a regional picture covering a considerable area around the cryptovolcanic disturbance (fig. 30). The transmitter site is underlain by Silurian strata. Signal intensity in the Kentland area is relatively weak (20 to 30 microvolts per meter) but increases in the vicinity of the cryptovolcanic structure. Field strength calculations for station WAAF in the Kentland area, using the Sommerfeld formula as modified by Van der Pol, indicate an expected field intensity of 20 microvolts per meter or less. This value is found in areas at some distance from the quarries, but at or near the quarries signal intensity reaches 50 micro- volts per meter. This intensity anomaly may be explained by reflection and excita- tion, and/or signal reinforcement through Silurian rocks connecting Kentland and Chicago. 50 RADIO WAVE TRANSMISSION GEOLOGIC FEATURES 51 R. 7 E ;.-, Point ;\ sBb J 9 • I s. "V r% ■*•*; ^^usessnMt>»t>^ 3eim ^. Wk ***•>*. SCAL£ 2 MILES Fig. 31.— Location of field intensity traverse across Hicks dome, Hardin County, Illinois. 52 RADIO WAVE TRANSMISSION It is of interest that high intensity values are found in an area north and slightly west of the exposed cryptovolcanic struc- ture (fig. 30). It is possible that future ex- ploration will prove that this high-intensity area overlies an extension, if not a major part, of the cryptovolcanic structure. Signal Intensity Versus Magnetic Intensity Shrock and Malott included a mag- netic intensity variation map 13 made by Justin Zinn, who used a Hotchkiss super- dip. A well-defined magnetic anomaly is shown in the area of the quarries and be- yond them. This anomaly may be caused by locally raised pre-Cambrian basement or intrusive magnetic rock. The radio field intensity anomaly ex- tends north and west beyond the quarries whereas the magnetic anomaly extends east and northeast. If the signal in- tensity anomaly were caused by a mag- netic component of the earth, the two anomaly patterns might be the same or similar. This is not the case; therefore it appears, at least around Kentland, that an anomaly in the magnetic field does not greatly influence the intensity of an electro- magnetic field in the ground-wave area. Signal Intensity Behavior Field hazard influence is easily recogniz- able on the map (fig. 30). The field in- tensity contour lines are pinched in toward the railroad track which cuts through the middle of the area. Signal intensity is ap- parently reduced considerably near the good metallic conductors (rails and associated wires), but the increased signal strength over the cryptovolcanic structure is strong enough to overcome this influence. An interesting observation can be made by comparing the field intensity contour maps on stations WAAF and WIND (figs. 29, 30). The intensity anomaly for WIND over the cryptovolcanic structure is relative- ly small compared to the strong anomaly for WAAF. The two stations are about equidistant 13 Shrock and Malott, op. cit., p. 366. from the surveyed area, the frequencies are roughly similar, the powers are 5000 and 1000 watts, yet the weaker station (WAAF) produces the greater intensity anomaly. This can possibly be explained by the geo- logic setting of the two transmitters. WAAF is on Silurian terrain ; WIND is on Devonian-Silurian terrain. Or perhaps the weaker signal is more sensitive to geologic and other influences than the stronger one. Cloos 14 observed a similar relationship near Baltimore, Md. Of the four broad- cast stations then in the Baltimore area, only WCAO (600 kc, 250 watts) showed clearly what he called "dead spots." The other three stations, WBAL (1060 kc, 10,000 watts), WFBR (1270 kc, 500 watts), and WCBM (1370 kc, 250 watts), appeared clear and undisturbed within the region investigated. Cloos says that where the signals are strong, anomalies are rare, but the same signals at a greater distance from their transmitters, and consequently weaker, are useful for detecting intensity anomalies. He qualifies his use of the word "strong" as being simply descriptive, with- out implication as to frequency or power of the station. Depth to Bedrock It would appear possible that some strata propagate or transmit radio waves through or along themselves so that, where the strata come to or near the surface, they add to signal intensity propagated along the air path. Thus, the depth from the ground surface to underlying bedrock might be an influencing factor. The disturbed Ordo- vician rocks, which are now being quarried, formerly cropped out in the flat glacial till plain. Shrock 15 quotes Gorby who gives the thickness of drift as 100 feet in a drill hole at the town of Kentland three miles west; more than 100 feet two miles south; 150 feet one mile north; and 30 feet only 200 yards east of the quarry. Radio waves appear to be more attenu- ated traveling in unconsolidated glacial drift 14 Cloos, Ernst, Auto-radio — an aid in geologic mapping: Am. Jour. Sci., ser. 5, vol. 28, pp. 2S6-261, 1934. 15 Shrock, op. cit., p. 472. GEOLOGIC FEATURES 53 MISSISSIPPI Msl St. Louis limestone Mw Warsaw limestone Mo Osage formation DEVONIAN Dc Chattanooga shale DIs Devonian limestone Msl Mvv Mo^Dc^^D]^___D^ Mo ^w^cmm??^^ ^^Uli^ SCALE 1 2 MILES ILLINOIS STATE GEOLOGICAL SURVEY A' Fig. 32. — Geologic map and cross section of the center of Hicks dome, Hardin County, Illinois {after Weller, Stuart, The geology of Hardin County: Illinois Geol. Survey Bull. 41, 1920). than in consolidated bedrock. 16 If this is true, increased intensity over the crypto- volcanic structure might be explained by the proximity of bedrock as the glacial till cover thins. However, depth to bedrock does not appear to be a major influence here because field intensity one mile north of the exposed cryptovolcanic structure, where the drift is 150 feet thick, is as strong as it is at the structure, where the drift is thin or absent. DOME STRUCTURE WITH A SUSPECTED IGNEOUS ORIGIN Geologic Setting Hicks dome in Hardin County, 111., a great doming of the rock strata, has been described by Weller. 17 At the center of the 18 Spieker, E. M., Radio transmission and geology: Bull. Am. Assoc. Petr. Geol., vol. 20, no. 8, pp. 1123-1124, Aug. 1936. 17 Weller, Stuart, The geology of Hardin County: Illi- nois Geol. Survey Bull. 41, 1920. dome Devonian limestone (thought to be Onondaga in age) crops out, and is encircled by successively younger beds, the outermost of which are Pennsylvanian in age. Weller 18 postulated igneous intrusion as the cause of structural deformation. Although the dome is analogous in many ways to the Omaha dome in Gallatin County, 111., which is known to be intruded, 19 surface examinations and drilling to date have failed to encounter any igneous material. Hicks dome is of interest for this type of study, in preference to the Omaha dome, because of the fewer hazards to radio fields and the more varied surface lithologic contrasts. The dome is cut in places by faults, and faulting is common in the surrounding area. Field strength surveys were made across the dome to explore the feasibility of geo- « Idem. 1; > English, R. M., and Grogan, R. M., Omaha pool and mica-peridotite intrusives, Gallatin County, Illinois: Illi- nois Geol. Survey Rept. Inv. 130, 1948. 54 RADIO WAVE TRANSMISSION logic mapping by signal intensity measure- ments. The area is part of the driftless section of southern Illinois, lying south of the margin of farthest Illinoian ice advance. Topographic relief along the traverse over the dome (fig. 31) is 225 feet. The core of Hicks dome (Devonian) is in sec. 30, T. 11 S., R. 8 E. Successively younger beds are encountered in all direc- tions away from the core of the dome. Penn- sylvanian strata occur about five miles north of the core. In that distance, along the north flank of the dome, the stratigraphic section ranges from Devonian limestone to Casey ville sandstone (Pennsylvanian). A cable tool well was drilled to a depth of 2345 feet in the southeast corner of sec. 30, T. 11 S., R. 8 E., in 1935. The hole started in the Chattanooga-New Albany formation and penetrated 1675 feet of De- vonian and Silurian strata and 570 feet of Ordovician beds. In 1944 the hole was deepened to a total depth of 3295 feet, pene- trating Ordovician strata still farther. The geologic map and cross section (fig. 32) show the attitude of the formations near the core of the dome. This map covers the same area and is drawn to the same scale as the topographic map (fig. 30). Field Hazards The only wires along the traverse are two-strand REA service on poles running along the road between Hicks and Hicks school (fig. 33). There is considerable difference in the curve recorded alongside the wires and in the curve where there were no wires. Its amplitude