ILLINOIS GEOLOGICAL SURVEY LIBRARY STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION GRAVITY BASE STATION NETWORK IN ILLINOIS Lyle D. McGinnis ILLINOIS STATE GEOLOGICAL SURVEY John C. Frye, Chief URBANA CIRCULAR 398 1966 (2200—28532—5/66) ^>v 10 GRAVITY BASE STATION NETWORK IN ILLINOIS Lyle D. McGinnis ABSTRACT A gravity base station network comprising 29 stations has been established with a LaCoste and Romberg gravity meter in Illinois . The stations are tied to a pendulum base at Washington University, St. Louis, Missouri, havinga value of 979 . 99863 gals . From a series of adjusted loops, the observed gravity value at the Champaign- Urbana, Illinois, base (Station 17) was found to be 980 . 12905 gals . All stations in the network are tied to within ±0.1 milligals of the St. Louis base . INTRODUCTION During the summer of 1965, 29 gravity base stations (fig. 1) were estab- lished in or near Illinois with a LaCoste and Romberg geodetic gravimeter (G-4) on loan from the United States Army Map Service, Gravity Division. The observations were made by members of the Illinois State Geological Survey to provide a common gravity base for state-wide regional surveys. Although a number of base stations had been established in Illinois prior to this work (see Behrendt and Woollard, 19 61), it proved necessary to develop a network of internally consistent points to which regional surveys in the state could be tied. Regional gravity coverage on approximately one-mile grids is nearly half completed (fig. 2). Stations in the regional surveys are occupied at bench marks and section corners located on U. S. Geological Survey topographic maps, where elevations are listed. Because of various limitations (Heigold et al., 1964), regional Bouguer data have an internal accuracy of ±0. 2 milligals . Gravity data published in Illinois up to the time of the present paper have not been tied to a common base; thus cor- rections are necessary in comparing one area with another. Those reports published in the future will be tied to the present network. When finished, the regional sur- veys will furnish aids to geologic interpretations throughout the state. The meter used in the gravity -control network has an extremely low and nearly linear drift rate (fig. 3) with a high reading precision of ±5 microgals. Resetting was not required with the instrument since it has a 6000-milligal range, 980.33076,1 (-7.9) Figure 1. Locations of gravity base stations in Illinois showing station numbers, observed gravity values, and free air gravity (in parentheses). Station 22 includes observed gravity at both the pendulum base (a) and the auxiliary site (b) . EXPLANATION [ggggj Oculors 354 and 369 KxnnH Circulars in preparation V/i^A Report of Investigations 219 ''///A Area to be surveyed, Summer 1966 Figure 2. Completed regional gravity surveys in Illinois with one -mile grid coverage. whereas the range observed in Illinois is only 430 milligals . The meter has a linear variation in scale constant as is shown in figure 4. It was calibrated twice prior to the survey with a small, but negligible, change in scale constant. SURVEY PROCEDURE AND RESULTS The gravity network is tied to a pendulum base station, which was estab- lished by University of Wisconsin geophysicists, in Room 2, Wilson Hall, 2.3 meters east of the west wall, 1.0 meters north of the south wall, Washington University, St. Louis, Missouri. The observed gravity at the pendulum site is 979.99863 cm sec" (gals). An auxiliary site at the eastentrance to Wilson Hall was established with a gravity meter for convenience of reoccupation. Observed gravity at the aux- iliary site is 979.99872 gals (Emil Mateker, Washington University, personal com- munication) . Stations of the network in Illinois were located mainly at U. S. Geological Survey bench marks with the majority of the sites in cities or towns. Field data for the survey are shown in table 1. The number of ties between stations, and errors and corrections applied to the ties, are shown in table 2. Loops of stations ranged from three stations about thirty miles apart to a large loop circling the state (table 3 and fig. 5). Errors of closure (E c ) around the loops ranged from .000 mil- ligals for the loop (11-9-8-11) to +.385 milligals for the loop (26-22-12-1 1-8-3- 4-6-10-13-17-20-21-23-24-25-26). Assuming corrections for earth tides (Goguel, 19 64) and meter drift (fig. 3) have been accounted for, the error of closure is due to set-up error; thus, the ac- curacy of a tie between two stations is independent of distance between them and is also independent of the time interval between readings. Gravity ties between stations around a loop must be adjusted according to the reliability of the ties so that closure equals zero. The accuracy of a tie between two stations is related directly to the number of times the tie was established (N) . Thus the error of closure (E c ) is E c = N lCl+ N 2 C 2+ N 3 C 3 + N 4 C 4 + + N n C n (1) where N n is the number of times a tie was established and C n is that fraction of the error of closure assigned to ties established N n times. The fractions of the error of closure weighted between stations are related as C Y = 2C 2 = 3C 3 = 4C 4 (2) and so on. An example of the method used to adjust ties is described below. In the loop, (18-14-12-18) E c is -.024 milligals. In figure 5, it is seen that two of the ties were established with only one reading, whereas the third tie was established with two readings. Equation 1 may be written then as 5Ci -.024 = 2Ci + ICo = 1 2 2 ' (3) 25 30 15 20 Days Figure 3. Meter drift (+ .2 milligals/month) during the month of August, 1966. 35 1035.50 1035.40 - c o 1035.30 U) > •o ^ (A o o> 1035.20 i *-" IO o X 1035.10 *: 1035.00 - 1034.90 (Divisions) Reading X 10 Figure 4. Linear variation of scale constant. Variation of K for LaCoste Romberg G-4 gravity meter. Figure 5. Station locations illustrating mean ties, number of ties-in brackets, and adjusted mean tie around gravity loops-in parentheses. Arrow indicates the direction of increasing potential field . Ci = -.009 6 milligals and C2 =-.0048 milligals. Applying these corrections first to the small loops and then to progressively larger ones and assuming those loops already adjusted to be correct, the closure around all loops is adjusted to zero. Observed mean ties and adjusted (in parentheses) ties between stations are shown in figure 5. The numbers (in brackets) that follow the observed mean tie indicate the number of ties used to establish the mean tie. Adjustments ranged from 0.000 milligals to 0.07 6 milligals. Observed gravity values adjusted to the base at Washington University (Station 22), St. Louis, are shown on figure 1. It was found that Station WA175, Janesville-Beloit (Behrendt and Woollard, 1961), which is Station 1 in the present network, had a value of 980. 33076 milligals or about 0.5 milligals greater than the Behrendt-Woollard value. The reason for this great a discrepancy is not known, and the respective accuracies of the two base readings cannot be determined from data available. The maximum adjustment required to close a loop was ±.076 mil- ligals . The network was also tied (Station 26) to U. S. Coast and Geodetic Survey Station number 1110 reported for Murphysboro in 1941 . Station 2 6 was found to be approximately 2 milligals less than the U. S. C.G.S . value. Behrendt and Woollard (1961) reported that much of the U.S. C.G.S. network was in error by over 3 mil- ligals, and it is concluded that the value determined for the present survey is more nearly correct. Principal facts, latitude, longitude, elevations of bench marks and the gravity meter, observed gravity, Bouguer gravity (p =2.35 and p =2.67 gm/cm^), and free air gravity are listed in table 4. Densities of 2.3 5 and 2.67 gm/cm^ are both used in calculation of Bouguer anomalies. In areas covered with glacial drift, the density 2.35 gm/cm^ is used; whereas, in nondrift areas, 2.67 gm/cm3 is as- sumed to be a representative density. ACCURACY OF READINGS The gravity network was tied to the St. Louis pendulum base station at which an absolute value for the earth' s gravitational field was determined. The remainder of the stations in the network were, as stated previously, established with