Geophysical Studies to 

Detect the Acme Underground 

Coal Mine, Wyoming 

Geophysical Studies to 

Detect the Acme Underground 

Coal Mine, Wyoming 

By C.H. MILLER 

Magnetic, electromagnetic, gravimetric, seismic refraction, 

and precise topographic surveys gave only subtle hints of 

underground openings, suggesting the need for further 

research in geophysical techniques 

U.S. GEOLOGICAL SURVEY BULLETIN 1677 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1988 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

Miller, C. H. (Carter H.) 

Geophysical studies to detect the Acme Underground Coal Mine, Wyoming. 

(U.S. Geological Survey bulletin ; 1677) 

Bibliography: p. 

Supt. of Docs. no.: 19.3:1677 

1. Acme Underground Coal Mine (Wyo.) 2. Geophysics-WyomingAcme Underground Coal Mine-Methodology. I. Title. II. Series. 

QE75.B9 no. 1677 557.3 s 86-600164 

[TN805.W8] [622'334'0978732] 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 2 

The Acme Mine 2 

Geophysical surveys 6 

Site 1 6 

Magnetic anomalies caused by heating rocks and by iron 6 

Magnetic survey and magnetic field characteristics 7 

Site 2 11 

Topographic survey 11 

Magnetic survey and magnetic field characteristics 11 

Electromagnetic survey and electromagnetic field characteristics 13 

Gravity survey and gravity field characteristics 15 

Seismic survey 20 

Soil arch (site 1) as model 22 

Seismic field of site 2 22 

Summary and conclusions 24 

References cited 27 

FIGURES 

1. Index map of the Powder River Basin, Wyo., showing location of 

Acme Mine 2 

2. Map of the Acme Mine showing geophysical survey sites 1 and 2 3 

3. Photographs of subsided Acme Mine 5 

4. Graph showing representative net magnetism of various rock types 7 

5. Graph showing first approximation of range of a maximized totalintensity magnetic anomaly as a function of distance from iron 

objects 8 

6. Map of contoured magnetic field of site 1 9 

7. Cross sections showing magnetic fields associated with buried junk and 

clinkers and subsided area at site 1 10 

8. Graphs showing 16-inch normal resistivity logs, velocity distribution, 

and core densities from two boreholes 12 

9. Map showing detailed topography of site 2 13 

10. Cross section C-C' through site 2 showing topographic surface and 

estimated position of mine openings with draw lines 14 

11. Map of contoured magnetic field of site 2 15 

12. Drawing showing magnetic anomalies calculated for mine-car rails in 

first and second entries and along cross section C-C' of site 2, compared to measured magnetic field 16 

13. Map showing electromagnetic field of site 2, in-phase component 17 

14. Map showing electromagnetic field of site 2, out-of-phase component 17 

15. Drawing showing the in-phase and out-of-phase electromagnetic components along cross section C-C' of site 2 18 

16. Drawing showing calculated effective depth of resolution for onefourth skin depth of penetration for an electromagnetic source as a 

function of resistivity along cross section C-C' 18 

Contents Ill 
IV Contents 

17-19. Map showing simple Bouguer gravity field of site 2, reduced at a density of: 

17. 1.70 g/cm3 19 

18. 2.00 g/cm3 19 

19. 2.67 g/cm3 20 

20. Drawing showing simple Bouguer gravity anomalies along C-C' for 

data reduced at three densities, compared with theoretical gravity 

anomalies computed for both air- and water-filled mine openings 21 

21. Map of a soil arch in site 1 showing topography, location of seismic 

lines, and time-distance graphs 23 

22. Location of seismic-refraction lines 1-5 along D-D' and E-E' at site 

2 24 

23. Time-distance curves and velocity-layering interpretation along seismic 

lines 1-3, of site 2 25 

24. Time-distance curves and velocity-layering interpretation along seismic 

lines 4-5, of site 2 26 

25. Time-distance graph and theoretical depth of penetration of a seismic 

wave 26 
Geophysical Studies to Detect the 

Acme Underground Coal Mine, Wyoming 

By C.H. Miller 

Abstract 

Adequate location maps are not available for some of the 

older underground coal mines in the Powder River Basin, 

Wyoming, nor for mines in other parts of the United States. 

These mines may subside and cause damage at the ground surface, or they may spontaneously ignite and consume coal 

deposits, polluting air and water. Consequently, techniques are 

needed for locating those potentially hazardous coal mines for 

which adequate maps do not exist. 

The Acme underground coal mine is just north of 

Sheridan, Wyoming. Mining was begun in 1911 and abandoned 

in 1940. Coal was extracted from the relatively flat lying lower 

Monarch bed, which is about 6 m thick with no more than 

12-20 m of overburden. The mine has burned and subsided 

over much of site 1, but site 2 was not subsided. 

A ground-run magnetic survey of site 1 defined some 

prehistoric clinker (rocks heated by burning coal and made both 

brittle and very magnetic) that extended a few tens of meters 

into an outcrop, as well as some buried junk. The magnetic 

survey also del ineated subsided ground, perhaps because of 

the mass effect of the broken terrain about the magnetometer 

or, perhaps, from magnetization of rocks by heat from burning coal escaping up subsidence fractures. 

At the nonsubsided site 2, the data from magnetic, electromagnetic, gravity, and precise topographic surveys were 

reduced to individual contoured planimetric maps. None of 

the contour maps indicate the outlines of underground 

openings. 

Theoretical magnetic calculations showed that although 

small iron objects such as tools could not be detected, large 

iron objects such as mine cars or heavy mine-car rails would 

produce much larger anomalies than were seen on the 

magnetic contour map. Calculations for the magnetic attraction of light mine-car rails at site 2, however, corresponded 

rather closely to one part of the measured magnetic field . 

A long straight metal conductor buried less than 0.25 skin 

depths will significantly disturb an electromagnetic field. A 

range of skin depths was calcu Ia ted for site 2 that essentially 

brackets the mine, but no significant equivalent electromagnetic 

anomalies were measured in the survey. Strong imaginary 

anomalies were measured over site 2, however, and these may 

possibly be attributed to ground-water variations between 

ground over the mine and ground away from it. 

Theoretical gravity anomalies were computed for both airand water-filled mine workings at site 2 assuming a rock density 

of 2.3 grams per cubic centimeter. The calculations for the 

water-filled mine produced anomalies of about 0.4 and 0.9 

milligal, considerably more than the possible error of 0.015 

milligal, but neither calculation corresponded to a measured 

gravity anomaly. 

Five seismic-refraction lines were run over site 2, and two 

velocity layers were determined: V 2 , which represents the 

velocity of bedrock, and VP1

, which ~epresents the velocity of 

the overlying soil. The VP 2 graph segments have a m~rked 

tendency to be nonlinear over the mined area and more lmear 

away from the mined area. Both the depth to bedrock 

calculated from the seismic data and that calculated from 

velocity gradients measured in similar lithologies indicate, 

however, that the measured refracted seismic waves do not 

penetrate into the bedrock as deep as the mine level. Therefore, 

the nonlinearity of the V 2 segments may be caused by deformation above the mine, ~!though deformation is not visible at 

the ground surface. 

These studies are valuable as an aid in further research. 

Although none of the above methods adequately located the 

mine, some of them hinted at detection. In fact, it was shown 

that magnetic surveys can detect both the depth to which 

clinkers extend into an outcrop and iron in sufficient quantity. 

Although the surveys with interpretation were done 

systematically and with great care, they were largely unsuccessful, and hence they need not be duplicated. Variations of 

these methods as well as the application of different techniques 

may be fruitful, however. 

INTRODUCTION 

Exploitation of vast coal and other resources in the 

Powder River Basin, Wyo. and Mont. (fig. 1), has caused 

a rapid increase in population and in commercial and 

residential development in the basin since the mid-1970's. 

Studies therefore were implemented by the U.S. Geological Survey to aid in intelligent land use during this sudden expansion, which is expected to continue. 

Abandoned underground coal mines in the Powder 

River Basin, as well as in other parts of the United States, 

present problems to land use. The land surface above 

these abandoned mines may eventually subside, thereby 

causing extensive depressions (Association of Engineering 

Introduction 
4BJ o,ao ______ 1:..:;.:06::..._ o____ _:_;.1 0;..:...4o _____ ---, 102o 

MONTANA 

0 MilesCity 

I 

I 

NORTH 

DAKOTA 

~------

I 

I 

SOUTH 

DAKOTA 

~-----

NEBRASKA 

42°-------_J------~------~ 

1 OIT 200 KILOMETERS 

Figure 1. Powder River Basin, Wyo., showing location of 

Acme Mine. 

Geologists, 1978; Dunrud and Osterwald, 1980). The 

resulting tension fractures around the depressions may 

also conduct air and water into the old mines and promote spontaneous combustion in the remaining coal. 

Underground fires pollute air and water, cause further 

subsidence, and may also burn into valuable unmined 

coal reserves. 

Maps showing the location of abandoned underground coal mines are not always available, and thus the 

locations of some mines are unknown until they are 

detected by surface subsidence. Land that is underlain 

by coal mines but has not yet subsided may therefore 

erroneously seem suitable for construction sites. 

The present report includes the results of geophysical surveys run during 1976 and 1977 over an underground coal mine in an attempt to provide geophysical 

techniques that may be used to locate these "lost" 

underground openings. 

Two sites (fig. 2) over the old, underground Acme 

Mine were chosen for the geophysical surveys. A ground 

magnetic survey and a short seismic-refraction line were 

run at site 1. Gravity, electromagnetic, seismic-refraction, 

and leveling surveys as well as a magnetic survey were 

run at site 2. In early 1978, the coal in the mine near site 

2 Geophysical Detection of the Acme Mine, Wyoming 

2 was burning so rapidly that a previously constructed 

firebreak strip mine was continued south and southwest 

of the site in order to save the unmined coal south of the 

break. This mining activity forced the cessation of further geophysical surveys at site 2. 

The findings from these present studies may have 

application to other searches for buried cavities by geophysical methods. Manmade cavities created by underground gasification of coal or by underground solution 

of salt deposits, for example, cannot accurately be located 

in many instances. The detection by earlier geophysical 

methods of naturally occurring underground cavities, 

including sinkholes and caverns in carbonate rocks and 

tubes in lava, has also met with only limited success (U.S. 

Army, 1977, 1981). 

ACKNOWLEDGMENTS 

Access to the survey sites was permitted by personnel of Peter Kiewit Sons' Co. and Big Horn Co., 

Sheridan, Wyo., and the Padlock Ranch, Dayton, Wyo. 

