1 I I 

Bureau of Mines Report of Investigations/l976 

An Electromagnetic System 

for Detecting and Locating 

Trapped Miners 

UNITED STATES DEPARTMENT OF THE lNTERIOR 
Report of Investigations 8159 

An Electromagnetic System 

for Detecting and Locating 

Trapped Miners 

By James A. Powell 

Pittsburgh Mining and Safety Research Center, Pittsburgh, Pa. 

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 

An electromagnetic system for detecting and locating 

trapped miners. [Washington] U.S. Bureau of Mines [I9761 

15 p, illus., cable. (U.S. Bureau of Mincs. Report of invcsrigarions 8159) 

1. Minc communication systems. 2. Minc safety. I. U.S. Bu. 

reau of Mincs. 11. Title. (Scrics) 

TN23.U7 no. 8159 622.06173 

U.S. Dcpt. of the 1°C. Library 

- i 
CONTENTS 

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

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

Acknowledgments .......................................................... 3 

Description of the system ................................................ 3 

Theoretical considerations ............................................... 3 

~ocation practice ........................................................ 6 

Basic equipment requirements ............................................. 8 

System refinements ....................................................... 13 

Field tests .............................................................. 14 

Summary and conclusions .................................................. 15 

ILLUSTRATIONS 

Schematic of EM location system ..................................... Flux lines and antenna position ..................................... Horizontal and vertical EM fields ..................... .............. Top view of flux lines .............................................. Determining "critical planes" ....................................... Location procedure .................................................. Preliminary version of manpack location transmitter in 

separate package .................................................. Preliminary version of manpack location transmitter. built into 

cap lamp battery .................................................. Deploying unit shown in figure 8 .................................... Preliminary version of receive unit ................................. Using receive unit shown in figure 10 ............................... Helicopter receiver ................................................. 

TABLE 

1 . Mines where EM location systems have been tested .................... 15 
AN ELECTROMAGNETIC SYSTEM FOR DETECTING AND LOCATING 

TRAPPED MINERS 

Jomes A. Powell 1 

ABSTRACT 

The theory of electromagnetic fields indicates such fields could be used 

to detect and locate trapped miners. To be useful, the hardware of the system 

must meet a number of requirements, including small size, intrinsic safety, 

and rugged construction. Such hardware has been built, and the system has 

been tested by the Bureau of Mines and its contractors. These tests indicate 

that the electromagnetic method provides a practical means to locate miners in 

emergencies. 

INTRODUCTION 

Soon after the radio came into comnon use, the Bureau of Mines recognized 

its potential as an aid in locating miners trapped by mine fires or explosions. 

The early experiments2 indicated that while through-the-earth electromagnetic 

(EM)communicationwas possible, the hardware requirements of a practical system could not be met by the technology available at that time. In 1968, 

however, the Farmington mine disaster resulted in a National Academy of Engineering recommendation3 that a postdisaster location system be developed. 

Thus, in 1970, the Bureau of Mines contracted with Westinghouse Electric 

Co. (contract H0101262)4 to develop through-the-earth communication techniques. 

lGeophysicist, Industrial Hazards and Communications (now with Sun Oil Co., 

Houston, Tex.). 

'llsley, L. C., H. B. Freeman, and D. H. Zellers. Experiments in Underground 

Communication Through Earth Strata. BuMines Tech. Paper 433, 1928, 60 pp. 

3~ational Academy of Engineering, Committee on Mine Rescue and Survival Techniques. Mine Rescue and Survival. National Technical Information Service, 

PB 191 691, 1969. 

4~estinghouse Electric Corp. Coal Mine Rescue and Survival, Volume 2, 

Communications/Location Subsystem. BuMines Open File Rept. 9(2)-72, 1971, 

258 pp.; available for consultation at the Bureau of Mines libraries in 

Pittsburgh, Pa., Denver, Colo. , Twin Cities, Minn., and Spokane, Wash. ; at 

the office of the Assistant Director--Mining and the Central Library, U.S. 

