RESEARCH ON THE TRANSMISSION 

OF ELECTROMAGNETIC SIGNALS 

BETWEEN MINE WORKINGS 

AND THE SURFACE 

Richard G. Geyer 

George V. Keller 

Takashi Ohya 

Prepared for: 

DEPARTMENT OF THE INTERIOR 

BUREAU OF MINES 

WASHINGTON, D.C. 

Colorado School of Mines 

Golden, Cobrado 80401 
USBM Contract No. H0101691 

"Research on the Transmission of Electromagnetic 

Signals Between Mine Workings and the Surface" 

Richard G. Geyer 

George V. Keller 

Takashi Ohya 

Colorado School of Mines 

USBM CONRACT REPORT H0101691 

Date: January 10, 1974 

DEPARTMENT OF THE INTERIOR 

BUREAU OF MINES 

WASHINGTON, D .C . 

NOTICE 

The views and conclusions contained in this document are those of the 

authors and should not be interpreted as necessarily representing the official 

policies or reccmendations of the Interior Department's Bureau of Mines or of 

the U .S. Government. 
FOREWORD 

This report was prepared by Richard G. Geyer under USBM Contract No. 

H0101691. The contract was initiated under the Coal Mine Health and Safety 

Research Program. It was administered under the technical direction of the 

Pittsburgh Mining and Safety Research Center with Mr. Howard E. Parkinson 

acting as the technical project officer. Mr. Alan Granruth was the contract 

administrator for the Bureau of Mines. 

This report is a sununary of the work recently completed as part of this 

contract during the period June 15, 1970 to October 1, 1973. 

This technical report has been reviewed and approved. 

We wish to acknowledge the advice, comments, and encouragement given over 

the past three years by Mr. Howard E. Parkinson, Dr. James Powell, and Mr. John 

Murphy of the Pittsburgh Mining Research Center relating to the work reported 

here. We especially wish to acknowledge the helpful advice and assistance given 

by Dr. James R. Wait of CIRES. 
ABSTRACT 

One aspect of a program to improve the chances of survival following coal 

mine disasters is the development of a communications system which will allow 

surviving miners to make their circumstances known to rescue teams. At present, 

it appears that an electromagnetic system may be most practical. 

The development and design of an effective electromagnetic communications 

link for signalling between mine workings and the surface requires knowledge of 

pertinent physical properties of the rocks overlying the mine workings and the 

characteristics of the ambient noise against which a communications signal must 

be recognized. Knowledge of the overburden electrical transmission properties 

is useful not only for determining a practical operating transmitter frequency 

at a given mine, but also for ensuring the best possible electromagnetic coupling 

for routine or emergency communication purposes. 

Field studies of the electrical transmission properties of the overburden 

of coal mines in Colorado, Illinois, and Pennsylvania generally show that a relatively uniform and unimodal electrical conductivity distribution exists provided 

that the region has not been subject to considerable structural or metamorphic 

deformation such as that observed in the Gary District of West Virginia. Measurements of ambient surface electromagnetic noise levels at several mines in Colorado, 

Illinois, and West Virginia were taken and analyzed and are presented in the form 

of amplitude histogram plots both as a function of frequency and time of day. 

These amplitude histograms may be used in conjunction with overburden resistivity 

data in implementing an uplink ccnununications beacon system which will yield a 

specified signal-to-noise ratio at the surface. 

Numerous C.W. field transmission tests were made in various coal mining 

environments with use made of either a buried vertical-axis transmitting loop or 

a short insulated grounded current line. These transmission tests were made from 

20 Hz to 20,000 Hz in frequency and at several geographic surface receiving sites. 

The results of these field measurements show, in general, good agreement with 

theoretically calculated results. Since the produced surface magnetic field from 

a buried vertical-axis loop source or the produced surface horizontal electric 

field from a buried horizontal current line show better agreement with theoretical 

results, they could be used for beacon location system. 

One criterion would be in detecting the equatorial and polar null lines of 

the horizontal electric field component which is normal to the direction of 

emplacement of a buried transmitting line current source. A second criterion 

would be in the surface mapping of the maximum in the vertical magnetic field 

directly over a vertical-axis loop transmitter and the associated null in the 

vertical magnetic field at a horizontal offset distance which is about 1.4 times 

the depth of burial of the transmitter. Finally, a third criterion would lie in 

the surface mapping of the null in the horizontal magnetic field directly over 

the vertical-axis beacon and the associated maximum in the horizontal magnetic 

field at an offset distance from the transmitter axis of half the depth of burial 

of the loop transmitter. 

In mapping the vertical magnetic field over a buried vertical-axis loop 

transmitter, the size effect of the transmitter loop must be taken into account 

i i 
when the separation distances between the transmitter and receiving sensor are 

less than ten times the effective radius of the source loop; increasing the size 

of the transmitting source relative to the source-receiver separation distance 

shifts the null in the vertical magnetic field to greater offsets. In mapping 

either the vertical or horizontal magnetic field over a buried vertical-axis loop 

transmitter, care should be exercised in ascertaining that measured nulls and 

maxima are along radials through the axis of the buried source loop. 

For the main part, coupling experiments show that ambient electromagnetic 

noise in coal mining districts is less of a problem above 1000 Hertz. But 

secondary and perhaps undesirable maxima and null phenomena (as well as penetration problems) would occur in many coal mine provinces in the behavior of the 

surface horizontal magnetic field (over a buried vertical-axis loop transmitter) 

if transmission frequencies as high as 10 kHertz were used. 

iii 
CONTENTS 

Page 

FOREWORD ................................. i 

ACKNOWLEDGMENTS ............................. i 

ABSTRACT ................................. ii 

INTRODUCTION ............................... 1 

ELECTRICAL PROPERTIES STUDIES ...................... 3 

Resistivity of Coal ........................ 3 

Resistivity of Overburden ..................... 4 

Galvanic Method .......................... 9 

Overburden Electrical Conductivity Distribution at 

Imperial and Eagle Coal Mines. Colorado ............ 9 

Overburden Electrical Conductivity Distribution at 

Montour No . 4 Coal Mine. Pennsylvania ............. 13 

Inductive Method .......................... 20 

NOISE ENVIRONMENT STUDIES ........................ 24 

Gary No . 14 Coal Mine. West Virginia ................ 26 

Peabody No . 10 Coal Mine. Illinois ................. 26 

Imperial and Eagle Coal Mines. Colorado .............. 26 

APPLICATION OF FIELD STUDIES ....................... 29 

Vertical-Axis Loop Transmitter ................... 29 

Grounded Horizontal Electric Line Transmitter ........... 30 

THROUGH-THE-EARTH TRANSMISSION TESTS ................... 37 

Inferences to Location ....................... 37 

Passive Detection of a Buried Loop fran the Surface ........ 42 

Downlink Pulse Signalling to Underground Mine Working ....... 54 

Theoretical Considerations ................... 54 

Experimental Downlink Pulse Signalling ............. 60 

Continuous Wave Field Transmission Tests .............. 62 

Horizontal Electric Field Measurements . Buried Line Source 

Transmitter . U.S. Bureau of Mines Experimental Coal 

Research Mine, Bruceton. Pennsylvania ............. 64 
Vertical Magnetic Field Measurements - Buried VerticalAxis Loop Transmitter - U.S. Bureau of Mines Experimental 

Coal Research Mine, Bruceton, Pennsylvania ........ Vertical Magnetic Field Measurements - Surface VerticalAxis Loop Transmitter - U.S. Steel Robena No. 4 Coal 

Mine, Uniontown, Pennsylvania. ............. Horizontal Magnetic Field Measurements - Buried VerticalAxis Loop Transmitter - U.S. Bureau of Mines Experimental 

Coal Research Mine, Bruceton, Pennsylvania ........ SUMMARY. .............................. APPENDICES 

A. Histograms/Gary No. 14 Coal Mine, Gary, West Virginia. .. B. ~istograms/Peabody No. 10 Coal Mine, Pawnee, Illinois. .. C. Histograms/Eagle Coal Mine, Erie, Colorado ....... REFERENCES ............................. 
Research on the Transmission of Electromgnetic Signals 

Between Mine Workings and the Surface 

INTRODUCTION 

Underground mining of coal remains one of the most hazardous of occupations, 

despite intensive efforts by government, industry, and labor to reduce the hazards. 

One of the most frustrating aspects of attempts to reduce the number of fatalities in coal mine operations is that of not being able to communicate with miners 

trapped underground imnediately after some type of catastrophic accident. When a 

gas explosion takes place, it is difficult to pinpoint the areas of a mine 

affected from the surface. Tens or even hundreds of miners may survive the 

initial effects of such a catastrophe, and their longer-term survival will depend 

on their taking the proper action, such as barricading themselves in a remote 

part of the mine, or seeking out an unobstructed exit from the mine. Normal communications are usually completely disrupted at the moment of the catastrophe, 

at a time when effective communication is essential. Therefore, there is a need 

for an emergency cormunications system for use under such circumstances, a system 

which would be sufficiently portable that a transceiver could be carried readily 

by groups of miners, or even individual miners at all times while they are underground. 

The problems related to the development of an emergency communications 

system are only partly technical in nature, but to a large extent, the design of 

a workable system will depend on human engineering factors which are sometimes 

difficult to formalize. The problem is that of providing a miner underground 

with a communications system which will fulfill one or both of two requirements; 

that of indicating the location of the trapped miners, and that of providing some 

transfer of information between the miner and other people at the surface of the 

earth. If a communications system with a high capacity for transmitting information could be provided to the miner, these problems might be solved at the same 

time. Therefore, an important aspect of a comunications system is the amount of 

information that can be transmitted over it per unit time. For voice transmission, 

data rates of about 3000 bits per second are necessary; while data rates as low as 

one bit per second or even one bit per minute might be used for the transmission 

of a simple code. Voice transmission would be more desirable than the use of a 

slow code, but as one might expect, the higher the data rate for the system, the 

greater will be the complexity, weight, and power requirements. Thus, in 

designing a workable system, we may expect there to be a tradeoff between data 

rate on the one hand, and portability on the other. 

A second important property of a communications system is whether it is oneway or two-way. A simple communications system would allow the trapped miner to 

make his presence known to people on the surface, but would not allow communication from the surface to underground (e.g., hammer tapping). In an emergency it 

is of prime importance that up-link communications, from the miner to the surface, be established first so that rescue teams may know if there is anyone 

alive underground and where they may be. However, it is also important to have 

a down-link so that information can be passed to the buried miner. Such information might advise him as to the wisest course of action to ensure survival, or 
would assure him that rescue attempts were proceeding, and so, prevent him from 

taking unwise actions on his own, Of particular interest is the possibility 

that the up-link and the down-lipk would not need to be symmetrical with respect 

to data rate. It would be quite reasonable to use a low-data-rate up-link 

system if a high-data-rate down-link system were available. In this way, 

specific questions along with instructions for a simply coded reply could be 

transmitted downward. 

In order to design a workable electromagnetic communications system coordinating both technical requirements and human engineering, it becomes necessary 

to know a variety of design parameters which may be grouped into three classes 

as follows: 

(1) the electrical properties of the rock overlying mine workings,. 

which determine the relationship between the amount of energy 

or parer applied to the transmitter and the strength of the 

signal as it propagates through the ground 

(2) the ambient background electromagnetic noise levels at receiver 

locations, which determine the minimum level of signal strength 

that can be recognized 

(3) the effect of mine workings and structures on the behavior of 

the communications signal. 

Some of the above design parameters may differ for every mine so that for 

ultimate optimization of emergency communications systems, it would be necessary 

to have such information for every drift of every mine working. 

Such an optimized system probably does not represent the desirable first 

step in developing an emergency communications system, but rather a longer range 

goal. At present, it would be preferable to specify a system which would not be 

sensitive to any but the most extreme differences in the character of a mine 

working, so that the same elements could be used by most mines. Thus, we may 

settle for near optimization based on the idea that similar mines in similar 

geological settings will be characterized by relatively narrow ranges for design 

parameters, and that these ranges may be determined with reasonable statistical 

reliability by making studies only over selected mines. 
ELECTRIC& PROPERTIES STUDIES 

The mathematical theory for the behavior of electromagnetic fields from an 

underground antenna, though tedious, has in recent years become somewhat well 

established (Wait and Campbell, 1953; Wait, 1962; Bannister, 1966; Sinha and 

Bhattacharya, 1966; Ban&, 1966; Wait, 1969; Wait, 1970; Wait, 1971c, 1971d; 

Wait and. Hill, 1972a; Hill, 1973a; Hill and Wait, 1973~). In all such theoretical studies the electrical properties of the earth affect signal strength in a 

way which is determined by the wavenumber of the medium or media through which 

propagation takes place. The wavenumber is given by the expression: 

where f is the frequency used in cycles per second, 1~, is the magnetic permeability of the medium in henrieslmeter, and E is the dielectric constant in faradsf 

meter (all quantities are expressed in MKS units in this report). Only one of 

these quantities, the frequency, is a design parameter. In order to specify 

values for the wavenumber in theoretical studies, it is reasonable to expect that 

statistical compilations of the other three factors might be needed, particularly 

as a function of fre.quency if any of the factors should be frequency-dependent. 