T.F. Bullard, S. Kanizay, W.M. Miller, J.K. 

Odum, F.W. Osterwald, and A.L. Ramirez participated 

in the geophysical field surveys and data reduction. All 

these participants except W .M. Miller were employees of 

the U.S. Geological Survey at the time of the surveys and 

data reduction. 

Most of the survey instruments were borrowed: 

magnetometer, E.R. Landis, U.S. Geological Survey; 

gravimeter, D.L. Healey, U.S. Geological Survey; 

seismograph, U.S. Bureau of Land Management; and 

electromagnetometer, G. V. Keller, Colorado School of 

Mines. 

Technical advice was provided by the following 

USGS personnel: H.D. Ackermann, seismic refraction; 

G.D. Bath, magnetics; R.R. Wahl, gravity; and R.D. 

Watts, electromagnetics. F.W. Osterwald, U.S. Geological Survey, provided information on principal facts 

and geology of the Acme Mine, and reviewed the 

manuscript both with the other advisors and after their 

critiques had been received and incorporated. 

The author thanks all of these people and organizations for their support. The accuracy of the final product, however, is the sole responsibility of the author. 

THE ACME MINE 

The Acme Mine is just north of the former town 

of Acme (figs. 1, 2), which is about 13 km north of 

Sheridan, Wyo., in the northwest part of the Powder 

River Basin. 

The major coal deposits in the area of the mine 

are in the Tongue River Member of the Fort Union 
100 200 METERS 

Figure 2. Acme Mine, showing geophysical survey sites 1 and 2. 

1

c 6.1m 

I 

I 

I 

I 

I 

I 

I 

II 

11· 

II 

II 

II 

II 

II 

II 

ROAD II 

=~~======================~~======== 

::: 

::: 

::: SPOIL ~ 

- PILE 

,, EXPLANATION 

Area known to be underlain by Acme 

coal mine 

~ Area of geophysical survey 

-1097- Topographic elevation in meters 

OB10.1m 

C 6.1m 

0 Drill holeOverburden thickness, m 

Coal thickness, m 

I; 

I; 

/; 

I; 

I; 

I; 

I; 

I; 

The Acme Mine 3 
Formation of Paleocene age (Mapel, 1959; Dunrud and 

Osterwald, 1980). The rocks of the Tongue River Member 

consist of soft mudstones, siltstones, and friable sandstones, as well as the coal beds. The coal beds vary in 

thickness from less than 1 m to as much as 17 m. 

Many coal beds in the Acme area, as well as in other 

parts of the Powder River Basin, were burned along their 

outcrops during prehistoric time, and the resulting intense 

heat has baked and fused the overlying sedimentary rocks 

into clinkers (Rogers, 1917; Bauer, 1972). In this report, 

clinker deposits are meant to include all baked and fused 

sedimentary rocks as well as any coke and ash residue 

from the burned coal. Clinkers are brittle and very 

magnetic. 

Surficial deposits at the survey sites include alluvium, colluvium, and regolith derived mainly from the 

relatively soft rocks of the Fort Union Formation. 

Thickness of the surficial deposits was observed in most 

scarps around subsidence pits to be less than 5 m. 

The Acme Mine was begun in 1911 and abandoned 

in 1940. During its life span, the mine was extended over 

about 5. 8 km2 • Coal was extracted from the lower 

Monarch bed, which is about 6 m thick with 10m of overburden at site 2, according to a drill-hole and coal-mine 

map furnished by the Big Horn Coal Company (written 

commun., 1982). This coal bed dips less than about 1° 

to the east and northeast according to elevations on the 

floor shown on an older mine map (Sheridan-Wyoming 

Coal Company, 1938). These floor elevations are assumed 

to be parallel with the lower contact of the coal bed. 

The coal was mined by the room-and-pillar method, 

and perhaps less than 50 percent of the coal shown in 

the plan view was extracted. The rooms tended to be 

about 3-4m high and 2.5-4 m wide. About 1-1.5 m of 

coal were left in both the ceiling and floor, evidently to 

provide stability. C. R. Dunrud (oral commun., 1983) 

related conversations with some of the original mine personnel, who pointed out that manmade ground support 

was not installed in the working entries except for supporting electric lines, but it probably was used in the main 

entryways. Some of the roofs were evidently flat, 

although old photographs of the mine interior show some 

arching. The roofs were presumably arched both naturally by slight caving and intentionally, for strength and mining convenience. 

K uzara ( 1977, p. 111-126) related that the coal was 

mined by undercutting the working face of coal with 

"short-wall" machines before the coal was shot down and 

loaded in cars by hand. The cars were then dragged to 

the main entryways by horses and taken out of the mine 

by motor cars that were evidently electrically powered. 

The main tunnels were electrically lighted, and the mine 

had a complete system of telephones with a phone at every 

point from which coal was being taken. The main tunnels also had "tracks for the motors and cars with 65-lb 

4 Geophysical Detection of the Acme Mine, Wyoming 

[per 1-yd length] rails throughout; the branch lines into 

the working chambers [were] temporary and lighter steel 

[was] used." 

Much of the mine has subsided and left broad 

topographic depressions (figs. 3A and 3B) that may span 

many rooms and entries (Dunrud and Osterwald, 1980). 

Localized pits and troughs are also included within the 

broad depressions, as well as along tension cracks around 

the large subsidence depressions. The main subsidence 

mechanism is apparently broad collapse of the mine roof 

with consequent stoping over individual underground 

openings. An original underground opening may 

"migrate" upward as far as the topographic surface and 

become manifested as a pit or trough. The pits and 

troughs seem to develop most easily over the intersections 

of underground openings, where the raveled and collapsed debris has migrated laterally into rooms, drifts, 

or crosscuts. Conversely, relative stability may occur over 

pillars or main entries that were still well supported after 

the mine was abandoned. The road that bears northeast 

from the portal and through site 1 (fig. 2), for example, 

is over the main haulageway of the mine. The manmade 

support in this main entry was apparently not removed 

before the mine was abandoned, and therefore the ground 

surface over it is still relatively unaffected by subsidence. 

Most of the topographic evidence of subsidence 

within the area of figure 2 is south of the 1128-m contour. Overburden above the mined coal bed in this area 

of subsidence is less than about 40 m, but the overburden 

north of the 1128-m contour shown in figure 2 may be 

as much as 70 m thick. Overburden thickness above the 

coal at the two geophysical survey sites (fig. 2) is estimated 

at 10-20 m, based on mine and topographic maps. 

Subsidence is evident over nearly all of site 1 where 

mining has been done. Any underground openings that 

remain at mine level are presumably filled with air rather 

than water because no water can be seen at the adit and 

also because any water introduced into the mine would 

tend to drain downdip to the east. 

Subsidence is not apparent at site 2 (fig. 2), except 

at the extreme northern tip of the area and outside site 

2 on the west, even though much of the site was undermined. The reason for the lack of subsidence over the 

mines at site 2 is unknown. If the mining at site 2 was 

the same age as that under the subsided area and the 

Figure 3 (facing page). Photographic views across subsided 

Acme Mine. A, An oblique view northeast across map of figure 

2; site 1 is in center of photo between power plant and large 

spoil pile and site 2 is in upper part of photo and immediately 

beyond northern tip of spoil pile. Photograph by F. W. Osterwald, April 1976. 8, Northeast view across part of mine that 

is about 2.5 km north of spoil pile of A; this part of mine was 

burning in 1980 both at depth and at ground surface with accompanying subsidence, explosions, and air pollution. Photograph by F. W. Osterwald and j. B. Bennetti, Jr., February 1980. 
The Acme Mine 5 
ground support was the same, then the ground at site 2 

should have likewise subsided. This is not the case, 

however. The mining at site 2 must have been done after 

the mining under the northern tip and to the west because 

the nonsubsided portion of site 2 is peripheral to the other 

mining. Most of the Acme Mine is, indeed, constrained 

by boundary lines of partial sections and ownership, but 

the mining at site 2 was not completed over all of its section. An older mined area would be more prone to failure 

than a significantly newer mined area. 

Another possible explanation for the lack of subsidence of site 2 is that the pillars may not have been 

mined as the miners "backed out" and abandoned that 

area. Yet another possibility is a change in geology. 

Relatively competent bedrock over site 2 may have contributed to its support and therefore retarded subsidence. 

The amount of water that occupied the original 

underground openings under site 2 at the time of the 

surveys is unknown. Any water introduced into the 

relatively nonsubsided area at site 2 might be trapped 

there because there is no outlet downdip to the east and 

northeast (fig. 2). Any water introduced into the subsided 

area, however, would probably be diverted downdip and 

into openings that are continuous to the east and northeast and out of the area. 

At the time of the geophysical surveys in 1976 and 

1977, a local inhabitant reported that snow readily melted 

over most of site 1 and that visible steam issued from 

some of the subsidence-caused fractures. Therefore either 

some of the unmined coal under site 1 was apparently 

burning, or warm air was conducted through the underground openings and to the ground surface by subsidence 

fractures. Site 2 showed these same heat characteristics 

at the subsided part at the northern tip of the site but 

not at the · nonsubsided southern portion. 

GEOPHYSICAL SURVEYS 

. Sites 1 and 2 (fig. 2) were both nearly free of manmade disturbances and objects such as powerlines, fences, 

and vehicular traffic that might interfere with geophysical 

surveys. 

Site 1 was selected for a magnetic survey because 

of the possible presence of abandoned iron objects in the 

mine. Also, most of the area was collapsed and much of 

the unmined coal was burning, with the possibility that 

the rocks along subsidence fractures would be magnetized 

by escaping heat. The undulating terrain of the subsided 

area would invalidate gravity, seismic, and electromagnetic surveys, which are sensitive to topographic relief. 

Site 2 was selected for detailed study because subsidence was visible only at the northern tip of the site. 

Topographic relief at site 2 is relativelylow, so the effect 

of either naturally rough terrain or subsided ground on 

6 Geophysical Detection of the Acme Mine, Wyoming 

the geophysical survey data would be minimized. The site 

was desirable also because the surveys would straddle the 

boundary of the known mined area. 

Both magnetic and electromagnetic surveys were 

run at site 2 because iron objects may have been abandoned in the mine and also because the magnetic and electromagnetic fields over the nonsubsided area may have 

been changed either by the underground openings or by 

burning coal. Gravity and seismic surveys were run to 

detect the underground openings; a leveling survey was 

run in conjunction with the gravity survey for use in the 

data reduction and to define any subtle subsidence trends 

that were not visible. 