is greatly increased by the effect of grounded REA poles (fig. 33). Also, signal intensity reaches its low- est level for the entire traverse near these wires, which have an attenuating effect. Topographic relief of 225 feet may pos- sibly influence signal intensity in some small way, but its effect is nowhere evident. Me- teorologic conditions were constant for the 30 minutes it took to complete the traverse. A real field hazard was the opportunity for error in maintaining the shielded loop an- tenna of the RCA 308-B (field intensity Fig. 33. — Signal intensity curve recorded across Hicks dome, Hardin County, Illinois. Intensity decrease (between Hicks and corner 506) is attributed to older strata at the center of the dome. GEOLOGIC FEATURES 55 meter) in a maximum signal orientation over the winding traverse. However, extra precaution was taken to assure proper loop orientation, and intensity anomalies because of misorientation are small and do not affect the overall intensity curve. This was dem- onstrated by repeating the traverse many times and essentially duplicating the re- corded curve each time. The curve shown in figure 33 was selected as representative of approximately 30 curves recorded during three days of field investigation of Hicks dome. Field Intensity Measurements The signal from station WEBQ, Harris- burg, 111. (1240 kc, 250 watts), 15 miles northwest, was used for the traverse. The traverse starts at station 634 (fig. 31), con- tinues along the indicated route to Hicks, Hicks school, B.M. 434, corner 537, corner 506, and ends at Hubbards store. The geological map and cross section (fig. 32) show where successive formations were crossed to reach the Devonian core of the dome and where they were recrossed going down the flank of the dome. The signal intensity curve (fig. 33) shows lower intensity associated with the older beds and higher with younger beds. The lowest signal intensity, between Hicks and corner 506, is found over Warsaw, Osage, and Devonian terrains. The highest occurs farther down the flanks of the dome (geo- logically, not topographically) over St. Louis and younger beds. The irregular and step-like character of the curve over the north flank of the dome (station 634 to Hicks) is interpreted to correspond to the characteristic conductivi- ties and dielectric constants of the Chester and lower Mississippian strata. Kerwin 20 demonstrated a relationship of high field intensity associated with high resistivities and lower intensities with lower resistivities of earth materials. A Schlumberger elec- tric log of a nearby drill hole (Ashland, No. 1 Lackey, sec. 1 1, T. 11 S., R. 9 E.), which penetrates the strata from the Vienna limestone to the St. Louis lime- stone, shows that the apparent resistivities of the formations differ considerably. The limestones are generally high (200 to 1400 ohm meters), the sandstones lower (80 to 190 ohm meters), and the shales lowest of all (10 to 40 ohm meters). A detailed field intensity survey on a detailed chart scale, tied in to formation boundaries and widths along the traverse, would show the relationship between the step-like character of the curve and the various rock strata. There is a small unexplained signal anomaly near B.M. 434 (indicated on the chart with a question mark). The transmitting site at WEBQ is underlain by Pennsylvanian strata. As Hicks dome is traversed and successive strata crossed (from younger to older beds), sig- nal intensity decreases. This, again, would appear to indicate that part of the energy may be propagated through or along strata energized beneath the transmitting antenna, for as successively older beds are crossed (farther removed from Pennsylvanian strata), signal intensity becomes lower. ORE BODIES Several hundred miles of radio field in- tensity traverses were run during two suc- cessive field seasons in the Galena lead and zinc area of northwestern Illinois in order to investigate the possible influence of ore on signal strength. Geologic Setting The Galena area lies almost entirely with- in the driftless region of northwestern Illi- nois. Surface materials consist chiefly of stream deposits, dune sand, loess, and glacial outwash deposits. 21 Topographic relief in the area is about 500 feet. Outcropping strata are Silurian dolomites, and Ordo- vician Maquoketa shale, Galena dolomite, Decorah dolomite, limestone, and shale, and Platteville limestone, dolomite, and shale. 22 These strata are more or less horizontal and parallel. Structure contours on top of the 20 Kerwin, op. cit. 412. 21 Trowbridge, A. C., and Shaw, E. W., Geology and geography of the Galena and Elizabeth quadrangles: Illinois Geol. Survey Bull. 26, pi. IV, 1916. 22 Idem. 56 RADIO WAVE TRANSMISSION Galena dolomite 23 indicate that the surface dips northeast at approximately 17 feet per mile. Structure contours on top of the Oil- rock (Guttenberg member of the Decorah formation) show this surface folded in places forming gently dipping anticlines and synclines. 24 Ore Deposits 25 The lead and zinc ore deposits of the Galena area are in the Galena, Decorah, and Platteville formations, in a zone about 140 feet thick. The mineralized zones range in depth from 100 to 650 feet but are com- monly between 250 and 350 feet. The ore deposits may be curved (arcuate) or long and relatively straight. The minable de- posits are usually 25 to 300 feet wide, thick- nesses are commonly up to 40 feet with a maximum of 125 feet, and length may be several thousand feet. The major zinc-lead deposits (lower-run deposits) are in "flats" (nearly horizontal sheets between or paral- lel to bedding planes) and in "pitches" (sheets cutting across bedding planes). The zinc mineral is largely sphalerite (zinc sulfide). Above water level the sul- fide becomes partially or entirely oxidized to carbonate to form the zinc carbonate min- eral smithsonite. The ore grade ranges from 3 to 20 percent zinc. The lower range of 3 to 5 percent is considered minable de- pending upon economic conditions. Pyrite and marcasite (iron sulfide) are associated with the sphalerite. Metallic iron ranges from 5 to 20 percent of the zinc ore. The mineral galena (lead sulfide) is usually less than 1 percent of the zinc ore but is found locally in rich pockets. The main gangue mineral is calcite (calcium car- bonate). Most top-run deposits are about 100 feet above the lower-run deposits in solution channels in the dolomite. The ore is usually entirely galena but grades laterally into ™ Idem. "Willman, H. B., and Reynolds, R. R., Geological struc- ture of the zinc-lead district of northwestern Illinois: Illi- nois Geol. Survey Rept. Inv. 124, pi. 7, 1947. 25 The geology in the following paragraphs is based on: Willman, H. B., Reynolds, R. R., and Herbert, Paul, Jr., Geological aspects of prospecting and areas for prospecting in the zinc-lead district of northwestern Illinois: Illinois Geol. Survey Rept. Inv. 116, 1946. Willman, H. B. and Reynolds, R. R., Geological structure of the zinc-lead district of northwestern Illinois: Illinois Geol. Survey Rept. Inv. 124, 1947. zinc and iron. The ore deposits are usually less than 25 feet wide, from 5 to 20 feet thick, and up to several hundred feet long. Between the top-run and lower-run deposits there are middle-run deposits which com- bine features of both. The middle-run de- posits are likely to be high in iron. AREAS OF WORKING AND ABANDONED MINES In areas of working and abandoned mines, field hazards are concentrated in the under- ground car tracks, machinery, electric mo- tors and pumps, mine wires and cables, and electric and telephone service wires. Be- cause of them, signal strength anomalies caused by ore deposits are nowhere clearly separable from those caused by field hazard influences. Field intensity surveys were run over the following mining areas: Bautsch, Gray, Pittsburg, Blewett, Ginte, Graham, Graham-Schneider, Vinegar Hill, North and South Unity, and Northwestern. An interesting observation of possible sig- nificance was made on a traverse over the Bautsch ore body before mining was started. The traverse was run across the south end of the ore deposit at the time a shaft was being dug approximately 200 feet north. The only nearby wires were three strands of elec- tric service coming in from the north. The recorded curves, run from southeast to northwest, decreased in signal strength near the ore body, and orientation of the maxi- mum signal direction shifted south approxi- mately 45 degrees. Similar changes in ori- entation were observed over other ore de- posits, but because of the structure, ma- chinery, cables, and guy wires associated with the shafts, no positive significance is attached to these observations. Various