Open File Report: 127-85 

THROUGH-THE-EARTH ELECTROMAGNETIC TRAPPED MINER LOCATION SYSTEMS. A REVIEW 

By Walter E. Pittman, Jr., Ronald H. Church, and J. T. McLendon 

Tuscaloosa Research Center, Tuscaloosa, Ala. 

UNITED STATES DEPARTMENT OF THE INTERIOR 

BUREAU OF MINES 

Research at the Tuscaloosa Research Center is carried out under a memorandum 

of agreement between the Bureau of Mines, U. S. Department of the Interior, 

and the University of Alabama. 
CONTENTS 

. 

Page 

List of abbreviations ............................................. 3 

Abstract .......................................................... 4 

Introduction ...................................................... 4 

Early efforts at through-the-earth communications ................. 5 

Background studies of earth electrical phenomena .................. 8 

~ational Academy of Engineering recommendations ................... 10 

Theoretical studies of through-the-earth transmissions ............ 11 

Electromagnetic noise studies .................................... 13 

Westinghouse - Bureau of Mines system ............................ 16 

First phase development and testing ............................. 16 

Second phase development and testing ............................ 17 

Frequency-shift keying (FSK) beacon signaler .................... 19 

Anomalous effects ................................................ 20 

Field testing and hardware evolution .............................. 22 

Research in communication techniques .............................. 24 

In-mine communication systems ..................................... 25 

Tangential research ............................................... 26 

Current research ................................................. 27 

DEVELCO system ................................................. 27 

Phase difference of arrival techniques .......................... 28 

Automated three-dimensional location system ..................... 28 

Adaptive noise cancellation ..................................... 29 

National Research Council evaluation .............................. 30 

Summary ........................................................... 31 

Bibliography ...................................................... 32 
AWG 

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kHz 

kW 

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LIST OF UNIT OF MEASURE 

American wire guide 

ampere per meter 

decibel 

degree Celcius 

hour 

hertz 

kilohertz 

kilowatt 

meter 

ABBREVIATIONS 

MHz 

ms 

mW 

PC t 

S 

SM'~ 

v 

W 

Yr 

megahertz 

millisecond 

milliwatt 

percent 

second 

siemenlmeter 

volt 

watt 
THROUGH-THE-EARTH ELECTROMAGNETIC TRAPPED MINER LOCATION 

SYSTEMS. A REVIEW 

1 ?. 2 

By Walter E. Pittman, Jr., Ronald H. Church, and J. T. McLendon, 

*** ABSTRACT 

In its role of providing technical assistance to the mining industry, 

the Bureau of Mines' Tuscaloosa Research Center has conducted research to 

develop trapped miner location systems which would aid in locating miners 

trapped by underground mining disasters. 

Efforts to produce electromagnetic systems for the location of trapped 

miners underground and to communicate with them were surveyed, from the 

1920's to 1981. Theoretical studies of through-the-earth electromagnetic 

transmissions are described as well as studies of the electrical 

characteristics of various rocks, minerals, and soil. Several trapped 

miner location systems are reviewed including the Westinghouse systems, 

the DEVELCO system, and an automated three-dimensional location system, 

and the phase difference of arrival technique. Tangential research, 

relevant to trapped miner location systems is described. A comprehensive 

bibliography covering all aspects of trapped miner, through-the-earth 

electromagnetic location and communications systems is appended. 

*** INTRODUCTION 

Mine disasters often leave survivors trapped underground. Would-be 

rescuers on the surface or underground who attempt to reach the victims 

are seriously inhibited in their work by their lack of knowledge of the 

actual location and condition of the victims. In order to mount an 

effective and timely rescue effort, rescuers need to have immediate 

knowledge of the precise location of any survivors and the precariousness 

of their situation. The nature and urgency of a rescue campaign needed 

in the case where the miners face a speedy death due to poisonous gas or 

fire is very different from the one in which the trapped miners have an 

adequate atmospheric environment. Some method of locating trapped 

survivors and communicating with them is essential. However, normal mine 

communications are usually disrupted by the same disaster that entrapped 

the miners and are not available to rescuers. Emergency systems of 

location and communication that can operate through the earth, either 

from the surface or through the rock and earth material of a cave-in is 

needed. Such a system must operate under many constraints. It must be 

reliable under adverse physical and environmental conditions in a mine; 

it must be inexpensive, and portable, it must be built within the 

"intrinsic" safety limits for mine use and it must be simple to operate. 

Furthermore, it must produce a signal whose source can be located with 

l~echnical information specialist, Tuscaloosa Research Center, Bureau 

of Mines, Tuscaloosa, Ma.; professor of the history of science, 

Mississippi University for Women, Columbus, Miss. 

 i in in^ engineer, Tuscaloosa Research Center, Bureau of Mines, 

Tuscaloosa, Ala. 
precision. The time constraints on rescue efforts are critical as was 

pointed out by K. H. Sacks in a 1977 analysis (25113 - of 43 coal mine 

disasters in which miners were trapped underground. The ability to 

locate trapped miners and to reestablish communications quickly is 

necessary for successful rescue efforts. 

A 1982 study (185) - done by Arthur D. Little, Inc., of 29 recent mine 

disasters, under a Bureau of Mines contract, revealed that generally, the 

time to death of survivors is so short that most search operations take 

too long. The study delineated the narrow time window, usually a maximum 

of 1 to 5 h, in which postdisaster mine rescue is necessary and possible. 

Timely knowledge of trapped miner location is critical to quick rescue. 

However, because of the strict time constraints involved, only rescue 

systems that are already in place have a chance of being successful. 

For many years research and development of such systems have been 

undertaken, usually with the support and guidance of the Bureau of Mines. 

These systems are rapidly approaching the point where they can be 

practically distributed in operating mines. 

*** EARLY EFFORTS AT THROUGH-THE-EARTH COMMUNICATIONS 

The earliest pioneers of radio were keenly interested in the 

possibility of using the earth for radio transmission. They were 

concerned with the mechanisms of radio transmission through-the-earth 

which was part of the initial efforts to understand the general 

mechanisms of radio transmission. As early as 1899, Nicola Tesla 

suggested the use of what are today described as extremely low 

frequencies (ELF) for worldwide communications using an earth medium 

(355). - The theoretical basis of radio propagation through the solid 

earth was elucidated by the great theoretical physicists, Arnold 

Somrnerfeld and Hermann Weyl. They derived reasonably accurate 

mathematical descriptions of the propagation of radio waves through earth 

by making two simplifying assumptions. These were to treat the earth as 

a uniform half-space and to assume that the earth had uniform electrical 

characteristics (264, 354). -7 

As usual under wartime conditions, WWI spurred technological 

development of all sorts including through-the-earth radio. In their 

search for secure communications, the Allies designed and deployed on a 

large scale the secret T.P.S. ("telegraphic par sol") communication 

system. Using low frequencies (500 to 1,800 Hz) the T.P.S. system 

utilized ground conduction paths to provide secure communications with 

the front lines. The maximum reliable range of the T.P.S. system was 

about a kilometer and transmissions had to be in code because of the 

limited information carrying capability of the very low frequencies used. 

In the system, two stakes or ground antennas were driven into the earth 

15 to 300 m apart and a battery-supplied direct current was allowed to 

flow between them. This current was interrupted by a code key which 

provided the signal and which could be detected at a distance somewhat 

over four times the separation distance between the antenna stakes (13). - 

5Underlined numbers in parentheses refer to the bibliography at the end 

of the report. 
Another technological innovation of the War was the geophone invented by 

French physicists to detect underground mining and tunnelling in the 

battle area. The geophone was designed to detect sounds of military 

mining operations and was essentially a sensitive microphone. The 

technology quickly spread to both sides and mining was effectively 

neutralized as a major technique of War (201). - 

When the United States belatedly entered the War in 1917, it seriously 

lagged the combatants in military technology. To overcome one area of 

deficiency, the Army turned to the U.S. Bureau of Mines to undertake 

studies of underground sound that might be applicable to offensive or 

defensive warfare. These tests also revealed the possibility of using 

military technology for peacetime purposes; specifically, the use of the 

geophone in mine rescue operations. After the War ended, Alan Leighton, 

a physicist who had worked on the Army project, devised an improved 

geophone and conducted a series of experiments that demonstrated the 

geophone's usefulness in mine rescue work (201). - 

Bureau of Mines' engineers quickly saw in the new technology an answer 

to the old problem of communicating with surviving miners trapped by a 

mine disaster. Although the geophone, in improved versions, has proven 

useful in mine rescue work to this day, its limitations are severe. It 

is short-ranged, can convey very little useful information and is subject 

to interference from many sources including mine rescue efforts. 

Recognizing these constraints, research interest soon turned to 

through-the-earth radio. During the 1920's intensive research was 

undertaken by Bureau of Mines' and independent scientists to develop 

through-the-earth radio. 

Ironically, the big question was whether or not radio waves penetrated 

the earth at all. The presence of any metallic conductor greatly 

enhanced the transmission of radio waves through-the-earth. Mines 

usually have pipes, power cables, rails, or structural metal in shafts 

leading to the surface. Radio receivers situated near such metallic 

carriers detected much stronger signals than those further removed. Many 

critics in the 1920's felt that all through-the-earth radio transmission 

was by means of such "anomalistic conduction." 

In 1922, Bureau of Mines' and Westinghouse engineers, in a mine near 

Pittsburgh, succeeded in detecting radio broadcasts from the first 

American radio station, KDKA. The signal, evidently carried by pipes and 

cables, was detectable for a few hundred meters into the mine. In an 

area further from metallic conductors it was detectable through only 15 m 

of overburden (59). These modest results encouraged much more research 

by government af;;i- independent engineers in the next few years. J. J. 

