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IC 
 
 8892 
 
 Bureau of Mines Information Circular/1982 
 
 A Feasibility Study of the Use of Surface 
 Redox Measurements To Detect 
 Subsurface Methane, Coal Burns, 
 and Hydrothermal Deposits 
 
 By Richard G. Burdick and Terry L. Radich 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
Information Circular 8892 
 
 A Feasibility Study of the Use of Surface 
 Redox Measurements To Detect 
 Subsurface Methane, Coal Burns, 
 and Hydrothermal Deposits 
 
 By Richard G. Burdick and Terry L. Radich 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 James G. Watt, Secretary 
 
 BUREAU OF MINES 
 Robert C. Norton, Director 
 

 This publication has been cataloged as follows: 
 
 Burdick, Richard G 
 
 A feasibility study of the use of surface redox measurements to 
 detect subsurface methane, coal burns, and hydrothermal deposits. 
 
 (Information circular ; 8892) 
 Includes bibliographical references. 
 Supt. of Docs, no.: 1 28.27:889^. 
 1. Mine safety. 2. Soils — Testing. 
 
 •5. Oxidation-retkict ion reac- 
 
 tion. 4. Prospecting— Geophysical methods. I. Radich, I'erry ].. !l. 
 Title. III. Series: Information circular (llnited Slates. Bureau of 
 Mines) : 889 2. 
 
 TN^^&A^A- 622s I622'.028'7j 82-600222 
 

 <i 
 
 u 
 
 CONTENTS 
 
 Abstract 1 
 
 Introduction 2 
 
 Acknowledgments 2 
 
 <ij Basis for use of redox method 2 
 
 "^ Description of redox probe 4 
 
 Field considerations for obtaining test data 6 
 
 Experimental field results 7 
 
 0- Case 1 7 
 
 Results, site A 8 
 
 Results, site B 9 
 
 Case 2 9 
 
 Case 3 12 
 
 Conclusions 13 
 
 Appendix 14 
 
 ILLUSTRATIONS 
 
 1. Comparison between laboratory and field redox measurements 4 
 
 2. Single-electrode, 1-1/2-ln-dlam probe 5 
 
 3. Double-electrode, 4-ln-dlam probe 5 
 
 4. Graph showing data readings plotted against time 6 
 
 5. Auger and bits used In field tests 7 
 
 6. Field Installation of probe 7 
 
 7 . SUFCo site A showing coal seam and burn area 8 
 
 8. Field curve of SUFCo site A 9 
 
 9. Field curve of SUFCo site B 10 
 
 10. 3-D computer model of Bear Mine 11 
 
 11. Field curve, Bear Mine site 11 
 
 12. Plat of Hazel A veins 12 
 
 13. Field curve. Hazel A Mine, line A 12 
 
 14. Field curve, Hazel A Mine, line B 12 
 
 A-1. Construction details of single-electrode probe 14 
 
 A-2. Construction details of double-electrode probe 15 
 
 ^ 
 
A FEASIBILITY STUDY OF THE USE OF SURFACE REDOX MEASUREMENTS 
 
 TO DETECT SUBSURFACE METHANE, COAL BURNS, 
 
 AND HYDROTHERMAL DEPOSITS 
 
 By Richard G. Burdick ' and Terry L. Radich^ 
 
 ABSTRACT 
 
 The Bureau of Mines conducted this research to determine the feasi- 
 bility of using soil redox measurements, a relatively new method in 
 mining geophysics, to locate or define a variety of conditions that 
 are hazardous or otherwise of interest to the mining industry. The 
 method was used in three widely separated areas in an attempt to de- 
 fine subsurface concentrations of methane, the edges of coal-burn 
 areas, and a hydrothermal vein set. Both a description of the method 
 used and test cases are given in this report. 
 
 ^Engineering technician. 
 ''Electronics technician. 
 Both authors are with the Denver Research Center, Bureau of Mines, Denver, Colo. 
 
INTRODUCTION 
 
 The concept of using geophysics for 
 premining investigations is based upon 
 the ability to locate or delineate haz- 
 ards or other areas of interest to the 
 mining community, so that their effects 
 may be considered during the advance 
 planning phases of the proposed mining. 
 
 Despite the rapid growth of the appli- 
 cation of geophysics in the mining in- 
 dustry during the past decade, many prob- 
 lems remain that are not readily solved 
 by current geophysical methods. The 
 oxidation-reduction potential (redox) 
 method, one of the less commonly used 
 methods, was selected as having potential 
 for solving some of these problems. 
 