other changes in signal strength over operat- ing or developed properties are not attribu- table solely to ore influence because of at- tendant field hazards. PROSPECTIVE ORE-BEARING AREAS Many miles of traverse were run, away from known ore deposits but within the boundaries of the principal mineralized areas, 26 and numerous changes in signal 26 Willman, Reynolds, and Herbert, op. cit., fig. 1. GEOLOGIC FEATURES 57 strength were observed. However, most of these changes are relatively small or so near wires and other field hazards that correlation between them and ore-deposit influence is uncertain. Several exceptions, one a strong intensity anomaly, merit mention. On an east-west traverse along the south edge of sec. 1, T. 28 N., R. 1 E., signal strength (station WKBB, Dubuque, Iowa, 16 miles northwest) decreases significantly and orientation of the maximum signal path shifts approximately 45 degrees near sta- tion 360 (fig. 34). Such a signal strength decrease is common in the presence of inter- mittent overhead wires; here, wire condi- tions (12 strands of telephone line) remain constant for the entire traverse, thus some other explanation for the anomaly is re- quired. It seemed possible that the anomaly might be caused by deposits of conductive ore. The mining company owning the lease drilled a test hole near station 360 (Fur- long lease) on the basis of the signal strength anomaly (fig. 34). Test drilling confirmed the presence of conductive minerals in the bedrock. Field examination of the well cuttings revealed abundant pyrite between 50 and 100 feet deep, some cuttings run- ning as high as an estimated 50 percent iron content. Some zinc was encountered just below 100 feet, but only small amounts of ore minerals were found between the zinc level and the bottom of the hole. The test hole failed to find ore in commercial quan- tities, but the pyrite-rich strata above 100 feet were uncommonly thick and rich for the area. It is possible that this shallow iron deposit is the direct cause of the record- ed attenuation of signal strength, or that associated ore deposits, as yet undiscovered, may exist in this vicinity. A sharp decrease in signal intensity was observed in a north-south traverse along the east edge of sec. 6, T. 28 N., R. 2 E. The decrease occurs approximately ]/g mile north of the southeast corner of sec. 6, and cannot definitely be accounted for by the associated field hazards. A similar sharp signal strength anomaly (increase) was ob- served in the NE J4 sec. 24, T. 28 N., R. ILLINOIS STATE GEOLOGICAL SURVEY Fig. 34. — Signal intensity curve recorded in the Galena, Illinois, area. The signal decrease, cen- tered at station number 360, is attributed to a rich concentration of iron sulphide from 50 to 100 feet deep. 1 E. This too cannot be definitely accounted for by associated field hazards. AREAS OF NEWLY DISCOVERED ORE BODIES To investigate further the relationship between signal intensity and ore deposits, traverses were made over proved ore re- serves where there were few cultural field hazards in the Shullburg, Wis., area. Ex- tensive traverses were made along secon- dary roads and in the fields and meadows away from obvious field hazards. Access to the land containing the ore deposits was facilitated and information on their location and geologic settings was kindly furnished by R. R. Reynolds, 27 who accompanied the field party and aided in making many of the traverses. 27 Geologist, The Calumet and Hecla Company, Shullburg, Wis. 58 RADIO WAVE TRANSMISSION Most of the ore deposits traversed are in or near synclinal axes and trend generally east-west. Three of the six ore deposits crossed have up to 140 feet of Maquoketa shale cover, two have thin Maquoketa shale cover (10 to 40 feet), and one, the Kittoe deposit, has no Maquoketa shale cover. The only clear signal strength anomaly was observed on the Kittoe property where signal strength decreases as the ore deposit is crossed (figs. 35, 36). A small stream, running approximately parallel to the trend of the ore, prevented crossing the ore de- posit completely and might be the cause of the observed anomaly. The Kittoe ore deposit contains rich amounts of galena and pyrite associated with and above the sphalerite. The recorded signal anomaly over the ore may be at least partly caused by lead and iron minerals. Other ore deposits, Hendrickson and Gensler, contain small to large percentages of lead and iron with zinc ore, but no changes in signal strength were observed in traverses across them. The Maquoketa shale has low electrical resistivity (where seen in electric logs of drill holes), is a relatively good conductor, and probably attenuates radio fields greatly. Perhaps the shale lowers the energy sufficiently so that it does not penetrate to the strata below. This factor might account for the anomaly on the Kittoe (with no Maquoketa shale cover) and the absence of anomalies on the other five ore deposits overlain by various thick- nesses of the shale. Many detailed meas- urements were made over these ore deposits using stations with different frequencies, powers, distances and directions of signal path, but strength curves were essentially similar. Summary Field intensity measurements in the Ga- lena area were made over ore deposits rang- ing from 100 to 600 feet in depth. Most of the known deposits traversed are chiefly sphalerite (poorly conductive zinc sulfide) with minor amounts of associated conduc- tive ore in the form of lead and iron sul- fides. No signal strength anomalies were ILLINOIS STATE GEOLOGICAL SURVEY Fig. 35. — Signal intensity curve of WMAQ re- corded south of Shullberg, Wisconsin. Decrease in signal intensity may be due in part to the in- fluence of the Kittoe ore deposit. observed which could be attributed solely to deposits of the ore mineral sphalerite. Signal strength anomalies recorded near working or abandoned mines were usually accompanied by cultural field hazards so that their significance is doubtful. In pros- pective ore-bearing areas away from known deposits several interesting anomalies were observed. One such anomaly appears to be accounted for, in part at least, by a rich concentration of conductive iron sulfide at shallow depths. On traverses over newly discovered and unmined ore deposits, only at Kittoe is there a signal anomaly. From observations of signal strength be- havior in air in the presence of linear metal conductors, such as wires and railroad tracks, it seems probable that linear or tab- GEOLOGIC FEATURES 59 ILLINOIS STATE GEOLOGICAL SURVEY Fig. 36. — Signal intensity curve of CHI recorded south of Shullberg, Wisconsin. Decrease in sig- nal intensity may be due in part to influence of the Kittoe ore deposit. ising where there are shallow conductive ores not covered by Maquoketa shale. On the basis of observations by Cloos 28 in the Baltimore, Md., area, steeply dipping bedrock contacts greatly reduce signal strength or even eliminate it. Thus an es- sentially nonmetallic tabular ore body might be recognized through field intensity meas- urements if the ore was steeply dipping or in an almost vertical position. In the Galena area, steeply dipping, poorly conduc- tive tabular ore bodies in pitches are usually associated with the lower- and sometimes with the middle-run ore deposits. No ob- served signal strength anomalies can be defi- nitely attributed to such structures. How- ever, it is possible that if the steeply dipping tabular ore deposits were of larger dimen- sions or came closer to the surface, field intensity measurements might reveal their presence. UNDERGROUND MINED-OUT AREAS Investigation of the influence of under- ground mined-out areas on signal strength was neither planned nor anticipated. It resulted from ordinary observation and co- incidence. Traverses were run looking for signal anomalies from the southern exten- 28 Cloos. op. cit. ular conductive ore deposits underground might exert similar influence. Most metal conductors in a radio field reduce the signal strength considerably, but occasionally they increase it, possibly through reradiation. It appears that in the absence of field hazards, or with recognition of their effects if pres- ent, abnormally low signal strength (or more rarely abnormally high strength) might be indicative of conductive ore in the bedrock. This is assuming that the radio field penetrates to the ore deposit. In the Galena area, the low-resistant Maquoketa shale appears to act as an electrical shield preventing appreciable amounts of radio fields from reaching underlying conductive ore deposits. Apparently here ore detection by the field intensity method is only prom- ILLINOIS STATE GEOLOGICAL SURVEY t Fig. 37. — Map showing route