Department of the Interior, Washington, D.C.; and from the National Technical Information Service, Springfield, Va., PB 208 267. 
Both seismic and EM methods were investigated. Originally, the EM work concentrated on large, more or less permanently placed units that would permit 

voice and/or code "conversations" between the mine and the surface. The early 

tests and theoretical studies carried out by J. R. Wait of the Institute for 

Telecomnunication Sciences (Bureau of Mines contract H0122061)" indicated that 

a location system that used portable "manpack" units was feasible. 

In such a system, the miners would carry a small transmitter that would 

be activated if the men were trapped. A team of rescuers on the surface would 

detect the transmission and would then approximately locate the point on the 

surface that was directly above the miners. 

Subsequent development work and tests by Westinghouse (Bureau of Mines 

contracts H0232049, H0242006, and ~0220073)~ and by Bureau of Mines personnel 

SGeyer, R. G. Thru-the-Earth Electromagnetics Workshop. BuMines Open File 

Rept. 16-74, 1973, 217 pp.; available for consultation at the Bureau of 

Mines libraries in Pittsburgh, Pa., Denver, Colo., Twin Cities, Minn., and 

Morgantown, W. Va.; at the Central Library, U.S. Department of the Interior, 

Washington, D.C.; and from the National Technical Information Service, 

Springfield, Va., PB 231 154/AS. 

"Farstad, A. J. Electromagnetic Location Experiments in a Deep Hardrock Mine. 

BuMines Open File Rept. 28-72, 1973, 54 pp.; available for consultation at 

the Bureau of Mines libraries in Pittsburgh, Pa., Denver, Colo., Spokane, 

Wash., and Twin Cities, Minn.; at the Central Library, U.S. Department of 

the Interior, Washington, D.C.; and from the National Technical Information 

Service, Springfield, Va., PB 232 880/AS. 

Farstad, A. J., C. Fisher, R. F. Linfield, and J. W. Allen. EM Location 

System Prototype and Comunication Station Modification. BuMines Open File 

Rept. 68-73, 1973, 107 pp., 44 figs.; available for consultation at the 

Bureau of Mines libraries in Pittsburgh, Pa., Twin Cities, Minn., Denver, 

Colo., and Spokane, Wash.; at the Central Library, U.S. Department of the 

Interior, Washington, D.C.; and from the National Technical Information 

Service, Springfield, Va. , PB 226 600/AS. 

Farstad, A. J., C. Fisher, Jr., R. F. Linfield, R. 0. Maes, and B. Lindeman. 

Trapped Miner Location and Communication System Development Program. 

Volume I--Development and Testing of an Electromagnetic Location System. 

BuMines Open File Rept. 41(1)-74, 1973, 181 pp.; available for consultation 

at the Bureau of Mines libraries in Pittsburgh, Pa., Denver, Colo., Spokane, 

Wash., Twin Cities, Minn., and Morgantown, W. Va.; at the Central Library, 

U.S. Department of the Interior, Washington, D.C.; and from the National 

Technical Information Service, Springfield, Va., PB 235 605/AS. 

Linfield, R. F., A. J. Farstad, and C. Fisher, Jr. Trapped Miner Location 

Development Program. Volume IV--Performance Test and Evaluation of a Full 

Wave Location Transmitter. BuMines Open File Rept. 41(4)-74, 1973, 52 pp.; 

available for consultation at the Bureau of Mines libraries in Pittsburgh, 

Pa., Denver, Colo., Spokane, Wash., Twin Cities, Minn., and Morgantown, 

W. Va.; at the Central Library, U.S. Department of the Interior, Washington, 

D.C.; and from the National Technical Information Service, Springfield, Va., 

PB 235 608/AS. 
demonstrated that the system worked in both coal and metal mines and contracts H0133045 and H0242010 were let to Collins Radio Co. for the development 

of preproduction prototypes of system hardware. As of January 1975, no insurmountable problems have been encountered in either hardware development or 

field testing of the units. This paper provides a survey of all these related 

developments. 