Extensive tables of physical properties data are available in the literature which my be used to reduce the magnitude of the problems in compilation of 

pertinent data (Clark, 1966; Parkhanenko, 1967). Based on such tabulations, the 

generalization may be made that for the rocks normally occurring in the vicinity 

of coal mines, the magnetic permeability is not variable, but assumes a value 

quite close to that of free space, or 477 x henrieslmeter. As a consequence, 

it is not necessary to carry out any statistical analysis of magnetic permeability 

data for the purpose of this research. 

However, the use of tabulated data on electrical conductivity and dielectric 

constant may not be entirely satisfactory. Coal mines are found rather near the 

surface in sedimentary sequences of rocks. Usually, the sedimentary rocks are 

those which were originally deposited in fresh or brackish water and, moreover, 

insofar as most coal mines are relatively near the surface, a significant fraction of the rock overlying a coal mine may have its properties modified by 

weathering. In a review of tabulated data on electrical properties, it becomes 

apparent that relatively few measurements have been made on such rock sequences. 

In considering the electrical properties of the rocks along a communications 

path between a mine working and the surface, it is convenient to consider coal 

and the overburden separately. 

Resistivity of Coal 

Studies of the electrical resistivity of coals have been carried out by 

many people (Parkhomenko, 1967; Krasnovakii, 1969; Baker et al, 1966; Tonkonogov 

and Veksler, 1971). Data on the resistivity of coals which may be found in 

literature do not always indicate the conditions under which measurements were 

made, such as the moisture content, temperature, measuring method, and so on. 

Baker and his coworkers reported that resistivity of anthracite depends on its 

moisture content, its temperature, the number of electrodes used to measure 
resistivity, and whether the sample is cut parallel to or perpendicular to the 

bedding plane (Baker and others, 1966). Data reported by various authors are 

listed in Table 1 for six types of coal. 

Of the values reported in Table 1, the highest resistivities were obtained 

for measurements made on samples obtained from a coal pile in a completely dry 

state. Lower values of resistivity are reported for naturally wet coal, while 

the resistivity of air-dried coal is about the same order of magnitude according 

to three references. However, A. F. Baker and others (1966) reported the resistivity of anthracite to be lower when it is dry than when it is wet. 

The range of resistivities reported for coals is extremely wide, extending 

from the semiconducting to the insulating regions. Resistivity is a function of 

the physical and petrographic character of the coal, the mineral composition and 

the degree of metamorphism. Generally, the resistivity of coal can be correlated 

with texture and composition, but is a function of a greater number of variables 

than the properties of other rocks. 

The basic petrographic constituents of coals are fusain, vitrain, durain, 

and clarain. The resistivity of dry samples of clarain and vitrain is m re than 8 3 x lo6 ohm-meter, while fusain has somewhat lower resistivity, 10 to 10 ohmmeter. As a result, the resistivity of a coal will depend on the relative 

proportions of these constituents. 

The relation between resistivity of coals and their ash content is shown 

in Figure 1. 

Dakhnov (see Parkhomenko, 1967) has analyzed data statistically, finding that 

the highest resistivities are found in gassing coal and greasy steam coal. 

According to Toporets (see Parkhomenko, 1967) long burning coals have the highest 

resistivities. Moreover, the amount of decrease in resistivity with increasing 

metamorphism depends on the type of metamorphism. A pronounced decrease in resistivity is found in coal fields subject to thermal metamorphism, particularly 

contact metamorphism. 

The role played by the carbon ratio in determining the resistivity of coal 

with various degrees of carbonization is not uniform. For long-burning coal, 

gassing coal and greasy steam coal, the character of the relationship is one of 

decreasing resistivity with increasing carbon ratio, while in the case of anthracite, on the other hand, increasing the carbon ratio increases the resistivity. 

It must be noted that not only is the carbon ratio important, but also the ash 

content, which is related to the carbon ratio. 

Resistivity of Overburden 

Conduction in near-surface rocks is almost entirely through the water filling 

the pore spaces of the rocks, and so, prediction of electrical properties for the 

overburden is somewhat simpler than for coal seams. In water-bearing rocks, 

there tends to be a correlation between resistivity and geologic age, inasmuch as 

the amount of water decreases with age, and salinity tends to increase with age. 

Most studies of the electrical properties of sedimentary rocks have dealt with 

deep-seated formations that serve as oil and gas reservoirs, but a large amount 

of data on the resistivities of near-surface rocks is available in National 
Coal quality 1 

I 

Source of data 

TABLE I. Resistivity of Coal in ohm-meter 

Brom coal 

Gassing coal 

Greasy steam coal 

(bitumenous) 

Coaking coal 

Source of data: 1. A. P. Kovalev, 2. V. ;.I. Daknov, 3. A. A. Agroskin and I. 

4. . 1. Shumilovskii, 5. F. 2. i<ransnovsl:ii, 6. Pi. P. Tonko

V. A. Veksler, 7. A. F. iiaker et al. 

1,ump.r steari coal 2.6 x 10 F: 

(super-bitunenous) 

Anthracite - 10 

- 

1 

-9 x 10 

1.7 x 10 11 

lo 4.9 x 10 

0.8 x 101° 

3 

- -. 

7 

10 -10 

2 x 10 8 

8 

10 

---e----- 

2 

10-2x10~ 

lo2 - 10 4 

-- 

4 

- 

6 8 10 - 10 

6 S 10 - 10 

- 

-- 

5 

- 

8x10-l.Ssl0 2 

- 

8 

8 
Fig. 1. Gtaph of the most probable values for the resistivity of coals af various grades 

as a function of quality. The ash content is plotted along the abscissa. 
Bureau of Standards Circular 546 (Kirby and others, 1954). These earth resistivity values were estimated from the rate of decay in field strength about 

radio stations. Such surveys are required of all broadcast stations by the 

Federal Communications Commission. Resistivities determined frm these fieldstrength surveys represent average values for rocks over an area of a few 

square miles about the broadcasting station to a depth of a few tens to a few 

hundreds of feet. The median value for slightly over 7000 measurements reported 

in Circular 546 is 143 ohm-meters. 

Selecting from the 7000-odd measurements only those made over sedimentary 

rocks, the correlation between resistivity and geologic age shown in Figure 2 

was obtained. Inasmuch as the age of the overburden over coal measures ranges 

from Carboniferous to Miocene, in most cases it is to be expected that the overburden will have resistivities lower than the median value of 143 ohm-meters 

mentioned above, probably near 60 to 70 ohm-meters. 

In developing a workable emergency comunications system, it is probably 

necessary to obtain statistical summaries of resistivity values for specific 

mines, rather than rely on the data presently in the literature. 

Laboratory measuranents on recovered samples may be subject to uncertain 

experimental errors, particularly since it is difficult to maintain or restore 

samples to their natural state with respect to water content - a factor which is 

quite important in making meaningful electrical properties measurements. Therefore it was decided that in this case -- in situ measurements of electrical properties would be preferable to laboratory studies. 

Two approaches to the determination of electrical properties with surface 

based equipment were used in our compilations. In one approach, current is 

introduced into the ground with electrode contacts and the potential established 

in the earth by this current is measured with a second pair of electrode contacts. In the other approach current is induced in the ground by a time-varying 

magnetic field with alternating current driven through a loop of wire or a 

grounded length of wire. The effect of the earth on this magnetic field is 

detected using a second loop of wire to measure the field at some moderate distance. 

If the earth over a coal mine were completely uniform in electrical properties, the values for electrical conductivity determined from these two techniques 

would be the same. However, if the earth is stratified with layers having different electrical properties, as is most probably the case, the two techniques 

will provide somewhat different values for the average conductivity of the ground. 

This difference is a consequence of the electrical anisotropy which is observed 

in layered materials (Van'ian, 1965). When the earth is made up of a sequence of 

layers with differing conductivity, the average conductivity detected with 

current flowing perpendicular to the bedding will be less than the average 

conductivity detected with current flowing parallel to the bedding. The differences will be greater when the individual conductivities in the beds cover a 

wider range. The difference may be only a few percent in rocks which are uniform, or it may amount to a factor of 2 to 3 in rocks made up of layers with 

widely different individual conductivities. If the difference is significant for 

the typical rocks over coal mines, it may be of importance in comparing the 

merits of electromagnetic fields generated with grounded wire antennas and those 
RESSTIVITY, ohm-mein. 

pi 9. Avenge rsaistivity for pup of sedimentary rocks with aimilsr 

ckactkutios. Esoh circle indieah pn average -tidy debnnhd from 

35 to 260 measurements about radio stations. The bar through esch circle 

indicates the mnge within which 95 per cent of the values for that group of 

date fall. 
generated with induction loops. Usually, the current field from a grounded wire 

will depend on the geometric average of the maximum and minimum conductivity 

measured along and across the bedding in a laminated sequence of rocks, while 

the field from a loop source lying in a plane parallel to the bedding of the 

rocks will depend on the maximum conductivity. 

Galvanic Method 

The system selected for the determination of electrical conductivity using 

electrode contacts is shown diagramatically in Figure 3. This particular arrange 

ment of electrodes is one which exhibits a relatively low inductive coupling 

between the cables used to introduce current into the ground and the cables used 

to connect the measuring electrodes to the receiving equipment. With such an 

array, it is possible to determine the dielectric constant at low frequencies, 

if it is large enough, as well as the conductivity. The use of this electrode 

array in geophysical exploration has been described by Madden and Cantwell (1967). 

In field experiments carried out over the Imperial and Eagle coal mines at 

Erie, Colorado, the Gary No. 14 coal mine at Gary, West Virginia, the Peabody No. 

10 coal mine in Illinois, and the Montour No. 4 coal mine in Pennsylvania, typical 

currents of 1 to 5 amperes were used with an "a-spacing" of 30.5 meters. The maximum separation between electrodes A and M in Figure 1 was 243.8 meters. Measurements were made along short traverses, so that computed values of apparent resistivity might be presented in the form of pseudo-section. 

With the dc measurement system described here, the current driven into the 

ground is in the form of square wave pulses; that is, the current is turned on 

to its dc level for a period of 10 seconds and then abruptly terminated. At the 

same time, the voltage between the measuring electrodes is recorded on a graphic 

recorder. A typical sample of acquired field data is shown in Figure 4. 

If the dielectric constant in the earth is large at low frequencies, the 

voltage will not disappear abruptly as the current flow is terminated, but rather, 

there will be a transient decay from the static level observed during current 

flow. The characteristics of this transient decay may then be used to estimate 

the tangent of the loss angle, from which the low-frequency dielectric constant 

can be determined. However, in the measurements made at the above coal mines, 

the transient decay was so quick so as to be not detectable within the accuracy 

of the recording equipment. This observation means, as was expected, that the 

dielectric constant is not large enough to be significant in affecting electromagnetic propagation at low frequencies. 

Overburden Electrical Conductivity Distribution at Imperial and Eagle Coal Mines, 

Colorado 

Surfacebased resistivity measurements using the axial dipole-dipole 

electrode configuration were made at the Imperial and Eagle Coal mines, 

Colorado, with a view toward the characterization of the geoelectric sections 

overlying the mine workings. Typical apparent resistivity sections along survey lines are given in Figures 5, 6, and 7. In plotting the sections, lines 

are drawn downward at angles of 45 degrees fran the center of each dipole location, and measured apparent resistivity values are plotted at the appropriate 

intersections . It is apparent that the geoelectric section overlying the mine 
DIPOLE - DIPOLE ARRAY 

nipole-dipole array used m field determinations of electrical Fig. 3 conductivity and dielectric constant. me spacing between electrodes A and M 19 increased stepwise to make a series of measurements with aiiferent effective penetrations into the earth. 
N - S RESISTIVITY SECTION AT IMPERIAL COAL 

N- MINE, COLORADO 

DIPOLE -DIPOLE ARRAY 

o p~p

/ 

b 

14 11 

19 

13 18 , .. ., 

HORIZONTAL SCALE 

0 

- 

200 FT 

pa ( ohm -meters) 

Fig. 5 

N-5 RESISTIVITY SECTION AT EAGLE 

COAL MINE, COLO. 

DIPOLE- DIPOLE ARRAY d N 

I pa (ohm -meters) 

500L 

HORIZONTAL SCALE 

0 200 FT 
E-W RESISTIVITY SECTION AT IMPERIAL 

COAL MINE, COLO. 

DIPOLE -DIPOLE ARRAY -E 

B' 

/ / / / / / / / 

4L SCALE 

0 

- 

200 FT 

pa ( ohm - meters) 

Fig. 7 
workings at the Imperial and Eagle mines is quite homogeneous, with an average 

resistivity of 12 ohm-meters. 