 The redox method is generally thought 
 of as a laboratory means to determine the 
 electrochemical balance of a chemical 
 reaction. Its use outside the laboratory 
 and in a geologic environment is not com- 
 monly considered. However, previous 
 work, predominant in the field of well- 
 logging, has shown that the method is 
 
 suitable in a geologic setting as well as 
 in the laboratory. 3 
 
 The main value of the redox method in 
 mining environments is its use as a rapid 
 reconnaissance tool for locating such 
 diverse geochemical anomalies as methane 
 pockets, coal-burn areas, hydrothermal 
 deposits, or other phenomena that may 
 cause a chemical reaction in the overly- 
 ing rocks. For this purpose, a traverse 
 or grid of shallow holes allows many 
 readings to be made, from which the ex- 
 tent of the anomaly may be interpreted. 
 
 Two different types of hazards were 
 originally chosen for this research — 
 methane pockets and underground coal 
 burns. It was later decided to test the 
 redox method's response to vein-type 
 deposits because of their interest to 
 parts of the mining industry. This re- 
 port describes the brief feasibility 
 tests performed in three different areas 
 and gives the test results and their cor- 
 relation with known conditions. 
 
 ACKNOWLEDGMENTS 
 
 The authors wish to express their 
 thanks to the SUFCo No. 1 Mine, the Bear 
 Mine, and the Hazel A Mine, for per- 
 mission to work on their properties and 
 for details on the various geologic 
 and mining problems in each area 
 that were necessary in performing these 
 
 investigations. In addition, we would 
 like to thank Kerry Frame and Dall Dimick 
 of SUFCo for giving of their time to 
 advise and assist in the fieldwork at 
 their mine, and we would like to thank 
 Bill Bear and Dave Herr of the Bear Mine 
 for their help and encouragement. 
 
 BASIS FOR USE OF REDOX METHOD 
 
 Redox measurements are analogous to 
 using earth materials as a half-cell. 
 The other part of the cell, or 
 electrochemical couple, is a satu- 
 rated calomel, or mercury -mercury 
 chloride (Hg-Hg2Cl2), half -cell of 
 known potential placed in contact 
 with with the soil through a porous 
 membrane. The algebraic difference 
 of the soil-calomel half-cells is 
 measured with a sensitive voltmeter; 
 the result is termed the redox poten- 
 tial of the soil. As the calomel cell 
 value is a constant, differences in 
 the measured values are attributable 
 
 to differences in the soil half-cell 
 potentials. 
 
 ■^Bacon, L. , and E. Charles. Redox Rem- 
 anent Magnetization. Geophys./ v. 46, 
 No. 8, August 1981, pp. 1168-1181. 
 
 Colombo, U. Differential Electric 
 Log. Geophys. Prospecting, v. 1, No. 7, 
 1959. 
 
 Karaoguz, D. An Experimental Study of 
 Redox Logging. Log Analyst, May-June 
 1970. 
 
 Pirson, S. J. Redox Log Interprets 
 Reservoir Potential. Oil and Gas J. , 
 v. 66, No. 27, July 1968, pp. 69-75. 
 
The term "soil," as used above, refers 
 to the upper few feet of surface materi- 
 als composed of clays, silica particles, 
 and various amounts of different minerals 
 that are susceptible to reversible 
 oxidation-reduction reactions. Of these, 
 the clay minerals are probably the most 
 prevalent and probably contribute the 
 most to the earth half-cell potential. 
 The clay micelles range from a maximum of 
 2 ym to colloidal in size and are plate- 
 like in shape, which gives them a very 
 large surface area in relation to their 
 size. These clay micelles are capa- 
 ble of holding both cations and anions 
 on their surfaces, which allows them to 
 exhibit a relatively high pseudo-chemical 
 activity. 
 
 Soil, by definiation, is the result of 
 weathering of rocks. Because of this 
 origin it may be considered to be of 
 reasonably uniform composition over 
 short distances and is assumed to have 
 been subjected to reasonably uniform 
 weathering processes. From this it is 
 assumed that the redox potential should 
 not vary significantly over short 
 lateral distances. This has been found 
 to be the case during the Bureau's field 
 studies. 
 
 If, after formation or emplacement of 
 the soil, the soil is exposed to differ- 
 ent cations or anions that attach to the 
 clay micelles, or if a chemical reaction 
 changes the valence state of the already 
 attached ions, a change in the redox 
 potential will occur. Field measurements 
 cannot determine what mechanism has taken 
 place, or if a change in potential has 
 occurred. They can only determine what 
 redox potential exists at that point at 
 the time of measurement. 
 
 The above explanation has dealt with 
 the major contributor to the soil redox 
 system, clay. Small silica particles 
 exhibit a similar, though less active, 
 role in the system, while other minerals 
 may exhibit similar phenomena or may 
 actually combine chemically with the var- 
 ious reactants. However, compared with 
 clay and fine silica, their contribution 
 
 to the redox potential may be considered 
 negligible in most cases. 
 
