Bureau of Mines Report of Investigations/l976 

Seismic ~etection of Trapped Miners 

Using In-Mine Geophones 

UNITED STATES DEPARTMENT OF THE INTERIOR 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Thomas S. Kleppe, Secretary . 

BUREAU OF MINES 

Thomas V. Falkie, Director 
This publication has been cataloged as follows : 

- 

Powell, James A 

Seismic detection of trapped miners using in-mine geophones, by James A. Powell and Richard A. Watson. [Washington] U.S. Bureau of Mines [I9761 

8 p. illus. (U,S. Bureau of Mines, Report of investigations 

8158) 

1. Mine safety. 2. Mine communication systems. I. Watson, 

hichard Alan, jt. auth. 11. U.S, Bureau of Mines. 111. Title. 

(Series) 

TN23.U7 no. 8158 622.06173 

US. Dept. of the Int. Library 
CONTENTS 

Page 

Abstract ................................................................. 1 

Introduction ............................................................. 1 

Acknowledgments .......................................................... 2 

The seismic system ....................................................... 2 

The tests ........................................................... 3 

Results .................... ......................................... 5 

Peak amplitudes and attenuation ..................................... 6 

Conclusions .............................................................. 8 

ILLUSTRATIONS 

1 . Schematic of recording system ........................................ 2 

2 . Schematic of playback system ......................................... 3 

3 . Schematic of deployed system ..................................... 4 

4 . Recording. source receiver distance 550 feet . ........................ 5 

5 . Observed and predicted amplitudes .................................... 7 
SEISMIC DETECTION OF TRAPPED MINERS USING IN-MINE GEOPHONES 

by 

James Am Powell 1 and Richard Am Watson 2 

ABSTRACT 

A seismic system which utilizes in-mine geophones to detect trapped 

miners has been developed. Tests at the Bureau of Mines Safety Research Mine 

at Bruceton, Pa., and at two operating mines indicate such a system has a maximum detection range in excess of 1,000 feet. The system is fieldworthy and 

portable and requires less than 30 minutes to set up and check. Real-time 

detection is possible. 

INTRODUCT ION 

In 1970, the National Academy of ~n~ineerin~~ reported that a seismic 

system might be capable of detecting and locating trapped miners. They 

described a system in which the miners would strike the floor, rib, or roof of 

the mine with a sledge, a roof beam, or any other large, heavy object they 

could find. The resulting vibrations would travel to the surf ace where they 

would be converted to electric signals by seismic transducers (geophones) . These signals would be amplified, filtered, and recorded. By comparing 

arrival times at several different geophone locations, the trapped miner would 

be located. 

In 1971 and 1972, under Bureau of Mines contracts H0101262 and H0210063, 

Westinghouse Electric Co. built and tested such a system. Under certain favorable combinations of geologic conditions, mine depth, and background noise, 

the system worked. However, when the system was taken to actual mine disasters, measurement at the disaster site indicated that the noise generated by 

surface rescue equipment would mask any signals the trapped miner could generate. Thus, times specifically designated for seismic "listening" must be 

allocated and enforced. 

l~eophysicist, Industrial Hazards and Communications Group (now with Sun Oil 

CO. , Houston, Tex. ) . '~lectronics technician, Industrial Hazards and Communications Group. 

~ational Academy of Engineering , Committee on Mine Rescue and Survival Techniques. Mines Rescue and Survival. National Technical Informat ion Ser - 

vice, PB 191 691, 1969. 
One possible way of reducing the effect of surface noise is to place the 

geophones in the mine, As with the surface system, the trapped miner would 

generate a signal by striking the mine rock. The resulting vibrations would 

be detected by geophones placed in the mine, This requires that geophones be 

installed in the mine prior to a disaster, or that they be installed by a 

rescue crew as part of the postdisaster procedure, This report describes the 

makeup and testing of an in-mine seismic system for the detection of trapped 

miners. No location has been attempted with this system. 

1 ACKNOWLEDGMENTS 

I The authors wish to thank Howard Parkinson and John Murphy of the Pittsburgh Mining and Safety Research Center, Bureau of Mines, for their frequent 

suggestions and assistance. The cooperation of Art Collis, superintendent of 

the Arkwright mine; of Mike Hudak, superintendent of the Loveridge mine; and 

of many other employees of the Consolidation Coal Co. is gratefully 

acknowledged. 

I THE SEISMIC SYSTEM 

Several geophone placement techniques have been tried. In one method a 

geophone was planted in the mine floor and buried under several inches of 

dirt. This burying reduced the effect of airborne sound and improved coupling 

of the geophone to the earth. In some tests several geophones were connected 

to give a single output, Series, parallel, and s e r i e s - p a r a l l e l combinations 

were used. This is referred to as a geophone "array"; in these tests most 

arrays consisted of six geophones in a hexagonal pattern. The hexagon has 

I Lockheed model 417 tape deck I 

I------ - - - _ - - - _ - - - - - - - - - - - - - - - - - - - 

I No. l geophone --' -NO. 2 through No. 7 geophones are between 50 ft and lOOft 1 

lplaced near source1 I from tape deck I 

I I Geospoce GSC-I10 

I I I 9 I geophones 

L ----- --J L- ----- ----- ----------------- ---- J 

Princeton applied 

research model 113 

record amplifiers 

I Tektronix model 324 

Note-The listing of brand names and model numbers is for informational purposes only, and is not 

intended to imply endorsement of any equipment 

~ FIGURE 1. - Schematic of recording system. 