a grav- ity meter and, therefore, observed gravity at these stations is based on gravity differences. The relative accuracy of each station is thus a function of the accu- racy of the St. Louis base and also a function of errors involved in the reading and setting up of a station and in the calibration of the gravity meter. Meter calibration by members of theU. S. Army Map Service was carried out prior to the establishment of the base network. If the calibration of the meter and the gravity value at the pendulum site are assumed to be correct, the accuracies of the observed gravity values in the network are a function only of set-up accuracy. A tie between two stations thus involves two possibilities for error. An estimate of the limits of possible error follows. As the number of ties between two stations increases, the probability that the mean of the gravity differences (Ag) represents the true difference (Ag^) a l so increases. Seven of the 38 ties have been tied three or four times, and the aver- ages of these represent the closest approximation to true gravity differences in the network. Ties should have a normal distribution about the true tie in the manner shown in figure 6. Figure 6. Expected normal distribution of ties about a true gravity tie. Ag The true gravity difference (Ag t ) between any two stations is unknown in the present survey. It may be approximated only by the normal distribution pattern about a sample mean (Ag). The distribution of ties about sample means (derived from the seven ties having three or more total ties) is illustrated in figure 7. A nor- mal distribution is apparent, having a standard deviation of ±0.047 milligals. The 99 percent confidence limit for the above data is ±.108 milligals, which is greater than the maximum adjustments applied to any tie. The fact that the error of closure around the largest loop (760 miles) in the state, comprising 16 ties, was only .385 milligals is evidence that errors are well within the limits described above . Because of the small error of closure, it is believed that, after adjustments, all base sta- tions in the state are tied to within ±0.1 milligals of the St. Louis base. -.9 - 6 -.5 -.2 Sg^-Ag Figure 7. The normal distribution of ties about a mean Ag. Four-tenths of a milligal frequency interval is utilized in order to produce a smoothed curve. CONCLUSIONS Gravity surveys in Illinois may be tied now to a common base station net- work of high precision. Incorporation of this data into a worldwide system is made easier. Interpretations of the geology in the state based on gravity data is facil- itated also by the use of a common system. Regional published surveys and all surveys to be published in the future by members of the Illinois Geological Survey will be adjusted to this network. 10 TABLE 1. GRAVITY BASE STATION NETWORK FIELD DATA, UNCORRECTED FOR METER DRIFT OR EARTH TIDES 1965 Month Time Counter reading Station no. 1965 Month Time Counter reading Station no. April August 27 2:20 PM 2756.528 17 25 9:00 AM 2621.914 21 3:20 2794.980 13 10:14 2665.732 20 4:20 2756.520 17 11:36 2719.203 17 28 9:29 AM 2756.403 17 2:00 PM 2719.228 17 10:29 2794.948 13 2:51 2719.631 15 10:35 2794.850 13 3:49 2707.972 18 11:58 2867.039 10 5:14 2711.345 14 1:10 PM 2794.771 13 6:16 2695.841 16 2:47 2867.068 10 7:42 2705.973 19 4:20 2912.072 6 26 8:58 AM 2705.925 19 5:28 2930.289 4 9:16 2695.755 16 7:43 2912.019 6 10:10 2711.312 14 8:32 2930.250 4 11:03 2749.645 12 29 10:31 11:32 AM 2930.130 2950.835 4 1 September 12:45 PM 2707.845 18 12:26 PM 2930.124 4 6 8:40 AM 2593.340 22 1:37 2911.902 6 2: 50 PM 2749.594 12 3:10 2866.991 10 4:03 2794.339 11 4:21 2794.778 13 6:00 2839.550 8 5:20 2756.209 17 8:10 2834.744 9 August (Daylight Sa ving) 10:15 2915.928 2 19 2:03 PM 2719.342 17 7 9:20 AM 2915.658 2 23 9:48' AM 2593.254 22 10:50 2890.222 3 12:34 PM 2541.164 26 11:45 2893.337 4 2:00 2548.596 27 2:40 PM 2877.690 5 4:07 2502.338 29 3:45 2867.658 7 4:46 2506.489 28 4: 55 2877.780 5 5:16 2502.336 29 6:15 2867.661 7 5:42 2506.511 28 7:40 2893.532 4 6:34 2548.778 27 9:15 2890.378 3 8:28 2541.359 26 8 9:45 AM 2839.375 8 24 9:52 AM 2 587.142 25 