A probability density plot of resistivity values of the overburden at 

Imperial and Eagle coal mines is inferred from Figures 5, 6, and 7 and is given 

in Figure 8. A uuimodal distribution of apparent resistivity is evident. 

Plotting Of the probability density functions in the manner portrayed by Figure 

8 is useful for the optimal design of a mine beacon electromagnetic communications system, because the electromagnetic wave decays as a function of the wave 

number of the medium (Equation 1). If differing mining districts can be characterized by such probabilistic distributions (with regard to resistivity and 

electromagnetic noise), inasmuch as what may be optimal for communications in one 

mining district may not be optimal in another district. 

For comparison, probability density plots of measured resistivity values or 

the overburden at Montour No. 4 mine, Peabody No. 10 mine, and Gary No. 14 mine 

are given in Figures 9, 10, and 11. All resistivity distributions appear unimodal in character; however, the spread in resistivity values measured at the 

Montour No. 4 mine is larger than that at the Imperial and Eagle mines and 

becopes even larger at the Gary No. 14 mine. On the other hand, the resistivity 

probability density plot for the Peabody No. 10 mine appears quite similar to that 

for the Imperial and Eagle mines. This comparison implies that electranagnetic 

transmissions through the earth, ambient noise and logistic factors being equal, 

would be less distorted by the resistivity distribution apparent at the Imperial 

and Eagle mines or at the Peabody No. 10 mine than by the resistivity distribution 

apparent at either the Montour No. 4 mine or the Gary No. 14 mine. Consequently, 

a theoretical model of a beacon source antenna immersed in an electrically and 

magnetically homogeneous halfspace (Wait, 1971d) would be applicable in mining 

provinces which have geoelectric sections similar to those measured at the 

Imperial and Eagle mines or the Peabody No. 10 mine and would yield insight into 

the behavior of electromagnetic transmissions. However, an analytic model 

applicable to a geoelectric section such as that encountered at the Gary No. 14 

mine (Geyer and Keller, 1971a) would have to account mt only for topography 

(Wait, 1971b), but also for the effect of electrical anisotropy and lateral 

inhomog enei ties. 

Overburden Electrical Conductivity Distribution at Montour No. 4 Coal Mine, 

Pennsylvania 

Galvanic resistivity measurements of the geologic section collected over 

various profiles at the Montour No. 4 mine, Pennsylvania, are shown in section 

in Figures 12, 13, 14, 15, 16, 17, and 18. From the latter figures it is 

observed that a relatively homogeneous geoelectric section exists over the 

Montour No. 4 mine workings. Only along profiles P-P', PI-P' I, A-A' , A' '-A' ' ' 

are there suggestions of lateral inhomogeneities at depths beneath the surface 

ranging from 200 to 350 feet. It is noteworthy that all of these anomalous 

resistivities are high; at present no correlation has been made associating the 

resistivdty highs with either the mine workings or perhaps resistive veins of 

subsurface coal. Since the average overburden conductivity at the Montour No. 4 

mine is an order of magnitude lower than that measured at the Imperial and Eagle 

mines, an upper limit on the maximum operating frequency of a practical beacon 

electromagnetic system at the Montour No. 4 mine would be an order of magnitude 

higher than an upper limit for the same system in a mining province such as that 
RESISTIVITY, ohm -meters 

Fig. 8 

(MONTOUR NO. 4 COAL MINE, PA.) 

Fig. 9 
Fig. 10 

60 

- $ 50- - 

t. 

'- 40- 

m 

z 

W 

n 

>- 

30- 

b- - 

4 

m 20- Q: 

0" 

E 10- 

O1 

GARY NO. 14 COAL 

MINE, W. VA. 

- 

10 100 1000 10000 

RESISTIVITY, ohm -meters 

Fig. 11 
NE-SW RESISTIVITY SECTION AT MONTOUR 

NO. 4 COAL MINE, PA. -NE 

U 

400L DIPOLE - DIPOLE ARRAY 

p ( ohm - meters) a 

HORIZONTAL SCALE 

0 

I 

200 FT 

Fig. 12 

SW -NE RESISTIVITY SECTION AT MONTOUR 

NO. 4 COAL MINE PA. 

DIPOLE -DIPOLE ARRAY - NE 

100 62 68 

33 

I 

ta 

w 400- 38 30- 

n 

500- 

HORIZONTAL SCALE 

0 200 FT 

Pel (ohm -meters) 

Fig. 13 
SW -NE RESISTIVITY SECTION AT MONTOUR 

NO. 4 COAL MINE, PA. 

- NE or' D~oLE;DIPO;E ATAY / , , ,ptl 

HORIZONTAL SCALE 

0 

- 

200 FT 

p, ( ohm -meters) 

Fig. 14 

SW - NE RESISTIVITY SECTION AT MONTOUR 

NO. 4 COAL MINE, PA. - NE 

DIPOLE -DIPOLE ARRAY A 

500L 

HORIZONTAL SCALE 

0 200 FT 

Pa (ohm -meters) 

Fig. 15 
W -E RESISTIVITY SECTION AT MONTOUR 

NO. 4 COAL MINE, PA. 

-E or'' /"IPOL;-D1P:LE "/"RAY / / 

, 

100 - 104 85 97 88 

W 

W 

- LL 4 7 36 

I 43 

In 30 45 

W 

n 400- 30 ,58 

24 

500- 

HORIZONTAL SCALE 

0 

- 

200 FT 

Pa (ohm -meters) 

Fig. 16 

NW - SE RESISTIVITY SECTION AT MONTOUR 

NO. 4 COAL MINE, PA. 

-SE YlP0L;- DIPFLE TRAY, , ,An 

52 68 

500 Pa (ohm -meters) 

HORIZONTAL SCALE 

0 

- 

200 FT 
NW -SE RESISTIVITY SECTION AT MONTOUR 

NO. 4 COAL MINE, PA. 

- S E of DFOLE;DIPO>E AYAY I I I ,K 

63 42 

500 

HORIZONTAL SCALE 

0 

7 

200 FT 

Pcl (ohm - meters) 
of the Imperial and Eagle mines, ambient noise, logistics, and through-the-earth 

communication distances being equal. In general, it is expected that the 

geologic regimen about other coal mines will be similar to that found in the 

experimental areas in which the geological character of the beds overlying a 

mine is simple. In other cases, where the character is more complex, it will 

probably be necessary to recognize the existence of several layers with different resistivities over a mine, and through which signals must propagate in an 

energency communications system. 

Inductive Method 

Another surface-based approach for measuring the conductivity distribution 

of the overburden is that in which current is induced in the earth by a timevarying magnetic field with alternating current driven through a loop. The magnetic field of this induced current, which is affected by the resistivity distribution of the earth, is then measured with a second loop at some moderate 

distance away from the first. By varying the frequency at the source loop, variations of conductivity with depth may be ascertained, the lower frequencies 

sampling greater depths. One advantage to an inductive as opposed to a galvanic 

method is that the source and receiver positions may be kept fixed while the 

frequency is varied. 

In dealing with such two-loop electromagnetic measurements it is convenient 

to use the concept of mutual impedances and ratios of mutual impedances. The 

mutual impedance Z between a source and a receiver is the ratio of the voltage 

induced in the receiver to the current I in the source. For two loops, 

where N is the number of turns of the receiving loop, A is the area of the 

receiving loop, and H is the magnetic field at the receiving loop. For two coplanar loops in free space separated by a distance R, the mutual coupling Zo is, 

neglecting displacement currents, 

where N1 and N2 are the number of turns on the loops, and A1 and A2 are the areas 

of the loops; Frischknecht (1967) gives the theoretical coupling ratio Z/Zo for 

two horizontal coplanar loops at the surface of a two-layer earth (see Figure 19), 

where (Z/Zo)' is the mutual coupling ratio for horizontal coplanar loops over a 

homogeneous earth given by (Wait, 1955), 

yL = iuUOa, a being the electrical conductivity in mhos per meter, uo the magnetic 

permeability of free space (4~ x 10-7 henrieslmeter), and w being the transmitter 

radian frequency. In the second term of (4), 
Fig. 19 "U >oTiZo"tal C0pl.n.r loop. at the surface of a rxo-layer eartn. 
where 

and Jo is the Bessel function of the first kind and of order zero. Frischknecht 

(1967) then evaluates both the amplitude and phase of the mutual coupling ratio 

Z/ZO as a function of B for a number of k and D/B values. 

Simple curve-matching techniques utilizing Frischlcnecht's reference curves 

may be used to interpret Z/Zo sounding curves made over a two-layer earth. The 

procedure is to plot the field data (after correction for the calibration response 

of the receiver-amplifier system) on transparent bilogarithmic paper with fk along 

the abscissa. The field data are then superimposed on the appropriate set of 

reference curves and shifted about, keeping the axes of the two sets of curves 

parallel until the best possible match is found. Once a suitable match has been 

found, the conductivity and thickness of the upper layer as well as the conductivity of the substratum are determined. 

Several electromagnetic depth soundings were made at the Imperial and Eagle 

coal mines in central Colorado using horizontal coplanar loops situated at the 

surface and separated by 1000 feet. The results of these soundings are portrayed 

in Figures 20 and 21; they generally show good agreement with galvanic resistivity 

data with the average resistivity of the first modeled layer being 20 ohm-meters 

and that of the second layer 65 ohm-meters. The determined thickness of the first 

of the two-layer earth model, about 80 meters, corresponds roughly to the present 

depths of the mine workings in this region. 
ELECTROMAGNETIC COUPLING FOR 

HORIZONTAL COPLANAR LOOPS 

o SURFACE 

ELECTROMAGNETIC COUPLING F

HORIZONTAL COPLANAR LOO

- SURFACE 

INTERPRETATION OF SOUNDING No. 1 FROM INTERPRETATION OF SOUNDING NO. 2 FROM IMPE

IMPERIAL MINE, COLORADO COLORADO 

Fig. 20 Fig. 21 
NOISE ENVIRONMENT STUDIES 

The second main line of effort with respect to research related to the 

development of an emergency mine communications system has been the development 

of a technique for characterizing the noise environment at a mine. Such information may be used to specify the minimum source strength required for a communications system and the receiver sensitivity that is useful. In practice, it 

will probably be found that the upper limit on transmitter strength will be 

determined by the weight allowed in the source, whereas the maximum receiver 

sensitivity will be dictated by the ambient noise field levels. 

The optimal design of a beacon electromagnetic comunications system for use 

in emergency situations after a mine disaster requires statistical information of 

ambient electromagnetic noise levels and of the resistivity distribution of rocks 

overlying coal measures. Analysis of the electromagnetic background noise is 

important, for it is against this noise background that the transmitting beacon 

signal must be recognized at the earth's surface. In order to analyze electromagnetic noise statistics, an 8-K Nova 4001 mini-computer (16 bit resolution) 

combined with an 8 channel analog-to-digital converter (13 bit resolution), multiplexing unit, and teletype for 110 handling was installed in a Dodge A-100 van. 

A block schematic of the field computer system is shown in Figure 22. 

The conventional approach to characterization of noise is through the use 

of power-density spectrums in which the average level of the noise field over 

some long term is determined at a series of frequencies. For the present study, 

the choice was made to use amplitude histograms over relatively broad frequency 

bands to characterize the noise. In so doing, the signal from an electric field 

sensor or a magnetic field sensor is first passed through a filter which rejects 

strongly below some lower frequency limit and above some upper frequency limit. 

Then, within these frequency bands, the percentage of time the noise from the 

sensors exceeds specified levels is determined. 

The advantage of the noise histogram approach is that it provides information on the design of a high-sensitivity receiver system which is not readily 

obtainable in the spectral approach. For example, different approaches would have 

to be used for noise rejection in the case in which the noise amplitude is 

relatively uniform in time than in the case in which it is impulsive, varying 

widely with time. In the first case, optimum noise rejection would be obtained 

by setting a threshold for signal detection just above the noise level. In the 

second case optimum noise rejection would be obtained by using redundant transmissions, and detecting the signal during quiet periods. 

Amplitude density studies of ambient electric and magnetic noise have been 

made at the Gary No. 14 mine, West Virginia, the Peabody No. 10 mine, Illinois, 

and at the Imperial and Eagle mines, Colorado. Hourly measurements were made 

over 8 frequency bands during the course of 24 hours, and for each frequency band, 

10,000 samples were taken. 

Measurements of the horizontal electric field background noise become 

important when measurements of electric field beacon signals are used for communication. Similarly, if measurements of wave tilt of the magnetic field are 

used for location purposes, it will be quite useful to know what percent of the 
INPUTS 

LOOP MAGNETIC FIELD 

ED WIRE 

ELECTRIC FIELD DETECTOR 

DETECTOR 

X 8 ANALOG 

AID CONVERTER 

13 BIT MULTIPLEXER 

RESOLUTION 

PROGRAMMED NOVA 8-K COMPUTER 

10 16 BIT RESOLUTION 

r-- 

DIGITAL PUNCH 1 DIGITAL PRINTOUT I 

Fig. 22 Block schematic of fiud eainputer sy8tem 
time certain background magnetic field noise levels are exceeded. 