Jakosky, a Bureau of Mines' engineer, was probably the most important 

figure. Many significant discoveries were made. For the first time the 

frequency dependency of earth penetrating radio became clear. Roughly, 

the higher the frequency the shorter the distance the radio could 

penetrate the earth. But lower frequencies could convey less 

information. The role of serendipitous metallic conductors was explored 

and found to be crucial to successful transmission and detection. The 

effects of differing soil and rock types and of the presence of water 
were studied. Power requirements were determined and the directional 

characteristics of antenna configurations were studied (165-168). -- Field 

tests were made in coal mines, Western hard-rock mines and Canadian 

railway tunnels (89). - 

Despite the money, time, work and widespread publicity, no practical 

through-the-earth radio resulted from the efforts of the 1920's. It was 

still unclear, in fact, if radio waves actually penetrated the earth, to 

any extent, in the absence of metallic conductors (162). - The latter 

point was finally cleared up by J. Wallace Joyce, a Bureau of Mines' 

geophysicist, in 1929 and 1930, when he undertook a series of carefully 

controlled experiments at Mammoth Cave in Kentucky. Joyce tested various 

shapes and sizes of loop antennas as well as the earth attenuation of 

radio waves at various frequencies from 500 Hz to 810 kHz, through the 

limestone and sandstone overburden. He also proved, conclusively, that 

radio waves did penetrate through rock and earth in the absence of 

metallic conductors. Low frequency (LF) signals gave the best results: 

at 500 Hz the signal was detectable at a depth of 274 m. On the other 

hand, Joyce obtained a maximum range of only 91 m using radio frequency 

waves in the 20 to 110 kHz range even at a power of 50 W. This 

attenuation was so great that Joyce concluded that radio waves could not 

penetrate the earth enough to be useful for mine rescue operations. 

Theoretical results that he derived from Maxwell and Sommerfeld 

reinforced his pessimissism (170). - It was a conclusion increasingly 

shared by most other researchers in the field. Interest in 

through-the-earth radio declined after 1930 and remained at a low ebb 

until after WWII. 

The development of through-the-earth radio for geophysical exploration 

continued. In 1928, Hans Lundberg, demonstrated a very successful system 

for locating underground metallic ore bodies based upon the transmission 

of audio frequency (50 to 10,000 Hz) current between two line electrodes 

on the surface, each 300 m long. In the next 20 yr the use of LF 

electromagnetic radiation for underground exploration became common, 

especially in oil and gas exploration (40, --- 156, 268). There was very 

little transfer of this new technology to communications, however, at 

least in the United States. 

The first successful through-the-earth communications system was 

developed by South African engineers. Facing a requirement for 

communications with their famous crack mobile fire fighting teams 

("Prototeam") in deep, hard rock, gold mines, the South Africans 

undertook investigations of electromagnetic propagation through rock 

strata. T. L. Wadley showed, in 1949, that communications through 

hundreds of meters of rock were possible if low radio frequencies were 

utilized. The South Africans achieved a useful range of 600 m at 300 kHz 

and a maximum range of 2,100 m (14, - -- 283-284). With this experience, the 

South Africans developed a practical operating apparatus which they 

successfully tested in 1961. Radio communications through-the-earth 

proved suitable both for ordinary communications and for emergency 

operations. But the original equipment proved to be too bulky for 

practical applications. Smaller equipment was developed in the 1960's 

utilizing the new miniaturization technology then becoming available. The 
South Africans also learned that equipment size could be reduced by 

lowering output power to as little as 1 W without seriously degrading 

detection ranges. The introduction of single-sideband modulation vastly 

improved performance of the equipment which operated at 903 kHz. 

Experimental units were so successful that over 100 were built between 

1970 and 1973. They were used to provide all the communications in some 

mines (279). - 

While the equipment fulfilled expectations, the South Africans soon 

discovered that operator training, which must involve all levels of mine 

personnel, was required if the system was to succeed in actual mining 

operations (14). A South African company currently markets a 

through-the-zrth radio system for in-mine use. 

In the United States, the Bureau of Mines continued its efforts to 

develop through-the-earth communication at a reduced level until after 

WWII when Coggeshall and Felegy (58, -- 104) undertook a series of 

underground experiments at two bituminous coal and three anthracite mines 

in Pennsylvania and at a salt and an iron mine in New York. They found 

that the high frequency "walkie talkie" radios of the Army failed to work 

underground but that a LF system gave adequate communication ranges 

through the earth. However, they were unable to separate conductor 

effects from ground transmission and finally concluded that all LF 

transmission was due to ground conduction or man-made metallic conductors 

serendipitously situated for radio pathways. Tests of voice, amplitude 

modulated, inductively coupled, radio frequency transmissions through the 

earth in several mines gave promising results. Operating in the 33 to 

220 kHz range, with a transmitter output below 2 W in most cases and 

below 3 W in every case, the Bureau of Mines' investigators established 

radio communication through a minimum of 320 m of overburden to a maximum 

of 622 m. Tests were conducted successfully through ground containing 

anthracite beds, ground without anthracite beds, and through ground 

containing mined out levels. It was found that extreme wetness, either 

at the level of communication or between it and the surface had no 

appreciable effect. Optimum frequencies for transmission efficiency were 

found to be a characteristic of each mine and to fall in the 33 to 188 

kHz range. Conductor effects were again found to be important. Any 

metallic conductor running along any portion of the radio wave 

transmission path, greatly enhanced transmission range. At one mine, a 

body of magnetite was utilized as a metallic conductor in a test. The 

researchers also concluded that loop antennas were unsatisfactory because 

of their high power requirement (224). - The experiments did not produce 

immediate concrete results. 

Interest in through-the-earth communication continued, however, outside 

the United States. French investigators transmitted to a depth of over 

400 m (110) - while in Austria, Bitterlich was able to locate an 

underground transmitter from the surface (35). - 

*** BACKGROUND STUDIES OF EARTH ELECTRICAL PHENOMENA 

Increasing use of electrical prospecting techniques and interest in 

making radio communications more effective led to basic studies of the 
electrical characteristics of the earth from the 1930's to the 1960's (3, 

------- 36, 56, 140, 178-179, 250, 261). Data were systematically collected 0:- 

the electrical properties of rocks, minerals, and soil types and made 

available through technical literature such as Parkhomenko's, "Electrical 

Properties of Rocks" (234), - and John G. Heacock's (ed.), "The Structure and 

Physical Properties of the Earth's Crust" (139). - Progress was also made 

toward an understanding of the physical mechanisms by which electrical 

charges pass through earth materials (60, --- 252, 254). B. J. Meakins (217), - 

Howell and Licastro (160) reported on the dielectric behavior of rocks 

while G. V. Keller (117) - studied the response of various rock types to 

transient electromagnetic fields. The latter is the basis of many 

electrical techniques used in geophysical work. Earth material electrical 

characteristics at radio frequencies were studied by Russell E. Griffin and 

Robert L. Marovelli (133) of - the Bureau of Mines. John C. Cook (64), 

working with electronics in mine environments in England reported-& the 

"RF Properties of Bituminous Coal Samples." Long-term investigations 

undertaken by the Bureau of Standards for radio propagation studies, made 

available large quantities of data on ground conductivity in most parts of 

the country (182). - 

Military interest in the possible use of through-the-earth transmissions 

for hardened and secure communications caused a flurry of new research, 

particularly with low or very low frequencies (VLF) or ELF (17, - - 72, - 80, 

------ 114, 124, 127, 136, 29, 103). Studies were undertaken of the radiation and 

reception of underground antennas and of the coupling of these antennas to 

the karth medium as well as the mutual coupling bf the (very large) antenna 

elements (113, ------ 115, 125-126, 135, 42). Numerous studies of electromagnetic 

noise were undertaken after it was realized that at VLF or ELF ambient 

noise was a major problem (45, ------ 76, 214, 230, 245, 347). The source of some 

of the noise was attributed to the interaction of ~articles of solar 

origin with the earth's electric and magnetic fields. Worldwide 

lightning is the source of most VLF and ELF radio interference. At the 

low frequencies involved, attenuation is so low that lightning 

interference from all over the world combines to provide a background of 

noise. The data collected on VLF-ELF electromagnetic noise was subjected 

to statistical analysis and from this mathematical models were derived to 

describe the noise background. The VLF-ELF noise background was 

discovered to exhibit a generally Gaussian distribution with impulse 

"spikes" or high points superimposed. The latter were caused by nearby 

lightning (128; ----- 215, 219, 348, 359). These computer models of LF noisk 

allowed the prediction of the performance of electronic systems at VLF or 

ELF (232, -- 251). 

Some investigators were quick to see the geophysical application of LF 

radio waves to underground exploration (1). Limitations in the use of 

radio methods were also quickly discoveryd. The same limitations would 

later be found to apply equally as well to through-the-earth signalling 

and location techniques using radio. It was found that penetration 

through the ground was best at low frequencies but because of the long 

wavelengths involved, the lower frequencies gave poor resolution. 

Interpretation of data proved difficult. Also, models based upon uniform 

earth electrical characteristics failed to produce representative 
results. Both two layer and multiple thin layer models were developed to 

account for radio wave data for through-the-earth transmissions (293). - 

It was also found, later, when actual field testing began, that 

electromagnetic transmission through-the-earth was a more complicated 

process than theory predicted. Conductivity proved to be a function of 

the type (or types) of rock or soil through which a signal passed as well 

as a function of the signal frequency. The conductivity could (and did) 

vary at individual mine sites; varying with depth, lateral displacement, 

and frequency; these variations were not necessarily uniform or 

predictable. Recognizing that the precise location of an underground 

transmitter depends upon an accurate knowledge of the local conductivity, 

in-mine tests were conducted. Efforts to determine conductivities at 

representative mine sites throughout the country have been undertaken by 

the Bureau of Mines and its contractors. J. Durkin (82) - of the Bureau of 

Mines has begun a systematic survey. 