of traverse (sta. 208—261—262) over mined-out area along Illinois Highway 13, Gallatin County. Contours are topographic, fault is extension of Ridgeway fault, and outcrops are of No. 6 and No. 5 coal beds. 60 RADIO WAVE TRANSMISSION sion of the Ridgeway fault in Gallatin County, 111. Along Illinois Highway 13, between Equality and Shawneetown, a sig- nal intensity high was repeated near the B. & W. coal mine, sec. 13, T. 9 S., R. 8 E. (fig. 37). The mine superintendent kindly permitted access to an up-to-date mine map which showed the extent of the mined-out area. This area corresponded almost pre- cisely with the field-intensity high. The Ridgeway fault had not been encountered in the mine. Therefore, inference was made (erroneously) postulating a cause-and-effect relationship between the mined-out area and the signal anomaly. Geologic Setting of the B. & W. Coal Mine The Harrisburg (No. 5) coal was being mined at a depth of 95 feet, 275 feet above sea level. Fifty-five to 65 feet of interbedded shale and siltstone and shaly siltstone lie above the coal bed, and 30 to 40 feet of cover overlies the bedrock surface. No. 6 coal crops out north of the mine and No. 5 crops out south (fig. 37). The mined-out area was "dry" and offered a geologic dis- continuity, some 95 feet below the surface, between bedrock and the air-filled mined- out area. At that time, early in the field work (June 1947), it was postulated that because the air in the mine was probably a better medium for radio wave propagation and transmission than the surrounding rocks, signal strength increased. This led to in- vestigation of other mined-out areas. Truax-Traer Coal Mines Through the courtesy of Walter Roe, engineer for Truax-Traer Coal Company, field investigations were made over a large, abandoned, watered mine at Hollidayboro, Jackson County, 111. The depth to the coal (No. 6) ranges from about 70 to 130 feet. A cap rock (Herrin limestone) up to 6 feet thick overlies the coal bed. The limestone is not continuous because of solution along jointing planes. Traverses run over the area used signals differing in power, fre- quency, distance, and direction of signal path, but no significant intensity anomalies were obtained. Radio transmitters, con- structed in the Illinois Geological Survey laboratory, were used first over the mined- out area, then off the mined-out area, but precise measurements showed no significant anomalies. The Hollidayboro and the B. & W. mines differ chiefly in conditions within the mined- out areas and in the immediate caprock. The mined-out area at the B. & W. mine is "dry," the caprock is interbedded shale and siltstone ; at Hollidayboro the mined-out area is filled with water, the caprock is lime- stone. The difference in effect on signal strength between the B. & W. mine and the Hollidayboro mine was tentatively explained by the differing conditions of the mined-out areas. It was thought that perhaps the electrical discontinuity between bedrock and Hollidayboro mine water (an electrolyte) was not as great as that between bedrock and air (assuming radio fields were pene- trating to these depths). However, in elec- tric logs of nearby drill holes, the shales above the coal have low electrical resistivi- ties and therefore could be acting as a shield to prevent an appreciable amount of a radio field from reaching the mine-out area. M. B. Buhle, Illinois State Geological Survey geologist, made collateral earth re- sistivity measurements in the Hollidayboro area. These measurements are made by in- serting four metal stakes into the ground (at various spacings to achieve various depths of penetration) along a line of trav- erse. From 10 to 200 volts at 18 cycles per second, at 10 to 200 milliamperes, are applied to the two outer stakes. Potential difference is measured between the two inner stakes and, from these readings, ap- parent resistivity in ohm-centimeters is cal- culated using the Wenner formula. 29 Measurements were taken for depths from 10 to 150 feet (maximum depth of coal 130 feet), at five- or ten-foot intervals. Re- sistivity values in ohm-centimeters at depths of 50 feet average about 4000, at 75 feet about 6000, at 100 feet about 7000, and at 150 feet about 5000. These values are relatively low and for the most part result from the high conductivities of the large 29 Wenner, Frank, A method of measuring earth resistivity: Bur. Standards Sci. Paper 258, July 15, 1914. GEOLOGIC FEATURES 61 amount of shale in the geologic section. If these shales act as a shield to radio fields preventing them from penetrating to the geologic discontinuity below, then of course the discontinuity cannot affect signal strength measured at the surface. Additional traverses were run over coal mines in Franklin and Williamson counties. No signal strength anomalies were recorded over these mines except in a few question- able instances where field hazards must be taken into consideration. Why a signal anomaly should be present over the B. & W. mined-out area and not over other mined- out areas needed to be explained. There- fore the B. & W. coal mine was re-examined. Re-examination of B. & W. Mine In the meantime, much had been learned about field hazards, and close examination of wire conditions along the traverse pro- vided a reasonable explanation, other than geologic, for the intensity anomaly. Examination of the field intensity chart recorded between stations 208 and 262 (figs. 37, 38) reveals a good intensity high over the mined-out area (labeled mine). Signal strength decreases to the west (toward sta- tion 208) and to the east (toward station 261), but increases farther east (toward station 262). Inspection of the wire condi- tions (noted to the left on the chart) reveals that four strands of REA wires south of the road and ten strands of telephone wires north of the road run along the entire traverse. Apparently the effect of the wires on the signal from WSON, Henderson, Ky., 38 miles east, is a general reduction in in- tensity, because at the two places where the wires bend away from the road, signal in- tensity increases noticeably. The wires, al- though they are constant for the entire length of the traverse, bend away from the road at the mine (increasing signal strength), and the significance of their tem- porary bending away from the road was not previously realized. The probable south- ern extension of the Ridgeway fault trends along the east edge of the mine (this is known from B. & W. mine test holes), ILLINOIS STATE GEOLOGICAL SURVEY Fig. 38. — Signal intensity curve run over mined- out area along Illinois Highway 13, Gallatin County. Mined-out area labeled mine, and places where wires bend away from the road are indicated. but it could not be confirmed by field inten- sity measurements because of the strong in- fluence of the wires. Because of the premature incorrect cor- relation between the mine and signal in- tensity, later anomalies of all types were carefully analyzed for influence of field hazards before the cause was attributed to geologic influence. At the B. & W. mine a comparatively small mined-out air-filled area, covered by 95 feet of bedrock and alluvium, has no recognizable influence on signal strength measured at the surface. It is conjecture whether the absence of a sig- nal anomaly results from the mined-out area's insignificant influence or on insuffi- cient depth penetration of the radio field. 62 RADIO WAVE TRANSMISSION / i Mahomet (Teays] bedrock valley Springfield SANGA Middletown bedrock valley MON I _l"~ i m ILLINOIS STATE GEOLOGICAL SURVEY 10 20 30 miles Fig. 39. — Location of 40-mile traverse along Illinois Highway 54 between Clinton and Springfield, Illinois. Bedrock valleys are after Horberg, Leland, Bedrock topography of Illinois: Illinois Geol. Survey Bull. 73, 1950. SOILS The Federal Communications Commis- sion regulations require that standard broad- cast transmitter locations comply with good engineering practice. The general require- ments include the following: 30 A map clearly showing: 1. Proposed location. 2. Surrounding business, industrial, residen- tial, and unpopulated areas. 3. Density and distribution of population. 4. Heights of all tall buildings and other structures. 5. Location of airports, airways, and other radio stations. 