10:37 2834.494 9 12:20 PM 2608.802 24 11:40 2794.180 11 1:37 2621.633 23 2:08 PM 2719.620 15 2:24 2608.924 24 3:20 2665.861 20 3:25 2587.352 25 4:45 2719.324 17 4:33 2609.048 24 5:24 2621.902 23 7:19 2622.092 21 TABLE 2. TIES BETWEEN INDIVIDUAL GRAVITY BASE STATIONS 11 Adjusted Total Ag min Ag max Ag min - Ag max Mean Ag mean Ag Tie ties milligals milligals milligals milligals milligals 17-13 4 39.789 39.945 .156 + 39.870 + 39.908 13-10 4 74.697 74.824 .127 + 74.773 + 74.811 10-6 2 46.532 46.568 .036 + 46.550 + 46.626 6-4 4 18.841 18.936 .095 + 18.885 + 18.923 4-1 2 21.427 21.444 .017 + 21.436 ___ 17-22 1 -130.344 -130.330 22-26 1 - 53.945 - 53.910 26-27 2 7.641 7.663 .022 + 7.652 ... 27-29 2 47.966 48.041 .075 - 48.004 ... 29-28 3 4.281 4.318 .037 + 4.300 ... 26-25 1 + 47.596 + 47.631 25-24 3 22.350 22.442 .092 + 22.391 + 22.403 24-23 3 13.153 13.303 .150 + 13.225 + 13.237 23-21 1 + 0.162 + 0.197 21-20 1 + 45.408 + 45.443 20-17 2 55.248 55.412 .164 + 55.330 + 55.329 17-15 2 .367 .432 .065 + 0.400 + 0.399 15-18 2 12.120 12.123 .003 - 12.122 - 12.145 18-14 2 3.436 3.484 .048 + 3.460 + 3.453 14-16 2 16.087 16.158 .071 - 16.123 _-_ 16-19 2 10.467 10.526 .059 + 10.497 ___ 14-12 1 + 39.734 + 39.719 12-18 1 - 43.218 - 43.172 18-22 1 -118.655 -118.584 22-12 1 +161.686 +161.757 12-11 1 + 46.284 + 46.238 11-8 2 46.757 46.764 .007 + 46.761 + 46.761 8-9 2 4.955 5.028 .063 - 4.992 - 4.992 9-2 2 83.977 84.079 .102 + 84.028 + 84.007 2-3 1 - 26.290 - 26.333 3-4 2 3.225 3.250 .025 + 3.238 + 3.162 4-5 2 16.294 16.308 .014 - 16.301 - 16.303 5-7 3 10.426 10.496 .070 - 10.455 - 10.457 7-4 2 26.720 26.804 .084 + 26.762 + 26.760 3-8 2 52.655 52.666 .011 - 52.661 - 52.682 9-11 2 41.729 41.809 .080 - 41.769 - 41.769 11-15 1 - 77.220 - 77.266 15-20 2 55.680 55.775 .095 _ 55.728 - 55.728 12 TABLE 3. 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01 3 o ■D ai e 00 O tp — c z CO ■<* o c 09 Q 3 =3 3 O C x r-< .,,* tp -H C •!-> O 10 « -H o vO J= o J= c- rJ O £ 0) CO ■H 0) p "* o s o • H c a.t- r-H p p j; Cfl -O JZ P 0> -c P TJ 01 01 v_- tp S p J3 o CM r-< in c r-< X 3 p >P CO rH p p •H ^ c U •£> •> O CO CJ1 0) 01 o< u rH O CO 01 a .2 o o o P 3 01 to tp -H cnr CO • bo tp CJ 0> P r-i p tC * p rH E _C §8 O to O C ul • •H Z P -a tp o • c bi 01 = CO CD IS 01 s *© 0) en J= ,01 CO p o to XJ CJ » p tp c E X CJ O 0) H P CJ\ •H •> 01 p 4_j c O S c •l-t P HH o O tp c C bo 01 r-\ e to c • TD 3 o c na oa 1 >H CO X 0] rH P P o x tp P bo JSi§ ►J tp CO o c o a> P tp Ip CO J) o CJ o -o c ID , O p CJ CO CJ • 1 •H a. • c 01 rH a X d C CM ■ -H r^ tp CO O CO cr •o p a =1 c t-J u o <-\ o > CO CO CJ O X) a, C CO 3 01 • 4) C rH e o c u CO CO CJ .,3 •h r- ^-^ WJ c •H 3 to P E > P 3 0) O CO tp CO , !-= CHU CJ 01 T3 r^ X tp CO O 01 o CD O p o p 01 CO •H r-\ o tp t "° . O CO a C -P CO LC o p rH CO p p 01 t~ J= ^ P tp rt) o 01 tp XJ « o S 3 CO 01 01 tp 01 •■c a bo p E P CJ CO o js 01 P x tt. a: P o 01 rH X) CO z O iH H o CJ J3 c c o* CD X. U a, to CO rH rH CO C JO * p ~^ CO o r~ O 4-i a* p o tp P p£ O 3 •H O -H E 3 Cfl'H O O C P tp tp * 01 10 * •H CO CO P c H cj o u ip Lfl O P C CJ CO CO P X rH CO O S o o o tp p ^- - s co 3 tp w CO 18 REFERENCES Behrendt, J. C. and Wool lard, G. P., 1961, An evaluation of the gravity control network in North America: Geophysics, v. 26, no. 1, p. 57-76. Goguel, Jean, 1964, Tidal gravity corrections for 1965: Geophysical Prospecting (European Assoc, of Exploration Geophysicists) , v. XII, supp. 1, 53 p. Heigold, P. C., McGinnis, L. D. and Howard, R. H., 1964, Geologic signifi- cance of the gravity field in the DeWitt- McLean County area, Illinois: 111. Geol. Survey C ire. 369, 16 p. McGinnis, L. D. , 1966, Crustal tectonics and Precambrian basement in northeastern Illinois: Illinois Geol. Survey Re pt. Inv. 219, 29 p. McGinnis, L. D. , Kempton, J. P. and Heigold, P. C. , 1963, Relationship of grav- ity anomalies to a drift-filled bedrock valley system in northern Illinois: 111. Geol. Survey Circ. 354, 23 p. Illinois State Geological Survey Circular 398 18 p. , 7 figs. , 4 tables, 1966 Printed by Authority of State of Illinois, Ch. 127, IRS, Par. 58.25. CIRCULAR 398 ILLINOIS STATE GEOLOGICAL SURVEY URBANA