 The source of the reactants that can 
 change the redox potential may be gases 
 formed from combustion of coal, ions 
 transported by hydrothermal action, hy- 
 drocarbon vapors and gases, or reactants 
 from surface sources. In addition, sul- 
 fide deposits adjacent to other rocks can 
 cause large redox potentials.'^ 
 
 When using the redox method for locat- 
 ing anomalous subsurface phenomena, the 
 following assumptions are made. The sur- 
 face soil-rock materials are composed of 
 clays, silica particles, and various 
 amounts of different minerals that are 
 subject to reversible oxidation-reduction 
 reactions. 5 The reactants involved in 
 the subsurface phenomena of interest are 
 capable of migration to the surface. And 
 finally, the oxidation or reduction 
 caused by the react ant is not subse- 
 quently altered by other reactions. This 
 oxidation or reduction may take place 
 over a period of months or years, or in 
 some cases even centuries, as the 
 oxidant-reductant may only consist of a 
 few molecules per year percolating upward 
 through many feet of rock. If the react- 
 ant material comes from a localized 
 source, such as a methane pocket or a 
 burned area in coal, the redox potential 
 of the rocks above will also show a 
 localization effect if the reactant move- 
 ment is vertical. In the case of coal 
 burns, the effect may be accelerated by 
 gas movement along cracks or fissures in 
 the overburden. 
 
 These basic assumptions must be kept in 
 mind when performing the tests and making 
 interpretations of the data. There is 
 not an extensive background of data or 
 results for the surface redox method as 
 applied to mining problems. Therefore, 
 the method and the interpretations should 
 be correlated with known information 
 about the anomalous conditions as fully 
 as possible. 
 
 ^First work cited in footnote 3. 
 ^Fourth work cited in footnote 3. 
 
DESCRIPTION OF REDOX PROBE 
 
 The redox probe is a ruggedized version 
 of a typical laboratory measurement sys- 
 tem in which a saturated calomel (Hg- 
 Hg2Cl2) cell with a known potential of 
 -0.2415 V is used as a reference standard 
 against which the soil is conqjared. The 
 reading obtained in the field is simply a 
 con^jarison of the soil redox potential at 
 any given place with this calomel stan- 
 dard. Figure 1 shows the comparison 
 between a laboratory measurement and the 
 field measurements using the redox probe. 
 The left portion shows a copper electrode 
 immersed in 1-molar sulfate solution 
 separated from the zinc electrode 
 in 1-molar zinc sulfate (ZnSO^) by a 
 permeable membrane. In this case, 
 the copper half-cell would have a 
 potential of -0.3448 V, while the zinc 
 half-cell would have 0.762 V. If the 
 circuit between the zinc and copper 
 electrodes were measured, the potential 
 would be found to be 0.762 -(-0.3448) 
 = 1.1068 V. 
 
 The right portion shows the redox probe 
 in place in the soil. In this case the 
 calomel half-cell has a fixed voltage of 
 -0.2415 V. The porous ceramic plug 
 allows electrical conductance between 
 this half-cell and the soil half-cell. 
 The gold electrode is inert and allows 
 measurements to be made without reacting 
 with anything in the soil. The redox 
 potential is thus measured between the 
 soil and calomel half -cells. ' 
 
 Several versions of the redox probe 
 have been field tested. Although the 
 probes have been of different shapes and 
 sizes, the essential elements have been 
 the same. 
 
 The probe shell is constructed of an 
 acrylic plastic, which is a good electri- 
 cal insulator and is not chemically reac- 
 tive in this use. A saturated calomel 
 cell (Hg-Hg2Cl2) is used as the reference 
 electrode, and a small gold disk is used 
 as the inert electrode. Conduction 
 between the calomel cell and the soil is 
 provided by a fritted ceramic disk and a 
 potassium chloride (KCl) solution salt 
 bridge. A nonconducting handle is used 
 for inserting and retrieving the probe 
 from holes. 
 
 Smaller probe versions have been con- 
 structed of 1-1/2-in acrylic rod with 
 single ceramic and gold electrodes for 
 use where portable gasoline augers or 
 hand augers were used for drilling the 
 test holes (fig. 2). 
 
 A larger, 4-in-diam probe has been con- 
 structed for use with a tractor-mounted 
 auger in order to increase the number of 
 tests that could be performed in a day. 
 This probe is large enough to accommo- 
 date double ceramic and gold elec- 
 trodes, which simplifies making probe 
 contact with the bottom of the hole 
 (fig. 3). 
 
 1.1068V 
 
 _! ZnS04 solution 
 
 Potential 0.762V 
 
 CuS04 solution 
 Potential -0.3448V 
 
 Laboratory 
 
 Measured voltage 
 
 Gold electrode 
 
 Calomel half-cell 0,245V 
 KCl solution for conductor 
 Porous ceramic plug 
 
 Field 
 
 FIGURE 1. - Comparison between laboratory (left) and field (right) redox measurements. 
 