6. The terrain and types of soil. 30 Standards of good engineering practice concerning stand- ard broadcast stations 550-1600 kw.: Federal Communica- tions Commission, U. S. Govt. Printing Office, Washington, D.C., pp. 29-30, 1940. Concerning types of soils the Commis- sion says: 31 The type and condition of the soil or earth immediately around a site is very important. Im- portant, to an equal extent, is the soil or earth between the site and the principal area to be served. Sandy soil is considered the worst type, with glacial deposits and mineral-ore areas next. Alluvial, marshy areas, and salt-water bogs have been found to have the least absorption of the signal. In the past, soils have been considered of prime importance in controlling the value of conductivity and dielectric constant of the earth along a signal path. In recent years, the amount of depth penetration of radio waves has been revised downward to ^Idem., p. 33. GEOLOGIC FEATURES 63 include not only the soils but earth material below. Terman says: 32 The value of conductivity and dielectric con- stant that is effective for radio waves represents the average value for a distance below the sur- face of the earth determined by the depth to which ground currents of appreciable amplitude exist. This depth of penetration is commonly on the order of 5 to 10 feet at frequencies used in short-wave communication, and 50 or more feet at broadcast and lower frequencies. As a result, the earth constants are not particularly sensitive to conditions existing at the very surface of the earth, as, for example, recent rainfall. If conditions, such as recent rainfall, at the very surface of the earth do not appre- ciably change the value of conductivity and dielectric constant, then soils, the thickness of which in temperate latitudes ranges from a few inches to several feet, should also cause little appreciable change. Soil Influence on Signal Strength Forty-mile traverses were run between Clinton and Springfield, 111., along Fed- eral Highway 54. Soil survey maps 33 are available for the three counties crossed by the traverse. The first traverse was run along a radial using the signal from WCVS, Springfield ; the second, approximately along an arc using WSIV, Pekin (fig. 39). More than 20 soil types, including sandy soils, are crossed and recrossed along the 40-mile traverses, but at no place is there an indication of a signal anomaly that can be construed to be related to the various soil types. Part of a recorded curve, chosen because it crosses several soil type bound- aries in a short distance and because no wires or other field hazards are recognized, shows an almost constant signal intensity (fig. 40). This curve, representing a dis- tance of 1 mile, runs 4 to 3 miles south- west of Kenny, DeWitt County, and is a segment of the continuous curve for 40 miles. The specific soil types traversed are indicated by patterns at the side of the chart. From the soil report of DeWitt County, 34 32 Terman. op. cit., p. 709. 33 Soil reports of DeWitt, Logan, and Sangamon counties : Univ. of Illinois Agr. Expt. Sta., Urbana. Illinois. 34 Smith, G. D., and Smith, L. H., DeWitt County soils: Soil Report 67, Univ. of Illinois Agr. Expt. Sta., Urbana, June 1940. ILLINOIS STATE GEOLOGICAL SURVEY Fig. 40. — Signal intensity curve, 3 to 4 miles south of Kenny, DeWitt County, Illinois. Signal intensity remains remarkably constant across different soil types (indicated to the left). the Drummer clay loam occurs in the nearly level or depressional areas in the upland, the surface soil (10 inches) is a well-granulated black clay loam, the subsurface (10 to 15 or 18 inches) is a brownish-gray or very dark-gray clay loam, the subsoil (15 to 18 to 35 inches) is a gray clay loam mottled with pale yellow, below 35 inches the material is rather friable ; the Osceola silt loam occurs on nearly level or slightly de- pressed areas on the outwash plains, the sur- face soil (7 to 8 inches) is a brownish silt loam with a gray cast, the subsurface (7 to 8 inches to 20 to 30 inches) is a dull-gray friable silt loam, the subsoil (20 to 30, to 50 to 60 inches) is a yellowish-gray clay, below 50 to 60 inches are stratified outwash sands; the Harpster clay loam occurs chiefly 64 RADIO WAVE TRANSMISSION in depressions and in association with Drum- mer clay loam, the surface soil (5 to 10 inches) is a grayish-black clay loam or a silty clay loam, the subsurface is a dark yellowish- gray loam, the subsoil is a yellow-mottled light-gray clay loam. The Brenton silt loam occurs on the undulatory glacial out- wash plains, the surface (8 to 10 inches) is a finely granular dark-brown silt loam, the subsurface (8 to 10, to 16 to 18 inches) is a light-brown silt loam, the subsoil (16 to 18, to 45 to 60 inches) is a yellowish-brown silty clay or clay loam, below 45 to 60 inch- es are beds of almost pure sand. Examination of the signal intensity curve recorded across these soil types (fig. 40), in the absence of known or suspected field hazards, shows a remarkably constant in- tensity. Crossing soil-type boundaries ap- pears to have little or no effect on signal intensity. Collateral earth resistivity surveys were made by M. B. Buhle in a nearby area (Elk- hart, Logan County). Resistivity values ranging from 2500 to 26,000 ohm-centi- meters at depths of five feet were recorded crossing Drummer clay-loam and Brenton silt-loam areas. Thus it appears that Ter- man is correct in saying that conditions ex- isting at the very surface of the earth do not appreciably change the value of effec- tive conductivity and dielectric constant. Other collateral earth resistivity measure- ments made in Macon County (Harris- town), Ford County (Gibson City), and Jackson County (Truax-Traer coal mines), show highly variable earth resistivities in soils of various types where signal intensity curves remain remarkably constant. It would appear, from traverses run in Illinois, that soil types developed on gla- cial drift have little or no influence on signal intensity. However, in view of the effect of bedrock on signal intensity, it would seem reasonable to expect that resid- ual soils in temperate latitudes, in ungla- ciated terrain, could influence signal in- tensity depending upon soil thickness and character of the bedrock. BEDROCK VALLEYS AND DEPTH TO BEDROCK Most of the field intensity surveys for the present work were run over terrain cov- ered with glacial drift. The exceptions are those run in the driftless area of north- western Illinois and adjacent Wisconsin, and south of the line of farthest ice advance in southern Illinois and adjacent Kentucky. NONGLACIATED AREAS Depth to bedrock in the nonglaciated areas is controlled mainly by the thicknesses of residual soil, wind-blown material (loess and sand), river and lake deposits, and in some areas glacial out wash. Residual soils in the areas investigated are relatively thin, reaching maximum thick- nesses on flats and valley bottoms and minimum thicknesses on the slopes. Bed- rock, either in outcrop or close to the sur- face, undoubtedly influences signal intensity far more than the soils (figs. 31, 32, 33). Loess and combinations of loess and sand attain thicknesses of more than 25 feet in some parts of Illinois. 35 These deposits, chiefly loess, cover glaciated as well as non- glaciated areas. Their influence on signal intensity is probably greatest where they are thickest. Traverses run over thick loess deposits on the Shawneetown Hills, Galla- tin County, 111. (figs. 18, 20), fail to pro- vide clues on signal intensity influence by loess. Intensity decreases slightly over the hills, but this may be because of the loess, vegetation, or the erosional remnant of bed- rock (the hills themselves) surrounded by the Ohio River alluvial plain. River and lake deposits attain consider- able thicknesses in parts of Illinois. Allu- vial terrain absorbs the signal much less than glacial and loess-covered glacial terrain. 36 M. B. Buhle found in his extensive earth resistivity surveys throughout Illinois that alluvial material, chiefly sands and gravels, 35 Smith, G. D., Illinois loess, variations in its properties, a pedologic interpretation: Univ. of Illinois Agr. Expt. Sta. Bull. 490, July 1943. 38 FCC, Standards of good engineering, op. cit., p. 33. GEOLOGIC FEATURES 65 usually has far greater electrical resistivity than glacial tills, that loess usually has low resistivity, and lake and river silts, even lower. 37 Field intensity traverses were run over parts of Gallatin County, 111., and Union County, Ky., covered by 100 to 150 feet of Ohio River alluvium and glacial out- wash (figs. 18, 27, 28). The curves re- corded along these traverses show signal strength anomalies which appear to be best explained by faulting in the bedrock. If this is true, then the radio fields are penetrating 100 to 150 feet of alluvial material to reach bedrock, and alluvial influence (if present) on signal strength is less than that of bed- rock structure. Glaciated Areas Where glacial drift is present in Illinois it ranges in thickness from a few inches to more than 600 feet where moraines cross deep bedrock valleys. 