At both the Gary No. 14 mine and the Peabody No. 10 mine measurements were 

taken at the surface directly above a working face. At the Imperial and Eagle 

mines measurements were taken about 1200 feet north of a working face. Only 

surface-based measurements were taken; thus the results described here are applicable 

for the design of an uplink electromagnetic transmission system. 

Gary No. 14 Coal Mine, West Virginia 

Ambient surface electromagnetic noise measurements were made at the Gary 

No. 14 mine approximately 754 feet directly above a working face. The Gary No. 

14 mine has a daily capacity of about 6,500 tons and is a D.C. faced mine. 

Amplitude probability density plots of the surface vertical magnetic field noise 

and horizontal electric field noise are given in Appendix A (see Figures A-1 

through A-16), a typical example of which is shown in Figure 23. Each plot consists of 6 graphs, each of which, in turn, comprise 10,000 data points. The time 

of day is indicated on each graph, and, for purposes of illustration, only every 

fourth hour is plotted. 

An immediate observation drawn from Figures A-1 through A-16 is that the 

maximum percent of events ranges from 30 to 55. In addition, it is markedly 

apparent that there is much more variance of ambient electranagnetic noise with 

frequency than with time of day. This is an important observation, for it 

indicates that one may design, in general, his transmitting antenna around the 

parameter of frequency, irrespective of time of day. Examples of how such summary 

electromagnetic noise measurements, together with electrical properties studies 

and knowledge of low-frequency coupling behavior between various source antennareceiver configurations, may be used for the practical implementation of a beacon 

electromagnetic canmunication system will follow shortly. 

Peabody No. 10 Coal Mine, Illinois 

Ambient surface electromagnetic noise measurements were made at the Peabody 

No. 10 mine approximately 354 feet directly above a working face. The Peabody 

No. 10 mine has a daily capacity of about 15,500 tons and is an A.G. faced mine. 

Amplitude probability density plots of the surface vertical magnetic field noise 

and horizontal electric field noise are given in Appendix B (See Figures B-1 

through B-16). These amplitude probability density plots demonstrate little 

change of the maxinun percent noise with time of day and clearly show the magnitudes of the higher noise levels at the lower frequencies. A 1:5:1 variance 

is seen at around 2200 and 800 hours, however, which my be attributable to 

shift changes within the mine. This variance is only apparent at frequencies 

less than 1000 Hz. 

Imperial and Eagle Mines, Colorado 

Ambient surface electromagnetic mise measurements were made midway 

between the nonoperating Imperial mine and the operating Eagle mine approximately 

300 feet above a working face but considerably offset (%lo00 feet) from the 

working face. The Eagle mine has a daily production of about 1800 tons and uses 

both 440 V.A.C. and 250 V.D.C. Amplitude probability density plots of the surface 

vertical magnetic field noise and horizontal electric field noise are given in 
R-SII HORI'LONTRL ELECTRIC FIEUI NOISE 
Appendix C (see Figures C-1 through C-16). 

From the plots of electric field noise, it is observed that for the 20-80 

cps bandwidth, 40% of the total number of events from 10,000 samples (samples 

were taken with the sampling time chosen such that the aliased frequency was 

above the upper limit of the frequency of concern) had amplitudes of 1 to 2 

millivolts per kilometer, 12% of the total number of events had amplitudes 

between 2 and 3 millivolts per kilometer and less than 0.1% of all events had 

magnitudes close to 10 millivolts per kilometer. For higher frequency ranges up 

to 5200 cps a broadening of the spectrum occurs for events of magnitude less than 

1 millivolt per kilometer. Thus, for example, in the frequency range of 320- 

640 cps, more than 85% of all events had amplitudes less than 1 millivolt per 

kilometer. For electric field noise frequencies above 5000 cps, however, a 

distinct narrowing of the spectrum again occurs with approximately 52% of all 

measured events having magnitudes in excess of 2 millivolts per kilometer. 

From the vertical magnetic field amplitude histogram plots, on the other 

hand, it is observed that for the 20-80 cps frequency range, most of the events 

occurring have magnitudes of 5 gammas per second (actually, the histograms are 

for the time derivative of the magnetic field, for which any induction coil 

receiving sensor wi1.l be sensitive) or greater; i.e., over 70% of all events have 

amplitudes of at least 5 gammas per second. For frequencies greater than 80 cps 

two phenomena are apparent. The first is similar to that observed for the N-S 

component of the horizontal electric field; namely, that of a broadening of the 

spectrum of the number of events occurring over a larger domain of amplitudes. 

For example, in the 640-1280 cps bandwidth 80% of all the measured events have 

magnitudes between 2.5 x gammas per second and 6 x 10-I gammas per second 

as opposed to only 49% of all measured events over the same amplitude range for 

the 160-320 cps bandwidth. The second phenomenon to be noted is a shift in the 

maximum percent of the total number of events to an order of magnitude lesser 

amplitude; i.e., the maxirmun percent of the total number of events in the 20-80 

cps frequency bandwidth is on the order of 10 gammas per second, whereas the 

maximum percent for the 5200-10,400 cps frequency bandwidth is on the order of 

1 gamma per second. 

In summary, the amplitude histogram plots of both the horizontal electric 

field and vertical magnetic field noise events illustrate more change with 

frequency (100:l for frequencies ranging from 1000 Hz to 40 Hz) than with time 

of day. The maximum percent of the horizontal electric field noise does display 

some variance with time of day at frequencies above 200 Hz; however, the magnitudes of the events occurring with greatest probability are so small that they 

do not seem to represent any communications problan, even at very low frequencies. 

Because of the surface offset distance relative to the working face at the 

Eagle mine, the summary noise tabulations do not represent the "worst-type" 

noise environment in this case. They do give information on ambient noise background levels for cmunications purposes at a considerable distance away from a 

working face, however. 
APPLICATION OF FIELD STUDIES 

Vertical-Axis Loop Transmitter 

As an example of how Appendices A, B, and C, together with measured resistivity values, might be used for the practical implementation of a beacon 

electromagnetic communications system in which the transmitter is a buried vertical magnetic dipole, consider the follauing. Suppose it is desired to communicate from a mine working to the surface where the overburden has a thickness of 250 

meters and a mean resistivity of 20 ohm-meters. A frequency which would give 

adequate penetration to the surface may be determined by the relation, 

where p is the resistivity in ohm-meters, 11 the magnetic permeability in henries/ 

meter (approximately equal to 471 x henries/meter) , and d the depth in meters 

to the mine working. As Wait (personal comunication) notes, equation (7), which 

is based on the attenuation of a plane wave in a conducting medium, gives an 

overly pessimistic upper limit for an operating frequency; however, such a frequency would be adequate provided a communications system is designed around 

noise constraints and the overburden resistivity distribution. 

With the above parameter values, 

Let the operating frequency, then, be 80 Hz and assume that the turns-area of 

the receiving coil is 1000 square meters (this is typical of geophysical-type 

coil receivers operating at this frequency). Impose a design constraint on the 

dipole transmitter-receiver system that the signal-to-noise ratio at the surface directly over the transmitter be 3:l. The question then becanes: what 

must the source transmitter moment be in order to ensure signal levels which are 

10 db above ambient noise levels most of the time. Suppose the noise statistics 

are similar to those measured at the Imuerial and Eaale coal mines, Colorado. 

Then, fran Figure C-1, it is noted that-70% of the vertical magnetic field noise 

lwels (actually the time derivative of the magnetic field, to which an induction 

coil is sensitive) at the surface range between 3 gammas per second and 10 gamas 

per second (2.4 x low3 amp-turns per meter-second and 8 x amp-turns per 

meter-second) and appear relatively independent of time of day. 

Now the e.m.f. induced in a loop receiver at the earth's surface due to a 

time-varying magnetic field is given by 

-4 AH 

"~eceiver = receiver {Effective ~reaj~~~~~~

where V Receiver is the induced e.m.f. in volts, AH/At is the time rate of change 
in the magnetic field in gammaslsecond, and the effective area of the receiver 

is given in square meters. Therefore, the probability is 70% that the e.m.f. 

induced in the receiving loop just from noise at the surface will be 

'Receiver = 8 x -12.56 . 10 - lo3 

that is, noise background will probably generate lOuv in the receiving coil at 

80 Hz for a coal mining province whose noise statistics are similar to those at 

Eagle and Imperial mines. Since a signal level is desired which is at least 10 

db above this noise background, transmitter signal strengths must be about 30 uv 

at the receiver. Now, for sufficiently low frequencies, the moment of the 

underground coil transmitter is related to the voltage seen at the surface 

receiver loop placed immediately above the transmitter by 

3 M = h V/({Eff ective ArealReceiver f.12.56-10-~) 

where h is the depth in meters to the transmitter coil from a receiver loop 

placed immediately over the transmitter, f is the transmitting frequency in Hz, 

V is the signal voltage desired at the receiver, and M is the required source 

moment of the transmitter. For signal strengths of 30w directly above the 

transmitter as seen with a receiver loop with an effective area of 1000 square 

meters, the manent required at 80 Hz would be 

3 = 4.68 x 10 amperem 

2 

This source moment could be obtained by a coil which consists of 290 turns of 

wire and which is laid out in a loop with dimensions of 2.0 meters by 2.0 meters 

(to conform to the shape of a mine opening). A current of 4 amperes would then 

be required to provide this manent. 

This example was chosen to illustrate the manner in which resistivity data 

may be used with ambient electrwagnetic noise data for a specific communications 

application (using a loop source antenna and a loop receiving antenna) in a mine. 

For the above example, the required source moment could be decreased by increasing the effective area of the receiving sensor. 

Grounded Horizontal Electric Line Transmitter 

At this point it would be appropriate to consider how horizontal electric 

field noise amplitude probability density plots may be used in the systems 

engineering design of an alternative underground antenna. An alternative 

underground antenna might be an insulated piece of wire which is grounded at 

distant ends (in which use of rails or possibly telephone lines within a mine 

working might be made) . Such a system is shown in Figure 24. 

The current "bipole" transmitting antenna could be tied into roof bolts at 

A and B with a typical separation, L, say on the order of 10 meters. Or the 

line source antenna in the mine could simply be tied into two grounded electrodes 

at A and B. 
Horizontal Electric 

Field Receiver on Surface 

\ - x 

P d 

\ \ 

(-1) B Q + rrcns:.is'.er in Kin< C or,, I, 

I 1 I I 

(x = -L/?) I (X = LIZ) -z 

Pig. ~hree-dimenslonsl configuration of a line so~rce anru?-.a in mine working and an arbitrarily lacstad sur.cace horizontal electric. 

field sensing receiver. F is the nidpoint of the current binole, and 0 is the~idpoict or the measuring dipale m rIxe rlrra'ce. The distance MN l~sufficiently snail suc*. that the potential 

difference across hl!i .is a2ggod measu e of-the lecrric field at o . = (x- + 1 ; R = (2 YL + 3j: . me *leCtrica: 

resistivity of the overburden is denoted 52. P . 
In most mining provinces the distance will be considerably smaller than 

the separation distance R between the midpoints of the buried transmitter and 

surface receiver (since a practical value of d is usually on the order of 100m). 

In this case we shall examine the static voltage response at the receiver point 0 

since it is largest for this antenna-receiver configuration and since it will 

afford us inmediate insight into the behavior of extremely low frequency electric 

fields around a buried horizontal electric line beacon transmitter. Thus, let 

us first consider the potential (voltage) that would be measurable at 0 due to a 

buried line source antenna of length L by which current of strength I is put 

into ground of resistivity p. For mathematical convenience we have oriented 

our reference coordinate system such that the x-axis is in the same direction as 

the transmitting antenna. 

The potential at 0 due to the current element AB is given by 

where - 2 2 % AO = {r +(~/2!~ - 2r(~/2)cosb + d 1 

- 2 % BO = {r2+(~/212 + 2r(~/2)cos+ + d I 

so that 

Consider now the various bipolebipole arrangements as shown in plan view in 

Figure 25. The two configurations which are relevant to the mine communications 

problem are the parallel and perpendicular, i.e., 
Mmslieq ;:' 

, . . . , . , : M _-. 