FIGURE 2, = Single=electrode, 1 = 1, 2=in=diam probe. 
 
 FIGURE 3, = Double-electrode, 4-in=diam probe. 
 
 The readings obtained from the probe 
 are in millivolts. Any meter capable of 
 reading -500 to +500 mV with a 2- to 4-mV 
 resolution should work, as long as the 
 input impedance is high (approximately 
 mately 10' 2 ohms). In addition, the 
 
 meter should be battery operated and tem- 
 perature stable. Bureau investigators 
 used a combination pH-millivolt meter, 
 which proved to be lightweight and stable 
 over long periods of time. 
 
FlfclLD CONSLDEtiATIONS FOR OBTAINING TEST DATA 
 
 Because an absolute redox value cannot 
 easily be assigned to a given geochemical 
 anomaly owing to differences in reactant 
 materials or soil makeup, a continuous 
 traverse of readings must be made across 
 both the suspected anomalous area as well 
 as adjacent areas where the soil is unal- 
 tered. In this manner, it is possible to 
 establish a baseline against which the 
 anomalous zone will be compared. 
 
 Because of the daily and seasonal 
 expansion and contraction of the soil due 
 to temperature changes, there is a thin 
 zone near the surface, from a few inches 
 to 1 to 2 ft deep, that is exposed to the 
 movement of atmospheric air. Redox anom- 
 alies should not be expected to show up 
 well in this zone of constant oxidation. 
 Therefore, the test holes should be 2 to 
 3 ft deep to get below this aereated 
 zone. The Bureau's test holes were be- 
 tween 4 and 6 ft deep where possible. 
 
 The test-hole diameter should match the 
 probe diameter as closely as practical. 
 The hole should be cleaned as well as 
 possible with the auger before inserting 
 the probe to assure that the soil being 
 measured is that representing the bottom 
 of the hole, and not some that has fallen 
 in from above. Also, the loose material 
 has been mixed with air during augering 
 and may not represent the undisturbed 
 materials. The probe should be inserted 
 as soon as possible after the hole has. 
 been dug to prevent air circulation and 
 excessive change in the redox potential 
 at the bottom of the hole. The short ex- 
 posure to air and the probe would affect 
 a highly poised oxidation-reduction sys- 
 tem such as a strong solution of chemi- 
 cals, but due to the large number of ions 
 involved per unit volume, the system 
 would "bounce back" quickly. However, a 
 soil-rock redox system is delicately 
 poised, and owing to the much smaller 
 number of reactive ions involved, the 
 disturbance caused by exposure to the 
 atmosphere and insertion of the probe 
 takes a considerable amount of time to 
 return to equilibrium. In the extremely 
 dry soil encountered in southern Utah, 
 
 this occasionally took as long as 40 to 
 45 min, while in the damper soil encoun- 
 tered in southwestern Colorado, equilib- 
 rium often returned within 10 to 12 min. 
 
 Rather than stating a fixed time for 
 the soil to return to equilibrium, it is 
 better to read the probe every 2 to 
 5 min, and from these readings determine 
 when the soil-probe system has stabi- 
 lized. Generally, two or three readings 
 on the flattening portion of an apparent 
 asymptote can be considered stable 
 for convenience sake and to expedite the 
 survey (fig. 4), 
 
 The auger and bits used in this study 
 are shown in figure 5. This tractor- 
 mounted auger uses a 4-in-diam auger and 
 has the capability of using both soil and 
 rock bits as shown. Figure 6 shows the 
 probe emplaced in the hole with the 
 millivoltmeter attached. A sponge- 
 rubber collar is used to prevent probe 
 movement in the hole and to minimize air 
 circulation. 
 
 
 Case where readings become identical 
 
 
 a 
 
 z ' 
 
 o 
 
 < 
 
 UJ 
 
 1 
 
 
 
 • 
 
 
 TIME — 
 
 
 Case where readings become asymptotic 
 
 
 C3 ( 
 
 Z 
 Q 
 < 
 UJ 
 
 oc 
 
 
 
 
 
 * 
 
 TIME -* 
 
 FIGURE 4. - Graphs showing data readings 
 plotted against time. 
 
FIGURE 5- = Auger and bits used infield tests. 
 
 EXPERIMENTAL FIELD RESULTS 
 
 The following test cases illustrate the 
 methods used in different environments. 
 Some of the field notes regarding test 
 conditions are included as a narrative 
 description of the areas, as are soil and 
 weather conditions that prevailed when 
 the tests were conducted, which may help 
 explain the results achieved. 
 