Geophysical techniques and principles are discussed 

in this report in the following order: magnetics is discussed in relation to site 1, although a magnetic survey 

was also run at site 2; electromagnetics, gravimetry, and 

seismology are discussed in relation to site 2 because these 

surveys are essentially unique to that site, except for a 

relatively small seismic survey at site 1 (fig. 2). 

Site 1 

Magnetic Anomalies Caused by Heating Rocks and by Iron 

Location surveys of abandoned coal mines have 

been run with magnetometers to locate rocks that have 

become magnetic when heated. D.E. Watson and D.B. 

Hoover (written commun., 1977) have surveyed burning 

coal mines, including part of the northern part of the 

Acme Mine, in an attempt to locate these burn fronts. 

When gray clastic rocks are heated, they tend to 

turn some shade of red or dark brown and become more 

magnetic. According to D.E. Watson (written commun., 

1976) the red hue mostly can be attributed to iron-oxide 

silicates, but the minerals that mainly contribute to the 

thermal-remanent magnetism are mostly ilmenohematites and titano-magnetites. The magnetic domains 

of these minerals are originally in a relatively random 

orientation, but heating reorients the domains so that they 

are parallel with the Earth's field at the time of heating 

and the net magnetization is greatly enhanced, along with 

other possible changes. The temperature does not have 

to exceed the Curie point in order for the rocks to be 

remagnetized. 

The distribution of magnetization in various rock 

types is compared in figure 4. The net magnetization of 

clastic rock is dramatically increased by several orders of 

magnitude when the rock is heated and the magnetization appears to be related to the degree of baking and 

fusing. The resulting clinkers are among the most 

magnetic rocks found in nature. The degree of magnetization caused by heating along subsidence fractures within 

clastic rocks, however, is unknown. 
I I I I I I 

COAL -

CARBONATE ROCKS -

NON-RED BED INDURATED AND 

NONINDURATED SANDSTONE 

___ ....... --.. ... -

NON-RED BED SHALE, CLAYSTONE 

OR MUDSTONE 

NONWELDED TO WELDED TUFF 

INTRUSIVE IGNEOUS ROCKS 

BAKED AND FUSED CLASTIC 

ROCKS (CLINKERS) I I l I I I 

10-6 10-5 10-4 10-3 

MAGNETISM, IN emu PER CUBIC CENTIMETER 

Figure 4. Representative net magnetism of various rock types (D.E. Watson, written commun., November 1976). 

Iron objects abandoned in mines also cause 

magnetic anomalies that may be detected at the ground 

surface. Figure 5 is a first approximation of the range 

of a maximized total-intensity magnetic anomaly as a 

function of distance from iron objects. 

Figure 5 indicates maximum anomalies for dipole 

objects that range in size from small tools to a mine car 

and for objects with lines of dipoles, such as mine-car 

rails. The magnetic attraction of dipole objects decreases 

as the inverse cube of the distance to the magnetometer. 

The first approximation formula (Briener, 1973) for this 

dipole case (objects of finite dimensions such as tools or 

a mine car) is 

(1) 

The magnetic attraction of an object represented by a 

long line of dipoles decreases as the inverse square of the 

distance. The first approximation formula for a line of 

dipoles (objects of one infinitely long dimension such as 

a mine-car rail) is 

(2) 

where F is the Earth's field, about 57,770 gammas at the 

time site 2 was surveyed; Tis the total magnetic intensity 

anomaly in gammas; k is magnetic susceptibility, about 

25 for "hard" steel, in cgs (centimeter gram second) units; 

V and A are volume and area, respectively, in cgs units; 

and r is the distance between the magnetometer and the 

iron object, in centimeters. 

Henshaw (1977) and Gallagher and others (1978) 

ran magnetic surveys as well as other geophysical surveys 

over abandoned vertical mine shafts. The shafts had been 

backfilled with iron objects as well as with soil of different magnetic properties than the surrounding rock, but 

no surface trace was found at the shafts. 'These surveys 

yielded positive results where the shaft fillings contained 

a significant amount of iron objects. The only known entry to the Acme Mine in the site areas is horizontal rather 

than vertical, but the subsidence pit over it has been 

backfilled with iron objects and soil. 

Magnetic Survey and Magnetic Field Characteristics 

A total magnetic intensity survey was run over the 

grid of site 1 (fig. 6) with a portable proton magnetometer. Readings were taken at about 10- or 30-m intervals 

with the sensor at a constant height of about 2.1 m above 

ground. Individual readings of total magnetic intensity 

were read to an accuracy of about ± 1 gamma ( ± 1 nT 

(nanotesla)). The readings were reduced to a base station 

value of 58,634 gammas. Base readings were repeated 

within 20 minutes of the initial reading at an accuracy 

of ± 4 gammas, and this base drift was linearly 

distributed over the station readings. The results are 

Geophysical Surveys 7 
Cii ::i 50 (/) 

UJ 

to 

z 

~ 

~40 

(/) 

~ 

~ 

~ 

~ 

(!) 30 z 

> ...J 

~ 

~ 

~20 

~ 

u 

i= 

UJ 

z 

~ 10 

~ 

OL_----~------_c~~==~======~----~ 

0 5 10 15 20 

DISTANCE FROM TOTAL INTENSITY 

MAGNETOMETER. IN METERS 

25 

Figure 5. First approximation of range of a maximized totalintensity magnetic anomaly as a function of distance from iron 

objects. Objects are assumed to be on floor of the Acme Mine, 

15-25 m (shaded area) below magnetometer at ground surface. Mine rail weights are in conventional units. 

shown in figure 6, a map of the contoured magnetic field, 

and figures 7A and 7B, cross sections showing anomalies, 

ground surface, buried materials, and mine workings. 

The magnetic field of site 1 shown in figures 6 and 

7 B is divided into three broad anomalies: 

1. The anomaly characterized by the 200-gamma contour interval, which is associated with the area of outcropping clinkers at the south end of the site (fig. 7B). 

This anomaly also includes a more localized anomaly caused by a junk pile (fig. 7A) in the southwest corner of the site. 

2. The anomaly characterized by the 50-gamma contour 

interval associated with the relatively major subsidence area in the central part of the site. 

3. The anomaly characterized by the 5-gamma contour 

interval associated with the relatively minor subsidence area at the north end of the site. 

Some localized magnetic anomalies are also evident 

around near-surface objects that are made of iron. The 

known objects made of iron include the steel drill-hole 

casings in the west-central part (fig. 6) and the junk in 

the southwest part of site 1. No rails, steel supports, or 

other objects made of iron could be seen within 6 m inside the main portal. 

8 Geophysical Detection of the Acme Mine, Wyoming 

The clinkers at the south end of site 1 are exposed 

at a scarp along the north side of the Tongue River (fig. 

2). The scarp is as much as 15 m high within site 1. 

Clinkers may crop out anywhere in the exposed section, 

although they are more evident in the upper part of the 

scarp. 

The magnetic anomaly (figs. 6 and 7B (B-B')) that 

is produced by these clinkers is characterized by the 

200-gamma contour interval. It has a magnitude of at 

least 600 gammas and is as much as 60 m wide. The width 

at the midpoints on the flanks of the anomaly, however, 

may more accurately measure the width of the mass of 

clinkers (Hasbrouck and Hadsell, 1976); this width is 

about 40 m. The anomaly does not extend over the subsided area, which implies that the clinkers extend only 

about 40 m into the outcrop and that the coal could only 

be mined as far south as the clinker contact. Subsidence, 

therefore, apparently begins approximately at the contact of the coal with the clinkers. 

Figure 7 A (A -A ') shows the magnetic anomaly 

associated with the buried junk pile in the southwest part 

of site 1 (fig. 6). The junk that is partly composed of iron 

includes one or two car bodies, some stoves and 

refrigerators, and various smaller objects such as cans, 

all of which were dumped in the subsidence pit over the 

entryway of the mine. The maximum amplitude of the 

anomaly over the junk pile is at least 4,800 gammas, and 

its width is about 3-4 m at the midpoints on its flank. 

The junk was visible during the original magnetic 

survey of site 1. It was buried later by no more than about 

4 m of soil, and then the magnetic profile was run. The 

magnetic profile along A-A' of figure 7A demonstrates 

the ease with which junk buried in these pits as well as 

other pits can be identified. The profile may also give 

some indication of the amount and proximity of iron objects in pits. 

Any iron used in mining but abandoned in an 

underground mine, however, is expected to be present in 

much smaller quantities and in less concentration than 

in the junk pile along A-A' (fig. 7A), and it would almost 

certainly be buried deeper unless it were in shafts. The 

resultant anomalies caused by iron abandoned in mines 

are discussed more fully in conjunction with site 2. 

The anomaly with a contour interval of 50 gammas in the central part of site 1 is of only slightly greater 

value (about 350 gammas) than the maximum residual 

of the 5-gamma-contour anomaly (about 110 gammas) 

at the north, although the latter anomaly may appear to 

be more complex because of its smaller contour interval 

(fig. 6). Neither of the broad central or northern 

anomalies appears to be controlled by mine drifts or 

rooms because the associated anomalies seem to be 

randomly oriented. The boundary between the central 

and northern anomalies, however, coincides with the 

general boundary between the areas of major and minor 
57 

/\ 

I (/) 

~ I 

I 

. 57,800~ 

. . ~ 

. 

EXPLANATION 

Magnetic contours-Showing 

~ total magnetic field inintensity in gammas. Hachured to indicate closed 

area of low magnetic intensity. Contours are referenced to 58,634 gammas 

Area in which contour interval is 5 gammas-Index 

contours every 25 gammas 

Area in which contour interval is 50 gammas-Index 

contours every 200 gammas 

Area in which contour interval is 200 gammas-Index 

contours every 1 000 gammas 

cs-iE;M; ....... ___ _... 

Area in which visible steam is issuing from subsidence cracks at temperatures less than about 

10°C 

Magnetic station 

0 

Drill hole with steel casing 

50 METERS 

150 FEET 

Figure 6. Map of contoured magnetic field of site 1. (See fig. 2 for location of site 1.) Section A-A', 8-8', figure 7. 

Geophysical Surveys 9 
A 

62,000 

61,500 

~ 61,000 enzen 

LU~ l-en 60,500 Zw 

-Ioo 

....lz 60,000 !:!::!<( 

u..z 

u-

~~ 59,500 

z~ 

(.!)~ 

<(<( 59,000 ~(.!) 

....Jz 

~- 58,500 

0 

1-

58,000 

57,500 

1120 

LU 

> g_J(J) 1110 

<(LUCX:: 

z~~ Q....JLU 1100 

j::<(~ 

<(LUz 

>en- LU 1090 

....J 

LU 

1080 

0 

B 

59,000 

57,000 

..... 