I 1 

Trace 

4 

Trace 

I 

Trace 

7 

Trace 

5 

Trace 

6 

Trace 

2 

Trace 

3 
sides about 6 feet long. The array geophones are planted in the floor and 

buried in the same manner as the individual floor geophone. 

Since mine floors are frequently muddy and/or are underlaid by incompe - tent rock, some geophones were attached to roof bolts. As will be discussed 

later, resin-grouted bolts were preferred but were not always available. The 

roof bolts provided a firm coupling of transducer to competent rock. 

The output of the geophone was recorded on a seven-track tape deck. 

Since geophone output is very small (usually much less than 1 millivolt), 

record amplifiers were required. Amplifier gains were varied from 20 db to 

80 db, depending on the expected strength of the signal. 

Figure 1 is a schematic of the recording system. The record amplifiers, 

tape deck, and oscilloscope are all battery powered. The geophones are passive transducers electrically isolated from ground. Hence, if capacitive 

coupling is negligible, the entire sys0 

tem is ungrounded. 

Lockheed model 417 

tape deck Figure 2 is a schematic of the 

playback system. Generally speaking the 

I recorded signal is already at the 

lthaco model 454 

prefilter amplifier 

appropriate level to drive the recording 

oscillograph. Thus the gains of both 

the prefilter and postfilter amplifiers 

are usually near unity and function primarily as impedance matching units. The 

filters are infinitely variable in the 

2- to 20,000-Hz band with a rolloff of 

I 24 db per octave. 

I Krohn-Hite model 3550R 

variable filter 

The Tests 

'-T- The experiments were made in a 

I quiet region of the mine during mainteI 

I nance shifts. Care was taken to reduce 

the effect of noise generated by the 

v test equipment and test personnel. Geo- Honeywel l Accudata model 112 

postfilter amplif ier phones were planted in the floor or 

clamped to roof bolts as described 

I above. The gains of the record ampliI fiers were adjusted so that the incoming 

signal was well above system background Hone~~ellV~s~c~rdermod~l2206 noise yet did not exceed the dynamic recording oscil loscope range of the system, 

Note- The listing of brand names and model 

numbers is for informational purposes only, Figure 3 is a schematic of the 

and is not intended to im~lv endorsement deployed seismic system- The block . . of any equipment labeled '?nstrument package" consists of 

six geophones or geophone arrays, six *' - Schematic of playback recording amplifiers, the tape deck, and 

system, 
Source- receiver distance 

( range 1 

FIGURE 3. - Schematic of deployed system. 

the oscilloscope. These are arranged as shown in figure 1. Figure 3 also 

shows that the output of a geophone placed near the source is wired back to 

track 1 of the tape deck. This geophone, referred to as the "source" geophone, is used to determine the time the seismic signal began. 

The actual experiments, once the instruments have been deployed and 

checked, consist of -- 

1. Measuring the distance from the geophone to the place where the 

trapped miner will generate the signal. 

2. Planting the source geophone within 15 feet of the trapped miner. 

3. Striking a roof bolt (or floor or rib) 10 times. Usually a 5- to 

10-pound draw bar or crow bar was used, although some tests were conducted 

using a 12-pound sledge, a timber, a geologist's pick, and even a hardhat. 

4, Moving to the next location and repeating these steps. 
Results 

Figure 4 is a high-speed (1 inch equals 0.05 sec) playback of a test in 

which the source-receiver distance was about 550 feet. This record contains 

many features common to most of the recordings so it will be discussed in some 

detail. The top trace (trace 1) represents the signal at the source geophone. 

The second trace (trace 2) kas recorded by a single geophone planted in the 

floor. Traces 3 and 4 were recorded from roof bolt geophones. Each of the 

last three traces represents the summed output of six geophones; that is, the 

arrays described above. 

Traces 3 and 4 show very pronounced ringing. Such ringing is common in 

geophones attached to roof bolts. On traces 2, 6, and 7 two distinct arrivals 

are noted. By noting the arrival times at the source (on trace 1) and at the 

recording site (on traces 2, 6, and 7) and by noting the distance between them 

(550 feet in this case), a velocity can be computed. The first arrival has a 

velocity near 12,500 fps; we postulate that it is a p-wave. The peak amplitude is in the second arrival and has a velocity near 5,000 fps. Since the 

second arrival has the higher amplitude, it is probably the arrival of greatest interest to those trying to detect signals. Its amplitude will be discussed later. 