 Case 1 
 
 At the SUFCo Mine No. 1, located in 
 south-central Utah, the elevations where 
 work, was performed ranged from 7,500 to 
 8,300 ft. Vegetation is medium to dense 
 cedar with sparse grass. The area has 
 moderate to heavy snow cover in winter 
 and dry, arid summers. At the time of 
 the field survey, in mid-July 1980, the 
 soil was extremely dry and powdery to a 
 depth of 4 to 5 ft. Daytime temperatures 
 were typically in the low to middle nine- 
 ties, and the relative humidity ranged 
 from 10% to 15%. 
 
 V'. 
 
 FIGURE 6. = Field installation of probe. 
 
 The target of interest at this site was 
 a partially burned coal seam. Moderately 
 good burn location control was available 
 from company personnel and visual obser- 
 vations, as well as existing mine maps. 
 Tests were conducted at two areas: the 
 first, site A, was adjacent to the mine 
 portal and offices. The approximate 
 depth to the coal seam was 40 ft at this 
 test site. The second, site B, was about 
 one-half mile west of the first, on top 
 
of a mesa and abou 
 seam. Initially, it 
 that a burn could be 
 much overburden, bu 
 sonnel reported that 
 during cold weather 
 mately 650 ft above 
 under the rim of the 
 was run. 
 
 t 800 ft above the 
 had not been thought 
 reflected through so 
 
 t reliable mine per- 
 vents are observable 
 at a point approxi- 
 the seam on a slope 
 mesa where the test 
 
 Results, Site A. 
 
 Measurements were made along the edge 
 of a road starting over an area known to 
 have been burned and extending past the 
 portal, which was not burned. The trav- 
 erse was 550 ft long, and measurements 
 were taken at 50-ft intervals to detail 
 the edges as closely as practical. In 
 this area, it was suspected by company 
 personnel that burn finger, or narrow 
 burn zone, existed between the known burn 
 and the known unburned coal near the por- 
 tal area. This was based upon the exis t- 
 ence of a small area of oxidized material 
 in outcrop beyond the limits of the known 
 burn. Hole depths were generally limited 
 
 to 3 to 4 ft owing to difficult augering 
 conditions, although in a few cases 
 deeper holes were possible. 
 
 The test site is shown in figure 7. 
 The coal seam shows just to the left of 
 the mine building, while the burned area 
 is to the right of the switchback in the 
 trail leading above the office. The red, 
 oxidized zone above the burn is exposed 
 more than 60 f t above the coal at some 
 places. The graph of the field data 
 (fig. 8) shows that the oxidized zone 
 above the burn has a redox potential of 
 140 to 150 mV, while that of the buff to 
 gray-colored rock above the unburned coal 
 is nominally about 100 mV. The rela- 
 tively more oxidized rock above the burn 
 also shows at about 200 ft and 300 to 
 350 ft from the start of the traverse. 
 The 200-ft readings correspond to the 
 approximate location of the suspected 
 burn finger. If this anomaly represents 
 a burn, then it is also inferred from the 
 data that a second finger may also exist 
 at 300 to 350 ft. 
 
 FIGURE 7. -' SUFCo site A showing coal seam and burn area. 
 
200 
 
 100 
 
 lu -100 - 
 
 -200 
 
 ^^V-^ 
 
 
 Jt 
 
 
 ^ 
 
 4) 
 
 1 (B 
 
 
 n 1 
 
 1 V 
 
 
 0) 1 
 
 c 1 
 
 1 c 
 
 
 a 1 
 
 3 
 
 
 o 
 
 = 
 
 
 c 
 
 I 3 
 
 
 - S 1 
 
 1 C 
 
 
 Kno 
 
 Know 
 
 
 - 
 
 100 
 
 200 300 400 
 
 DISTANCE, ft 
 
 500 
 
 600 
 
 FIGURE 8. - Field curveofSUFCo site A. 
 
 Results, Site B 
 
 This series of measurements started 
 about 50 ft from the cliff edge at the 
 top of the mesa over a known burn area 
 and extended 2,100 ft westward to an area 
 known to be unburned. The projected burn 
 edge came from old mine maps and records, 
 but as it is suspected that the burn is 
 still active in this area, the edge may 
 well be migrating westward from where 
 shown. The depth to bedrock is generally 
 less than 4 ft, and in many cases it is 
 exposed at the surface. In those cases, 
 where the sandstone was less than 2 ft 
 deep, an attempt was made to auger 6 to 
 12 in. into the rock with a rock bit in 
 order to get below the zone of air oxida- 
 tion. The soil in this area was ex- 
 tremely dry, and times to soil-probe 
 equilibrium were quite long, in some 
 cases as much as 45 min. 
 
 The graph of the field data from site B 
 (fig. 9) is more difficult to interpret 
 than site A and contains some apparent 
 ambiguities. 
 