*** NATIONAL ACADEMY OF ENGINEERING RECOMMENDATIONS 

The problem of through-the-earth signalling and location techniques 

gained a new urgency as the result of a mine disaster at Farmington, 

W. Va., November 20, 1968. A dust and gas explosion at Mountaineer Coal 

Co.'s Consol No. 9 mine killed 78 miners. In the sealed mine, the fate 

of the trapped miners was unknown for some time. Out of this tragedy 

came numerous programs intended to make mining safer. At the request of 

the Bureau of Mines, the National Academy of Engineering put together the 

Committee on Mine Rescue and Survival Techniques to "conduct a study 

program to assess the technological capabilities that can be applied to 

survival and rescue techniques following mine disasters." Particularly 

the Committee was asked to consider new devices and technology that might 

be applicable to the problems of mine safety. Their recommendations led 

to several programs to develop through-the-earth communications and 

location systems (227). - 

Moved by the same events, Congress, on December 30, 1969, passed the 

Federal Coal Mine Health and Safety Act of 1969 (Public Law 91-173). 

This Act made the health and safety of miners "the first priority and 

concern" of all in the industry. The Act also required the Secretaries 

of Interior, and Health, Education and Welfare to develop, promulgate and 

enforce mandatory health and safety standards. In turn, this committment 

led to expanded research efforts by the Bureau of Mines in the area of 

mine safety including items designed for postdisaster mine rescue 

applications. This included the development of trapped miner location 

and communication systems. 

The Committee on Mine Rescue and Survival Techniques suggested a 

three-phase program to the Bureau of Mines. In the first, an emergency 

beacon was to be built and tested. It was to be a simple device, rugged 

and inexpensive, with a narrow bandwidth suitable for one-way signalling 

in the 500-1,000 Hz range and consisting of only a battery, a buzzer and 

a key. Stored in refuge chambers and work areas, the beacon would 

provide a method for through-the-earth location of trapped miners from 

the surface. In the second phase, individual, mobile transceivers were 

to be developed that could tie the individual miner into the regular mine 

communication system. The functions of the first two phases would be 
combined in the third; the development of an in-mine mobile communication 

system for individual miners that had through-the-earth capabilities for 

emergency use. In the third phase, the effective signal strength would 

have to be increased 100-fold from the beacon signal to transmit voice. 

It was recognized that low frequencies would have to be used for 

sufficient earth penetration; but that presented another problem, that of 

a low rate of data transmission. Higher frequency signals could transmit 

more useful information, even voice, but were quickly attenuated in rock 

and dirt. Some sort of trade-off would be required. The same was true 

of antennas. Two antenna types, the grounded long wire and the loop, 

were recognized as potentially effective in mine usage at low 

frequencies. Tn the grounded long wire the signal strength was 

proportional to its length times the current intensity. It would usually 

be possible to lay out the long wire in a mine. Coupling of the antenna 

to the rock overburden would be no problem in damp mines but might be one 

in dry- or well-ventilated mines. In a loop antenna the signal strength - 

is proportional to the area of the loop times the current intensity. 

Here the loop size is the limiting factor, larger loop sizes needed for 

lower frequencies are impractical due to bulk. Loop antennas can be 

located more easily and accurately than long wire (227). - 

*** THEORETICAL STUDIES OF THROUGH-THE-EARTH TRANSMISSION 

Theoretical understanding of electrical wave propagation through the 

earth was advanced enough to allow rapid laboratory development of 

hardware systems. As early as 1952, James R. Wait, (319) - whose studies, 

and those of his associates, underly most work on through-the-earth 

electromagnetic probing or signalling, derived expressions for the 

electric field of a small loop buried in a dissipative medium, the earth. 

Wait's earth was considered to have a finite conductivity and the loop 

was energized by a step-function current. Approximate solutions were 

also found for the magnetic fields. Wait also considered the case of a 

conducting sphere in a transient magnetic field; an example of interest 

for geophysicists (285). - About the same time, R. H. Lien (203) - derived 

the expressions for the electric field produced by an oscillating 

horizontal dipole in a dissipative medium when the radiating frequency is 

low. In 1970, Wait (318) - derived the expression for the radiation into 

the earth of a line source of current on the surface when the source is 

an impulse function. Wait and D. A. Hill (325, -- 330) found expressions 

for the subsurface electric fields of a grounded cable on the surface 

carrying a time harmonic varying current under transient and steady state 

conditions. They also calculated the electromagnetic fields on the 

surface produced by a pulse excited loop buried in the earth (147, -- 325, 

-- 328, 330). In 1976, Wait and K. P. Spies (336) - investigated the case of 

a small loop radiating in the earth which was considered to be 

stratified. They found that the ratio of the horizontal to vertical 

magnetic field strength at the surface was not appreciably affected by 

the geometry of the earth at sufficiently low frequencies. Hill and Wait 

determined the theoretical response of a small wire loop, buried in the 

earth (considered homogeneous) to a transient electromagnetic field 

(147). - Wait and Spies derived the expressions for the impedance of a 

circular loop, over conducting ground to a LF signal; a case that 

resembled actual search coils (338). - They also derived equations 
describing the attenuation of radio waves in the earth's crust from ELF 

to VLF (334). - Hill (142) - derived expressions for the case in which the 

buried radiator, a cable of finite length, was oriented at an incline to 

the surface. In each case it was found that radio waves induced in the 

earth are affected by the electrical properties of the earth so as to 

cause the current density within the earth to decrease with depth (293). - 

As R. G. Geyer and G. V. Keller (122) - pointed out only the frequency is 

within the control of the experimenter and designer. Permeability, 

conductivity, and the dielectric constant may also be frequency dependent, 

and at least partially susceptible to design parameters. 

To study through-the-earth electromagnetic wave propagation, S. M. Shope 

developed an earth model, in 1982, incorporating a magnetic dipole buried 

in a three-layered earth (255). - Applying appropriate limiting values, 

Shope was able to reduce the three-layered model to two separate 

two-layered models and then to a homogeneous half-space model. Shope's 

solutions were in the form of infinite integrals. Numerical analysis was 

carried out and a computer program devised to allow the estimation of the 

surface magnetic fields above the buried source. 

Wait and his colleagues also did considerable work describing the theory 

of transmission of radio waves in mine tunnels and along - metallic 

conductors such as pipes, conduits, wire ropes, etc. (315, -- 332). Such 

cases were of more use in normal mine operations than under emergency 

conditions and were centered in the middle frequency range. shortly-after 

the National Academy of Engineering recommendations were published, Wait 

(290) - pointed out the possibility of precisely locating a trapped miner if 

he were able to broadcast a LF electromagnetic signal from a source loop 

underground. Location would be determined by comparing the magnitude of 

the ratios of the horizontal and vertical magnetic fields on the surface 

or the difference in phase between the vertical and horizontal magnetic 

fields at different surface positions. Wait and Hill (323) then (1972) 

worked out the explicit express ions describing the waveforms of transient 

magnetic fields produced by a step-function-excited loop buried in the 

earth and observed on the surface. They suggested that these waveform 

functions could be used to interpret signals received on the surface in a 

manner that might lead to the location of the source from observations at 

a single point on the surface. The accuracy of these methods depended - 

greatly upon the accurate orientation of the underground transmitting 

loop parallel to the surface. If, as D. A. Hill (142) pointed out in 

1973, the loop is not exactly horizontal then the magnetic dipole moment 

would have an additional horizontal component that would change both the 

waveform and the apparent location of the source. He derived expressions 

for a horizontal magnetic dipole, primarily to consider the effects of 

possible random antenna orientation but he also suggested that the 

vertical magnetic field of a horizontal loop could be used for location 

purposes. 

The question of precisely locating an underground signal, once it was 

detected, was studied by Wait and C. H. Stoyer. They identified the 

possible errors inherent in locating an underground signal (a magnetic 

dipole) located in an inhomogeneous earth which varied laterally from the 

source to the receiver (271). - T. W. H. Caffey and L. Romero in a recent 
study derived expressions for the location of a similar underground 

magnetic dipole (44). - 

*** ELECTROMAGNETIC NOISE STUDIES 

The electromagnetic background noise environment of mines is highly 

complex and derives from many sources. The primary source of 

interference is the omnipresent 60 Hz and its harmonics. Mine machinery, 

lighting, transmission lines and nearby inhabited areas all contribute to 

the 60 Hz haze which prevents the use of many low frequencies for 

through-the-earth communications. Heavy DC machinery often used in 

mining also produces strong, LF interference when it is starting up or 

being switched. Natural interference is also present and highly 

significant at the low signal strengths involved in through-the-earth 

electronics. For nearly a century LF interference has been noted on 

telephones, radios, etc. These are variously described as "whistlers ," 

"growlers," or a steady hiss. It is believed that the world-wide pattern 

of thunderstorms produces most of these noises; at these low frequencies 

attenuation is so low that almost the entire world can contribute. The 

"hiss" and other noises are believed to be caused by solar particles 

falling into the earth's magnetic envelope. Furthermore, in recent 

years, several LF and VLF radio stations, mostly military, have gone into 

operation, contributing further interference. The nature, distribution 

and magnitude of these electromagnetic noises were largely unknown 

quantites in 1970. 

The next phase of the effort to create through-the-earth trapped miner 

location systems was the measurement of the electromagnetic noise field 

within mines and on the surface above mines. At the same time efforts 

were being made to produce effective communication systems for use within 

the mine during normal operations using wire or radio. The studies were 

useful for both purposes. Under Bureau of Mines' contracts several 

research groups began, in 1971, to systematically study electromagnetic 

noise in mines (4-5, -- - 12, -- 26-27). The Colorado School of Mines group, 

which included R. J. Geyer and G. V. Keller (123). - took measurements at a 

number of coal mines in Colorado, Illinois, and West Virginia. The data 

were deliberately taken above the working faces and working sections and 

included measurements taken near a power station. A wide frequency band 

(20 Hz to 10 kHz) was taken to sample, and data were collected over the 

full 24-h period spanning all work shifts in the mine. 

The Institute for Telecommunications Sciences under the direction of 

J. H. Crary (69) undertook a study for the Bureau of Mines of the 

background electromagnetic noise, natural and manmade, in the frequency 

range 20 Hz to 20 kHz. The magnitude of the electromagnetic noise field 

as a function of frequency, above and below the surface was recorded. 