1120 Q) ::::. 

Ci: 

1110 ~ Outcropping 1:7) 

1100 1: eli~ ~ 

1090 

1080 B 

1070 

0 50 

Outcropping fine-grained sedimentary rocks 

with some clinkers 

\_ci_:.o ·. ·.P· ... _: .: . .. iJ/ . Artific ial fill,.......-,<: . . )). ·2t. .c:?. / A~proxlmate old 

. d . k ' · · · · ·/ subs1ded land surface d1rt, an JUn 

Mine opening 

A 

5 10 15 

LENGTH OF SECTION, IN METERS 

Approximate location of mine 

100 150 200 250 300 

LENGTH OF SECTION, IN METERS 

20 

A' 

Stream 

bed 

350 400 

· Figure 7. Magnetic fields associated with A, buried junk (A-A') and B, clinkers and subsided area (8-B') at site 1. 

subsidence (figs. 6, 7B). The slightly higher amplitude of 

the 50-gamma-contour anomaly in the central part of site 

1, therefore, appears to be related to the area of major 

subsidence. The cause of this greater anomaly over the 

subsided area is unknown. Perhaps any heat in the mine 

10 Geophysical Detection of the Acme Mine, Wyoming 

below the area of greater subsidence could more easily 

escape up subsidence fractures and magnetize nearby 

rocks, or perhaps a terrain effect caused by the undulating 

topography of the subsidence pits influenced the magnetic 

measurements. 
Site 2 

The Acme Mine surveys at site 2 include measurements of topography, magnetics, electromagnetics, gravity, and seismic velocity. Some of these measurements 

essentially can be assessed independently; others need supporting information. Separate measurements or at least 

reasonable estimates of resistivity for the electromagnetic 

surveys are needed, and a vertical profile of velocities is 

desirable for the seismic surveys. These supporting data 

are commonly collected from nearby boreholes, but unfortunately no borehole information of this type was 

available near the Acme Mine. 

Geophysical data were available, however, from 

two boreholes away from the Acme Mine but drilled in 

similar rocks. The data from these holes are reproduced 

in figures SA (borehole B-1 (Farrow, 1976)) and 8B 

(borehole S-2 (Farrow, 1980)). Borehole B-1 penetrated 

the Wasatch Formation and borehole S-2 penetrated the 

same geologic formation (Fort Union) as was mined at 

site 2. The lithology of the Wasatch and Fort Union Formations is very similar. Borehole S-2 is 13 km south of 

the mine at Sheridan, Wyo., and B-1 is 67 km south. 

Because the lithologies at the boreholes and at the undisturbed ground at the mine are similar, the geophysical 

data seem to be at least indicative of some properties at 

site 2. 

Figure 2 shows the general outline of the Acme 

Mine in 1938 (Sheridan-Wyoming Coal Company, 1938) 

and the generalized topography of site 2. Figure 9 shows 

the detailed plan of the mine at site 2 as well as the detailed topography, and figure 10 shows a cross section 

through figure 9. The horizontal accuracy of the mine 

map is apparently within 10 m. The mine map presumably 

was updated to include all mining at site 2. The mine 

rooms are estimated to be about 3-4 m high, and their 

average depth of burial at the southern boundary along 

the cross section of figure 10 is estimated to be about 

11m. 

The ground-surface area affected by subsidence is commonly greater than the mined area where all the coal has 

been removed. The draw angle (fig. 10) is measured from 

the vertical and includes all the subsidence-caused deformation at the ground surface down to the outer boundary of the mine opening that caused the subsidence. The 

angle of break is also measured from the vertical. The 

break line connects points of maximum tensile stress and 

strain from the ground surface down to the edge of the 

mine workings. The draw angle may range from 0° to 

35 ° in many coal mines in the West, depending on overburden thickness, mining methods, geology, and topography. However, the draw angle for abandoned mines 

around Acme, where the overburden is less than about 

60 m, is steep to near vertical (Dunrud and Osterwald, 

1980). 

Topographic Survey 

The topography of site 2 (figs. 9, 10) was precisely 

surveyed in order to accurately reduce gravity data and 

to define any subtle patterns in the topography caused 

by subsidence over the mined area. The grid of site 2 was 

laid out with a transit and tape, and its horizontal accuracy is about ±0.3 m. The elevations of the grid were 

leveled to an accuracy of about ±0.015 m. 

According to A.A. Matilla (U.S. Geological 

Survey, oral commun., 1976), aerial photographs were 

taken of the Acme Mine in the early to middle 1940's. 

He recalled that in 1949 he could clearly see the outline 

of drifts, pillars, and rooms on the photographs over 

some parts of the Acme Mine; but when he drove over 

the mine in the same year, he could see no surface trace 

of subsidence on the ground. The detailed topography 

of figures 9 and 10 shows a relatively featureless slope 

to the southeast. There is no particular pattern to the contours, however, that suggests that they are controlled 

either by subsidence or by small and local intermittent 

streambeds. 

Magnetic Survey and Magnetic Field Characteristics 

A magnetic survey was run over the grid of site 2 

using the same portable magnetometer and the same field 

techniques that were used at site 1. The readings at site 

2 were also corrected for drift and referenced to the same 

base-station value of 58,634 gammas as the readings at 

site 1. 

The contoured magnetic field of site 2 is shown in 

figure 11, and a cross section (C-C') through the site is 

shown in figure 12. There is no prominent isogamma contour pattern that suggests subsurface control of the 

magnetic field. 

A relatively prominent residual magnetic low 

(57,620 gammas) of less than about 15 gammas exists over 

the "working rooms" (in the center of the area of fig. 

11) of the nonsubsided area. A very general high (57 ,620 

gammas) of about 10 gammas is positioned over the main 

and bypass entries in the older workings (subsided) to the 

northwest of the low, and another high (57 ,620) of 

perhaps 15 gammas is over the first and second entries 

in the newer workings (nonsubsided) to the southeast of 

the low. These two general magnetic highs may be caused 

by abandoned iron objects in the first and second entries 

and (or) changes in the amount of magnetite in the intervening sedimentary rocks of the overburden. In addition, 

the anomalous high over the subsided older workings may 

be influenced by magnetization caused by heat from mine 

fires escaping up subsidence fractures. 

The graph of the first approximation of magnetic 

intensity (fig. 5) indicates that at least two 21-lb/yd rails 

must be present in the mine at site 2 in order to be reliably 

Geophysical Surveys 11 
.... 

N 

~ 

I'D 

0 

"C 

:r 

-<! 

fl) 

;:;· 

~ 

0 

I'D 

it 

Q. 

cs· 

:::J 

s.. 

;. 

I'D 

> I") 

3 

I'D 

~ :;· 

~I'D 

:; 

-<! 

0 

3 

:;· 

ao 

en 40 

0 z z 30 

-0 

uiu 20 

~~ 

i= j 10 

~ 

10 20 30 

B-1 

CORE 

HOLE 

~ 0 

DISTANCE, IN METERS S-2 

CORE 

HOLE 

0 

10 ~--~~-1 10 

20 

I 20 

en 

a: 30 

w 

~ .... 40 

w 50 

60 z 

70 

:i 80 .... 

30 

.... 40 

.... 50 

60 

z 70 

80 

90 

< . ,~\\ . ·\ ...... .. \~ • CD 

\No 

·-·~<. :-·~\ .\ ~ . \\ \' .. (1,) \ •• 

'""' N .. · 

t fl.. 90 fl.. 

w W100 

0100 

110 

120 

120 

130 

130 c____c:::>.__ _ _J____ __ _i__ 

0 50 100 8 0 

RESISTIVITY, IN oo 8 0 0 0 0 0 0 0 N '<:t CD N N N N 

OHM-METERS VELOCITY, IN 

METERS PER 

SECOND 

A 

50 en 

0 z z 40 

-0 

w u 30 

~~ 

i= j 20 

~ 10 

0 

0 10 20 30 

50 100 

RESISTIVITY, IN 

OHM-METERS 

0 

0 

0 

N 

DISTANCE, IN METERS 

0 

0 

N 

N 

0 

0 

'<:t 

N 

< 

~~= 

~ \\. 

~:; .~\~' \No 0 

< \.~· 

·.~ \ 

\\,\ 

w• 

. \• -.I. 

W\ 

N\ + . ~ ,. ~ l : 

0 0 0 0 0 0 CD 00 0 N N ("') 

VELOCITY, IN 

0 

0 

N 

("') 

METERS PER SECOND 

B 

0 0 ...... <"! ("') '<:t U? ~ ,.... 0 c--i c--i N c--i c--i N N c--i '<:t 

("') DENSITY, IN GRAMS 

PER CUBIC CENTIMETER 

EXPLANATION 

UTHOLOGY 

Alluvium 

Sandstone 

Mudstone 

Siltstone 

Claystone 

Shale 

Coal 

LEAST-SQUARE 

FIT 

Linear 

Logarithmic 

Figure 8. Sixteen-inch normal resistivity logs, velocity distribution (least-squared data from boreholes and data from surface-run seismic surveys at the boreholes), and 

core densities from two boreholes: A, B- 1 at Buffalo, Wyo., and B, S- 2 at Sheridan, Wyo. Although both boreholes are some distance from Acme Mine, S-2 borehole 

is in same unit as mine (Fort Union Formation), and B- 1 borehole is in a lithologically very similar formation (Wasatch Formation). On least-squared fit, V , compressional 

wave velocity in meters per second; z, depth in meters. On surface-run seismic surveys, numerals on curves, compressional-wave velocities in meters per second; x, o, 

arrival times. These basic data are used in reduction and interpretation of electromagnetic surveys. 
detected; however, it also indicates that just one 65-lb/yd 

rail or one 1 ,000-kg mine car would create relatively very 

large anomalies. Because the magnetic anomaly map of 

figure 11 does not depict such large anomalies, two 

21-lb/yd rails in both the first and second entries of site 

2 have been assumed to be a more reasonable situation 

for modeling purposes. 

Magnetic anomalies were calculated for mine-car 

rails in both the first and second entries of cross section 

C-C' in figure 12. The calculations assume that there is 

only one set of continuously joined rails per drift and that 

each set can be represented by one solid, horizontal steel 

cylinder. A horizontal cylinder magnetized by a steeply 

dipping (about 75 ') Earth's field would behave as a 

monopole, although the rails are a continuous line of 

dipoles. Equation (2) was used to approximate rails along 

cross section C-C' by substituting the same magnetic and 

cylinder values used to compute the graph of figure 5. 