38 Horberg 39 has iden- tified Kansan and possibly Nebraskan gla- cial deposits in samples from drill holes in the Mahomet bedrock valley in Champaign County. He recognizes three soils; the lowermost or Aftonian is underlain by sand and gravel. The Illinoian glacial deposit covers nearly two-thirds of the state and is 5 to 50 feet or more thick. 40 This drift con- sists largely of bluish-gray clayey till which has been weathered to depths of 15 feet or more. 41 Sangamon interglacial deposits sep- arate the Illinoian from the overlying early Wisconsin drift which covers nearly a third of the state (east, central, and north). De- posits of middle Wisconsin age cover a small part of the state in the Chicago area and north. Many miles of traverses of field intensity measurement in Illinois were run over gla- cial drift. A seven-mile traverse was run 37 Buhle, M. B., Illinois Geol. Survey, personal communi- cation, 1949. 38 Horberg, Leland, op. cit., Illinois Geol. Survey Bull. 73, 1950. 39 Horberg, Leland, A major buried valley in east-central Illinois and its regional relationships: Jour. Geol., vol. 53, no. 5, 1945; reprinted as Illinois Geol. Survey Rept. Inv. 106, p. 353, 1945. 40 Alden, W. C., Glacial geology of the central states : Sixteenth Int. Geol. Cong., Guidebook 26, Excursion C-3, p. 7, 1933. 41 Leighton, M. M., and MacClintock, Paul, Weathered zones of the drift-sheets of Illinois: Jour. Geology, vol. 38, no. 1, pp. 28-53, 1930; reprinted as Illinois Geol. Survey Rept. Inv. 20, 1930. over the buried Mahomet bedrock valley, starting at the northwest corner of sec. 35, T. 21 N., R. 6 E., Piatt County, III, and running due east to the northeast corner of sec. 35, T. 21 N., R. 7 E., Champaign County, 111. The traverse starts high on the west slope of the buried valley, crosses the west slope, the valley bottom, and ends half a mile up the east wall. Depth to bedrock at the starting point is 280 feet; the west slope is about five miles long; depth to bedrock at the foot of the west slope is 430 feet ; the valley bottom is about one mile wide and depth to bedrock is greater than 430 feet ; half a mile up the east valley wall, depth to bedrock is 375 feet; topo- graphic relief along the traverse is approxi- mately 80 feet. The signal used was 1000 watts at 1020 kc, originating about 50 miles west and slightly north. The traverse is essentially along a radial from the transmitting station. Signal intensity on the recorded curves de- creases as bedrock becomes deeper along the traverse, reaches a minimum over the valley floor, and increases again as the bedrock be- comes shallower along the east wall. The overall signal intensity decreases gradually from west to east. This is to be expected along a radial away from the signal source. REA and telephone wires parallel the road along most of the traverse. There is a grounded-pole effect across the valley bot- tom (fig. 17). Here also, the present Sanga- mon River valley crosses the bedrock Ma- homet valley, and signal decrease may be due in part to the influence of the water course. The signal decrease coincides with the place of greatest drift thickness (over the bedrock valley floor). However, field haz- ards (wires, grounded poles, the Sangamon River, and the radial traverse) make it difficult to prove that the signal anomaly is solely the result of increased depth to bed- rock. A traverse across the south wall toward the center of Mahomet bedrock valley (Area IV, fig. 21) is apparently not influ- enced by the valley. The 3960-foot trav- erse crosses at least 100 feet of valley slope without noticeable signal strength variation that can be attributed to depth of bedrock. 66 RADIO WAVE TRANSMISSION However, this is hardly a fair test because the woods influence signal intensity along part of the traverse. Other traverses cross a smaller bedrock valley, tributary to the Mahomet bedrock valley, near Fisher, Champaign County, 111., and near Gibson City, Ford County, 111., but because of numerous field hazards the recorded signal anomalies cannot be at- tributed with assurance to variations in drift thickness. The 40-mile traverse between Clinton and Springfield, 111., crosses the buried bed- rock Mahomet valley and its tributary, the Middletown bedrock valley 42 (fig. 39). Depth to bedrock is more than 200 feet in the Middletown valley, more than 450 feet in the Mahomet valley, and less than 200 feet elsewhere along the traverse. Neither valley visibly affects signal intensity. This may be due to interference from field haz- ards, although it is considered unlikely be- cause field hazards are few ; or possibly the variation in depth to bedrock did not affect 42 Horberg, Leland, op. cit., Illinois Geol. Survey Bull. 73. the field intensity of the two particular sig- nals used along this traverse. There is a gradual thinning of the drift from 155 feet at Cornland to 10 feet or less at Springfield (fig. 39) accompanied by a corresponding decrease in signal intensity (from Pekin) towards Springfield. This signal decrease may result largely from drift thinning. Several traverses cross the buried bedrock Saline valley in Saline County, 111. Some signal anomalies were recorded near the po- sition of the bedrock valley and its tribu- taries, but present drainage, geologic struc- tures, and field hazards nearly coincide with the bedrock valleys and leave the true source of the anomalies uncertain. Additional investigation is needed to de- termine the effect on signal intensity of thick- ness of cover over bedrock. A possible ap- proach might be to energize electromag- netically a selected bedrock stratum and then measure the signal intensity over the energized area to find out if there are differ- ences in signal strength caused by differences in thickness of the overburden. CHAPTER 9 — SUMMARY AND CONCLUSIONS This is a report on a preliminary investi- gation of features that affect radio field intensity in the ground-wave area at stand- ard broadcast frequencies. Theoretical con- siderations of field intensity behavior have been treated only superficially, and pri- mary attention has been given to sig- nal strength behavior in the field. Hun- dreds of miles of traverses were run in Illi- nois and immediately adjacent areas, meas- uring and automatically recording signal in- tensity. These traverses were run chiefly in areas of known field hazard and geologic conditions. Experimental data were gath- ered on field strength behavior in the pres- ence of cultural and natural features. These data were examined and evaluated for a cause-and-effect relationship between cul- tural and natural features and signal in- tensity anomalies. The conclusions may be summarized as follows : 1. From the present and previous work it appears that radio waves penetrate bed- rock. Numerous instances of penetration up to 1000 feet have been reported, and in one instance 6000 feet of quartzite was re- ported to have been penetrated by a signal from a 10-watt transmitter between 100 and 300 kc. 2. Satisfactory reconnaissance field in- tensity surveys can be made using battery portable and automobile radios by meas- uring variations of the intermediate fre- quency voltage. 3. Portable field equipment, capable of reliable detailed field intensity measure- ments, was developed for this investigation and proved satisfactory. It consists of an RCA 308-B field intensity meter with a bidirectional shielded loop antenna, a vibra- tor power supply for the 308-B meter, and an Esterline-Angus model A-W graphic re- corder, driven by a Clark cable drive from a tee in the speedometer cable of the trans- porting vehicle. This equipment, suitably shock-mounted in a wooden-bodied station wagon, withstands rough field treatment. An improvement in this instrumentation would be incorporation of a device that automatically orients the shielded loop an- tenna in the direction of maximum signal intensity. 4. Continuous signal intensity measure- ments in the field have shown that spot read- ings, taken at large intervals along a trav- erse and without regard for field hazards, can be misleading. 5. The laboratory attenuation tests on diamond drill cores, although possibly indic- ative of electromagnetic conductivities of rocks, are not conclusive because of the great difference between laboratory and field conditions. 6. In field measurements, relative signal intensity curves, which are similar to ac- tual field intensity curves, were used throughout most of the investigation be- cause they require less calibration and set- up time and thus make possible the survey- ing of greater areas more rapidly. 