Figure 4 represents the results of playing back through the 20- to 200-Hz 

bandpass setting on the filters. Tests have been made using no filters and 

I I I1 Ill1 IIIIIIII I1 Ill' 

Trace 2 

Trace 3 

Trace 4 

Trace 5 

- Trace 6 

Trace 7 

0.01 

I I I I I I I IIIIIIIII I I Ill 

TIME SCALE, sec 

FIGURE 4. - Recording, source receiver distance 550 feet. 
also using other filter settings. The 20- to 200-Hz setting seems to give the 

highest quality records. 

Three trips to operating coal mines (two trips to the Arkwright mine, 

near Morgantown, W. Va., and one trip to the Loveridge mine, near Fairview, 

W. Va.) were made during this project. Typically 30 or 40 recordings were 

made during each visit. In lieu of individually describing each record, the 

results of all three trips are summarized below. 

1. The records made when the source is not in the same entry as the 

receiver do not differ from those made when source and receiver are in the 

same entry. Hence it is concluded that the signals are not guided by the mine 

t unne 1. 

2. P-wave and second arrival velocities at the Arkwright mine are near 

15,000 fps and 5,000 fps, respectively. These are somewhat higher than the 

velocities (12,500 fps and 5,000 fps) recorded at Loveridge mine. 

3. Hoof bolts held in place by an expansion anchor tend to resonate with 

a frequency slightly over 100 Hz. These bolts may ring for up to 1 sec following a single impulse. At the Bruceton Safety Research Mine a geophone was 

attached to a resin-grouted roof bolt. No ringing was encountered with that 

roof bolt. No resin-grouted roof bolts were available at the operating coal 

mines. 

4. A 20- to 200-Hz bandpass filter setting gave the best results at both 

mines. 

5. Arrays of six geophones did not perform noticeably better than single 

geophones. It is noted that arrays are useful in discriminating between signal and noise on the basis of the directional characteristics of these components. For example, in oil exploration geophone arrays are used to reduce 

horizontally propagating noise while reinforcing vertically propagating signal. 

We hypothesize that both signal and noise are generated in the mine and hence 

have the same directional characteristics. Thus the array does not improve 

signal -noise ratio. 

6. The geophones used in the Loveridge mine were placed within 100 feet 

of an area undergoing retreat mining. Despite the considerable roof working 

and noise of roof falls, the system worked well. 

7. At both operating mines, signals are clearly detectable at distances 

of more than 1,000 feet from the source and cannot be detected at distances of 

1,500 feet. Hence the range of the system is between 1,000 and 1,500 feet. 

Peak Amplitudes and Attenuation 

To date we have seen no theoretical explanation of the second arrival in 

figure 4. We do not intend to provide such an explanation here. Instead, two 

empirical formulas describing signal attenuation will be discussed. 
In describing the attenuation of seismic waves proDISTANCE FROM SOURCE, ft 

FIGURE 5. - Observed and predicted amplitudes. expressed in feet, c is near 

0.004. 

Figure 5 shows the observed and predicted amplitudes at the Loveridge 

mine for a single roof-bolt-mounted geophone. The dashed line indicates decay 

by an R'a rule, where a = 1.98 with a standard deviation of 0.11. The solid -c R 

line indicates decay by the rule, where c = 0.0045 with a standard deviation of 0.06. The residual variance using the first rule is 31.2; using the 

second rule it is 16.0. This indicates that the second rule is more accurate 

than the first. 

duced by explosives, several 

Bureau of Mines investigators4 used a relation in 

which peak particle velocity 

is assumed to decay as R'a, 

where R is distance from the 

source and a is a constant 

in the range 1.1-1.6. The 

exact value of a in our 

tests depends on the mine 

and the geophone location, 

but least-squares fits indicate values for a of between 

1.9 and 2.6 with a standard 

deviation on the order of 

0.1. These values are 

appreciably higher than 

those in the explosive 

testing. 

A better fit to experimental data was obtained 

assuming decay is propor- -c R 

tional to , where c is 

a constant which characterizes the mine geology and 

the geophone placement. 

When peak particle velocity K, is expressed in inches per 

second and range R is 

4Condon, J. L., J. N. Murphy, and D. E. Fogelson. Seismic Effects Associated 

With an Underwater Explosive Research Facility. BuMines RI 7387, 1970, 

20 PP. 
CONCLUSIONS 

The range of this system is between 1,000 and 1,500 feet. An emergency 

version of the system could be set up and checked in less than 30 minutes. 

Such a system would weigh about 100 pounds and fit in a 1- by 2- by 4-foot 

box. No elaborate signal processing would be required; real-time results are 

possible. An operational version of the test equipment would require one 

trunk that would be taken into the mine, one trunk containing spare parts and 

repair equipment, and a third trunk that would contain filters and other 

equipment to allow more detailed data processing, 

Geophones should be attached to resin-grouted roof boltswhenavailable,or 

to firmly set conventional bolts when necessary. The best signal-generating 

technique is to strike a roof bolt with a 5- to 10-pound metal bar. If it 

appears dangerous to strike the roof bolts, a competent portion of the mine 

roof, floor, or rib should be struck. When a metal bar is not available, 

objects such as timbers, hammers, or even hardhats can be used as signal 

sources.