ACKNOWLEDGMENTS 

The author wishes to thank Richard Watson, Howard Parkinson, and 

John Murphy of the Bureau of Mines Pittsburgh Mining and Safety Research 

Center for their frequent suggestions and assistance. The cooperation of personnel at the many mines where the field tests were made is gratefully 

acknowledged. 

DESCRIPTION OF THE SYSTEM 

Figure 1 shows the trapped miner location system. The miner's cap lamp 

battery powers a small transmitter, which generates a signal in the transmit 

antenna. This signal is picked up by the receive antenna, amplified and filtered by the receiver, and then detected through earphones. This simple 

transmission-reception is adequate for detection of trapped miners. To under 

stand how location is achieved, it is useful to consider the fields produced 

by the transmit antenna. 

Axis THEORETICAL CONSIDERATIONS 

This section explains 

the technical details; less 

theoretically inclined 

Receive Receiver readers may wish to skip to 

antenna the "Location Practice" 

section. 

* .. -,, , . ,- ~, - \ ,. , /A=// %?//.>7. .A\*, ., Before proceeding, a 

% ,*N i~ $y,~//\ I definition must be introCap lamp 

battery 

duced. The "axis" of a circular antenna is a line 

running through the center 

of the antenna and perpendicular to the plane of the 

circle. Hence, in figure 1, 

the axis of the received 

antenna is through the center of the antenna and perpendicular to the plane of 

Electromoqnetic the figure. 

transmitter Transmit 

antenna The transmit antenna's 

FIGURE 1. - Schematic of EM location system. field is shown in figure 2A. - The solid line represents 

the antenna, which is a circular loop of wire lying in the horizontal plane. 

The dashed lines, which are referred to as flux lines, represent the field 

produced by this antenna. The intensity of the signal on a loop receive 

antenna is proportional to the number of the flux lines that pass through the 

loop. The flux lines tend to be concentrated directly above the center of the 

transmit antenna, and to be less dense as distance from this center point 

increases. Thus, in figure 2B, a maximum signal on the vertical axis receive 

antenna is expected when that antenna is at position 1; that is, when it is 

directly above the center of the transmit antenna. As the receive antenna is 

moved away from the center point, for example, position 2, the signal becomes 

weaker. 

The situation is somewhat different if the receive antenna is rotated 90' 

so that its axis lies in a vertical plane which passes through the center of 

the transmit antenna. Directly above the center of the transmit antenna, at 

position 1 of figure 2C, the receive antenna is parallel to the flux lines. 

Hence, no flux lines pass through the receive antenna, and no signal is 

received. At points away from the center point, for example, at position 2 of 

figure 2C, the flux lines are no longer vertical. The flux lines now pass 

through the receive antenna, and a signal is received. As the receive antenna 

is moved still farther away, flux lines become less dense and the signal 

becomes weaker. 

Figure 3 graphically summarizes these phenomena. As figure 3A_ shows, we 

again assume the transmit antenna is a circular loop lying in the horizontal 

plane. Signal strengths are measured along a horizontal surface above the 

transmitter. The receive antenna is located on this surface a distance d from 

the surface point directly over the center of the antenna. 

Figure 3B shows vertical field strength (which is the strength of the 

signal detected on a vertical axis receive antenna) versus d. The signal 

strength peaks directly over the center of the transmit antenna, and then 

drops off as distance from this point increases. The horizontal field (that 

detected on a horizontal axis receive antenna and shown in figure 35) is a 

minimum at this center point, increases with distance to some critical point, 

and then decreases. 