Studies were made of the attenuation of radio waves through the earth at 

frequencies from 20 Hz to 100 kHz, and tests were made of the accuracy of 

location of an underground signal source. Various loop antenna 

combinations of characteristics were tested; shielded and unshielded, 

horizontal and vertical orientations, varying number of turns, etc. For 

each location and antenna loop configuration, a frequency band that was 

exceptionally quiet was selected for further analysis as well as an 
unusually noisy frequency band. The quiet band was chosen because it 

gave promise of being useful for through-the-earth communications and the 

noisy one could provide evidence as to the source of electromagnetic 

noise. The noise patterns proved to be very complex and it proved 

impossible to separate temporary local noises from the general background 

except in a few isolated cases (12). - 

These results showed generally good agreement with an earlier National 

Bureau of Standards study (25). For their work, the Bureau of Standards 

group put together a versatxe, semi-portable integrated unit which was 

capable of the calibration and measurement of the entire noise recording 

system they used. On tape, they recorded the wideband magnetic field 

noise waveforms over the frequency range 40 Hz to 10 KHz. This data was 

later subjected to computer-assisted spectral and statistical analysis in 

the laboratory. Tests were made at three operating coal mines in 

Colorado which were considered representative under every working 

condition; full production, shift change, etc. The Bureau of Standards 

group found that the magnetic field strength of the background noise 

varied from a low of 20 dB (relative to 1 ampm'l at 60 Hz) at a quiet 

location to a high of 138 dB at a frequency of 500 Hz near an AC power 

buss. At locations distant from arcing DC machinery and at frequencies 

between 60 Hz harmonics, noise minima of over 60 dB were found, giving 

promise of being suitable for through-the-earth transmission usage (12). - 

A West Virginia University group under the direction of M. D. Aldridge 

and W. W. Cannon (9) undertook, for the Bureau of Mines, studies to 

measure electromag~etic noise from 2 kHz to 200 kHz in operating mines. 

The West Virginia University group was primarily interested in developing 

a wireless paging system for use in mines and in improving mine telephone 

communication. Data were taken from eight coal mines in the West 

Virginia-Pittsburgh area at 20 frequency steps between 2 kHz and 200 kHz. 

The West Virginia University group measured the horizontal and vertical 

field components of the electromangetic noise near DC and AC equipment 

and transmission lines. The conducted noise on trolley and telephone 

lines was also measured. Some attention was given to through-the-earth 

transmissions and from the experimental results and theoretical 

calculations an optimum system was proposed. This system transmitted 10 

W at 870 Hz from a 100 turn, 15 AWG wire antenna, 1 m in radius and was 

picked up by a 29 turn 0.4 m radius antenna. A range of 600 m should be 

realized but to increase the range to 750 m would require 100 W; to 

increase it to 1,200 m would require 10 kW (12). - 

As part of its ongoing work in subsurface location and communications 

the Westinghouse Georesearch Laboratory collected some data on 

electromagnetic noise in the frequency range 20 Hz to 5 kHz. The data 

was collected at four coal mines that were either closed down or only 

partly operational in order to simulate the quiet conditions of a 

postdisaster period. A portable minicomputer originally designed for 

seismic signal processing allowed real-time analysis of noise at the mine 

site. The Westinghouse group also collected, on a long-term basis, 

background electromagnetic noise produced by atmospheric sources and 

human activity at Boulder, Colorado. 
Review of these studies done by Bureau of Mines' contractors in 

electromagnetic noise were conducted by Arthur D. Little, Inc., (12) and 

the Institute for Telecommunication Sciences (6). Both studies were 

found deficient to a greater or lesser degree and the reviewers 

recommended a further program of research to correct the deficiencies and 

to provide coverage of frequencies not studied in the earlier work. 

Following these guidelines a National Bureau of Standards research 

group undertook the systematic study of the electromangetic noise 

environment of representative mines, six coal and two hardrock, to 

compile a "comprehensive library" of electromagnetic noise data. Data 

were recorded under a wide variety of combinations of mining conditions, 

equipment, and power supplies. It was found that for mines deeper than 

300 m, and an average conductivity of SM'~, low frequencies (100-500 

Hz) would have to be used to achieve adequate penetration. For depths 

approaching 3,000 m even lower frequencies were necessary. But these 

very low frequencies can transmit little useful information and 60 Hz 

interference becomes serious. Shallower depths or more conductive ground 

(approximately 10-I SM-') could allow the use of frequencies up to 5 kHz 

or even higher (184). - Signal power requirements for various depths, 

noise levels, and conductivities were calculated and the signal to noise 

ratios were predicted for various conditions (192). - The noise data 

recorded in the mines were random and required statistical analysis to be 

meaningful. 

Motohisa Kanda (172) - showed that five statistical measures had meaning 

in the analysis of electromagnetic noise in mines. The Allan Variance 

Analysis was used to determine how much data was needed. The Interpulse 

Spacing Distributions gave the probability distributions for the spacing 

of successive pulses in the received noise. These distributions are 

functions of the noise amplitude level. The Pulse Duration Distribution 

gave the probability distribution for pulse widths which are measured in 

terms of the percentage of pulses exceeding an arbitrary width in a given 

time period. The Average Crossing Rate gave the average number of times 

the noise amplitude level "crosses" an arbitrary level. The Amplitude 

Probability Distribution provided the fraction of a given time period 

that the electromagnetic noise envelope exceeded various arbitrary 

levels. This is the most common and useful measure of noise for the 

design of electronic communication systems. 

E. C. Field, Jr. and M. Lewinstein (105) - produced an 

"Amplitude-Probability Model for VLF/ELF Atmospheric Noise." They found 

that the amplitude-probability distributions could be represented in 

terms of two parameters. One of these characterized the "impulsivity" of 

noise from local noise sources. The other represented the ratio of 

energy in the impulsive-noise component to the energy in the 

background-noise component. Both of these can be measured at the 

receiver. Each of these concepts had potential applicability to trapped 

miner location systems because the very low signal strengths received on 

the surface were highly sensitive to background noise interference. 

A Colorado School of Mines group (122) - proposed in 1974 the possibility 

of using a system based upon the detection of a buried, passive loop 
excited from the surface as a means of trapped miner location. The loop 

could either be worn by or deployed by the miner. However, calculations 

showed that the ambient electromagnetic noise levels on the surface of 

most operating mines exceeded the fields radiated by a buried passive 

loop. The group suggested that a passive loop system be used only as a 

backup to an actively powered transmitter for trapped miner locat ion. 

*** WESTINGHOUSE - BUREAU OF MINES SYSTEM 

FIRST PHASE DEVELOPMENT AND TESTING 

Sponsored by a Bureau of Mines' contract and based upon the National 

Academy of Engineering's recommendations, the Special Systems Division of 

Westinghouse undertook the development and testing of an electromagnetic 

communications/location system in 1970-1971 (101). - Utilizing 

off-the-shelf technology as far as possible, the Westinghouse group 

developed a low power (4 W) beacon transmitter for mine refuge chambers 

with simple, keyed, signalling capabilities that could also be located 

from the surface. A powerful (1 amp) voice transmitter, using a long 

horizontal wire antenna laid on the surface, was built for downlink 

transmissions. A manpack voice receiver, with very low battery power 

drain was designed to be attached to the miner's cap light battery pack. 

Using this equipment, Westinghouse engineers tested the concept of 

through-the-earth communications at five coal mines in Colorado, 

Pennsylvania, and West Virginia at depths of 88 to 259 m. 

Preliminary work included determining the effective local earth 

conductivity at each site as well as measurement of the background noise, 

natural and man-made, in operating and in closed down mines. Voice 

transmission tests at various depths, and at various horizontal surface 

offset distances were made at frequencies from 300 to 3,000 Hz. The 

beacon uplink transmitter was operated at 1,375 or 2,750 Hz. Reliable 

uplink communications were established through a distance of 457 m, but 

only simple coded signals were possible. Downlink "intelligible voice 

communications" proved possible in every case to an underground mine 

shaft through a slant range of 443 m but not to one 867% distant. Very 

high quality communications were established through a 311% slant range. 

Depending upon local earth condictivity these results indicate a maximum 

usable depth for the Westinghouse equipment of 488 m using the manpack 

unit and 975 m using a heavier, high power, refuge or "Habitat" receiver. 

Little field degradation was found when the signal from the beacon uplink 

transmitter was detected on the surface at points distant from the spot 

directly above the transmitter. The equipment functioned properly even 

under adverse (below 0" F) weather conditions. The tests proved the 

possibility of through-the-earth location and communications systems 

using (then) current technology. About 90 pct of the coal mines in the 

United States were within the range of the Westinghouse equipment. There 

were some unresolved problems. The tests had shown the need for 

extensive earth conductivity studies in the areas of coal mines as well 

as a need to survey the electromagnetic background noise environment at 

various frequencies of interest around and within coal mines. It also 

appears that conductor effects were ignored in the early Westinghouse 

tests (353). - 
SECOND PHASE DEVELOPMENT AND TESTING 

In the second phase, Westinghouse engineers working under Bureau of 

Mines contracts, developed and tested a prototype through-the-earth 

location and communication system in 1972 and 1973 (101, --- 204, 236). The 

location system was based upon the detection, at the surface, of the null 

(minimum signal) of the horizontal magnetic field produced by a buried 

transmitter. Electromagnetic pulse as well as continuous wave, half 

wave, and full wave transmitters were tested. Numerous limitations had 

to be placed upon the underground electromagnetic transmitter design 

parameters. Fewer were required for the surface units where size, 

weight, simplicity of operation, intrinsic safety, and power consumption 

were less critical factors. 

The system used the miner's 4-V cap lamp battery pack as a power source 

and therefore the power drain was a critical factor. The design 

attempted to achieve the greatest transmission efficiency (power drain 

for a given signal strength) as well as the maximum possible transmitting 

moment (INA - where I = current, N = number of turns of a loop antenna, A 

= area of the loop) to weight ratio. The aim was to build a transmitter 

capable of producing a 10 dB signal to atmospheric noise ratio at the 

surface. The continuous wave transmitters proved to be much more 

efficient. It gave a 10 dB enhancement of signal strength which, for 

example, would allow a four-fold reduction in antenna weight for the same 

battery drain. Full wave continuous wave was safer too. Because the DC 

component delivered to the antenna was lower, there was less danger that 

the antenna lead power level could exceed intrinsically safe levels. 