- 

Max "a, 

Line ~ransmitter in ~ine 

Fig. 25 various buried transmitter and surface receiver configurations shown in plan view. M N - axial configuration ov sv -A 

M~ N~ - parallel configuration 

b$ N~ - Perpendicular configuration 

M~ N]. - radial eonfiguration 

'Mv N" - azimuthal configuretion 

M~~N~~ - equatorlal configuration 
Ip9.L { 

2x/L - 1 

'Rec eiver , 

(x,y;d) = - 2 312 

Parallel 4rd3 (l+(r/d)2+(~/2d)2-Lx/d ) 

2x/L + 1 

2 312' - (l+(r/d)'+(~/~d)'+~x/d ) 

PARALLEL CONFIGURATION (11) 

where 9. is the length of the receiving electric field sensor. We note that for 

4 = 0, 

IvQL { 2x/L - 1 (x,o;d) = - - 

2x/L + 1 

"Receiver, 2 312' 

Parallel 47rd3 (l+(x/d -L/2d) ) 3/2 (l+(x/d +L/2d) ) 

AXIAL CONFIGURATION 

We also note that for 4 = n/2, 

Ip 9.L 1 

'~eceiver, (0,y;d) = - 2 312 

Parallel 2nd3 (~+(~/d) '+(~/2d) ) 

EQUATORIAL CONFIGURATION 

Similarly, 

IV a 2 2 2 2 2 2 -% E (x,y;d) = - g;; {(x +y +(L/2) -Lx+d ) Y 

-' - (x +y +(LIZ) '+Lx+d ) 1 

so that 

rpay{ 1 

'Receiver , (x,Y;~) = 2 312 

Perpendicular 2nd3 (l+(r/d) '+(L/ 2d) '-~x/d ) 

PERPENDICULAR CONFIGURATION (14) 

We observe that when x = 0 or when y = 0 we are along null lines for the perpendicular electric field, a phenomenon which could be used for location purposes! 

Let us now consider another example in systems design in which an electrical field is generated underground by passing current through a wire of 

length L. The voltage measured at the surface of the earth (in the axial 

configuration) arising from this wire can then be calculated £ran formula (12) 

where p is the resistivity of the overburden and x is the horizontal offset of 

the receiver from the transmitter site (in the axial direction of the transmitter). When the receiver is directly above the buried line transmitter 

(x = 0) the magnitude of the received voltage is 
Suppose now that it is desired to communicate from a mine working to the 

surface where the overburden has a thickness of about 100 meters and an average 

resistivity of 24 ohm-meters; i. e., the same parameter problem as that at 

Peabody No. 10 coal mine, Illinois. A frequency which would give adequate penetration to the surface may be determined by equation (7) so that an operating 

frequency of 

may be taken. It should be noted at this juncture that we are already in violation of the assumed static behavior of the electric field, upon which formulas 

(10) through (14) are based, when we choose an operating frequency of 610 Hz 

(ELF region). It would still be useful, however, to proceed with the static 

derivations to illustrate how one may gain ready insight into design parameters. 

Assume the length of the surface wire receiver is about 10 meters, and 

impose a design constraint on the buried line transmitter-receiver system that 

the signal levels at the surface directly above the transmitting line are 10 db 

greater than maximum ambient noise levels. The question then becomes: what must 

the source transmitter moment (product of current, I, and length L) be in order 

to ensure a signal-to-noise ratio of 3:l at the surface directly above the 

transmitting antenna most of the time? Suppose also that the noise statistics 

for the horizontal electric field are similar to those measured at the Peabody 

No. 10 coal mine, Illinois. Then from Figure B-12 it is noted that most of the 

time ambient horizontal electric field noise levels at the surface are about 

10pvIm at f = 610 Hz. 

Now the e.m.f. induced in a wire receiver at the surface whose length is 

about 10 m just from noise is 0.1 millivolt. Therefore transmitter signal 

strengths must be about 0.3 millivolts at the receiver. From equation (15) 

an adequate source manent may be approximately determined on the basis of whether 

the length of the transmitter wire is large or small relative to the depth of 

burial. If the length of the transmitter wire, L, is considerably smaller than 

the depth of burial, an adequate source manent in this case would be about 7 

ampmeters. If the length of the insulated grounded transmitter wire in the 

mine is 30.5 meters, the current required would be about 0.2 amperes. 

This example was chosen to illustrate the manner in which resistivity data 

may be used with ambient electromagnetic noise data and theoretical considerations for a specific communications application in a mine. For the above 

example, the required current could be decreased by increasing the length of the 

transmitter wire. Similarly, if the resistivity of the overburden were larger 

with the use of the same operating frequency, a smaller source moment would be 

required by roughly the factor l/p. 

A more exact analysis using the nonstatic solution for a buried grounded 

insulated current element could be made with the results recently obtained in 

research carried out by Hill (1973a). Hill considers the electromagnetic surface fields for an inclined buried cable of finite length and evaluates the 

magnetic field canponents at the surface for various tilts of the transmitting 

line. It would be a ready extension to evaluate the surface electric field 
produced (in general) by an inclined buried current element and to perform the 

above system design analysis making use of available electrical properties 

data and ambient background noise data. 
THROUGH-THE-EARTH TXANSMI SSI ON TESTS 

Numerous through-the-earth electromagnetic transmission tests, both of 

the pulse and continuous wave type, employing either an electric line source or 

a vertical-axis loop source were made in several mining provinces (Peabody No. 

10 coal mine in southern Illinois, U.S. Bureau of Mines Experimental Mine in 

Pennsylvania, and the Robena No. 4 coal mine in Pennsylvania). The results of 

these field transmission tests enable one to examine the limitations of various 

source-receiver antenna configuration under the constraints of everyday mining 

operations. The results also help one to draw inferences as to the relative 

merits of possible location schemes which could be used to locate underground 

workers at beacon antenna transmitting sites. Location schemes would be of two 

types. For example, the transmitting antenna might be located on the surface 

above the mine workings, be either a loop antenna (whose axis is oriented vertically or horizontally) or a grounded insulated line antenna, and be excited 

either with continuous wave or pulse current. The underground worker may then 

wish to position himself relative to the surface antenna for location or possible evacuation. A perhaps more expedient location scheme, however, would not 

be a downlink one, as suggested above, but an uplink one; namely, the situation 

where the transmitting beacon antenna is underground and carried by the miner 

(or immediately accessible to him) and where the receiver site(s) is (are) on 

the surface. 

Inferences to Location 

One approach already mentioned for a through-the-earth beacon location 

scheme is the detection of (extremely low frequency) nulls at the surface in 

the electric field normal to the orientation of a buried line source antenna. 

By noting such null lines one can ascertain the location of the transmitter 

within the mine working. 

Another technique, which makes use of the magnetic field produced at the 

surface from a buried vertical-axis loop antenna, is to detect the null in the 

horizontal magnetic field directly above the transmitting loop (which exists at 

all frequencies). The null in the vertical magnetic field is at a surface horizontal 

offset distance from the transmitter axis of 1.4 times the transmitter depth of 

burial (null becomes sharper as frequency decreases). The maximum in the vertical 

magnetic field is directly over the transmitting loop antenna or the maximum 

in the horizontal magnetic field is at a horizontal offset of approximately half 

the transmitter burial depth. These approaches to location have been discussed 

theoretically by Wait (1970) and, during the summers of 1971 and 1972, were 

field tested, both at the Peabody No. 10 mine in Illinois and at the U.S. Bureau 

of Mines Experimental Mine in Bruceton, Pennsylvania. 

An electrmagnetic beacon propagation feasibility test was conducted at 

the Peabody No. 10 coal mine on June 11, 1971. The Peabody No. 10 mine is 

located near Pawnee in Ellis and Christian counties, Illinois, and lies under 

relatively flat topography. The Illinois No. 6 coal seam is being mined, and 

Peabody No. 10 mine has an approximate annual production of 7,000,000 tons. 

During the time when measurements were taken, the Peabody No. 10 mine was 

operating. Because the mine was i n f u l l operation, and ambient electromsgnetic 

noise levels were large, the feasibility of an electromagnetic subsurface-surface 
beacon comnunication link was rigorously examined. Noise levels during the 

test, which were described in an earlier section of this report, were probably 

representative of a "worst-type" situation insofar as a disaster situation is 

concerned. 

The average resistivity of the surface rocks through which electromagnetic 

propagation took place was 24 ohm-meters. A beacon loop transmitter with an 

effective area of 230 square meters was installed in the vicinity of the &fain 

South Tunnel (see Figure 26) at a depth from the surface of 108 meters. The 

buried vertical magnetic dipole source carried a current of 3 amperes at a frequency of 310 Hertz and was driven by a General Radio power oscillator (200 

watts maximum, 5 amperes maximum current). At the Peabody No. 10 mine a frequency not exceeding 310 Hertz would be desirable since one skin depth, 6, or 

the distance by which the electromagnetic fields would be severely attenuated 

by propagation through the overburden, is 

6 = A A= 24 = 106 meters 

31.0-3.14.12.56'10-~ 

This depth corresponds approximately to the depth of burial of the source transmitter. 

The vertical and horizontal magnetic field components produced on the surface were measured at three sites, one directly over the source transmitter and 

two offset from the buried antenna (see Figure 26 for location of surface sites). 

The horizontal magnetic field components at the surface were measured in this case 

with a high mumetal induction coil whose effective area, impedance, and coil 

sensitivity were calibrated before the field test (see Figures 27 and 28); the 

vertical magnetic field at the surface was measured with a loop laid out on the 

surface with a 1000-foot length of 26-turn cable whose impedance versus frequency 

was also measured before the field test (see Table 11). Impedance versus frequency measurements were made so as also to ascertain the resonant frequency of 

the mumetal coil and the required input impedance of the measuring equipment. 

The results of the field transmission tests are illustrated in Figure 29 

where the behavior of the measured magnetic field components produced on the 

surface as a function of offset distance is plotted. In Figure 29 the measured 

magnetic field components are normalized by IHzol, the magnitude of the produced 

vertical magnetic field directly over the buried transmitter. 

It is noted that the vertical magnetic field produced at the surface 

remains essentially constant out to an offset distance of 6/5, where 6 is one 

(plane-wave) skin depth (m) . At an of £set distance of one skin depth the 

magnitude of the vertical magnetic field is down by only 3 db. The horizontal 

magnetic field produced at the surface, on the other hand, exhibits a sharp 

null directly over the beacon and reaches a maximum at an offset distance 

roughly equivalent to one skin depth away from the source. 

These results indicate that measurements of the horizontal magnetic field 

produced at the surface from the buried magnetic dipole antenna could be used to 

locate the buried dipole source by determining the null directly over the subsurface antenna and the maximum at one skin depth offset distance from the 
--- 

I a ".~- .. - I! ,- i Fig. 26 

Effec!tve orem and :rnpedonce of "".of Texa

Fig. 27 
~ensili.fy or "u.oITemi' roil rr Irwuncy. 

Fig, 28 
TABLE I1 

Impedance x 

1.72 

1.82 

1.82 

1.71 

1.80 

1.76 

1.73 

1.73 

1.71 

1.71 

1.76 

1.79 

1.86 

1.98 

2.20 

2.30 

2.15 

1.60 

1.11 

.81 

.61 

.46 

.36 

Impedance of seismic cable used for surface measurements in the electromagnetic 

subsurface-surface beacon comunication test at Peabody No. 10 coal mine, 

Illinois. Impedance (ohms) is given as a function of frequency. 
antenna. In addition, the propagation test results demonstrate that for downlink communications, it would not be too critical where a surface loop transmitter were positioned relative to a receiver in a mine working. If 

it is known that the receiver (vertical-axis induction coil) in the mine is not 

offset greater than one skin depth from the surface dipole transmitter, signal 

strengths within the mine at the receiver site should not be significantly 

affected. 

It is interesting to canpare the field results obtained at the Peabody No. 

10 mine to the theoretical considerations employed by Wait (1969, 1970) and 

Ban& (1966). Wait shows that when the earth may be represented as an electrically and magnetically homogeneous halfspace of conductivity o and permeability P, and for the situation illustrated in Figure 30 where the earth's surface 

is the plane z = 0 and the loop is located at z = -h, the horizontal and vertical magnetic field components produced at the surf ace are, respectively, 

and 

where the buried magnetic dipole of area A carries current I and where 

u = (h2+iwpo)%. Upon numerical evaluation of the integrals in equations (16) 

and (17) above, Wait observes that has a sharp null for D x/h = 0 and 

rises to a maximum for D 0.6. This observation was closely verified in the 

field in the test at Peabody No. 10 mine. In addition, the behavior of the 

magnitude of the vertical magnetic field in the Peabody test substantiates Wait's 

results. At the Peabody No. 10 mine, 

so that when the receiver site is located at an offset distance 'x' equivalent 

to the depth to the transmitter h, the normalized field strength HZ/Hzo should 

fall to approximately 75% of its peak value. This behavior is illustrated in 

Figure 29. 

Passive Detection of a Buried Loop from the Surface 

Several investigators (Parkinson, Powell, Wait, 1972) have suggested that 

one possible method of locating a trapped miner would be to passively excite a 

closed insulated loop of wire frw the surface. The miner would have the 

target loop which could be any closed conductor, such as a single turn loop of 

AWG #10 wire of radius on the order of 1 meter. Both the source antenna and 

receiver loops would be at the surface (Figure 31). One distinct advantage of 

such a scheme is in the simplicity for the underground miner; namely, he would 
z RECEIVER 

SOURCE I LOOP 

Fig. 30 ~~~j.~~~ti~~ dipole buried in the earth. 