So far we have assumed that the horizontal axis of the receive antenna 

was in the plane containing the center of the transmit antenna. In figure 4, 

which is a top view of the flux lines, the antenna with orientation 1 satisfies this condition. That figure also indicates that an antenna with its axis 

horizontal and perpendicular to the flux lines (orientation 2) does not cut 

any flux lines. Hence, no signals are received at such an orientation. 
1 Orientotion 2 LOCATION PRACTICE 

\ I 

\ I The first problem in 

\ I locating the signal source 

\ \ I is to detect a signal. At 

\ \' / any given point there is an 

\ / / orientation of the receive 

/ Orientotion 1 antenna which gives maximum 

signal; this occurs when the 

axis of the antenna is parellel to the flux lines. 

This most favorable orienta- 

/ tion is a function of the 

' / \ location of the transmitter; / \ 

/ \ hence it is unknown. At 

/ \ most points within the range / 

/ I \ \ \ of the system, however, ori- 

/ 1 \ enting the axis of the 

/ I \ receive antenna vertically 

\ gives an adequate signal. 

I Hence, when initially searching for a trapped miner, the 

receive antenna is oriented 

FIGURE 4. - Top view of flux lines. so that its axis is vertical. 

Once a signal is detected, location can begin. The receive antenna is 

oriented so that its axis is horizontal. Thus, with the axis in the horizontal plane, the receive antenna is rotated. Let us define the vertical 

plane containing both the transmit and receive antenna as the "critical" 

plane. When the axis of the receive antenna is in this critical plane, a maximum signal is received; that is, when the antenna is in orientation 1 of 

figure 4. When the axis of the receive antenna is perpendicular to the critical plane (orientation 2 of figure 4), a minimum signal is received. 

Figure 5 illustrates the procedure. In 5A the man is searching for the 

signal. The 18-inch-diameter loop in his hands is the receive antenna; in 

figure 5A the antenna axis is vertical. In 5B he has oriented the receive 

antenna so that its axis is in the critical piane and a peak signal is 

received. In figure 5C the axis is perpendicular to the critical plane so no 

signal is received. TK~ lower portions of figures 5B and 5s show the top 

views at these latter two orientations. 

In practice, it is easier to detect the orientation where the signal disappears than the orientation where the signal peaks. Thus, the searcher 

rotates the antenna to find a minimum signal and determines the critical plane. 

He sights down the antenna plane with the earth's surface. He then moves 

about 20 yards perpendicular to the critical plane and repeats the process. 

The intersection of the two critical planes contains the center of the transmit antenna. On the surface, this intersection of planes is the point where 

the two imaginary lines cross. Figure 6 illustrates this principle. 
5A , 58 5C 

SEARCH LOCATE 

Antenna axis vertical Antenna axis horizontal Antenna axis horizontal 

and in critical plane and perpendicular 

to critical plane 

Receive ontenno 

Receive antenna 

receive antenna 

(critical plane) 

FIGURE 5. - Determining "critical 

Directly over the center of the transmit antenna, no signal is received 

when the axis of the receive antenna is horizontal, because regardless of what 

horizontal direction the axis is pointing, the receive antenna is always in a 

critical plane. Thus, to get an exact location, the rescuer searches for the 

point at which he can rotate his antenna (always keeping the axis horizontal) 

and receive a minimum signal. He should check this location by verifying that 

the signal strength on a vertical axis receive antenna is a maximum. The 

location is the point at which the vertical axis antenna signal peaks and all 

horizontal axis antenna signals null. 
Second critical 

plane 

/ 

BASIC EQUIPMENT 

REQUIREMENTS 

/ - / 

/ Much of the preceding 

Position 1 / discussion concerns location 

/' 

Receive antenna when /' theory. This section dis- - ------- 3f. -------- First cusses the practical prob- min~mum s~gnal received // plane 

/ Center of lems of developing useful 

, transmit antenna hardware for the system. 

The most demanding hardware requirements are those 

for the transmitter. This 

unit must be readily available to a miner whenever 

FIGURE 6. - Location procedure (top view). and wherever the emergency 

occurs. Such availability 

is assured only if the miner carries the transmitter with him at all times. 