The magnetic anomalies for the main and bypass 

drifts were each computed separately as well as together, 

and the results are plotted over the respective drifts and 

compared to the measured total-Earth's field in figure 12. The configuration and amplitude of the computed anomalies generally match those of the measured 

magnetic field. Furthermore, the peaks are positioned 

so that they could be caused by iron-made objects in the 

two drifts. There is enough ambiguity in the magnetic 

prospecting method, however, that other evidence 

should be collected and interpreted before the presence 

of iron objects in the main and bypass drifts can be 

concluded. 

Electromagnetic Survey and 

Electromagnetic Field Characteristics 

An electromagnetic survey was run over the grid 

at site 2 using a Slingram apparatus (Keller and 

Frischknecht, 1970). This method uses two portable coils 

that are held horizontally and in the same plane. One coil 

is a transmitter and the other is a receiver; they are connected by an electric wire. When the coil of the transmitter is energized, its primary electromagnetic field induces 

eddy currents into the ground. The resulting eddy currents produce a secondary electromagnetic field. The 

primary field is compensated for by referencing along the 

electric wire, and the resultant secondary field is detected 

and resolved into two components with 90 ° separation: 

(1) the in-phase or real component and (2) the out-ofphase or "imaginary" component. The components are 

expressed arbitrarily in percentage of secondary field 

strength; the real component is referenced to 100 and the 

imaginary component is referenced to 0. The ratio of the 

in-phase anomaly to the out-of-phase anomaly, as 

measured in homogeneous earth, is small for poorly conductive earth and large for conductive earth; however, 

this ratio also depends on the geometry and conductivity 

N 

l 

50 METERS 

R 84 W 

EXPLANATION 

-3.50- Topographic contour-Interval is 

0.1 m and datum is arbitrary 

w 

z 

::J 

z 

0 

6 

w 

(f) 

• Survey point 

Figure 9. Detailed topography of site 2, Acme Mine, showing passages (mine map, Sheridan, Wyoming Coal Company, 

1938), limits (hachured line) of visible broad surface subsidence 

(shaded) and detailed topographic survey at site 2, which is 

away from the visibly subsided area. Limits of theoretical deformation (draw line) above mine passages are nearly vertical for 

these rocks but detailed topography does not indicate actual 

subsidence. Section C-C', figure 10. 

Geophysical Surveys 13 
c 

1110 

Cf) 

a: 

w 

G:i 1100 

~ 

~ 

z 

0 

~ 1090 

> w 

...J 

w 

1080 

Second 

entry 

\o 

\~ 

\ 

I 

First 

entry 

I 

C' 

0 30 60 90 120 150 180 210 240 270 300 

LENGTH OF SECTION, IN METERS 

Figure 10. Cross section C-C' (fig. 9) through site 2 (fig. 9) showing topographic surface and estimated position of mine 

openings with draw lines (theoretical limits of subsidence) . Subsidence was not indicated by detailed measurements of 

topographic surface. 

of layers as well as the frequency of the primary electromagnetic field (Parasnis, 1966). 

The apparatus had ostensibly been calibrated over 

"neutral ground" prior to being loaned to the investigators, but an approximate check was still made at 

site 2. This check was done over smooth terrain but away 

from possible effects of mine subsidence. Essentially no 

adjustments were needed as indicated by the check. 

Readings taken during the mine survey were taken at input frequencies of both 440 and 1, 760Hz; the readings 

taken at the low frequencies exhibited instrument noise, 

so the higher frequency data were used in the analyses. 

The survey was advanced in a series of straight lines 

that were mutually parallel with north headings and with 

data points at 7.6-m intervals. The terrain was flat enough 

that no spacing corrections needed to be made. Slingram 

data are "bifocal" in regard to sensitivity, and, consequently, inhomogeneities near either the transmitter or 

receiver may have a greater effect than do inhomogeneities in between. Nevertheless, there was no practical way 

to delineate the exact location of the inhomogeneities. 

Therefore, the standard practice was followed of plotting the data at the center of wherever the 60-m line was 

positioned at the time of the reading. 

Figures 13 and 14 show the electromagnetic fields 

in plan view of the three profiles for both the in-phase 

and out-of-phase components, respectively, of the site 2 

survey. The in-phase field is considerably less complex 

than the out-of-phase field. The in-phase field of figure 

13 has a generally broad gradient that increases toward 

the west and perhaps the north. Low anomalies in the inphase field include a general low over the northeast end of 

the second entry of the nonsubsided area and a small 

unexplained low in the south-central part of the area. The 

out-of-phase component map (fig. 14) also shows a broad 

relatively high anomaly over the nonsubsided mine 

14 Geophysical Detection of the Acme Mine, Wyoming 

workings and a low anomaly over the northeast end of 

the second entry of the nonsubsided area. 

A cross section along C-C' (figs. 13, 14) of the electromagnetic components is shown in figure 15. The outof-phase component shows its lowest anomaly over the 

second entry of the newer workings. The cause of the 

anomaly is unknown, but two possibilities seem eminent: 

(1) metal objects abandoned in the mine and (2) resistivity boundaries, which correspond to mechanical boundaries between disturbed and undisturbed ground. 

Computations of electromagnetic fields are complicated, but according to Watts (1978) a long, straight metal 

conductor, for example, that is buried less than one-fourth 

skin depths will significantly disturb the ambient electromagnetic fields. Skin depth (the effective depth of penetration of electromagnetic energy in a conducting medium 

(Sheriff, 1974)) is equal to (2 pi p.w'h) where p is resistivity, in ohm-meters, p. is magnetic permeability in henries/ 

meter, and w is angular frequency. One-fourth skin depth 

was computed for site 2 as a function of resistivity. The 

relatively low magnetic permeability of the magnetitebearing sedimentary rocks and coal is only slightly more 

than free space; it was assumed to be 13 X 10-7 H/m. 

Angular frequency was obtained from 1,760 Hz, which 

was the survey frequency. 

Resistivities of the sedimentary rocks and coal of the 

Fort Union Formation at site 2 were not available in the 

area of the Acme Mine. An indication of representative 

resistivities in undisturbed ground was available, however, 

from borehole S-2 (fig. 8B) in the same geologic formation some 13 km south at Sheridan, Wyo., and from borehole B-1 (fig. 8A) in the similar Wasatch Formation some 

67 km to the south, at Buffalo, Wyo. The resistivity logs 

of boreholes B-1 and S-2 of figure 8 both indicate resistivities that range from about 10 to 100 ohm-meters. The 

clays, shales, and mudstones tend to be responsible for the 
T 

lower resistivities; the siltstones and sandstones show intermediate resistivities; and the coals show the higher 

resistivities. 

Skin depth varies with resistivity as well as with frequency. The depth to which one-fourth skin depth is 

affected by the beds at site 2 is unknown, but the ''effective'' resistivity at that site is estimated to range from 

about 10 to 70 ohm-meters. The corresponding range of 

one-fourth skin depth (fig. 16) is about 10-22 m. This 

range of depth essentially includes all the possible 

thicknesses of overburden as well as the workings of the 

Acme Mine at site 2. It indicates that relatively long, 

straight metallic conductors might be detected by this electromagnetic survey. Note, however, that these conditions 

require rails that are connected and not badly rustedconditions that may not be realistic in the Acme Mine. 

An alternative interpretation does not require abandoned metal in the mine. Strong imaginary-component 

anomalies, such as those indicated in figure 15, are often 

seen over resistivity boundaries, which in this case would 

probably correspond to the ground-water variations between disturbed and undisturbed ground. 

57 Gravity Survey and Gravity Field Characteristics 

N 

N 

1 

50 METERS 

R 84 W 

EXPLANATION 

/":._630) Magnetic field Intensity in gammas-

\Q__/ For total value, add datum of 57,000 

gammas; contour hachured to indicate 

closed area of lower magnitude; contour 

interval 5 or 10 gammas 

w 

z 

::J 

z 

0 

~ w 

en 

• Station point 

Figure 11. Acme Mine, showing contours of magnetic field 

of site 2, in relation to passages (mine map, Sheridan, Wyoming Coal Company, 1938), and limits (hachured line) of visible broad surface subsidence (shaded). 

A gravity survey was run over the grid at site 2 using a temperature-compensated precise Worden Master 1 

gravity meter. The observed gravity values at the site were 

referenced to International Gravity Base IGB-15546E at 

the new Sheridan, Wyo., Post Office (E.J. Hauer, Chief 

of Department of Defense Gravity Services Division, 

Defense Mapping Agency, St. Louis Air Force Station, 

MO 63118, written commun., Aug. 1976), which is 

980,228.81 mGal. 

A temporary gravity base was also set near the grid 

of site 2. Traverses of several stations were made at one 

time from the temporary base, and individual traverses 

were done in less than an hour. Error of closure was no 

more than about 0.1 mGal, averaging about 0.07 mGal. 

Instrumental drift was considered linear and all closure 

errors were distributed with respect to time to the stations established during the traverse. Each station was 

read at least three times, or until the scale readings agreed 

within the equivalent of about 0.005 mGal. The three or 

more readings were completed within about 2 minutes and 

a complete traverse was done within about 15 minutes. 

The individual traverses were also tied together by 

repeating individual stations. 

Simple Bouguer values were reduced to sea level 

and were computed for each of the gravity stations over 

site 2 by adding the observed gravity and elevation 

1

The use of trade names is for descriptive purposes only and does 

not constitute endorsement by the U.S. Geological Survey. 

Geophysical Surveys 15 
Vi 58,680 :3 uen 

-w I-two 

Zz 

t!J<( 

<(z 

:E- 58,660 ...~en <(<( 

t-:E 

O:E I-<( 

0t!) 

~z 58,640 

::::>-:. 

en>- <(tw-

:E~ w 

I-

~ 58,620 

1110 

en 

a: 

w 

Iw 1100 

:E 

~ 

z 

0 

i= 

~ 1090 

w 

...J 

w 

c 

Steel 

post 

Second 

entry 

I 

First 

entry 

I 

C' 

0 30 60 90 1 20 1 50 1 80 21 0 240 270 300 

LENGTH OF SECTION, IN METERS 

Figure 12. Magnetic anomalies calculated for mine-car rails in first and second entries and along cross section C-C' (fig. 11) 

of site 2 and compared to measured magnetic field. Draw lines are theoretical limits of subsidence; mine openings are estimated. 

correction and subtracting the latitude correction. Only 

simple Bouguer values were computed because the 

topography of site 2 is very flat and all terrain corrections within a few hundred meters of each station would 

be approximately the same. There may be a relatively 

broad regional effect in the gravity field, however, 

because of the higher topography north of the grid. 