Active Receiver 

Passive Target 

Fig. 31 ~eomekrica~ configuration for pa8-i~- 

loop detection scheme. source antenna 

ha. radius a with n turns. Passive target loop has radius at with nt 

turn*. 
not have to possess (either within a mine working or on his person) any power 

source and all that he would need to do in a disaster situation is lay out a 

closed loop of insulated wire of maximum area. In this part of the report we 

discuss the practicality of such a scheme. In the sequel, reference is made to 

Figure 31. 

The transmitting loop antenna is located at the surface at the origin of 

our reference coordinate system and has a radius "a" with "n" turns. The passive target loop is buried at a depth -h, is offset a horizontal distance of 

Po, and has a radius "at" with 'Int" turns. The active transmitter loop will 

carry sane current I and will induce a current in the buried target loop which 

will be denoted by It. This target loop, then, will act as a secondary, passively excited source which will generate a back e.m.f. in the transmitting, 

active antenna. This back e.m.f . will cause a current of smaller magnitude Is 

to flow in the active loop at the surface. We now examine the ratio of the 

secondary current, Is, due to the presence of the buried target loop to the primary current, I, for it is against both the primary source field and the ambient 

noise background that Is must be detected. 

Wait and Spies (1971a) give the expression for the vertical magnetic field 

at the target observation point P at a depth h and offset distance po £ran the 

Source antenna: 

where 

and where A = a/h, Z = ho/h, D = p/h, H = (opow)'h. (Note that the only difference between equations (19) and (17) is that the fact that the loop antenna may 

not be a dipole antenna is accounted for; i.e., it may be a finite loop which is 

taken into account by the factor J1(Ax)/(Ax/2). Note that lim J1(Ax)/(Ax/2) = 1.) a+o - - 

In the above formulas Wait allows the source loop antenna to be elevated at a 

distance ho above the ground and also allows for a finite size of the transmitting loop instead of treating the source antenna as a vertical magnetic 

dipole in the strict sense. The symbol o denotes the conductivity (assumed 

uniform and isotropic) of the overburden, w ( = 2nf) denotes the radian frequency 

of the transmitter, and u denotes the magnetic permeability of the overburden 

(= 4n. lo-' henriesheter). 

In our case, the source loop antenna is on the surface so that Z = 0. 

We now wish to calculate the current induced in the passive loop, It. This is 

accmplished by first considering the e.m.f. generated in the target loop, Et, 

due to the time rateof-change of magnetic flux through it and then dividing 

that e.m.f. by the resistance Rt of the target loop wire. The e.m.f. generated 

in the target loop is found by a straightforward application of Faraday's law 

and Stoke's Theorem; namely, 
and 

where the surface integral is aver the effective area of the target loop and 

where the line integral is the total e.m.f. generated in the target loop. 

Equation (21) yields 

2 21ra n E = -p n Ira - (po -h) ttt ot tat 

so that 

and 

from (18) above. Since I(w) in the source loop varies sinusoidally with time, 

equation (23) may be written as 

L 

1-1 = - "oata w I 6 

t target 4R th 3 

The next step is to compute the magnetic field Hz,, at the point (0,O) due 

to the current It induced in the small passive loop. This secondary magnetic 

field may be found from (Wait, 1970): " 

where 

2 5 rn 3 e-(x2+~) 

Q=/ 

X J (Dx)dx 

0 x+(x2+in2)% 0 

or, by making use of equation (24), -" 

If Is is the induced current in the active source loop on the surface due to the 

current flow It in the passive loop and if R is the resistance per unit length of 

the active source loop, then 
22 332- 

1 =-- I-roa n vontata f QQI 

S 

aH /at = 2R 2,s 4R tRh 6 

Thus 

A 

The maximum value for Q or Q is where the offset distance po is zero; this maximum 

value for Q and 6 is unity so that 

Is 4~ n, - = 

I , maximum R~R~~ 

where R and Rt are given in ohms; a, at, and h in meters; and f in cycles per 

second. 

Equation (29) demonstrates that the secondary fields radiated by the passive 

loop antenna are quite small unless: (1) the radii a and/or at are made quite 

large relative to the depth of burial h, (2) the transmitting frequency f is 

markedly increased, or (3) the resistance R of the source loop and/or the 

resistance Rt of the passive loop is made small. Generally, weight is not a 

severe limitation for the source loop antenna at the surface so that a loop 

transmitting antenna might typically consist of one turn of AWG #1 insulated 

copper wire with a circular radius of 50 meters (requiring about 1000 feet of 

wire). This loop antenna would have a resistance of about 0.10 ohm. If one 

were willing to double the weight of wire, he could approximately halve the 

resistance. 

Because the secondary fields radiated by the passive loop are proportional 

to the third power of radii whereas they are in inverse proportion to only the 

first power of the resistances of the loops, it follows that it would be better 

to increase the area(s), even if in order to keep the weight of the passive loop 

low, Rt increases, As Wait and Spies (1971a) show, however, the maximum value 

chosen for Q and Q occurs at low frequencies, at a zero offset distance from the 

transmitter loop, and where the radii of the source loop and passive loop are 

much smaller than the overburden thickness. Thus, as a and/or at are increased 

in relation to h, a finite loop evaluation, not a dipolar one, must be made. 

For example, when the radius a of the source antenna is half the depth of burial 

to the passive loop, the coaxial vertical magnetic field at the passive loop 

(i. e., 4) falls from a value of unity to 0.72, a drop of 28%. This finite loop 

evaluation, when the areas of the loops become large relative to the overburden 

thickness, is not a severe limitation, although it must be kept in mind when 

considering actual coupling magnitudes. 

Another consideration is that the target passive loop and/or the receiving 

loop at the surface (here chosen to be at the same position as the surface 

transmitting loop) may be designed so as to have a resonant peak in the effective area at the transmitting frequency (which is judiciously chosen for 
adequate penetration and lower power spectrum noise levels). An example of such 

a peak in effective area (number of turns times area) of a typical air-core loop 

used for transmission tests at the U.S. Bureau of Mines Experimental Mine is 

illustrated in Figure 32 (this loop weighs about 4 lb and is about 1 meter in 

diameter) . 

Still another possibility for increasing the ratio of I IS/1I is to 

increase the transmitting frequency of the source loop. As stated above, however, this may be done efficiently only to a certain limit in order to obtain 

adequate penetration within the earth. An illustration of the attenuation of 

the subsurface vertical magnetic field of a surface vertical-axis loop antenna 

is given in Figure 33. In Figure 33 the attenuation in decibels is plotted as a 

function of the dimensionless depth ratio h/6 = h-, where 6 is a skin depth 

in the earth equal to I/=, o being the conductivity of the overburden and p 

being the magnetic permeability (= 41r.10-~ henrieslmeter). On the basis of 

electrical properties measurements of the overburden made thus far (Geyer and 

Keller, 1971b) an average value of o for the overburden of most coal mines would 

be about mhos/m and, at most, h would be 400 m (an overburden thickness of 

about 1350 feet) . In this case 

A typical transmitting frequency would be 1000 Hz so that h/6 = hm -= 2.5. 

Thus, referring to Figure 33, for most practical purposes the attenuation of the 

vertical magnetic field at 1000 Hz would be less than 10 db for even the deepest 

mine. However, if we increase the transmitter frequency, attenuation becomes 

greater so that in order to keep the same signal levels at the passive loop 

site, the power output of the transmitter would have to be increased. In addition, as Wait and Spies (1971d) note, the values of Q and 4 above become less 

than unity with increase of the transmitter frequency. 

In Tables 111-VIII below, the maximum value of the ratio of the secondary 

to primary current at the source transmitter location due to passive excitation 

of a buried loop is given at a number of frequencies and depths of burial and 

under the constraint that the passive loop weight is to be kept to about 2 lb. 

Tables IX-XI give the same ratio as a function of frequency if an increased 

weight to about 6 lb for the passive loop would be acceptable. (It should be 

noted that the values given for I 1 are maximum possible values, particularly 

for the higher frequencies since the values of Q and Q were taken to be unity. 

At higher frequencies Q and 6 would actually be less than unity. For example, 

at 10,000 Hz, an overburden resistivity of 100 ohm-meters, and an overburden 

thickness of 100 m, the value of Q and 6 would be closer to 0.9.) 

The above analysis has been based on a C.W. type transmission. Recently, 

Wait and Hill (197 2a) have considered the electromagnetic transient response of 

a small wire loop buried in a homogeneous conducting earth for impulsive 

excitation and have obsemed that the time delay and difference in wave shape 

of the target response offers some hope for detection and direction-finding. 

However, on the basis of experimental field transmission tests (downlink and uplink) performed at the U.S. Bureau of Mines Experimental Coal Research Mine as 

well as on the basis of ambient noise measurements taken at a number of large 

operating coal mines (Geyer and Keller, 1971b) it seems that it would be more 
LZ EFFECTIYE A,WR OF TYPICAL AIR-CORE LOOP VS. FREciUEllCY ATTENUATION OF VERTIC

FIELD OF SURFACE VER

ANTENNA VS. NORMALIZ

h 1 b 

FREQUENCY (Hz) 

Fig. 32 Fig. 33 
TABLE 111. Ratio of Secondary to Primary Current at Source Transmitter Location 

Due to Passive Excitation of Buried Loop. 

f 

(Hz) I 'sf1 I ,maximum 

Passive loop buried 100 m and consists of 1 turn of AWG #5 insulated copper 

wire with circular radius of 1 m. Passive loop weighs 2 lb and has a resistance 

Rt of 0.006 ohm. Source loop at surface consists of 1 turn of AWG ill insulated 

copper wire and has a circular radius of 50 m and resistance R of 0.10 ohm. 

TABLE IV. Ratio of Secondary to Primary Current at Source Transmitter Location 

Due to Passive Excitation of Buried Loop. 

f 

(Hz) I Isf1 I ,maximum 

Passive loop buried 200 m and consists of 1 turn of AWG /I5 insulated copper 

wire with circular radius of 1 m. Passive loop weighs 2 lb and has a resistance 

Rt of 0.006 ohm. Source loop at surface consists of lturn of AWG 111 insulated 

copper wire and has a circular radius of 50 m and resistance R of 0.10 ohm. 
TABLE V. Ratio of Secondary to Primary Current at Source Transmitter Location 

Due to Passive Excitation of Buried Loop. 

Passive loop buried 300 m and consists of 1 turn of AWG /I5 insulated copper 

wire with circular radius of 1 m. Passive loop weighs 2 lb and has a resistance 

Rt of 0.006 ohm. Source loop at surface consists of 1 turn of AWG /I1 insulated 

copper wire and has a circular radius of 50 m and resistance R of 0.10 ohm. 

TABLE VI. Ratio of Secondary to Primary Current at Source Transmitter Location 

Due to Passive Excitation of Buried Loop. 

Passive loop buried 100 m and consists of lturn of AWG 814 insulated copper 

wire with circular radius of 10 m. Passive loop weighs 2 lb and has a resistance 

Rt of 0.50 ohm. Source loop at surface consists of 1 turn of AWG #l insulated 

copper wire and has a circular radius of 50 m and resistance R of 0.10 ohm. 
TABLE VII. Ratio of Secondary to Primary Current at Source Transmitter Location 

Due to Passive Excitation of Buried Loop. 

f 

(HZ) ('sf1 I ,maximum 

Passive loop buried 200 m and consists of 1 turn of AWG /I14 insulated copper 

wire with circular radius of 10 m. Passive loop weighs 2 lb and has a resistance 

Rt of 0.50 ohm. Source loop at surface consists of 1 turn of AWG /I1 insulated 

copper wire and has a circular radius of 50 m and resistance of 0.10 ohm. 

TABLE VIII. Ratio of Secondary to Primary Current at Source Transmitter Location 

Due to Passive Excitation of Buried Loop. 

Passive loop buried 300 m and consists of 1 turn of AWG #14 insulated copper 

wire with circular radius of 10 m. Passive loop weighs 2 lb and has a resistance 

Rt of 0.50 ohm. Source loop at surface consists of 1 turn of AWG #l insulated 

copper wire and has a circular radius of 50 m and resistance of 0.10 ohm. 
TABLE IX. Ratio of Secondary to Primary Current at Source Transmitter Location 

Due to Passive Excitation of Buried Loop. 

Passive loop buried 100 m and consists of 1 turn of AWG i\10 insulated copper 

wire with circular radius of 10 m. Passive loop weighs 6 lb and has a resistance 

Rt of 0.20 ohm. Source loop at surface consists of 1 turn of AWG 1\1 insulated 

copper wire and has a circular radius of 50 m and resistance of 0.10 ohm. 