The transmitter itself was miniaturized by using a printed circuit with 

a tuning fork oscillator and four switching transistors to obtain full 

wave signal to a resonated loop antenna. A 200 ms tone burst was 

broadcast every 2 s for a duty cycle of about 9 pct. In a test, a fully 

charged battery pack gave over 57 h of transmission. The transmitter was 

also tested for durability under extreme temperature conditions (-20" C 

to + 60" C) and for frequency stability (drift >2 Hz at 2,010 Hz). 

Recognizing that lower frequences gave superior through-the-earth 

penetration, the Westinghouse transmitter was designed to utilize the 

frequency range 300 Hz to 3,000 Hz. Because of man-made noise, the 

frequency domain below 1,000 Hz was not actually used. Instead, the 

transmitters were designed to operate at one preset frequency each in the 

1,000 Hz to 3,000 Hz range. In actual field tests at three mines, 

signals broadcast at frequencies near 1,000 Hz (1,050 Hz) were more 

severely affected by man-made noise than those between 2,000 and 3,030 Hz 

(2,010, 2,070, 3,030 HZ) and the maximum signal to noise ratio was 

obtained in this higher frequency range. 

Antenna design proved troublesome because of weight and intrinsic 

safety considerations. The main determinant of antenna performance was 

the question of intrinsic safety because it imposed limits on the output 

power. Weight was another major factor. Westinghouse attempted to 

maximize the antenna moment (INA) to weight ratio. But the output power 
is a function of the size and length (the weight) of the antenna and is 

itself limited to permissible values to be intrinsically safe. No 

perfect combination was possible; several were investigated. The best 

proved to be a horizontal loop antenna deployed in the mine. 

Westinghouse designers envisioned loop antennas semi-permanently mounted 

around coal pillars where they would be available to a trapped miner in 

case of a disaster. A portable, deployable long wire antenna was also 

designed. It was housed in a molded plastic package that also contained 

the transmitting unit. The container which was to be worn by the miner 

on his belt was built to be rugged enough to withstand the severe mine 

environments. 

Westinghouse designed two variations, fixed and variable frequency, of 

the surface receivers. The fixed frequency receiver came to be preferred 

because of its relative simplicity of operation, relatively low cost and 

portabilit Its sensitivity was far better than its design requirements 

(0.1 ampm-ij and its batteries were sufficient for 40 h of continuous 

operation. The antenna or receiving loop was a small (0.45 m diameter) 

multi-turn, hand held loop. An approximate underground location was 

first achieved on the surface by measuring the strength of the 

underground signal. The strongest signal would be detected on the 

surface above the source of the underground transmission. After roughly 

determining this point on the surface it was located precisely by 

rotating by hand the receiving loop to the position of minimum signal. 

This is normally done from at least two separate directions and the 

intersection of the two (or more) planes, defined by the receiving loop 

orientation, in the null position directly above the buried transmitter. 

Tests were performed by the Westinghouse group at three Eastern coal 

mines and one Western hard-rock mine. Conductivities at the coal mines 

were of the order of 1.1 x SM'~ and the maximum depth for a 

successful test was 300 m. The Western hard-rock mine had an average 

conductivity of 3.4 x SM'~. Tests at this mine proved the 

capability of the system to pinpoint a transmitter location through 119 m 

of overburden to an accuracy of 5 m. Severe weather conditions and steep 

hillsides above the test mines added difficulties and realism to the 

tests. At each mine, ground conductivities were determined and the 

electromagnetic noise in the frequency range of interest was recorded. 

The tests consisted of surface location exercises in which the position 

of a buried transmitter was determined from the position of the surface 

null and compared to that obtained by land survey. Field intensity 

measurements of the transmitted signal were made at varying radial 

distances from the surface point directly above the buried transmitter as 

far as the signal could be detected. A test was also made to determine 

if the location of one underground transmitter could be determined in the 

presence of another one, nearby, also broadcasting. The signals were 

easily separable and identifiable. The effects of sloping surface ground 

above the transmitter were also found to be insignificant in affecting 

location accuracies. An experiment was designed to test the effect of 

tilting the underground antenna from its normal horizontal orientation as 

could readily occur in a mine disaster. The antenna (at a depth of 

116 m) was laid upon a pile of rubble at approximately a 20' inclination. 
This shifted the apparent surface null position by 13 m, which could be a 

significant distance in mine rescue operations. Anomalous results were 

also obtained when metallic conductors (metal pipes, power cables, a 

fence line) were in the path of the through-the-earth transmissions. 

Attempts to detect the signal from a moving helicopter gave poor 

results. No signals could be detected from within either a reciprocating 

or a jet engine aircraft. However, when an antenna loop was hung 11 m 

beneath the jet powered helicopter it proved possible to detect the 

transmitter signal at an altitude of 38 to 76 m and at a position 30 m 

from the surface location. This opens the possibility of using 

helicopters for a rapid area survey to locate trapped miners whose 

posit ion is unknown. 

These Westinghouse tests proved the practicability of the designs. 

Some questions remained. Exactly what were the optimum frequencies? 

Could a two-wag communication system be designed? What could be done 

about the anomalous propagation effects of metallic conductors? Antenna 

design was an area open to further development. Finally, some method of 

location that was more precise and more predictable than homing on the 

null signal point was highly desirable. Could one be designed and built? 

FREQUENCY-SHIFT KEYING (FSK) BEACON SIGNALLER 

A related project was simultaneously undertaken by another group of 

Westinghouse engineers (10). The objective was to develop a durable, 

fixed location and commu~cations system which would have 

through-the-earth capabilitp and be available for normal use as well as 

for postdisaster mine rescue operations. It was to be connected to the 

mine phone system, to be capable of simultaneous through-the-earth and 

phone line transmission and to have paging capability. The system was 

also to be able to integrate a number and variety of underground sensors. 

All of this (up to 20 underground stations) was to come to a single 

surface station where the data were to be displayed, with visible and 

audible alarms and where centralized mine communications came under the 

control of one center for both normal and emergency use. 

Each underground station could be connected to the mine phone system 

but each was also capable of through-the-earth voice transmissions to the 

surface (uplink) using a voltage controlled oscillator to generate 

frequency modulated (FM) carrier frequency of 7 kHz. Under normal 

conditions the subsurface FSK beacon dignaler broadcasts coded signals 

from a variety of sensors. These operate on 1 of 20 assigned frequencies 

between 3,390 and 4,530 Hz. There is an emergency feature whereby an 

operator may manually depress one of six push buttons on the FSK Beacon 

generating automatically one of six emergency signals (Yes, No, Don't 

know, Repeat, Wait and Morse Code Follows). There is a separate Morse 

code button. There is also an emergency feature that, when turned on, 

produces a continuous wave signal for 20 s then cuts off for 20 s as long 

as the switch is on. This allows identification (it broadcasts at the 

local subsurface station frequency) and location from the surface. The 

FSK beacon utilizes the same antennas, surface and subsurface as the 

voice uplink. 
The surface stat ion contains receiving equipment for the voice uplink 

and display consoles for the subsurface condition monitoring system. It 

also has a voice downlink transmitter which uses a direct audio system at 

200 W. Special subsurface receivers were designed for this system. All 

underground equipment was hardened to approximately twice the 

instantaneous shock and temperature levels that a human can be expected 

to survive but it was not tested. The system itself was tested, 

satisfactorily, in January 1973 at the Bureau of Mines, Bruceton, Pa., 

experimental mine (10). The Westinghouse designed equipment was also 

tested (95) in a veF deep (915 m and 1,311 m) hard-rock metal mine 

(Galena Mine in Idaho). The system, designed primarily for coal mines, 

proved to be useful in the deeper metal mine. Location accuracies were 

46 m for the long wire antenna, 150 m for the horizontal loop and 140 m 

for the vertical loop at 915 m depth. An important anomalous effect was 

produced on transmissions by the deep mine shaft which was lined with 

various metallic conductors. Signals passing near the shaft utilized 

this route with a resulting enhancement of signal strength by a full 

30 dB. 

*** ANOMALOUS EFFECTS 

Field tests of through-the-earth transmissions conducted by a Colorado 

School of Mines research group at several mines with various combinations 

of antennas and frequencies often resulted in anamolous data because of 

the presence of underground conductors (108). - Sometimes these 

serendipitous conductors enhanced transmissions greatly but the 

scattering effects could also hinder efforts at precise location of the 

source of a transmission. 

The problem was an old one. Anomalous conduction effects had been 

found as early as the 1920's. B. A. Austin (14) pointed out that the 

South Africans with 20 to 30 yr experience in-&iderground radio had found 

that when metallic conductors (pipes, rails, power lines, etc.) were 

present, anomalous effects were the rule rather than the exception. 

Transmissions in the presence of metallic conductors were sometimes 

enhanced as much as 10-fold. The existence of conductors also enhanced 

transmission along intermittent or broken or buried conductors. Austin 

also pointed out that mine tunnels had a wave guide effect on 

electromagnetic transmissions which could be very significant and which 

were utilized by the South Africans (14). - 

Other investigators, particularly James R. Wait (291, -- 326) and D. A. 

Hill (142, -- 144) had already begun theoretical investigations of the 

problems created by the presence of buried conductors, particularly as 

they related to the problem of precisely locating an underground 

transmission source. In a series of studies Wait and Hill derived 

mathematical solutions to describe the effects of buried conductors of 

various shapes, sizes, and inclinations upon electromagnetic fields in 

the earth as well as for transmission along such conductors. They 

concluded that detection and location from the surface of a trapped mine] 

equipped with a special transmitter was feasible within certain limits 

and that the experimental data being amassed was in good agreement with 

theoretical predictions. 
Raymond D. Watts (352) of the U.S. Geological Survey used the Fast 

Fourier Transform (~m to compute the electromagnetic fields scattered 

by a long straight conductor (pipe, cable, rails, etc.) within the earth. 