Hence, the unit must be small and lightweight. 

Emergencies are uncommon events, so any given unit will be used rarely, 

if ever. However, the unit must work that one time it is needed. An 

FIGURE 7. - Preliminary version of manpack location transmitter in separate package. 
extremely reliable unit is 

required. Anything carried 

by a miner is subject to 

considerable wear and abuse, 

so the transmitter must be 

very rugged. 

Such a unit exists. 

The transmitter is intrinsically safe. It can be 

encased in a 1.4- by 0.35- 

by 3.45-inch molded package. 

Thus encased, it can be 

shocked, immersed in water, 

subjected to temperature 

extremes, etc., with no 

damage. 

Two antenna types have 

been tested. Under favorable conditions that will be 

defined later, a 90-foot 

coil of No. 16 wire is adequate. An antenna this size 

weighs about half a pound; 

it can be put into an easyFIGURE 8; - Preliminary version of manpack location to-carry package. 

transmitter, built into cap lamp battery. An alternative is to 

use a previously installed 

antenna. In most coal mines this would be several hundred feet of No. 10 or 

No. 12 wire wrapped around a pillar of coal. In an emergency, any available 

wire can be used. 

A problem that becomes increasingly relevant as we approach implementation of a practical system is the packaging of the transmitter and antenna. 

We have studied a separate unit (fig. 7) and units packaged in the miner's cap 

lamp battery (figs. 8 and 9). Considerable attention is given to using sturdy, 

reliable switches and connectors in these units. 

The transmitter is ower red by the miner's cap Iamp battery. A fully 

charged battery will power the transmitter for over a day. Even at the end of 

an 8-hour shift, the battery should have enough power to drive the unit for 

another 8 hours. 

The receiver presents fewer problems. We have developed a sturdy, fieldworthy unit. Since it may be useful to take the receiver into a mine, it is 

intrinsically safe. As shown in figure 10, the total receive system consists 

of a loop antenna about 18 inches in diameter, a tuning unit which can be 

carried on the rescuer's belt, and a headset. As figure 11 indicates, the 

receive unit is easy to carry, even in rugged terrain. 

RECEIVER 

HEADPHONE 

ANTENNA 

FIGURE 10. - Preliminary version of receive unit. 
FIGURE 11. - Using receive unit shown in figure 10. 
SYSTEM REFINEMENTS 

Previous sections have outlined how the EM location system works and 

described the basic hardware for that system. In practice, certain problems 

are encountered with such a basic system, and certain refinements are useful. 

For example, consider the situation in which more than one group of 

miners is trapped. Will their transmitters interfere with one another, making 

location impossible? This problem can be avoided by assigning each group of 

miners a different transmitter frequency; generally the best frequencies to 

use are those midway between 60-Hz harmonics in the 1,000- to 3,000-Hz band; 

that is, 1,050 Hz, 1,110 Hz, 1,170 Hz, . . . 2,910 Hz, 2,970 Hz. Thus, we 

have a 33-channel system. Each transmitter is fixed to one frequency. A 

tunable receiver for surface use is being built. 

A second problem is trying to determine if a reception is the signal of a 

trapped miner, or is just noise. Since the receiver is very sharply tuned, 

any reception (signal or noise) that passes through the tuning section to the 

earphone will have the same frequency. However, controlling the rate and 

duration of the transmitted signal provides a way to distinguish a signal from 

noise. A pulse about 0.2 second long transmitted every 2 seconds is optimum 

for location work. This means the receiver earphone will emit a short ''beep'' 

every 2 seconds. It is extremely unlikely that a noise source would show such 

a uniform cycle. The intermittent tone means the transmitter is switched on 

only 10 percent of the time. This extends the battery life to the 24 hours 

mentioned above. 