TABLE X. Ratio of Secondary to Primary Current at Source Transmitter Location 

Due to Passive Excitation of Buried Loop. 

f 

(Hz) / IS'' / ,maximm 

Passive loop buried 200 m and consists of 1 turn of AWG 810 insulated copper 

wire with circular radius of 10 m. Passive loop weighs 6 lb and has a resistance 

Rt of 0.20 ohm. Source loop at surface consists of 1 turn of AWG #1 insulated 

copper wire and has a circular radius of 50 m and resistance of 0.10 ohm. 
TABLE XI. Ratio of Secondary to Primary Current at Source Transmitter Location 

Due to Passive Excitation of Buried Loop. 

f 

(HZ) ('s"( ,maximum - 

Passive loop buried 300 m and consists of 1 turn of AWG 810 insulated copper 

wire with circular radius of 10 m. Passive loop weighs 6 lb and has a resistance 

Rt of 0.20 ohm. Source loop at surface consists of 1 turn of AWG #1 insulated 

copper wire and has a circular radius of 50 m and resistance of 0.10 ohm. 
expedient, if possible, to use an active source at depth for direction-finding 

and only to consider passive detection as a backup method of location. It is 

because ambient noise levels at the surface of most operating coal mines are in 

excess of the secondary fields radiated by a buried passive loop. 

Downlink Pulse Signalling to Underground Mine Working 

Theoretical Considerations 

Another approach for through- the-earth camnunications, in this case downlink from the surface to an arbitrary subsurface position within a mine working, 

may be effected by a long insulated wire surface antenna excited by an impulsive or step current. This insulated wire at the surface is grounded at distant 

ends (see Figure 34), and with this type of antenna at the surface, a number of 

different source-receiver sensor configurations are possible (see Figure 35). 

In a previous report (Geyer, 1972a) the mutual coupling (voltage induced in the 

receiving antenna normalized by the current in the source antenna) for impulsive 

excitation of the line source at the sulface and for a vertical loop receiver was 

found to be 

2 2 

Po 1 POZO -P x 

~"ertical(xo~'o;t) = - 2 { 1 - -)=p \- 

2rt 2P t O O) 4pt 

Loop 

2 

iioZo 2 POXO vox0 

=P(-~~X~I~P~) + + (ZT K exp (- -) 

26 pt 4pt 

1 !-'o % 2 

- - 2 (-) rpt ) exp (-vozo/4pt)} 

where the receiving sensor is at (xo,zo), p is the resistivity of the overburden, 

vo the magnetic permeability (= 471 x lo-' henrieslm), and where K is known as 

Dawson's integral or 

Similarly, the mutual coupling for a horizontal loop was found to be, 
LONG LINE 

EARTH'S TRANSMITTER 

SURFACE 

1 RECEIVER 

z POINT 

Fig. 34 oeometry Of a long insolated grounded wire at the earrh,. surface and an arbitrarily located broadside receiver position. 

DOWNLINK SOURCE - RECEIVER SENSOR 

CONFIGURATIONS 

,Line Source , T \ 

Grounded .-I. 

Line 

,Line Source \ \ \ -, 

Horizontal Loop 

,Line Source \ \ \- \ 

vertical Loop 

Fig. 35 
and the electric field measured with a grounded line receiver, 

00 1 E (x ,z 't) = - u 0220 2 

y 0 0' 2 {T (1 - -1 exp (-uoxo/4~t) 2P t 

2P t 

Numerous theoretical coupling curves for any of the above receiving sensors 

at a number of different depths of burial and offset distances and for several 

resistivity distributions of the overburden were calculated under the assumption 

that the source line antenna was impulsively excited. These results were extensively discussed in a previous comunication (Geyer, 1972). Several typical 

transient coupling responses for the propagated electric and magnetic field components are given in Figures 36, 37, 38, 39, 40, and 41. 

As noted earlier, the evaluated transient mutual coupling responses at a 

buried receiver site for impulsive input to the source antenna suffer no loss in 

generality and may be put to use for any type of current input to the source 

antenna. This fact is an important one, for the practical use of the time-domain 

electranagnetic method frequently requires the transmission of a step current 

(as distinct from an impulsive current) into the ground and the corresponding 

recording of a transient at the receiver location. If the harmonic function 

F(iw) corresponding to an impulsive input 6(t) is known for all frequencies, then 

the response for other inputs can also be determined. Consider, for example, a 

unit impulse into the earth as a linear system: 

6 (t) EARTH f(t) 

where f (t) is the impulse response of the earth linear system. Next, consider 

a step function u(t) as input into the same earth: 

~(t) EARTH f (t) * u(t) 
p = 10 ohm - m 

ZQ ' 100 m 

I 

0 1 2 3 4 

Fig. 36 7, PILL,^,' 

-I 1 BURIED HORIZONTAL LOOP 

p = 10 ohm - m 

Zo= 100 rn 

I I I 

0 1 2 3 4 

7 4t n 111 7 2 

Fig. 37 
-16, A BURIED VERTICAL LOOP 

'1 \ p = 10 ohm - m 

I 

0 1 2 3 4 

2 

Fig. 38 r, 4t~/~~z~ 

p = 100 ohm-m 

Zo = 100 m 

I 

0 1 2 3 4 

Fig. 39 

2 

r, ~~PIP~Z~ 
-I I BURIED HORIZONTAL LOOP 

p = 100 ohm -m 

2, = 100 rn 

I I 

0 1 2 3 4 

Fig. 40 7, 4tp/~,zf 

-16 - BURIED VERTICAL LOOP 

p = 100 ohm-m 

Zo ' 100 m 

4 ! I 

0 1 2 3 4 I 

Fig. 41 T, 4t~111,~: 
where f(t) * u(t) is the impulse response of the earth convolved with the step 

input function. In the frequency domain this mathematical operation can be 

written as F(iw)/iw where F(iw) represents the system function of the earth and 

l/iw is the operator that will transform F(iw) into the response for a step 

input function. Thus, if one uses a current input to the source antenna which 

is different from a sharp impulse 6(t), all he need do is to convolve that current input with the transient coupling response evaluated for impulsive excitation to analyze the resultant waveforms. 

Several important points are deduced from theoretical considerations for 

downlink pulse signalling using a long insulated line transmitter grounded at 

the surface (Geyer, 1972a). For example, analysis of a variety of depths of 

burial and horizontal offsets of the receiving sensor from the surface line 

antenna indicates that doubling the depth to an electric field sensor reduces 

the coupling transient peak by approximately one order of magnitude, whereas the 

same increase in depth to a vertical loop receiver changes the magnitude of the 

coupling response by about two orders of magnitude. As the horizontal sourcereceiver offset distance becomes larger, the coupling transients for all field 

canponents decay less rapidly and positive signal peaks for the electric field 

occur later in time. All coupling transient responses increase as the square 

of the resistivity of the earth. Finally, all of these phe~mena, as well as 

distinctions in amplitude and shape of the coupling transient responses, may be 

ascertained with a recorder which has a relatively fast rise time. 

Experimental Downlink Pulse Signalling 

The above theoretical considerations and calculations were made so that 

they could be used in the design and interpretation of an actual field transmission test. Prior to a downlink pulse transmission experiment performed at 

the U.S. Bureau of Mines Experimental Coal Mine, a study was made of the resistivity distribution of the overburden. This study was made so as to ensure 

proper interpretation of experimental coupling data with calculated theoretical 

coupling curves. A typical resistivity profile of the overburden at the 

Experimental Coal Mine is given in Figure 42. From this figure we may ascertain 

that the electrical properties of the overburden are relatively homogeneous and 

uniform, a situation not unexpected in light of the nature of the geology of 

the overburden which consists primarily of highly weathered shale with some 

silty laminae of sandstone. The average resistivity of the section shown in 

Profile Line 1NW-SE is about 177 ohm-meters. 

In the pulse transmission experiment the line antenna at the surface was 

excited with a step current and the transient signal at the receiver location 

was sensed with a grounded insulated line receiver, amplified, displayed on a 

small Tektronix oscilloscope, and photographed with a camera. The length of the 

surface antenna was approximately 223 m, and the magnitude of the exciting step 

current 1 ampere. In this test the horizontal electrical field at zero offset 

distance was measured. The source antenna, which was separated from the buried 

receiver by approximately 23 m, was excited by driving a General Ratio 1308-A 

Power Amplifier (200 watts, 5 amperes maximum output) with a square-wave function generator. 
PROFILE LINE INW-SE 

U.S.B.M. EXP. MINE 

NW 

6 177 175 

177 159 1 

Fig. 4 2 rpic.l ..~i.~i~itr pr.t&l. (in OI,~-~~IC,,) -1 .v. burden .t U. 5. BU,~." OF 

Mino Experimentoi Cool Mina, Brucrnmn, Pcnn~ylvoni~. Eltron goIvoniC 

.rrq "led in r.,;,,ivity d.lermin.fi.n. 

4 k0.1 rns 

Fig. 43 T~~~~~~~~L for horirnntal elcctric field exciiltion of the line svrfrce 

as traced from a photograph of 

oscillasc~pe Face. Gain rs 30 db. 
The transient coupling for the horizontal electric field without excitation of the line surface antenna is shown in Figure 43 (traced from a photograph 

of the coupling signal on the oscilloscope). The received transient coupling 

signal with excitation of the line surface antenna is shown in Figures 44 and 45, 

where Figure 45 displays the signal corrected for the gain of the amplifier 

and length of the receiver sensor. Figure 45 also illustrates the theoretical 

step response at the receiver location and shows reasonably close agreement with 

the field data. The differences evident in the theoretical step response and the 

actual response measured are interpreted to be the result of sane factors; i.e. 

(1) the theoretical coupling curve used for comparison was that calculated 

assuming an infinitely long surface antenna rather than a finite horizontal 

electric dipole and (2) the system response of the amplifier system was not 

deconvolved out of the data illustrated in Figures 44 and 45. 

At any rate, the results of the downlink pulse-type communication test (as 

opposed to C.W. tests) demonstrate the feasibility of a transient technique for 

through-the-earth signalling. One obvious advantage to a pulse-type technique 

is that a relatively large amount of frequency information is inherent in one 

pulse. However, a distinct disadvantage of a pulse-type communication system 

for signalling between the surface and a mine working is that ambient noise 

backgrounds, particularly in the "late-time" part of the transient coupling 

response, will lead to reduced signal/noise ratios at practical transmitter 

power output levels (between 10 and 100 watts). At the U.S. Bureau of Mines 

Experimental Mine, this constraint was nor a serious limitation; however, in a 

large operating coal mine, measured electrical noise environment levels (Geyer, 

-- et al, 1971b) suggest that a receiver system for a pulse-type system would have 

to be designed so as to account for decreased signal-to-noise ratios in the 

late-time (after the zero crossings) part of the transient response. 

Continuous Wave Field Transmission Tests 

The implications of the use of either a short line source or a loop source 

buried at depth within a mine working have already been discussed with regard to 

possible location schemes. In this part of the report we examine in greater detail 

uplink comunication techniques and actual tests from a mine working to the surface so that we may be able to gain insight into which source-receiver configuration might be best to use under a given set of circumstances. It was examined that 

how well the coupling signals correspond to what theory predicts, how many distortion effects (due to sources of noise and secondary scattering; i.e. power lines, 

buried pipes and rails, mine machinery, etc.) are seen which affect communications, 

and how one may avoid distorting structures by a judicious choice of the type of 

measurements he will make. In addition, we will be able to observe in firsthand fashion what the effect of large transmitting loops (i.e., finite loops) 

have on the nature of propagated electromagnetic fields measured at the earth's 

surface; thus something can be said about wavetilt, actual magnitudes of the 

measured field components, and maxima and null phenomena in the surface field 

components as they relate to the size of the transmitting antenna loop. 

One technique, then, for canmunications from a mine working to the surface 

would involve the excitation of an insulated loop of wire with continuous wave 

audio-frequency current, where the loop of wire might be that which is wound 

about a mine pillar. One would then measure the produced electromagnetic field 

at the earth's surface. For example, the produced magnetic field could be 
Fig. 44 ~ransient coupling for horizontal electric field with excitation of the line surface antenna as traced from a ohotoara~h oscrllascope face. Oain LS 30-db: 

TRANSIENT HORIZONTAL ELECTRIC 

FIELD RESPONSE 

-4 k0.1 rns 

experimental 

.------- theoretical 

Fig. 45 Transie". h..i.."..l e1ectric field 

signal corrected for gain of amplifier system and length of receiver sensor. 
measured by using two small induction loops; by noting spatial changes in the 

amplitude voltage response through either induction loop, one can, in addition 

to establishing a communications link, triangulate on the position of the buried 

transmitter. Actual field tests were made using this source-receiver configuration, which is illustrated in Figure 46. 