Elevation corrections (combined Free Air and 

Bouguer corrections) of 2.67, 2.00, and 1.70 g/cm3, corresponding to crustal densities above sea level, were 

applied. Latitude corrections were applied using the international gravity formula: g = 978031.85(1 + 0.005278895 

sin2¢+0.000023462 sin4¢) mGal (E.J. Hauer, written 

commun., 1976). 

Possible error in the simple Bouguer survey is attributed to error in determination of station elevations 

and to measurement inaccuracy with the gravity meter. 

The elevations of the grid were leveled to an accuracy of 

about 0.015 m, as mentioned previously, which corresponds to an error in the simple Bouguer anomaly of 

about 0.003 mGal. Inaccuracies in the horizontal positioning of the survey are negligible. Accuracy of reading 

the gravity meter is about 0.005 mGal, and errors due 

to an assumption of linear drift in the meter and in small 

16 Geophysical Detection of the Acme Mine, Wyoming 

Earth-tide variations are estimated to be no more than 

a few thousandths of milligals. Total accuracy of the 

simple Bouguer anomalies is, therefore, within about 

0.015 mGal. 

Densities of the sedimentary rocks and coal of the 

Fort Union Formation were not available from the Acme 

Mine. Representative core densities were available, 

however, from the S-2 borehole (Farrow, 1980) in the 

Fort Union, the formation that contains the Acme Mine. 

The densities are plotted in figure 8 along with the 

lithologic log of the borehole. Twenty-six densities of 

mainly sedimentary rocks average 2.3 g/cm3• Coal densities are not well represented, however, because the cores 

disintegrate easily. The density of sub bituminous coal is 

somewhat lower than that of the sedimentary rocks (Farrow, 1976). 

Three simple Bouguer gravity maps were prepared 

for site 2 using densities for the Bouguer corrections in 

the data reductions of 1. 70 g/cm3 (fig. 17), 2.00 g/cm3 

(fig. 18), and 2.67 g/cm3 (fig. 19). There is no strong 

contour pattern that suggests that the mine openings control the simple Bouguer gravity field. On the contrary, 

each of the three contour maps shows a broad gravity 

high over the nonsubsided mined area and general gravity 
N 

l 

....,.a 'fl' • 

) 

R 84 W 

EXPLANATION 

Electromagnetic contour-Showing 

in-phase component, referenced to 

100 percent; hachured to indicate 

area of low magnitude 

. 

c·9 w 

z 

~ ::::i 

z 

0 

~ u 

w 

1/) 

• Survey point 

T 

57 

N 

Figure 13. Electromagnetic field (Siingram) of site 2, in-phase 

component; input frequency is 1,760 Hz and there is 60-m 

separation between transmitter and receiver. Shown in relation to passages and to limits (hachured line) of visible broad 

surface subsidence (shaded). 

N 

l '---6 

50 METER! 

. ~4 

R 84 W 

EXPLANATION 

Electromagnetic contour-Showing 

out-of-phase component, referenced 

to 0 percent; hachured to indicate 

area of low magnitude 

• Survey point 

T 

57 

N 

Figure 14. Electromagnetic field (Siingram) of site 2; out-ofphase component; input frequency is 1,760 Hz and there is 

60-m separation between transmitter and receiver. Shown in 

relation to passages and to limits (hachured line) of visible broad 

surface subsidence (shaded). 

Geophysical Surveys 17 
en 

a: 

UJ 

1-

z 

UJ 

u 

a: 

UJ 

0.. 

z 

~ 

z 

UJ 

z 

0 

0.. 

~ 

0 

u 

UJ 

10 

5 

0 

~ -5 

:z:: 

0.. 

-10 

1110 

~1100 

~ 

~ 

z 

0 

~ 1090 

> UJ 

_J 

UJ 

1080 

c 

0 

/ - ..--._;Out-of-phase component -" ------- ' ' __ / \ _./ \ 

'-

Second 

entry 

I 

First 

entry 

I 

30 60 90 120 150 180 210 

LENGTH OF SECTION, IN METERS 

240 

C' 

90 

100 

105 

Figure 15. In-phase and out-of-phase electromagnetic components along cross section C-C' of site 2 (figs. 13 and 14). 

RESISTIVITY, 

IN OHM-METERS c C' 

0 50 100 Second First 

0 entry 1110 

I 

:i 

1- en 0.. \o a: UJ UJ 

Q 1-

zen 10 \~ 1100 UJ 

-a: ~ 

~UJ en!- \ ~ 

:z::W z ~--~ 0 a:z 

:::>- f= 1090 0 20 <( 

u. > u... UJ 

_J 

z UJ 

0 

30 1080 

120 150 180 210 240 270 300 

LENGTH OF SECTION. IN METERS 

Figure 16. Calculated effective depth of resolution for one-fourth skin depth of penetration for an electromagnetic source 

as a function of resistivity (1 0-70 ohm-meters) along cross section C-C'. Input frequency is 1,760 Hz and magnetic 

permeability is 13 X 1 o- 7 H/m. 

18 Geophysical Detection of the Acme Mine, Wyoming 
N 

1 

50 METERS 

R 84 W 

EXPLANATION 

Gravity contour-Shown in milligals; 

add -107 mGal for simple Bouguer 

gravity anomaly; hachured to 

indicate area of low magnitude 

Contour interval 0.05 mGal 

w 

z 

::i 

z 

0 

i= 

u 

w 

CIJ 

• Survey point 

T 

57 

N 

Figure 17. Simple Bouguer gravity field of site 2, reduced at 

a density of 1.70 g/cm3. Shown in relation to passages and to 

limits (hachured line) of visible broad surface deformation 

(shaded). 

N 

1 

50 METERS 

R 84 W 

EXPLANATION 

Gravity contour-Shown in milligals; 

add -121 mGal for simple Bouguer 

gravity anomaly: contour hachured 

to indicate closed area of lower 

magnitude. Contour interval 

0.05 mGal 

w 

z 

::i 

z 

0 

i= 

u 

w 

CIJ 

• Survey point 

Figure 18. Simple Bouguer gravity field of site 2, reduced at 

a density of 2.00 g/cm3. Shown in relation to passages and to 

limits (hachured line) of visible broad surface subsidence 

(shaded). 

Geophysical Surveys 19 
N 

t 

50 METERS 

A 84 W 

EXPLANATION 

Gravity contour-Shown in milligals; 

add -147 mGal for simple Bouguer 

gravity anomaly; contour hachured 

to indicate closed area of lower 

magnitude. Contour interval 

0.05 mGal 

w 

z 

::J 

z 

0 

B 

w 

Ill 

• Survey point 

T 

57 

N 

Figure 19. Simple Bouguer gravity field of site 2, reduced at 

a density of 2.67 g/cm3 . Shown in relation to passages and to 

limits (hachured line) of visible broad surface deformation 

(shaded). 

20 Geophysical Detection of the Acme Mine, Wyoming 

lows along the southeast side but away from the mined 

area. An anomaly with an amplitude of less than 

0.1 mGal and of relatively small areal extent occurs over 

the most northeasterly crosscut between the first and 

second entries. 

Figure 20 shows theoretical two-dimensional gravity 

anomalies caused by the mine openings compared to the 

measured simple Bouguer gravity anomalies slightly west 

of cross section C-C' (figs. 17, 18, 19). The mine openings to the north of cross section C-C' also were considered for the theoretical computations, and the gravity 

anomalies due to any mine openings were assumed to extend several times their width at right angles to the cross 

section. Theoretical gravity anomalies were computed for 

both air- and water-filled mine workings with a rock density of 2.30 g/cm3• The regional gradient was assumed 

to be flat. 

The measured simple Bouguer profiles along C-C' 

show a broad low anomaly south of the nonsubsided part 

of the mine, instead of over it, and a broad high anomaly over the entries, increasing in magnitude to the north. 

The reason for these anomalies is unclear; perhaps there 

are broad lateral changes in the densities of the underlying sedimentary rocks. The cause of the anomaly of small 

areal extent over the crosscut between the first and second 

entries is likewise unknown. The anomaly is actually composed of a relatively low portion on the south and a high 

portion on the north. The low portion could be caused 

by a relatively near surface cavity caused by upward stoping during subsidence, but the high portion must have 

another cause. Either portion could be caused by error 

or a relatively rapid lateral change in near-surface density. 

Both the water- and air-filled profiles of the 

theoretical gravity anomalies are lowest north of the newly mined area because of the influence of the subsided 

mines on the north. The computed gravity profile exhibits 

a maximum amplitude of about 0.4 mGal for the waterfilled mine and about 0.9 mGal for the air-filled mineboth considerably greater than the estimated possible error in the gravity survey of 0.015 mGal. However, neither 

computed gravity profile is similar in configuration to the 

measured gravity along profile C-C'. 

Seismic Survey 

Seismic-refraction surveys were run over the nonsubsided portion of the mine at site 2 (fig. 2). Background 

information for the interpretation of these surveys in site 

2 was provided by (I) some in-hole seismic measurements 

done by Birdwell, Inc., Tulsa, Okla., on rocks of similar 

composition as those at the mine along with seismicrefraction measurements along the ground surface near 

the boreholes and (2) some seismic-refraction measurements over a soil arch that bridged subsided bedrock and 

coal. 
c C' 

5 o.o r--------------==============--

i= 

~ 0.05 

(/) 0 _J UJ 

Rock density 

2.30 g/cm3 

~ ~ 0.1~~-~-~--~--~--~-~--L-- --L-~-- ~-_L- _....J__~_L_ __ ~~ :J 

_J 1.0 

~ 

~ 0.9 

~ _J 

<( 0.8 ~ 0 0 UJ 

z a: <( ::::> 0.7 

~ ~ ~ ~ 0.6 a: 

(!) 

0.5 

0.4 ~~-~--~ --~---L--~-L-~~-L_~~ -~--_L~~--_L-- --~--~--~ 

(/) 

a: 

UJ 

1110 

tu 1100 ~ 

~ 

z· 

0 

~ 1090 

> 

UJ 

_J 

UJ 

Second 

entry 

I 

0 30 60 90 120 150 180 210 240 270 300 

LENGTH OF SECTION, IN METERS 

Figure 20. Simple Bouguer gravity anomalies along C-C' for data reduced at three densities (1.70 g/cm3, 2.00 g/cm3, 

and 2.67 g/cm3) compared with theoretical gravity anomalies computed for both air- and water-filled mine openings. 

Draw lines are theoretical limits of subsidence; mine openings are estimated. 