 It had been hypothesized, before the 
 coal-burn tests were started, that the 
 fire gases would cause a reduction anom- 
 aly over the burn, and that the unburned 
 area would show a more oxidized baseline. 
 However, the field data tend to show more 
 oxidized readings over the burn areas, 
 both at site A, where the tests were run 
 under favorable conditions as far as 
 proximity to the burn and known unburned 
 
 areas were concerned, and also near the 
 estimated burn front of site B. The data 
 from site B, however, show a different 
 behavior near where the tests were 
 started that cannot be explained with 
 current knowledge and the behavior of the 
 data at site A. 
 
 At the start of the traverse, a nega- 
 tive potential exists. The potential 
 goes positive within 75 ft and becomes 
 still more positive until the estimated 
 edge of the burn is reached, after which, 
 an "unburned" baseline was reached. The 
 area below the starting point of the 
 traverse is the outcrop of the burned 
 coal, and it is assumed that this is 
 where the coal burned initially. If this 
 is the case, the negative and more mildly 
 oxidized readings should not occur if the 
 same phenomena are responsible for the 
 data at both sites A and B. The change 
 from highly to less oxidized zones near 
 the estimated fire front might be more 
 difficult to interpret in an area where 
 little was known about the burn location. 
 Further studies will be made in the fu- 
 ture to try to identify the various pro- 
 cesses involved that caused such data as 
 at site B. 
 
 Case 2 
 
 Bear Mine is located in southwestern 
 Colorado at an elevation of 6,500 ft. 
 Climate is wet in winter and spring, with 
 dry, arid summers. The survey was run in 
 a north-south-trending canyon parallel to 
 the eastern edge of the mine. The soil 
 materials are colluvium and valley fill 
 and contain bits and pieces of sandstone, 
 shale, and pieces of coal from the steep 
 slopes and cliffs above. At no point in 
 the survey did the drillholes encounter 
 bedrock. The soil materials were con- 
 sistently damp, and augering was very 
 easy during field operations in mid-July 
 1980. Two months earlier, an attempt was 
 made to perform this survey, but free 
 water was encountered in about 30% of the 
 test holes, which caused erratic read- 
 ings. The presence of free water flowing 
 through the loosely consolidated materi- 
 als apparently does not permanently alter 
 
10 
 
 200 
 
 > 
 E 
 
 u 
 
 I- 
 O 
 
 o. 
 
 X 
 
 o 
 
 o 
 
 UJ 
 
 flC 
 
 ■200- 
 
 -400 
 
 ^^^/^-^T-v^ 
 
 Estimated burned coal Estimated unburned coal 
 
 1 
 
 1 1 1 II 1 
 
 300 600 900 1,200 
 
 DISTANCE, ft 
 
 FIGURE 9. = Field curve of SUFCo site B. 
 
 1,500 1,800 2,100 
 
 the redox characteristics of the soil, as 
 the overall test results of this later 
 experiment closely duplicated those run 
 in earlier years. 
 
 The target of interest in this area was 
 a known concentration of methane caused 
 by an accumulation at the crest of a 
 small anticlinelike structure in the coal 
 200 ft below the canyon floor. This 
 methane had been a problem for the mine 
 for many years. When working this area, 
 it was not unusual, where entries were 
 driven through the area, to have to leave 
 the mine temporarily and use extra venti- 
 lation before returning. During the 
 1960's, the Bureau attempted to inter- 
 cept the crest of the structure with a 
 well to bleed off the methane in advance 
 of future mining; but the well, based 
 upon a theorized straight axis to the 
 structure, was drilled to the south of 
 the true location of the curved axis. 
 Figure 10 is a 3-D computer model 
 of the structure, which was generated 
 from surveyed points in the coal seam. 
 The test well and the anomaly loca- 
 tion have been plotted onto this model. 
 This methane anomaly had been used 
 for other unpublished tests in 1970 and 
 
 1977, and 
 well known. 
 
 its surface location 
 
 was 
 
 Measurements were made along a trail 
 running up the floor of the canyon. The 
 traverse was set up to cover an area 
 approximately 500 ft each side of the 
 anomaly. The test holes were nominally 4 
 to 5 ft deep and were normally placed on 
 the uphill side of the trail to stay away 
 from disturbed materials where the trail 
 had been cut into the hillside and filled 
 towards the creek in the center of the 
 canyon. The probe generally came to 
 equilibrium within 10 min. The field 
 data curve (fig. H), shows the peak of 
 the anomaly about 500 to 550 ft down the 
 canyon from the old Bureau of Mines test 
 well. This is the same point where it 
 was located in 1970 and 1977 during early 
 to middle autumn, with much drier ground 
 conditions. It may be seen from the fig- 
 ure that the redox values are con- 
 sistently higher than the previous data, 
 but the anomaly is shown at the same 
 location by both plots. The smoothness 
 of the composite 1970-77 curves is due to 
 the averaging effect of a much larger 
 number of data points from these two se- 
 ries of tests. 
 