Watts considered the currents induced in the buried conductors by a plane 

wave electromagnetic source field. The induced currents give rise to 

secondary radiated fields from which the position and depth of the 

conductor can be determined. Watts also derived a method for determining 

the effect of more than one buried conductor and their possible 

interaction. 

C. H. Stoyer (270) - analyzed the case in which the buried conductor 

presented two long dimensions to the surface such as in the case of a 

buried mass of conducting metallic ore or a trapped miner with a loop 

antenna. The loop is realized as a vertical magnetic dipole if the 

surface is far enough away. A conductor (ore body, pipe, rail, cable, 

etc.) in its path will have its field on the surface distorted. Stoyer 

used a finite-dif ference method to predict the surface magnetic fields in 

terms of the magnetic field polarization ellipse parameter. The error 

involved in locating the source of the transmission is "quite small." 

But R. J. Greenfield (132) - pointed out that small errors could have 

substantial effects particularly in the case where there were large 

lateral changes in ground conductivity along a transmission path. An 

anomalous effect of vertically varying conductivity could be useful in 

both normal and emergency mine communications as R. L. Lagace and A. G. 

Emslie pointed out. A coal seam, of relatively low conductivity, bounded 

by more highly conductive rock can serve as a mode of transmission for 

medium frequency radio waves. While the signal attenuates rapidly in 

coal, if there is a metallic conductor nearby, the signal will be coupled 

to it. It then follows a low attenuation mode guided by the metallic 

conductor which is the same effect noticed earlier by other 

investigations. 

Investigation of three types of anomalies became part of a later study 

by T.S. Cory (67) undertaken for the Bureau of Mines primarily to gather 

data for the dGelopment of middle frequency for mine face area 

communications. Cory found that wireless communications in the absence 

of nearby metallic conductors was equivalent to lateral through-the-earth 

communication. The transmission efficiency was enhanced by nearby 

conductors. Ranges of 600 m were regularly obtained if metallic 

conductors were located near the signal source or receiver. In their 

absence, the reliable ranges were reduced to 100 to 300 m. If both 

transmitter and receiver were close to metallic conductors (the same 

entry, for example, as a mine trolley) then ranges of several kilometers 

become possible. Cory found that the optimum frequency (in the middle 

frequency range) for conductor-free transmission was 520 kHz and for 

conductor-assisted transmissions 700 to 1,000 kHz. 

'In a related study a group from West Virginia University, C. A. Balanis 

and others, (15-16) studied the conductivity of coal as a function of 

frequency in the microwave range. Wait (303) - pointed out that 

theoretically a coal seam can be visualized as a resistive layer bounded 

by conductive rock. If the source is a vertical electric dipole then the 

dominant mode of propagation has relatively low attenuation. The 
possibility then was clearly open, of using the coal seam itself as a 

medium of transmission of radio frequency signals. 

The effect of adjacent conductors on the propagation of radio waves in 

a coal seam was treated theoretically by D. C. Chang and J. R. Wait in 

1980 (51). They found, as had been discovered in actual mines, that 

attenuzion was lower in the presence of metallic conductors and derived 

mathematical approximations for low and medium frequencies. 

Practical use of serendipitously situated conductors is also beginning. 

ARF Products, Inc., has developed (1981) a voice communication system for 

within mine use (102). - The ARF system uses low and medium frequencies 

which are coupled into adjacent conductors such as mine wiring, pipes, 

cables, and rails. Commercial systems are expected to be available from 

ARF in the future. 

*** FIELD TESTING AND HARDWARE EVOLUTION 

Some actual field tests seemed to give promising results. D. Kalvels 

of Westinghouse (171) - under a contract for the Bureau of Mines tested an 

earlier developed trapped miner location device in over 36 mines at 

depths ranging from 60 to 430 m at the frequencies, 630, 1,050, 1,950, 

and 3,030 Hz. Voice signals were successfully transmitted through 213 m 

of rock. The research program measured the transmission loss of signals 

through the overburden and a data base of this information was compiled. 

As predicted, the transmission loss increased with the length of the 

overburden path and with the frequency. The study also found that uplink 

transmission loss characteristics could not be extrapolated from downlink 

values probably because of the difference in the types of antennas used 

on the surface and within a mine. 

Another Westinghouse group (A. J. Farstad) (99) attempted to apply the 

through-the-earth communications system develoFd for the Bureau of Mines 

to metal and nonmetal mines. Tn these mines the depths are usually much 

greater and to achieve coverage of 90 pct of the mines a 

through-the-earth capability of 900 m was needed. There is also extreme 

variability in the conductivity of the rock strata of these non-coal 

mines. In particular, metal and salt mines are likely to have much 

higher than normal conductivities. Furthermore, the underground geology 

and topography are likely to be much more complex and less predictable. 

Penetration of the earth can be achieved using VLF (10 Hz) and very long 

(long compared to the overburden) wire receiving antennas on the surface. 

Direction finding, Farstad suggested, could be done by deploying a grid 

of long wire antennas on the surface. 

Other tests and analyses were made of the Bureau of Mines1 system. Tn 

1974, Collins Radio investigators (144) - determined that through-the-earth 

systems were impractical. They required too much power to penetrate the 

earth. The large power supplies also may not be intrinsically safe. The 

needed antenna size is too large to be practical in any portable form and 

too large to be readily deployed. The "prime" use, said the Collins 

Radio group, was one-way paging of individual miners from the surface 

where large fixed transmitters and antennas could 
be used. They suggested that a loop antenna (20.3 cm diameter) could be 

wound in the miner's hard hat and they also suggested the systematic use 

of anomalous conduction by metallic conductors such as rails, pipes, etc. 

Another suggestion called for the creation of an emergency mine 

communication system designed exactly like the normal mine communications 

and located in the same places for familiarity under the stress of a mine 

emergency. 

Concurrent with the Westinghouse studies, a research group from the 

Colorado School of Mines was conducting similar studies for the Bureau of 

Mines (122). - At a variety of mine test sites the effective earth 

conductivity was measured for various depths and locations. The 

electromagnetic noise environment was recorded and analyzed both within 

the mines and on the surface. A pulse transmitter was also developed and 

tested for downlink communication through the earth, using a step current 

which excited a long wire (223 m) antenna. IJnderground, the transient 

signal was picked up on a long wire antenna at a depth of 23 m. While 

the pulse system gave advantages in signal compression, the ambient 

electromagnetic noise levels were a problem. Field tests were made of 

the transmission through the earth of LF signals (20 Hz to 20 kHz) by 

vertical axis loops and horizontal wire antenna and the results compared 

with theoretical predictions. 

Hardware development also continued. Collins Radio engineers under a 

Bureau of Mines' contract (62-63) -- built 50 miniature waveform generators. 

These were designed to drive a LF loop antenna and to be a part of the 

electromagnetic trapped miner location system. The miniature (3.6 by 

0.9 by 8.8 cm) waveform generators were securely encased ("potted"), 

could operate under water, and were capable of being produced 

economically on a large scale. They were designed for a single preset 

frequency each in the range 1,050 to 3,030 Hz and had a 10-pct duty 

cycle; that is, they generated signals of 100-ms duration about 10 pct of 

the time and were off the rest (58). 

R. L. Lagace and others of Arthur D. Little (187-188) -- did substantial 

work before 1978 in the development of antennas for in-mine use, both for 

through-the-earth communications and for normal use in mining operations. 

The problem of antenna development was complicated by the fact that at 

the frequencies of most interest (VLF to MF) the wavelengths are 

extremely long while the antennas, for practical reasons, must be 

physically small. Trade-offs among the various constraints were 

necessary to achieve the maximum possible transmission efficiency. 

Other trapped miner transmitters and transceivers were developed and 

tested by General Instrument Corporation under a Bureau of Mines' 

contract from 1978 to 1981. Breadboard units and prototypes were built 

and tested and finally 95 transmitters and 12 transcievers were produced. 

The frequencies of the units were 630, 1,050, 1,950 or 3,030-Hz. Some of 

the units were built with aluminum wire instead of copper to save weight 

and several were built integrally as part of the miner's cap and battery 

pack. Representative units were tested environmentally. An improved 

version was developed by General Instrument Corporation (48) for the 

Bureau of Mines and subjected to extensive testing. ~histra~~ed miner 
electromagnetic signalling and detection system was based upon the 

belt-worn miniature transmitter, which, in an emergency, would be 

attached to the cap lamp battery for power. The transmitter would drive 

a loop antenna which consisted of 91 m of #18 copper wire, normally 

carried on the belt, and deployed in an emergency around a coal pillar or 

in a passageway. The Collins Radio-General Instrument Corporation system 

was field tested by the Bureau of Mines (J. Durkin) and Westinghouse 

(R. F. Kehrman, A. J. Farstad, and D. Kalvels) under a Bureau research 

contract . 

The Bureau of Mines' researchers (82, -- 189) tested the system in coal 

mines throughout the United States and used the data to develop models of 

the signal and noise distributions that would be found on the surface 

above coal mines throughout the country. These signal and noise 

distributions were then used to estimate the probable performance of 

through-the-earth electromagnetic communications. The expected 

probability of detecting a miner's signal from a depth of 305 m is 54 

pct; this depth exceeds 90 pct of all American coal mines. The expected 

probability of detecting a signal from 153 m is 95 pct; this is deeper 

than 50 pct of American coal mines. Improvements in the signal to noise 

ratio (sNR) greatly enhances the probability of detection. For every 

3-dB improvement in the SNR, the probability of detection increases 6 to 

8 pct. The Westinghouse researchers (176) - generally obtained better 

results which presented a more optimistic view of the "reliability and 

effectiveness" of the system tested. Successful communications were 

established in 91 of the 93 Eastern and Western coal mines, where tests 

were conducted through overburden thicknesses ranging from 21 to 472 m. 