7. Field notes are best kept on the paper chart of the recorder so that all the perti- nent data are in one convenient place. Field notes should include date, time, weather, station used (frequency, power, direction, and distance from traverse), notes on geo- graphical orientation, on associated field hazards, and on signal anomalies, whether caused by obvious field hazards or by un- known features. 8. Signals to be measured should be se- lected well within their ground-wave areas. A weak (250 to 1000 watt) signal from nearby (5 to 50 miles, depending upon fre- quency) is to be preferred to a strong (5000 to 50,000 watt) signal because the weaker signal is more affected by a geologic discon- tinuity than a strong signal, which tends to overcome the discontinuity and minimize the signal anomaly. 9. The most common field hazards in Illinois are wires; overhead electric (REA) and telephone wires almost invariably cause [67] 68 RADIO WAVE TRANSMISSION strong signal anomalies. These anomalies are usually sharp decreases in intensity, but rarely they are sharp increases. Wires paral- lel to a traverse commonly decrease the general signal intensity level along the en- tire traverse. Other field hazards that affect signal intensity similarly are bridges, rail- road tracks, metal structures (towers and buildings), fences, and pipelines. Anomalies caused by streams are usually sharp (either increase or decrease), but of relatively small magnitude. Individual trees have little if any effect on signal intensity at broadcast frequencies. Wooded areas, if large enough, some- times cause shadow effects, and signal level decreases when the woods lie between the radio station and the point of measurement if the point of measurement is within 100 feet of the woods. Woods affect high fre- quency signals more than signals at low frequencies. Topography as a field hazard in Illinois can be almost disregarded. Large hills are the only important topographic features that influence signal intensity. This influ- ence is observed only when the hill is inter- posed directly between the radio station and the point of measurement, and if the point of measurement is within a few tens of feet from the hill. Many field hazards that can seriously affect field intensity measurements may be recognized, therefore making it possible to evaluate the amount of geologic influence. 10. For any period up to six hours during the daytime, under relatively constant weather conditions, meteorological effects on ground-wave signal intensity appear to be negligible. 11. The Shawneetown - Rough Creek fault in Illinois and Kentucky and the In- man East fault in Illinois are apparently responsible for signal strength anomalies on traverses across them. Lithologic discon- tinuity probably causes a discontinuity in electromagnetic waves and is reflected in signal strength behavior. It would therefore appear feasible to map similar features by this technique. 12. Signal strength is affected near the Kentland quarries in the cryptovolcanic structure in northwestern Indiana. It in- creases for signals originating on Ordovi- cian, Silurian, or Devonian terrain, but does not change for signals originating on Penn- sylvanian terrain. 13. Signal intensity decreases over the core of Hicks dome in Hardin County, 111. The signal, transmitted from Pennsylvanian terrain, decreased in strength as the dip- ping rocks were traversed from younger to older towards the core. It is expected that similar lithologic contrasts elsewhere can be mapped by this method. 14. Data from the traverses across known geologic features (11, 12, 13) sug- gest that part of a radio field may be trans- mitted or propagated by or along bedrock strata. Signal intensity is high over the Inman East fault. Here, the West Frank- lin limestone may be carrying some of the radio field from Evansville, Ind. In the Kentland area in Indiana, part of the radio field appears to enter the cryptovolcanic structure along Ordovician, Silurian, and Devonian bedrock, thus in- creasing signal intensity measurably over the regional level. At Hicks dome, signal intensity decreases over the older beds across the core of the dome. A signal, originating on Pennsjl- vanian terrain, was stronger over Pennsyl- vanian rocks around the flanks of the dome than it was over the Devonian and Mis- sissippian rocks across the dome. Signal strength behavior becomes more understandable if, in addition to atmos- pheric propagation, transmission along bed- rock is postulated. Apparently limestone with high electrical resistivity offers a path for radio energy along which attenuation is less than it is along shales, siltstones, or sandstones. It seems probable that certain bedrock strata may act as wave-guides. 15. In the Galena area in Illinois no sig- nal anomalies can be attributed solely to the influence of the poorly conductive zinc ore mineral sphalerite. A rich concentration of pyrite and marcasite at shallow depths caused at least one strong signal anomaly (decrease in strength). Signal anomalies over the Kittoe ore body are attributed at least in part to the conductive minerals SUMMARY AND CONCLUSIONS 69 iiipillt^ ^ '->";•<»> ^"i m ^W^Vs: ililiiiiiMll 70 RADIO WAVE TRANSMISSION associated with the ore at shallow depths. The Maquoketa shale in the Galena area and some Pennsylvanian shales in southern Illinois appear to act as shields to radio fields, thereby preventing penetration to the strata beneath the shales. 16. Soils developed on glacial till, or on loess overlying glacial till, appear to have little if any influence on signal intensity. 17. Depth to bedrock may influence sig- nal strength in some places, but the kind and amount of influence has not yet been satisfactorily determined. Several traverses crossed bedrock valleys where depth to bed- rock is known to be variable, but the signal anomalies cannot be attributed solely to changes in depth to bedrock. The same is true of sand and gravel de- posits in glacial drift ; traverses run over known deposits sometimes produce signal anomalies but cannot be attributed solely to the influence of the deposits on signal in- tensity. 18. Where the geology offers discontinu- ities to electromagnetic waves, and field hazards are few, radio field intensity sur- veying offers promise of a rapid and eco- nomical means of getting data to construct reconnaissance geological maps. The step- like character of the intensity curve record- ed across the flank of Hicks Dome is an example, and this kind of surveying should be applicable in any area where bedrock is within 50-100 feet of the surface and field hazards are few. If overwhelming field hazards along roads are unavoidable, the method could be tried by boat down rivers and streams. Intensity surveying from a slow, low-flying aircraft, above field haz- ards, could be tried. 19. In general, when transmitting and receiving antennas are on (or close to) the same rock strata, reception is best. 20. Direct electromagnetic energization of highly electrically resistant strata might be attempted. A section thus energized might be traced (in drill holes if necessary) to determine areal extent, structure, con- ductive ore deposits, and lithologic changes such as reefs. It is possible that a system of underground communication could be es- tablished which would prove practical under special circumstances. 21. Knowledge of the behavior of radio waves over, along, and through bedrock can aid in the selection of good locations for various types of radio communication equip- ment. 22. Further study of radio wave inten- sity and polarization, related to geologic structure, is needed to help resolve the method. APPENDIX A FCC GROUND CONDUCTIVITY MAP OF THE U.S. A ground conductivity map 1 prepared in 1938 by the Broadcast Division of the Bureau of Engineering of the Federal Com- munications Commission, agrees remarkably well with the geological map of the United States (fig. 41). This agreement led to conjecture about the relationship between geology and radio wave transmission and was the starting point of the present work. Communication with T. J. Slowie 2 re- vealed how the map was made. Field in- tensity measurements, taken by major broad- casting companies, individual stations, and the FCC staff, were examined for their asso- ciation with soil types, geologic formations, topography, and vegetation. At that time there were comparatively few measurements, particularly over large distances, and ground conductivities for such areas were approxi- mated from the values that soil types, geo- logic formations, topography, and vegeta- tion exhibited in other areas where conduc- tivities were known by actual measurement. In the preparation of the map, reference was made to the U. S. Geological Survey's Geologic Map of the United States and several "Soil Regions" maps of the U. S. Department of Agriculture. The FCC has found discrepancies in cer- tain areas. The most significant errors have been where conductivity estimates were 1 Standards of good engineering practice, op. cit., Fig. 3. 