Figure 8 shows velocity data interpreted from depth 

versus velocity for two boreholes for both downhole and 

surface-run surveys. Borehole S-2 (fig. SB; Farrow, 19SO) 

was cored into the Fort Union Formation, which also includes the coal and rocks of the Acme Mine, and borehole 

B-1 (fig. SA; Farrow, 1976) was cored into the Wasatch 

Formation. Although S-2 and B-1 are distant from the 

Acme Mine and from each other, they are cored in similar 

lithologies, and their measured velocities are very similar. 

Because the lithology of these two boreholes is quite 

similar to that of the undisturbed ground of site 2, it 

follows that their velocity data may also be indicative of 

those at site 2. 

The seismic measurements at boreholes S-2 and 

B-1 show a low-velocity layer that overlies a high-velocity 

layer. The low-velocity layer is defined by the first segment of the time-distance graphs; its seismic velocity 

ranges from about 360 to 4SO m/s and its thickness from 

about 2.4 to 5 m. The underlying high-velocity layer is 

defined by both the segment of the time-distance graphs 

and by the in-hole velocity graphs. The seismic velocity 

of the high-velocity layer exceeds 1,520 m/s and is infinitely thick for the purposes of this study. 

The in-hole velocities of figures SA and B were interpreted from sonic logs that were run within 12 m of the 

ground surface. Therefore, measurements were taken only 

in the bedrock (high-velocity layer). The in-hole velocities 

of both boreholes S-2 and B-1 were best-least-square fitted for both linear and logarithmic functions of depth. 

The best fit for the velocities of each hole is probably the 

logarithmic curve, but the velocity gradients of the linear 

fits are more easily compared; these are 3.7 m/s/m of 

depth in borehole S-2 and 3.4 m/s/m in borehole B-1. 

Most seismic-refraction lines run for engineering 

purposes elsewhere in the northern Powder River Basin 

indicate a correlation between the low-velocity layer and 

undivided weathered bedrock and overlying surficial 

deposits. On the other hand, the high-velocity layer is 

characteristic of nonweathered sedimentary bedrock of 

either the Fort Union or Wasatch Formation. 

Geophysical Surveys 21 
Although borehole velocity data were not available 

at the Acme sites, a comparison between the surface-run 

velocities at boreholes S-2 and B-1 and elsewhere in the 

Powder River Basin shows that, within broad limits, 

either the bedrock or overburden can be identified from 

velocity data. 

Soil Arch (Site 1) as Model 

Subsidence in at least one place in site 1 created a 

soil arch that can be visually verified (fig. 21). The arch 

is at least 7 m in vertical thickness at its center and apparently is developed only in the mantle of soil and not 

in the underlying bedrock. The bedrock has apparently 

stoped and raveled, and the loose debris has migrated into 

the mine openings below. Seismic line 1 of figure 21 was 

run directly over the bridge and seismic line 2 was run 

nearby, where other near-surface underground openings 

also apparently existed (fig. 2). 

The time-distance curves for seismic lines 1 and 2 

across the soil arch of site 1 are also included in figure 

21 for 11 geophones with spacings of 3.35 m. An explosive energy source was used on line 1 and repeated 

sledgehammer blows were used on line 2 as energy sources 

in conjunction with a signal-enhancing seismograph. 

Record quality was good. The low-velocity segments of 

these graphs range from 350 to 580 m/s, which is close 

to the range of low-velocity segments of 347 to 475 m/s 

measured at boreholes B-1 and S-2 (fig. 8). A second 

segment with higher velocity than the low-velocity 

segments is shown on each time-distance graph in figure 

21 for site 1. These second segments do not represent 

seismic velocities for bedrock because (1) the second segment velocities are far less than those bedrock velocities 

measured in boreholes S-2 and B-1, (2) the seismic lines 

are not long enough to exceed the critical distance required by the thickness of the soil, and (3) the second 

segments indicate atypical data that are probably caused 

by deformation in the soil. 

Seismic Field of Site 2 

Figure 22 shows five seismic-refraction lines along 

two traverses in plan view that were run in site 2. The 

lines are each about 100.5 m long with geophone spacings of 9.1 m. Adjacent lines overlap by about 10m. The 

energy sources for the resulting seismograms were the 

equivalents of about three-quarter-pound charges of 

dynamite. They were detonated at depths of about 1.5 m 

in boreholes. Record quality was very good. 

Figures 23 and 24 show the time-distance graphs 

as well as the velocity layering at depth along cross sections D-D' and E-E' (fig. 22) for the five seismic lines. 

The velocities of the VP1 segments of the graphs range 

22 Geophysical Detection of the Acme Mine, Wyoming 

from 340 to 520 m/s, averaging about 400 m/s; the 

velocities of the VP2 segments range from 1,830 to 

2,870 m/s. The VP2 segments of those lines over the 

mined area have a marked tendency to be nonlinear, 

whereas those lines away from the mined area tend to 

be more linear. The VP2 segments that are apparently affected by the mining include all of lines 3, 4, and 5. Those 

lines that are seemingly not affected include line 1 and 

possibly line 2. 

The depth and configuration of the VP1

- VP2 interface were computed by the method of differences for a 

two-layer case (Laby, 1931; J. C. Hollister, written commun., 1957; Redpath, 1973). This method of computing 

accommodates the variable thickness of the low-velocity 

layer. These variations may be due to changes in the surface topography and (or) in the topography of the buried 

contact between the low-velocity and high-velocity layers. 

There is little topographic relief at the ground surface 

along cross sections D-D' and E-E', however, nor does 

the VP1

- VP2 interface change elevation very rapidly. The 

very irregular velocity segments, therefore, seem to be attributable to the mining and subsidence. 

Most seismic-refraction lines run for engineering 

purposes elsewhere in the northern Powder River Basin 

indicate a correlation between the VP1 layer and surficial 

deposits (alluvium, colluvium, eolian deposits, etc.) and 

between the VP2 layer and bedrock of the Tertiary, 

Wasatch, and Fort Union Formations (Miller, 1979). The 

thicknesses of these vp! (low-velocity) layers are as great 

as 12m, but 3-6m is more typical. The thicknesses of 

surficial deposits along lines D-D' and E-E' are 

unknown, but bedrock is exposed at depths of less than 

4 m in the subsidence pits just west of line 4 (fig. 22). 

The equivalent depths to the apparent VP1- VP2 interface 

are as much as 10-12 m. Because the depth to the apparent VP1

- VP2 interface along D-D' and E-E' is as 

much as twice that of the nearby surficial deposits or even 

that of most other measured surficial deposits across the 

Powder River Basin, we suspect that the low-velocity layer 

( vp!) may be apparently thickened by deformation in the 

bedrock above the mine. 

Seismic-refraction theory requires that the part of 

a refracted wave that travels in the VP2 layer and that 

contributes to the first arrivals at the seismometers will 

travel just below the VP1- VP2 interface. Velocities 

measured in the bedrock of the Fort Union and Wasatch 

Formations, however, increase with depth (fig. 8). The 

average linear increase of velocity with depth of boreholes 

B-1 and S-2 is about 3.6 m/s/m of depth. Figure 25 

demonstrates the theoretical travel path in a medium of 

two layers: vp! (overburden) is 400 m/s, does not increase with depth, and has a thickness of 10 m; VP2 

(bedrock) is 2100 m/s just under the interface, increases 

with depth, and has infinite thickness. The VP2 segment 

of the time-distance graph is hyperbolic and imperceptibly 
\( 

15 

·:~ 

80 

LINE1 

70 

60 / C/) C/) 

0 0 

z z 0 50 / 0 u I u UJ UJ C/) C/) 

::i ::i ....J 40 I ....J 

~ ~ 

z z 

u..i 30 ~"" X u..i ~ :7/ ~ 

i= ' ~ i= 

20 ~~" 

10 

0 

0 5 10 15 20 25 30 35 

LENGTH OF SEISMIC LINE, 

IN METERS 

N 

80 

70 

60 

50 

40 

30 

20 

10 

0 

0 

Vpl. Vp2 

-1108-

EXPLANATION 

Compressional-wave velocity 

Topographic contour-Showing elevation, in meters. Dashed contours are 

subsurface 

--2- Seismic line-Showing number 

30 METERS 

LINE2 

~ 

0£ >f ;::; I 

II / 

;Y' 

/</ 

v c 

t;;r.q) 

~ 

~~" / z 

5 10 15 20 25 30 35 

LENGTH OF SEISMIC LINE, 

IN METERS 

Figure 21. Map of a soil arch in site 1 showing topography, location of seismic lines, and time-distance graphs. 

Geophysical Surveys 23 
N 

1 

50 METERS 

A 84 W 

EXPLANATION 

I 

-t Seismic line and number 

I 

w 

z 

:::i 

z 

0 

i= 

u 

w 

(/) 

T 

57 

N 

Figure 22. Location of seismic-refraction lines 1- 5 along 

0 - 0' and E- E' at site 2. Shown in relation to passages and 

to limits (hachured line) of visible broad surface deformation 

(shaded). 

24 Geophysical Detection of the Acme Mine, Wyoming 

concave downward. The wave paths are linear in the VP1 

layer and segments of circles in the VP2 layer (Ewing and 

Leet, 1932). Curvature along the hyperbolic segment in 

the graph is so slight for the 200m or so shown in figure 

25 that it cannot be detected by eye. Because of this very 

slight curvature, straight-line segments commonly are 

made instead of curves in time-distance graphs for the 

VP2 segment (Palmer, 1983). 

Theoretical travel paths in the VP2 layer are shown 

in figure 25 for horizontal intervals of approximately 

50 m to about 200 m. The maximum depth of penetration for seismic waves along a seismometer line of about 

the same length as those run in site 2 (105m) is only about 

2.3 minto "bedrock." By analogy, the first arrival times 

on the time-distance graphs of lines 1-5 are contributed 

by wave fronts that do not penetrate as deep as the mine 

roof of average depth of burial shown in figures 23 and 

24, nor even as deep as the mine roof with the least overburden. Therefore, the VP2 segments of those seismic 

lines that are affected by the mines may be caused by 

deformation in the bedrock above the mine drifts. 

SUMMARY AND CONCLUSIONS 

One potential hazard in underground coal mining 

areas in the Powder River Basin, as well as in other parts 

of the United States, is abandoned coal mines. These 

mines are hazardous because they may subside and cause 

damage to the landscape or structures, and the mines may 

spontaneously ignite and pollute air and water. A further problem with abandoned underground mines is that 

adequate maps of the underground openings are not 

always available; hence, the location of some mines may 

be unknown to local residents until the mines are 

manifested by surface subsidence. Ways of detecting 

abandoned but nonsubsided mines are, therefore, very 

useful and important, and geophysical techniques may 

offer some solutions. 