The frequencies used were 630, 1,050, 1,950, and 3,030 Hz. The 

conclusion was drawn that the comunications system would work in 

practically all mine environments. The Westinghouse resarchers were not 

so sanguine about the trapped miner location system whose reliability and 

accuracy depended upon the ambient electromagnetic noise, the depth 

involved and the possible presence of metallic conductors which might 

distort the signal. The strength of the through-the-earth signals 

reported by the Westinghouse scientists proved to be higher than those of 

the Bureau of Mines; running 10 to 20 dB higher at the higher (1,950 and 

3,030 Hz) frequencies. An independent evaluation of the conflicting data 

was made by workers at Arthur D. Little, Inc., from 1978 to 1980 (190). - 

They concluded that the Bureau of Mines measurements had been more 

accurate due to a systematic bias introduced by Westinghouse equipment or 

procedures and that the expected probability of detection derived by 

Bureau scientists was realistic. 

*** RESEARCH IN COMMUNICATION TECHNIQIJE S 

Work has also been done under Bureau of Mines sponsorship to improve 

the detectability of through-the-earth signals. Enhancement in detection 

can mean greater depths through which a system may operate as well as 

greater reliability. 

Voice bandwidth compression techniques were applied in 1975 to mine 

wireless communications by R. H. Spencer and W. G. Render (265) - with 

disappointing results. Narrower bandwidths would allow less 
electromagnetic noise interference with the received signal. But it 

required a higher SNR for intelligibility and therefore more transmitted 

signal power. Sophisticated techniques to overcome this problem are 

expensive. In a related study, Penn State investigators in 1977 (218) - 

evaluated the Bureau of Mines-developed downlink through-the-earth 

communications system by testing the intelligibility of human speech 

broadcast through the system. They used a modified rhyme test in which 

tapes recorded with standardized word patterns recorded at known, 

calibrated levels were broadcast and the received signals evaluated for 

intelligibility. Computer simulations were used rather than actual 

in-mine tests. These simulations indicated that the system would be of 

dubious reliability at a depth of over 300 m even at optimum frequency. 

A group of researchers at the University of Michigan's Cooley 

Laboratory developed in 1979 and 1980 an automated emergency mine 

communication system under a Bureau of Mines contract (248). - The system 

utilizes a unique (repeating pseudonoise) signal and a sensitive 

detection system that includes matched filter detectors. This is coupled 

with sophisticated signal processing techniques including the use of a 

new decision-variable algorithm to estimate (and allow elimination of) 

the noise between the signal peaks. The new system proved capable of 

operating, in the laboratory, with an input SNR of -34 dB. The system 

developed by the Bureau of Mines required an input SNR of +6 dB. The 

difference is 40 dB. It was also determined by the investigators from 

earlier Bureau of Mines propagation studies that the received signal 

power for through-the-earth transmissions decreases to the tenth power of 

distance. Hence, a 40-dB improvement in needed input strength would mean 

an increase in range by a factor of 2.5. The greatest improvement comes 

from the fact that the Bureau of Mines system uses the human ear as a 

detector, while this prototype uses an automated system. The new 

technique, however, was a laboratory evaluation and demonstration which 

did not undergo actual field tests. 

*** IN-MINE COMMUNICATION SYSTEMS 

Dual purpose in-mine communication systems have been suggested and 

their use investigated by the Bureau of Mines (53). Such a system would 

function as a normal in-mine working communication system and as an 

emergency system in a postdisaster scenario. These systems usually 

operate at higher frequencies than the trapped-miner through-the-earth 

systems and they often deliberately utilize the anomalous conduction 

paths provided by metallic conductors along the line of propagation. 

Recognizing the need for a broader range of in-mine communications, 

Nelson and Johnson Engineering in 1981 proposed a more comprehensive 

system (102). - Among the functions that a complete mine communication 

system could perform are monitoring (for example, methane, rock bursts, 

flooding, carbon monoxide), point to point, postdisaster and point to 

mining face communications (or communications with remote sections). A 

computer controlled system was designed which was connected to a network 

of underground terminals located at strategic points. These terminals 

routinely emit radio frequency pulses to interrogate the miniature 

transponders worn by miners or attached to equipment or vehicles. The 

transponders would be constantly powered by float charged equipment 

batteries or by the miners 4-V cap lamp battery. The transponders would 
respond to the interrogation with a uniquely programmed coded signal so 

that the underground radio frequency terminal can identify the individual 

transponders. The underground terminals are then interrogated by the 

main computer on the surface which keeps a running account of underground 

conditions and personnel. There was a favorable reaction to the proposal 

from most of the nine mine operators contacted, particularly from the 

larger operations, and the Mine Health and Safety Administration (MHSA), 

but there was some opposition expressed due to the cost and complexity of 

the proposed system. 

Propagation studies under a Bureau of Mines' contract were undertaken 

in several low (91 to 122 cm) coal mines at medium frequencies (98 to 

4,050 kHz) by a Collins Radio communications switching system team in 

late 1977 and 1978 (66). The primary work was the measurement of the 

magnetic field strenzh which was taken so that it characterized the 

radio transmission properties of each of the five mines and four coal 

seams tested in Pennsylvania, West Virginia, and Kentucky. The magnetic 

field strength measurements were taken as a function of range and 

frequency. Tests were conducted both in "quasi-conductor free mine 

areas'' and in proximity to metallic conductors. The tests were run under 

conditions of negligible ambient electromagnetic noise from mine 

operations because the mines were not operating. Maximum communication 

ranges were calculated from the magnetic field strength versus range and 

frequency data under assumption of low ambient electromagnetic noise 

(set-noise-limited) and the levels of noise (median mine noise) expected 

during normal mine operat ions. The communication ranges for the 

set-noise-limited example varied from 180 to 260 m in the frequency range 

200 to 300 kHz; the ranges varied from 115 to 140 m between 400 and 

800 kHz. 

*** TANGENTIAL RESEARCH 

Research in areas tangential to through-the-earth communications often 

proved valuable to investigators. A group from Stanford Research 

Institute (771, in an effort to determine if there were hidden chambers 

in the ~~~~xan pyramids, and for other archeological uses, developed a 

high frequency (16 to 50 MHz) electromagnetic sounder. This was tested 

in a California dolomite mine before being used in Egypt. Using the 

Wawona Tunnel at Yosemite National Park, R. J. Lytle and D. L. Lager 

(209) - studied the transmission of high frequency (3 to 50 MHz) radio 

waves through 300 m of hard rock and determined the bulk conductivity of 

the rock (2 to 5 x 10'~ SM-~). T. Y. H. Caffey (43) devised a miniature 

telemetry link capable of receiving data from sensors buried up to 50 m 

below the surface. He also built a control link capable of turning the 

telemetry transmitter on or off from the surface. A matched filter 

detection system designed by University of Wisconsin engineers (57) to 

correct distort ions in through-the-earth electromagnetic transiez 

signals used in geophysical work offered the possibility of being useful 

for the precise location of trapped miners. 

There was other work in areas common to both electromagnetic 

geophysical methods and through-the-earth communications and location. 

N. Harthill (137) - reported on time domain electromagnetic sounding and 
T. Lee (200) - on the "Estimation of Depth to Conductors by Use of 

~lectromagnetic Transients," both of which had dual applicability. 

D. F. Moore and E. A. Quincy (221) - wrote on the statistical (Bayes) 

classification of subsurface electromagnetic responses while Ling to 

Szeto devised a minicomputer program for analyzing 

time-difference-of-arrival data. 

Another area was the use of supersensitive receiving equipment on the 

surface. One, which grew out of antisubmarine warfare research, was the 

cyrogenic SOUID (superconducting quantum interference device) 

magnetometer. J. E. Zimmerman and W. H. Campbell (362) - developed several 

varieties of SQUID in semi-portable forms. Field tests and 

standardization tests showed the system to be "reliable, accurate, 

portable, and simple to operate in geophysical operations. Directional 

measurements of natural magnetic field fluctuations as small as 0.0001 

gamma with periods from 0.5 seconds to several hours" were obtained. 

Liquid helium, used to cool the SOUID to superconducting ranges, 

presented problems of storing and handling. Zimmerman and Campbell 

concluded that present (1975) and nearly achieved technology will allow 

the storage, within SQUID devices of liquid helium for a period of weeks 

or months; and thus make it a practical geophysical tool of extreme 

sensitivity. Because of its sensitivity it held particular interest for 

through-the-earth location efforts particularly in the case of great 

depths, or if passive devices were to be used underground to signal the 

trapped miner's presence. 

Military interest in LF communications had application to mine 

through-the-earth location and communication systems. Research for the 

U.S. Navy's Seafarer Project SANGUINE, included the design of a noise 

processor for the SANGUINE receiver. Basing their design on extensive 

world-wide recordings of ELF (3 to 300 Hz) atmospheric noise, J. E. Evans 

and Andrew S. Griffiths (87) in 1974 suggested using nonlinear processing 

to suppress the effect of'-iarge, random noise "spikes" in the ELF range 

as well as "notch" or narrow band filters at the frequencies of man-made 

interference. A computer simulation of the noise processor gave an 

enhanced performance of 15 to 20 dB over a linear receiver in high Level 

noise periods. 

*** CURRENT RESEARCH 

Presently, research is progressing into separate areas for 

through-the-earth location and communication systems. Much of it is 

based upon past research and earlier recommendations for future research. 

Each of the methods suggested utilizes a microcomputer as an essential 

element. 

DEVELCO SYSTEM 

DEVELCO engineers (249) - in 1979 began the development of a location 

system for trapped miners in deep mines that would give accurate location 

results through 1,000 m of overburden. The DEVELCO group recognized that 

the previously developed loop antenna was nearly of optimum design 

because of the weight, cost and intrinsic safety factors that limited its 
size. Improvement could possibly come through better beacon design 

including the use of a high efficiencv transmitter, of larger diameter 

wire for the loop antenna, lighter metal (aluminum, for example) for the 

antenna, and the development of better and lighter batteries (lithium). 