11 
 
 FIGURE 10- = 3-D computer model of Bear Mine. 
 
 100 
 
 > 
 E 
 
 _i 
 < 
 
 I- 
 z 
 liJ 
 
 h- 
 O 
 Q. 
 
 X 
 
 o 
 
 a 
 
 lU 
 
 tr 
 
 100 
 
 -200 
 
 Composite 1970 
 and 1977 tests 
 
 200 400 600 800 
 
 DISTANCE, ft 
 
 FIGURE 11. - Field curve. Bear Mine site 
 
 1,000 1,200 1,400 
 
12 
 
 Case 3 
 
 Hazel A Mine is Located in central Col- 
 orado in the Front Range Mineral Belt. 
 The area is typical of the Front Range, 
 where elevations range from 7,000 to 
 9,000 ft. The soil mantle is thin and 
 quite rocky over an igneous-metamorphic 
 con^slex of rock, which is predominantly 
 granitic in character. The annual pre- 
 cipitation is about 15 in, a considerable 
 portion of which occurs as snow. This 
 series of tests was run during September 
 1980 when the soil conditions were dry. 
 There were two known hydrothermal veins 
 occurring on this property, and the redox 
 traverses were run across these known 
 veins. 
 
 The composition of these veins is not 
 known exactly by the Bureau. Float rock 
 in the vicinity of the veins showed clay 
 alteration and solution vugs as well as 
 limonite and hematite deposition in and 
 around the solutioned zones. Sulfide 
 minerals observed in pieces of vein were 
 sparse. From these observations, the 
 oxidized anomalies found would seem to 
 correspond to the character of the veins 
 at the surface. Figure 12 is a plat of 
 this test area. 
 
 Test line A was a generally north-south 
 line run across an east-west trending 
 vein. The soil encountered was very 
 
 Scale, ft 
 
 Hazel A veins 
 9/28/80 
 
 rocky and extremely dry. Test holes 
 averaged between 2 and 3 ft deep. The 
 angering was quite difficult, and because 
 of the dryness of the soil, it was nearly 
 impossible to clean the hole with the 
 auger. Generally, the bottom foot or so 
 of the hole had to be cleared out by 
 hand. 
 
 The original traverse (line A) was made 
 with test holes on 50-ft centers. The 
 intermediate points near the vein were 
 run after the anomaly was located. Fig- 
 ure 13 shows the field curve for this 
 site. An anomaly at 250 ft corresponds 
 with the known vein, while the smaller 
 perturbation in the curve between 75 
 and 100 ft may be a possible paral- 
 lel vein. The anomaly near 250 ft was 
 later corroborated by a geologic plat 
 of the area in possession of the mining 
 company. 
 
 JUU 
 
 — , , 
 
 c 
 
 1 
 
 c 
 
 I 
 
 1 
 
 
 
 
 > 
 
 0) 
 
 
 
 
 
 
 9 
 
 c 
 
 
 
 
 
 
 
 o 
 c 
 
 
 
 
 
 
 
 
 
 
 
 
 200 
 
 a. 
 
 
 
 
 ^^ 
 
 
 100 
 
 1 
 
 1 
 
 
 Line A 
 
 1 
 
 
 
 200 300 
 
 DISTANCE, It 
 
 FIGURE 13. ' Field curve, Hazel A Mine, Line A, 
 
 250 
 
 100 
 
 100 200 
 
 DISTANCE, ft 
 
 300 
 
 FIGURE 12. - Plat of Hazel A veins. 
 
 FIGURE 14. - Field curve. Hazel AMine, Line B. 
 
Test line B was run about 850 ft west 
 of line A across a generally north-south 
 trending vein. This vein was quite easy 
 to locate visually by a continuous line 
 of prospect holes along 200 to 300 ft of 
 
 13 
 
 its length. The Bureau's test holes were 
 35 ft apart, 2 to 3 ft deep in a decom- 
 
 posed granite soil, 
 figure 14, the vein 
 the baseline. 
 
 As may be seen from 
 shows well against 
 
 CONCLaSIONS 
 
 From the results of this feasibility 
 study, it appears that the redox method 
 shows a potential as a means of locating 
 geochemical phenomena associated with 
 mining. 
 
 The study left some questions unan- 
 swered in regard to the interpretation of 
 data from coal-burn areas. It is hoped 
 that future investigations will concen- 
 trate more heavily in this area, with 
 a view to answering some of these 
 questions. 
 
 As it would be virtually impossible to 
 predict a numeric value for any given 
 redox phenomena in an area, a suitably 
 long traverse must be run to assure that 
 both unaltered ground as well as the 
 
 anomalous ground are covered. This 
 assures that an unaltered baseline is 
 present with which to compare the anom- 
 aly. In practice, this should present no 
 great problem as the method will probably 
 be used in areas where a general problem 
 is suspected, rather than as a large- 
 scale reconnaissance method. 
 