2 Slowie, T. J., Secretary of the FCC, Washington, D.C., 1949. made for restricted areas. Thus, conduc- tivity along ridges in a mountainous region may be considerably less than estimated on the map; along the axes of valleys conduc- tivity may be far greater than estimated. In general the map has been accurate, and the abundant conductivity data collected be- tween 1938-1949 has not required any ma- jor revision. Analysis of Map Ground conductivity is expressed in elec- tromagnetic units (emu) ; 1 X 10" 14 emu represents poor conductivity and 30 X 10~ 14 emu, good conductivity. Analysis of the ground conductivity areas of the map was made for the age of rocks outcropping in them. Ground conductivity values for rocks of all ages were tabulated by outcrop areas, and an average conductivity value was esti- mated for each geological period. For pre- Cambrian and intrusive rocks, conductivity is low (3 X 10" 14 emu) ; conductivity in- creases through the successive periods of the Paleozoic reaching a high of 15 X 10" 14 emu in Permian outcrop areas ; averages 7 X 10~ 14 emu for Triassic and Jurassic rocks ; is 10 X 10 14 emu for Lower Cretaceous and reaches a high of 20 X 10 14 emu for Upper Cretace- ous rocks; Eocene rocks average 14 X 10" 14 emu, Oligocene 5 X 10 14 emu, and Miocene and Pliocene average 7 X 10" 14 emu in their outcrop areas. [71] APPENDIX B GLOSSARY OF RADIO TERMS Absorption — The loss of energy from a wave by dissipation in propagation through or adja- cent to a dissipative medium. Atmospheric noise — Noise caused by natural elec- trical discharges in the atmosphere (also called "static"). Attenuation — Of a wave, the decrease in dis- placement with distance in the direction of propagation. If the attenuation varies with frequency, it is defined for a sinusoidal wave of a certain frequency and of constant ampli- tude at any point. The attenuation of a wave may be defined relative to the attenuation in some ideal conditions such as in free space or over a perfectly conducting plane. Conduction of current — 1. Metallic: conduction due to the movement of free electrons. 2. Electrolytic: conduction from the transport of ions in electrolytes. 3. Dielectric: no free electrons available. Conductivity of earth materials — Good, 10 x 10" 14 emu and above; intermediate, about 5 x 10 14 emu; poor, 1.0 x 10" 14 emu. Specific rocks and minerals may be divided into three groups as to resistivity: Good conductivity, 10" 6 to 10 ohm-centimeters; intermediate, 10 2 to 10° ohm-centimeters; poor, 10 10 to 10 17 ohm-centi- meters. db. — Zero decibel is the threshold of hearing, 60 db. is the level of ordinary conversation, and 120 db. is the level of thunder. Dielectric — Nonconducting for direct current, an insulating medium such as air, glass, oil, or mica, but will conduct alternating current. Dielectric constant — The presence of a dielectric other than a vacuum raises the capacity of a condenser in comparison to its capacity in the absence of the dielectric by a factor known as the dielectric constant. The dielectric constant of air is 1 and sea water is 81. Displacement — A change in a medium, propor- tional to the square root of the stored energy of a certain kind. It is exemplified by compres- sion in a sound wave and by electric or mag- netic flux density in an electromagnetic wave. Electromagnetic wave — A wave in which there are both electric and magnetic displacements. Electromagnetic waves are known as radio waves, heat rays, light, X-rays, etc., depending on the frequency. emu — Electromagnetic cgs units. The electro- magnetic system of cgs units (abbreviated emu) results if one uses centimeters, grams, and seconds, and then arbitrarily assumes that the magnetic permeance of a centimeter cube in a vacuum is unity. Fading — The variation of radio field intensity caused by changes in the transmission medium. Field — Open country, woods, swamps, hills, rolling land, place of outdoor operations in geologic and geophysical investigations. Field — A portion of space controlled or affected by a force. Ground reflected wave — The component of the ground-wave that is reflected from the ground. Ground-wave — A radio wave that is propagated over the earth and is ordinarily affected by the presence of the ground. The ground-wave in- cludes all components of a radio wave over the earth except ionospheric waves and trop- ospheric waves. The ground-wave is some- what refracted by the normal gradient of the dielectric constant of the lower atmosphere. Guided wave — A wave whose propagation is concentrated in certain directions within or near boundaries between materials of different properties located in a path between two places. I.F. — Intermediate frequency. Ionosphere — That part of the earth's atmosphere above the lowest level at which the ionization is large compared with that at the ground, so that it affects the transmission of radio waves. (Experiments indicate that this lowest level is about 50 kilometers above the earth's sur- face.) Ionospheric wave (sky-wave) — A radio wave that is propagated by reflection from the ionosphere ; sometimes called a sky-wave. Noise — Rushing, crackling, popping sound heard in a receiver. Noise level — Amount of noise with relation to the signal being received ; signal to noise ratio. Plane-earth attenuation — The attenuation over an imperfectly conducting plane-earth in ex- cess of that over a perfectly conducting plane. Radio field — Wave energy from an antenna with the following properties measurable: The po- tential of the field, the potential gradient or intensity of the field, and the direction and polarization of the field. Radio field intensity, radio wave intensity, field strength, signal strength — The electric or mag- netic field intensity at a given location resulting from the passage of radio waves. It is com- monly expressed in terms of the electric field intensity. Unless otherwise stated, it is taken in the direction of maximum field intensity. Radio frequency — A frequency at which electro- magnetic radiation of energy is useful for communication purposes. (The present useful limits of radio frequencies are roughly 10 kilo- cycles to 10,000 megacycles.) [72] APPENDIX B 73 Radio interference — An undesired disturbance in reception, or that which causes the disturbance. It may be a disturbance in the radio trans- mitter, the transmission medium, or the radio receiver. Examples are: Background inter- ference in the transmitter, undesired electro- magnetic disturbance in the transmission me- dium as by lightning or undesired radio waves, and hum or thermal agitation in the receiver. Radio wave propagation — The transfer of energy by electromagnetic radiation at radio frequen- cies. Reflected wave — The wave caused by the reflec- tion of part of an incident wave back into the first medium. Refracted wave — The wave caused by the re- fraction of the part of an incident wave which travels into the second medium. Resistance — The opposition to a steady electron flow. Secondary fields — Eddy currents. Sinusoidal wave — A wave whose displacement is the sine (or cosine) of an angle propor- tional to time or distance or both. Spherical-earth attenuation — The attenuation over a perfectly conducting spherical-earth in excess of that over a perfectly conducting plane. Transverse electromagnetic wave — An electro- magnetic wave in which both electric and mag- netic displacements are transverse to the direc- tion of propagation; called a TEM wave. Troposphere — That part of the earth's atmos- phere in which temperature generally de- creases with altitude, clouds form, and con- vection is active. (Experiments indicate that the troposphere occupies the space above the earth's surface to a height of about 10 kilo- meters.) Tropospheric wave — A radio wave that is prop- agated by reflection from a place of abrupt change in the dielectric constant or its gradient with position in the troposphere. Wave — A disturbance propagated through a medium. Also, the graphical representation of a wave or of any periodic variation. Wave duct — A wave guide with tabular bound- aries capable of concentrating the propagation of waves within its boundaries. Wave-guide — A system of material boundaries capable of guiding waves. Wave length — In a periodic wave, the distance between corresponding phases of two consecu- tive cycles. It is equal to the quotient of phase velocity by frequency. Illinois State Geological Survey Report of Investigations No. 162 1953