The system proposed by DEVELCO included taking vector magnetic field 

measurements of the surf ace magnetic field produced by the underground 

transmitter at three or more surface locations. Then using iterative 

techniques the data can be solved for the transmitter location. DEVELCO 

would use a microcomputer with necessary peripherals for this. Accurate 

location measurements require "static, precise field sensors in a 

carefully surveyed array," which can be either permanent or 

semi-portable. The key problem, according to DEVELCO engineers, is that 

of detecting very small signals in the presence of high noise levels and 

getting enough information from those signals for accurate location 

measurements. It can be done, DEVELCO says, by using sophisticated 

communication techniques including noise processing, coherent detection 

methods and special modulation. 

PHASE DIFFERENCE OF ARRIVAL TECHNIQUES 

A research group at the U. S. Bureau of Mines' Tuscaloosa Research 

Center developed a through-the-earth location system based upon phase 

difference of arrival techniques. Operating with continuous wave radio 

signals, whose phase differences can be accurately measured even when the 

signal is weak, location is automatically determined by a microcomputer 

which is an integral part of the system. The system is based on 

multiplexed transmission frequencies, an array of surface receivers to 

measure the phase differences of the transmitted signal and a 

microcomputer for real time location calculations. In the first phase, 

computer modeling of the propagation of electromagnetic fields 

through-the-earth was undertaken. Equations were derived, based on phase 

difference of arrival techniques, to enable the location of a trapped 

miner, assuming uniform conduction of the overburden and a point source 

transmitter. Sufficient field testing was not conducted prior to the 

project ' s terminat ion. 

AUTOMATED THREE-DIMENSIONAL LOCATION SYSTEM 

Another suggested method of locating trapped miners gives promise of a 

high degree of accuracy in location position. A system developed by 

F. H. Raab and associates (242) - used a three-axis magnetic-dipole source 

and a three-axis magnetic sensor to determine the position and 

orientation of the three-axis magnetic dipole source. The magnetic 

source generates a LF or quasi-static dipole field exciting the 

three-axis magnetic sensors which give sufficient information to allow 

the determination of both the position and orientation of the sensors 

relative to the source. This signal is amplified and fed into a computer 

where it is used to update previous coordinate measurements of the 

position and orientation of the three-axis magnetic dipole source. 

Raab's group recognized that the presence of nearby metallic conductors 

could seriously affect the accuracy and reliability of their system but 

their calculations indicated that the problem was of manageable 

proportions. 
Another new automated trapped miner location system was developed by 

Polhemus Navigation Sciences in 1978 and 1979 under a Bureau of Mines 

contract (243-244). -- It was based upon the phase quadrature technique and 

involved the retransmission of an ELF signal by an underground 

transceiver. In this system, three traneportable three &is 

magnetic-dipole transmitters are deployed on the surface. The three 

transmitters receive timing and control signals from a centrally located 

van-mounted control system through a very high frquency (VHF)/FM radio 

link. Each transmitting antenna contains three mutually orthogonal wire 

loop elements which when properly excited produce a quasi-static magnetic 

field whose phase depends upon the position and orientation of the 

signal. The ELF (13 Hz for typical coal mine depths) downlink signal is 

received by the subsurface receiver and used to modulate the uplink 

carrier with the received downlink signal. The subsurface 

receiver/transmitter is similar to the beacon transmitter previously 

developed for the Bureau of Mines by Westinghouse. It operates in the 

ultra low frequency range (600 Hz to 3 kHz) for typical coal mine depths. 

On the surface, the uplink receiver uses a three-axis antenna to receive 

the signal and extensive digital signal processing based on a small 

computer to integrate data, estimate the subsurface position and control 

the system operation through the VHF/FM ratio net. Having demonstrated 

the feasibility of their system, Raab, Hansen, and their collaborators, 

plan to build and test breadboard models and then field test the system. 

ADAPTIVE NOISE CANCELLATION 

LF radio signals, especially in the ELF-VLF region, can penetrate the 

earth much more readily than those of higher frequency. However, in the 

ELF-VLF region atmospheric noise becomes a major problem. Most of the 

noise comes from thunderstorms around the world. Because of the 

extremely low attenuation of ELF-VLF radio waves, even far away 

thunderstorms make a contribution to the background radio noise. Nearby 

thunderstorms have a stronger effect. The background electromagnetic 

noise at ELF and VLF is distributed according to a Gaussian distribution 

with sharp impulses, caused by nearby thunderstorms, superimposed on this 

background. Signal processing must therefore contend with both the 

steady background and the impulsive noise. 

By the 1970's much basic research had been done in the ELF-VLF range 

(see above) particularly in regard to the U.S. Navy's Seafarer Project 

SANGUINE and by geophysicists (163, -- 273). The geophysicists were 

interested in using the low radio frequency ranges for subsurface 

exploration and, in fact, the Adaptive Noise Cancellation (ANC) system is 

based on magnetotelluric techniques used in geophysical exploration. In 

the magnetotelluric method typically a remote receiver and antenna are 

used to provide a reference signal to be used for noise cancellation in 

signal processing. Because these wavelengths are extremely long the 

background noise waveforms are essentially the same at considerable 

distances of sevaration between the reference receiver and the actual 

receiver (107, - -- i16-117, - 273). The coincidental noise signals, 

representing - the background electromagnetic noise can - be made to cancel, 

using computer assisted signal processing. This leaves the desired 

signal, greatly enhanced. 
However, because the background electromagnetic noise contains 

impulsive elements a simple integration (cancellation) of the signal and 

reference signal will not produce an adequate signal-to-noise ratio over 

an appreciable time period. Instead, ANC techniques have been developed 

and applied. 

ANC techniques are based upon the use of a distant reference receiver 

to provide a reference signal that is essentially free of the desired 

signal but comprised almost exlusively of background noise. This is used 

in a sort of feed back mode to cancel that portion of the signal that is 

due to background noise. The signal comparison is carried out 

dynamically by a computer. The result is, usually, a vastly increased 

gain in the signal strength of the desired signal (28, ---- 73, 75, 118, 

------ 129-130, 220, 240, 246, 357). 

Green Mountain Radio Research Company (F. H. Raab and associates) 

under a Bureau of Mines' contract undertook to apply ANC techniques to 

the through-the-earth communication and location problem (239). - The 

first phase of the study, now complete, involved developing a computer 

model for the multicomponent (Gaussian plus impulsive) ELF 

electromagnetic noise and the development of the computer software needed 

for noise correlation-cancellation and the review of ANC algorithms. 

Raab and his associates recognized that the ELF needed to penetrate 

the earth imposed special conditions and afforded special opportunities. 

It meant, for example, that the field produced at the surface by a buried 

transmitter was essentially a pure quasi-static magnetic dipole field and 

that the associated electric field was negligible. A local electric 

field antenna then would receive no signal but only propagating noise. 

Thus it could be used as a reference source of correlated noise for ANC 

(241). - Conversely, the amplitude of the magnetic field falls off very 

rapidly with distance (signal loss of 60 dB for a distance change of 1 to 

10 km), so that a remote magnetic field sensor can provide an essentially 

signal-free source of correlated noise to use in ANC. The researchers 

predicted that noise reductions in the order of 20 to 30 dB are possible 

by ANC techniques (239). - Hill and Wait also derived related theoretical 

noise and propagat ion models (154). - 

*** NATIONAL RESEARCH COUNCIL EVALUATION 

Ten years after the original 1970 National Academy of Engineering 

report ("Mine Rescue and Survival") an evaluation of the progress made 

was undertaken by the National Research Council. The Council consists of 

representatives from the National Academy of Engineering, the National 

Academy of Science and the Institute of Medicine. In surveying the 

effectiveness of the work done by the Bureau of Mines and its contractors 

in postdisaster survival research, the Council report was generally very 

favorable. 

While the postdisaster work of the Bureau of Mines made up only a 

small part of its research budget (5.5 pet) during the 19701s, its work 

had "clearly shown the practicality of communicating with and locating 

trapped miners." There had been "significantu advances in theoretical, 
analytical, and experimental aspects and in design of hardware for 

postdisaster use. In general, the Council concluded that the Bureau of 

Mines' research effort "has more than met the recommendations of the 1970 

NAE report ." 

The Research Council report found some problem areas. There was a lag 

between the research and development process and implement at ion in 

industry. Hardware and techniques developed bv research are not being 

usefully applied in the mining industry. There is a "lack of a clear 

policy or methodology for the transition from basic research to applied 

research, development, demonstration, testing, and ultimately, 

implementation." The Council added that it was unclear how much of this 

transition was considered to be a responsibility of the Bureau of Mines. 

The Research Council also suggested that, in the future, more emphasis 

be placed on exploratory or basic research and less on equipment 

development and demonstration. It also said that insufficient research 

attention had been paid to the problem of communications between trapped 

miners and underground rescue teams. Finally, the Research Council 

recognized that the development of inexpensive digital circuits and dense 

packaging techniques in the 1970's has made possible the use of 

specialized microcomputers in through-the-earth systems. The Council 

suggested that these systems can be adapted, on short notice, to 

conditions in individual mines. They could provide sophisticated signal 

processing techniques to minimize ambient noise and to compensate for 

peculiar local conditions that affect radio transmission and reception 

(228). -

*** SUMMARY 

Trapped miner, through-the-earth location and communication systems 

have been under development for many years. Researchers have succeeded 

in deriving theoretical models which describe the transmission of 

electromagnetic energy through the earth accurately. The mechanisms of 

the ambient background noise have been investigated and a large body of 

data has been accumulated describing the noise under actual mine 

conditions. There also has been a large amount of research done to 

accumulate a large body of knowledge on the variable earth electrical 

characteristics such as conductivity. Prototypes of location systems 

have been built and tested successfully and from these tests a second 

generation of trapped-miner location systems is evolving. 

Current research seems to be approaching the successful development of 

a workable system which is small, low powered, inexpensive to produce, 

intrinsically safe, rugged and reliable, and which has vastly superior 

earth penetrating capabilities. Much more sensitive receiving equipment 

and integrated minicomputers have made the requirements for underground 

equipment much less stringent. Coupling trapped miner location systems 

to small computers makes possible rapid and accurate location 

computations and is a vigorous field of research at present. 
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