 The Bureau's testing indicates that the 
 smaller diameter probe would probably be 
 preferable for most applications. This 
 generally simplifies the drilling of the 
 holes as a small, portable hand auger may 
 be used rather than a tractor-mounted 
 rig, which is easier in rough topography. 
 In addition, the smaller probe seems less 
 susceptible to rough handling than the 
 larger probes. 
 
14 
 
 APPENDIX 
 
 The construction of the redox probe is 
 fairly simple. The probe shell requires 
 some light machining, but the dimensions 
 are not critical. 
 
 In effect, the probe shell is simply an 
 electrically and chemically inert device 
 to hold the calomel electrode and the 
 gold electrode in a constant relationship 
 to one another. Its other function is to 
 allow the electrodes to be lowered into a 
 hole and be pressed against the soil in 
 the bottom of the hole. 
 
 Because the potassium chloride (KCl) is 
 an electrical conductor, the chamber con- 
 taining the calomel cell and this solu- 
 tion must have a watertight seal at the 
 top to prevent an internal short circuit 
 between the chamber and the gold elec- 
 trode. The last element to the probe is 
 a tube through which the KCl solution can 
 be replenished when necessary. The 
 Bureau used a small-diameter hole drilled 
 
 into the calomel cell chamber and fitted 
 with a tight-fitting, flush rubber stop- 
 per to prevent dirt intrusion. 
 
 Construction Notes 
 
 1. The general shape of the probe tip 
 as shown on the shop drawings (figs. A-1 
 and A-2) has been found to be superior to 
 flat or conical shapes for making contact 
 with the bottom of the hole. 
 
 2. The dimensions of the calomel cell 
 chamber must be such that the cell can be 
 sealed tightly at the top to prevent 
 solution leakage if the probe is tipped 
 or inverted. 
 
 3. Both the ceramic and the gold elec- 
 trodes should be sanded to conform 
 smoothly with the probe shell to prevent 
 dirt entrapment at this point. This also 
 allows the probe shell to protect the 
 ceramic from chipping or breaking. 
 
 Ceramic frit 
 
 Calomel cell chamber 
 
 Ox ,x OC-<^0s.X < 
 
 '/s-in hole, V32-in deep for gold electrode 
 
 PROBE 
 
 To fit connector 
 (cemented) 
 
 To fit 1V2-ln-0D 
 handle (5 ft long) 
 
 CONNECTOR 
 
 FIGURE A-1. - Construction details of single-electrode probe. 
 
15 
 
 Ceramic frit .^s/'l-- 
 
 \ \ Calomel cell / / i 
 
 .\ \ \ / chamber / / | 
 
 \\ \4 "^"-"-"^. 
 
 ^\ ^^ ^ A____A^ 
 
 10 threads 
 per inch 
 
 Gold electrode 
 
 8- 
 
 PROBE BODY 
 
 ^^I^^X" 
 
 10 threads 
 per inch 
 
 Drill V. 
 
 PROBE CONNECTOR 
 
 FIGURE A-2. - Construction details of double-electrode probe. 
 
 4. Bot:h the ceramic and gold elec- 
 trodes should be mounted to the probe 
 shell with silicone rubber to prevent ex- 
 cessive solution leakage or dirt and 
 water intrusion into the probe. 
 
 5. The mounting lug on the probe 
 should be lathed to a snug fit to what- 
 ever type handle is to be en5)loyed. Nor- 
 mally, a rigid plastic or PVC tube works 
 satisfactorily for a handle. 
 
 Parts List 
 
 Fritted Ceramic Material 
 
 Coors P 1/2 BC (A fritted ceramic with a 
 very slow flow rate) (Some calomel cells 
 are equipped with a fritted ceramic, 
 which may be removed and used in the 
 probe. ) 
 
 Calomel Cells (Capable of disassembly and 
 insertion into probe) 
 
 <rU.S GOVERNMENT PRINTING OFFICE: 1982 - 505 - 002/81 
 
 Van Waters and Rogers 
 
 Beckman 
 
 Ingold 
 Probe Shell 
 
 34106-148 
 
 34119-183 
 
 40459 
 
 34105-666 
 
 5502-01 
 
 Acrylic or polycarbonate rod of suitable 
 diameter. 
 
 Note: 
 
 The above parts are listed as an aid to 
 procuring the necessary con5)onents for a 
 probe, and in no way should be construed 
 as an endorsement by the Bureau of Mines. 
 Other similar materials are available 
 from chemical supply houses, but an ex- 
 haustive search has not been made by the 
 authors. 
 
 INT.-BU.OF MINES,PGH.,P A. 26329 
 

 
 
 

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