So far, we have implied that the rescuer must walk over the trapped men 

before they can be located. But what happens at a large mine which is spread 

over tens of square miles and may be covered by heavily wooded mountains? To 

search such an area on foot could take several days. Thus, we have developed 

a helicopter-borne receiver (fig. 12). This unit enables us to monitor six 

different frequencies at once while flying over the area. The final, most 

accurate location must still be done on foot, but the men can be approximately 

located from the air. 

The location system could be very useful in an emergency, but a true twoway comunication system would be even better. For this reason a combination 

voice receiver-code transmitter is being developed for use in the miner's 

unit. It will be recalled that the location transmitter is turned on only 

about 10 percent of the time. With the voice receiver, the trapped miner 

would listen for voice messages from the rescue crew during the remaining 

time. A continuous transmit switch will be added to the miner's unit so that 

he can send coded replies to questions asked by the rescuers. Testing of this 

system should begin in 1975. 
FIGURE 12. - Helicopter receiver. 

FIELD TESTS 

As table 1 indicates, the system has been tested at a variety of mines, 

both coal and hardrock, throughout the United States. In these tests a number 

of experiments with various antennas, transmitter types, etc., were conducted. 

The most important fact about these tests, however, is that the system has 

performed well a t a l l these mines. 

At both the mines with more than 2,000 feet of overburden (Grace and 

Galena) the fields were severely distorted by electric conductors in or above 

the mine. At these mines, no location was attempted; it was simply noted that 

signals could be detected. At the other mines the Locations of several different transmitters were determined. All location errors were Less than 60 

feet, and most were Less than 30 feet. At mines with less than 600 feet overburden, the easily carried 90-foot-long antenna is adequate. At greater 

depths, a longer, permanently emplaced antenna is used. 
TABLE 1. - Mines where EM location systems have been tested 

These results were obtained under mock-emergency conditions. That is, 

the transmitter is deployed at an unknown point, and the surface personnel do 

not require any "hints" to perform the location. No fine tuning, no conversation between trapped miners and rescuers, and no additional adjustments are 

required. 

SUMMARY AND CONCLUSIONS 

Mine 

Bureau of Mines Safety Research. 

Rainbow No. 7................... 

Latrobe Construction..........,. 

Inexco No. I.................... 

Camp No. 2..... ................. U.S. Tunnel..................... 

Copper Queen. ................... Guyan No. I..................... 

Robena No. 1. ................... Putnam.......................... 

Somerset. ....................... Robena No. ..................... 

Geneva.......................... 

Grace........................... 

Galena.... ...................... 

An EM system has been built and tested that permits the detection and 

location of trapped miners. The hardware required is compact, sturdy, and in 

general practical for use in mines. Successful field tests of the system have 

been conducted at a wide variety of mines. 

Location 

Bruceton, Pa........ 

Rock Springs, Wyo. .. Latrobe, Pa......... 

Jamestown, Colo..... 

Morganfield, Ky..... 

Idaho Springs, Colo. 

Bisbee, Ariz........ 

Amherstdale, W. Va.. 

Waynesburg, Pa...... 

Elmwood, W. Va...... 

Somerset, Colo...... 

Waynesburg, Pa...... 

Dragerton, Utah.. ... Morgantm, Pa...... 

Wallace, Idaho...... 

Mine type 

Coal... ..... ... do....... 

Limestone... 

Fluorspar... 

Coal........ 

Hardrock 

tunnel. 

Copper ...... Coal........ 

... do....... 

... do.. ..... ... do....... 

... do...... . ... do....... 

Iron........ 

Lead, zinc.. 

W.S. GOVERNMENT PRlNTlNG OFFICE: 1976.601.7~1/1% 

1NT.-BU.OF MINES,PGH..PA. 21376 

Overburden, 

feet 

80 

14 0 

325 

350 

3 75 

390 

4 00 

400 

400 

460 

500 

990 

1,500 

2,400 

4,300