The Acme Mine was begun in 1911 and abandoned 

in 1940. Coal was mined by the room-and-pillar method 

from a coal bed, which is about 6 m thick and with overburden thickness at two geophysical sites estimated to be 

less than 12-20 m. The surface above much of the mine 

has subsided and left broad topographic depressions, 

some of which span many rooms, entries, and pillars. 

Two sites that were nearly free of man-made 

disturbances were chosen for the geophysical surveys over 

the Acme Mine. Site 1 was selected for a magnetic survey 

for two main reasons. There is a reasonable chance that 

abandoned iron is located there. Also, there exists the 

possibility that heat from the burning mine magnetized 

the subsidence fractures that extend over most of that site. 

Site 2 was selected because it had not subsided and the 

relatively low topographic relief permited gravity, seismic, 
D D' 

120 r-------UNE 1---------. - --------''LINE 3 ----------. 

j4--+------LINE ----+--+~ 

100 

(IJ 

o · z 80 0 

u 

w 

(IJ 

:::i 60 = 

~ 

~ 

u.i 40 

~ 

i= 

20 

0 

(IJ 1110 a: 

w 

Iw 

~ 1100 ~ 

z -- ~ 1090 

<( 

> w 

...J 

w 1080 

0 

I xl 

9'2.10, I/ I 

_.,\ A' I 

-.J~'2-""_)</ I I 

)(-""' I I 

y/ : I / : I :tx : o \\ I I X ..._ I '11'- t l : // I " I I ~~ 

I 

Vp1 =400 

~~c _________ _ 

Vp2 = 2100 

50 100 

I I 

I I 

ro?JO A-+ -::.\~_.-X I I 

-.J~'2-~ I );/ I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

150 200 250 

LENGTH OF SECTION, IN METERS 

Figure 23. Time-distance curves and velocity-layering interpretation along seismic lines 1-3 (cross section 0-0'; fig. 

22) of site 2. V 1 and V 2 are compressional-wave velocity in meters per second; x, o, arrival times; dashed line interval, inferred v~locity l;yering. 

and electromagnetic surveys, which are sensitive to 

topographic relief, to be carried out unimpeded. Site 2 

also allowed a comparison of the geophysical data obtained over the mine with data obtained away from the 

mine. 

None of the methods employed at Acme Mine adequately located the mine, although some of them hinted 

at detection. At site 1, the magnetic surveys could 

positively identify the buried contact of clinkers, which 

are highly magnetic, and coal that had been mined and 

the mined portion that had subsided. The magnetic 

surveys could also positively identify iron-bearing objects 

that had been dumped into some subsidence pits at site 

1 and backfilled with soil. Neither the magnetic nor electromagnetic surveys indicated that any iron-bearing objects such as rails, tools in quantity, or mine cars haa been 

abandoned in the mine at site 2. The electromagnetic 

surveys indicated the general boundary between the mined 

and nonmined ground; these electromagnetic indications 

may have been caused by a higher water content in the 

bulked material over the mine. The seismic refraction 

surveys indicated irregular and atypical second-layer 

velocity segments over both a visible soil arch at site 1 

and the nonsubsided mine at site 2; these irregular 

segments may have been caused by deformation in the 

overburden above the mine. Not only did the gravity 

survey fail to delineate the mine, it also detected a broad 

high anomaly rather than the expected low situated over 

the mined area. One speculation for this discrepancy is 

that the deformed overburden may be more dense, and 

hence cause greater gravity values because bulking in the 

overburden may have created greater porosity with consequent water content increase. 

The studies have some value in further research. 

Although the surveys with interpretations were done 

systematically and with great care, they were largely unsuccessful and, hence, need not be duplicated. However, 

variations of these survey techniques may be fruitful. A 

closer spacing of measurement points might be helpful, 

for example, for all the surveys. The seismic-refraction 

lines might be restructured so that more lateral information can be ascertained and deep penetration can be obtained. Tidal corrections to increase the accuracy of the 

gravity survey and electromagnetic equipment that 

operated at 400 Hz would, of course, be helpful. 

Other relatively inexpensive geophysical techniques 

also can be applied to detecting underground coal mines. 

Such techniques include the application of shallow seismic 

reflection or even ground-run radar for very shallow 

targets. 

Summary and Conclusions 25 
en 

c 

z 

E E' 

120 ...------LINE 5- --- -----. 

----- LINE4----~~ 

100 

en 

c 

z 80 0 

u 

w 

en 

::J 

...J 60 

~ 

~ 

LLi 40 ~ 

i= 

20 

oL-~ L-~~L--L~-~~-~~L_~~-~~-~~-~~~ 

1110 

~~ 1100 - w f-f-

<(W 

ir; ~ 1090 ...JZ 

w0 

Second 

entry 

50 100 150 

LENGTH OF SECTION, IN METERS 

Figure 24. Time-distance curves and velocity-layering interpretation along seismic 

lines 4-5 (cross section E-E', fig. 22) of site 2. V 1 and V 2 are compressional-wave 

velocity in meters per second; x, o, arrival time~. P 

8 100 w 

en 

::::i 

_J 

~ 

~ 

u.i 

~ 

i= 

50 

0 ~~-~~-~-L__L_~~-~-L--L-~~-~~--L-~~-~~L-~ 

Shot point 

~~,:~~ I I~ 

~~ ::{ 0 ' ' ~-------------=--- 50 100 --150 , ~[ 200 

DISTANCE, IN METERS 

Figure 25. Time-distance graph and theoretical depth of penetration of a seismic wave through 

a 1O-m-thick low-velocity (400 m/s) layer and refracted into a layer of initial velocity 2,100 m/s, 

which increases linearly with depth at a rate of 3.6 m/s/m. 

26 Geophysical Detection of the Acme Mine, Wyoming 
REFERENCES CITED 

Association of Engineering Geologists, 1978, Seminar on coal 

mining activities and concepts of multiple land use, selected 

papers: Association of Engineering Geologists Bulletin, v. 

15, no. 2, p. 145-251. 

Bauer, L.P., 1972, The alteration of sedimentary rocks due to 

burning coal in the Powder River Basin, Campbell County, Wyoming: Texas Technical University M.S. thesis, 65 p. 

Briener, Sheldon, 1973, Applications manual for portable 

magnetometers: Geometries, Sunnyvale, Calif., 62 p. 

Dunrud, C.R., and Osterwald, F.W., 1980, Effects of coal mine 

subsidence in the western Powder River Basin, Wyoming: 

U.S. Geological Survey Professional Paper 1164, 49 p. 

Ewing, W.M., and Leet, L.D., 1932, Seismic propagation paths, 

in Geophysical prospecting: American Institute of Mining 

and Metallurgical Engineers Transactions, v. 97, p. 

245-262. 

Farrow, R.A., 1976, Preliminary report on the geotechnical 

properties of the Wasatch Formation at Buffalo, Wyoming: 

U.S. Geological Survey Open-File Report 76-877, 76 p. 

___ 1980, Preliminary report on the geotechnical properties 

of the Fort Union Formation at Sheridan, Wyoming: U.S. 

Geological Survey Open-File Report 80-1246, 175 p. 

Gallagher, C.P., Henshaw, A.C., Money, M.S., and Tarling, 

D.H., 1978, The location of abandoned mine shafts in rural 

and urban environments: Bulletin of the International 

Association of Engineering Geology, no. 18, p. 179-185. 

Hasbrouck, W.P., and Hadsell, F.A., 1976, Geophysical exploration techniques applied to Western United States coal 

deposits, in Coal Exploration: International Coal Exploration Symposium, 1st, London, England, 1976, Proceedings, p. 256-287. 

Henshaw, A. C., 1977, The location of abandoned mine shafts 

using a proton magnetometer: University of Newcastle 

upon Tyne M.Sc. dissertation, p. 82. 

Keller, G.V., and Frischknecht, F.C., 1970, Electrical methods 

in geophysical prospecting: New York, Pergamon Press, 

p. 278-353. 

Kuzara, S.A., 1977, Black diamonds of Sheridan-a facet of 

Wyoming history: Cheyenne, Wyo., 1st ed., September, 

Pioneer Printing and Stationery Co.,125 p. 

* U.S. GPO: 1988 573-{)47 /66,079 

Laby, T.H., 1931, The principles and practice of geophysical 

prospecting: Cambridge, England, Imperial Geophysical 

Survey Report, p. 337-341. 

Mapel, W.J., 1959, Geology and coal resources of the BuffaloLake De Smet area, Johnson and Sheridan Counties, 

Wyoming: U.S. Geological Survey Bulletin 1078, 148 p. 

Miller, C.H., 1979, Seismic, magnetic and geotechnical properties of a landslide and clinker deposits, Powder River Basin, 

Wyoming and Montana: U.S. Geological Survey Open-File 

Report 79-952, 47 p. 

Palmer, D., 1983, Comment on "Curvec}.ray path interpretation of seismic refraction data," by S. A. Greenhalgh and 

D. W. King: Geophysical Prospecting [Blackwell Scientific 

Publications Ltd., P.O. Box 88, Oxford, England], v. 31, 

no. 3, p. 542-543. 

Parasnis, D.S., 1966, Mining geophysics, Ser. 3 of Methods 

in geochemistry and geophysics: New York, Elsevier, 

356 p. 

Redpath, B.B., 1973, Seismic refraction for engineering site investigations: U.S. Army Corps of Engineers Waterways 

Experiment Station, Explosive Excavation Research 

Laboratory, Livermore, Calif., 55 p. 

Rogers, G.S., 1917, Baked shale and slag formed by the burning of coal beds: U.S. Geological Survey Professional Paper 

108, 10 p. 

Sheridan-Wyoming Coal Company, 1938, Coal mine map of 

the Acme Mine-Map of mine no. 42: U.S. Bureau of 

Mines, Building 20, Denver Federal Center, Denver, 

Colorado. 

Sheriff, R.E., 1974, Encyclopedic dictionary of exploration 

geophysics: Society of Exploration Geophysicists, 266 p. 

U.S. Army, 1977, Symposium on detection of subsurface 

cavities, Vicksburg, Mississippi, July 12-15, 1977: U.S. 

Army Chief of Engineers, U.S. Army Engineer Waterways 

Experiment Station, Soils and Pavements Laboratory, 

191 p. 

___ 1981, Symposium on tunnel detection: U.S. Army 

Mobility Equipment Research and Development Command 

and Colorado School of Mines, Geophysics Department, 

Golden, CO 80401, 142 p. 

Watts, R.D., 1978, Electromagnetic scattering from buried 

wires: Geophysics, v. 43, no. 4, p. 767-781. 

References Cited 27