The other alternative for through-the-earth uplink or downlink voice 

transmission involves the use of an insulated wire which is grounded at both 

ends and which is excited with C.W. audio-frequency current or with a strong 

impulsive current. Excitation of a line source with a strong impulsive current 

together with experimental results has already been discussed. Here the first 

mode of antenna excitation -- that of C.W. audio-frequency current -- is discussed. 

During the summer months of 1972 (June - September) numerous transmission 

tests, both uplink and downlink, were made at the United States Bureau of Mines 

Experimental Coal REsearch Mine in Bruceton, Pennsylvania, and the U.S. Steel 

Rubina No. 4 Coal Mine in Uniontown, Pennsylvania, using various source-receiver 

configurations. This section of the report discusses the results of several of 

those transmission tests in which a line current source was used as the transmitting antenna. Such a configuration is shown in Figure 47. The transmission 

tests were made over a relatively broad band of frequencies and at a number of 

different gepgraphic setups. 

Horizontal Electric Field Measurements - Buried Line Source Transmitter - U.S. 

Bureau of Mines Experimental Coal Research Mine, Bruceton, Pennsylvania 

In Figures 48, 49, 50, 51, and 52 are summarized typical C.W. mutual 

coupling transmission data taken at the U.S. Bureau of Mines Experimental Coal 

Research Mine at Bruceton, Pennsylvania, where measurements of the horizontal 

electric field at the surface over a broad range in audio frequency were made. 

In these transmission tests only the component of the surface horizontal electric field which is parallel to the direction of the buried line transmitting 

antenna was measured. These data are corrected for the calibration response of 

a 60 Hertz filter-amplifier system and are normalized by the source strength 

moment of the line transmitter (i.e., the product of the current output to the 

transmitting antenna and the length of the transmitting antenna). Note that the 

normalization is by the current output (rather than by the power output) since 

the amount of current with which one is able to power the transmitter changes 

with frequency, normally decreasing with frequency. Because the strength of the 

source transmitter is proportional to the current (the current may often 

decrease while driving voltages increase, thus keeping the output power levels 

constant), it is probably best that mutual coupling data be presented in this 

fashion. 

The location of the buried line source transmitter, which consisted of a 

45 meter length of AWG #18 gauge insulated copper wire grounded at distant ends 

with copper-clad steel stakes driven into the ground (roof bolts could also be 

used for the ground return electrodes, but in this case care should be taken to 

reduce the contact resistance of the roof bolts by either filing of the normally 

accumulated layers of rust or by using screw-set clamps), is shown in Figure 53. 

The line source transmitter was powered by a General Ratio Model 130E-A Power 

Oscillator (200 watt, 5 ampere maximum output). Output current levels from the 
4 KECElVER SENSORS 

NINE LOOP TPANSIKTTER WORKING iN I+ --I-- 

a 

Fig. 46 vertical-axis loop transmitter antenna 

and induction loop receiving sensors far uplink C.W. trnnsnissi.n. 

, RECEIVER SENSOR 

EIIRTH'S SURFACE Y--- 

X 

r, r-, 6- 

h 

LINE CURReNT TWSMITTER IX NINE NORt(lNG 

Fig. 47 Three-dinmrional "iev of transmitter antenna and rccerver sensor under consideration far 

"pllnk C.W. transmission. 
. Pield ~ata hi23m 

x-om 

dl , , , u 

uo 

G) 10 100 1000 rl 10000 

w Fig. 48 FREQUENCY. HZ 

. Field Data 

U. S. DUI<IIM Or MINE5 CXRCRIWNTAS 

MINE, BRUCETON. Pli. 
I<L!CY IYCT 

Lx + U. S. JIIIUSIIU OP MINES EXPERIMCNTAL 

, , MINE. BIIUCBTON. PA. ,, 

h 0. = 0.01 mhos/m 

Line Transmitter STATION 3 

. Field Data 

I I 

100 1000 I loo00 

Fig. 50 PREWENCY. HZ 

IICTI. I Yor U.!:. IIUI<IIM 01' NIIII:S L:YL'I:lllMl.hTAl. 

, k:~ MINE. UItUCCTON. PA. 

STRTTON 6 

r, 6.01 "hos/m 

Line Tcansm h-23m 

. Field Data Xi60rn 

@ = 88 

I k I 

100 1000 10000 

Fig. jl FREQUENCY. HZ 
, ,,cccivcr U. 5. DUI4LNI OP MINES EXPERIMCNTAL , x MINE. BRVCETON. PA. 

f 0.01 mhos/m STATION 7 

. I I I 

100 ID00 loo00 

Fig. 52 PIIEQUENCY. HZ 
line transmitter into the ground varied from about 0.8 ampere at 20 Hertz to 

about 0.2 ampere at 20,000 Hertz. The surface locations of the various receiver 

sites are also shown in Figure 53, where the angles between the center of the 

surface projection of the line transmitter and the centers of the various 

electric-field receiving electrode setups are indicated. 

Surface electric field measurements were made with non-polarizing electrodes 

(metal immersed in a solution of one of its salts) oriented in a direction 

parallel to the buried line transmitter. Such stable, non-polarizing electrodes 

were used so that electrolytic potentials would not be built up between the 

electrodes and the solutions in soil pores. The potential difference between 

the surface receiver electrodes was measured with a Hewlett-Packard Model 3410A 

Tunable Phase-Lock Microvoltmeter; this microvoltmeter has an input impedance 

of 10 megohms and is capable of measuring coherent signal levels 10 db below 

random ambient electromagnetic noise levels (white noise). A typical crosssection of field-collected electrical resistivity data over the U.S. Bureau of 

Mines Bruceton Mine was illustrated in Figure 42. 

The C.W. coupling transmission data given in Figures 48-52 shw, in general, good agrewent with what might be predicted theoretically for an average 

resistivity of the overburden of about 100 ohm-meters. Namely, two features 

of the field data clearly stand out, as illustrated in Figure 5h: (1) the 

parallel electric field strengths decay exponentially with increasing offset 

from the buried line transmitter, and (2) parallel electric field signals produced at the earth's surface per unit source moment are larger at lower frequenc ies . 

A peculiar feature not predicted by any theoretical considerations which 

is particularly evident in the nature of the received electric field signals at 

station setups 2 and 3 is the observation that the coupling transmission signals 

(normalized by the strength of the source antenna) actually increased with 

increasing frequency. Moreover, there is almost negligible scatter in this data! 

The exact reason for this field result (which is opposite to what would be 

predicted by theory) is not known. However, it is thought that the power.line 

which ran directly overhead parallel to the buried transmitter (NE-SW), as well 

as the surface pipe lines which were present, could have acted as secondary noise 

and scattering sources with efficiency of signal coupling in this case increasing with frequency. 

Vertical Magnetic Field Measurements - Buried Vertical-Axis Loop Transmitter - 

U.S. Bureau of Mines Experimental Coal Research Mine, Bruceton, Pennsylvania 

According to theoretical considerations (Wait, 1970; Wait and Spies, 1971d) 

the amplitudes of t h e v e r t i c a l and horizontal magnetic field components, respectively, produced at the earth's surface by a buried vertical-axis insulated loop 

carrying alternating current are given by the following expressions: 

and - 
- 

rn 

2 

0.5 1.0 1.5 2.0 1.5 3.0 1.5 4.0 6.5 0 

Fig. 54 NORMRLTZED OFFSET FROM BURTED LINE TRMISMITTER. ~fh 
where the attenuation factors IQI and JMJ are not simply exponential functions 

of the separation between the source antenna and receiving sensor, but are 

rather given by the dimensionless expressions, 

Here Jo and J1 are Bessel functions, D is the horizontal offset distance 

p divided by the depth of burial h, and H is the product h(21~fou~)%. The symbol 

A represents the ratio of the radius of the source loop to the depth of burial 

of the source loop; as noted previously (Wait and Spies, 1971d; Geyer, et al, 

197213) the theoretical offset positions of the amplitude nulls in the surface 

vertical magnetic field as well as the peak in the surface horizontal magnetic 

field depend on the parameter A. For example, as the radius of the source loop 

transmitter becomes large relative to its depth of burial (say twice as large), 

the offset null position in the vertical magnetic field can shift by approximately 

70% at near static conditions to about 100% at higher frequencies (see Figure 55). 

In addition, the actual magnitude of I QI directly over the source antenna 

decreases as A a/h increases. Figure 56 illustrates percent changes in I QI 

relative to static coupling directly over the buried vertical-axis loop transmitter as the radius of the transmitting loop becomes equal to the depth of 

burial (a/h = 1) or becomes equal to twice the depth of burial (a/h = 2). 

These phenomena in themselves are not detrimental to a communications scheme. 

They be recognized, however, if one wishes to map the nulls in the surface 

vertical magnetic field and use them as location criteria. 

For static conditions the size of the loop does not affect the sharpness 

of the null in the vertical magnetic field (which for a very small loop 

(a/h - 0) occurs at an offset distance from the transmitter equal to about 1.4 

times the depth of burial of the transmitter), ambient electromagnetic noise 

factors being equal. The effect of increasing the frequency of transmission 

tends to blur or make less distinct these nulls. For field purposes, however, 

the offset positioning of these nulls (though less distinct) is not significantly 

affected. In a similar fashion, an increase in the electrical conductivity a of 

the overburden tends to blur the distinctness of these nulls while not significantly affecting the offset position of the null. This phenomenon is to be 

expected, since for a given depth of burial h to the transmitter and a given 

size A = a/h of the transmitter, it is really the product (a£)% which deternines 

the sharpness of the null. 

The above discussion leads to several deductions. For one, if an attempt 

is made to measure absolute amplitudes of the vertical magnetic field with a 

vertical-axis induction loop as a means for locating a buried transmitter in a 

mine working, there is a necessity for knowing quite accurately both the effective electrical conductivity of the overburden as well as the size of the 

transmitting loop antenna. Generally, a priori knowledge is at hand on the depth 

of burial and frequency of transmission, but it may not always be practical to 

know what the effective size of the transmitting antenna is unless a standard is 
OFFSET XS 

NOmIZED RADIUS OF LOOP TRIWSNITTER. A - 

Fig. 55 

Fig. 56 
both set and implemented. As a general rule (Geyer and Keller, 1971b) it was 

found that electrical conductivities within any given coal mining district do 

not change by more than 10:l. Changes in electrical conductivity of the overburden from one coal mining region to another (e.g., from eastern Colorado to 

West Virginia), however, can vary as much as 100:l. Of course, exceptions to 

the above rule-of-thumb exist, and the generalizations made above would probably 

not hold in metal mine districts, metal mine regions being usually quite inhomogeneous electrically. At this juncture, a trade-off exists: namely, the effect 

of electrical inhomogeneities on the field behavior may be minimized by decreasing the frequency of transmission; however, by using lower frequencies and keeping output transmitter power constraints the same, one generally has lower 

signallnoise ratios. The latter effect often overshadows any effects of electrical inhomogenei ties. 

In Figures 57-61 are summarized typical C.W. mutual coupling data taken at 

the U.S. Bureau of Mines Experimental Coal Research Mine at Bruceton, Pennsylvania, where measurements of the vertical magnetic field at the surface over a 

broad range in audio frequency were made. These data are corrected for the 

calibration response of a 60 Hertz filter-amplifier system, for change in effective area of the receiving loop sensor (e.m.f. induced in loop = effective area 

X magnetic permeability of freespace X 21rf X vertical magnetic field) and by 

the current output to the transmitting antenna since the amount of current with 

which one is able to power the transmitter changes with frequency (normally 

decreasing with increasing frequency as noted earlier). Because the strength of 

the source transmitter is proportional to the current, it is probably best that 

mutual coupling data be presented in this fashion. Moreover, the form of presentation of the empirical data (attenuation function IQI in Figures 57-61 also 

corresponds to that used by Wait (1971a) and thus provides for ready comparison 

with theoretical calculations over prespecified models. 

The location of the buried vertical-axis loop transmitter, which consisted 

of 5 turns of AWG ill8 gauge insulated copper wire wound around a pillar beneath 

station 1 and powered by a General Radio Model 1308-A power oscillator (200 watt, 

5 ampere maximum output), is shown in Figure 53. Surface measurements were made 

with an air-core loop sensor whose effective area and impedance were measured in 

the laboratory (see Figure 62) and whose diameter is 1 meter. E.m.f.'s induced 

in the induction loop sensor were measured with a Hewlett-Packard Model 3410-A 

Phase-Lock Microvoltmeter which could measure coherent signal levels 10 db below 

ambient electromagnetic noise levels. The effective radius of the source loop 

antenna in this case for uplink transmissions was approximately 0.39 the depth 

of burial. An average conductivity of 0.01 mhoslmeter was taken for the overburden for theoretical comparisons. This choice was made from the field resistivity data collected on the surface (Geyer and Keller, 1972b); a typical profile over the mine workings at the U.S. Bureau of Mines Bruceton Mine is shown 

in Figure 42. 

The mutual coupling curve for the vertical magnetic field at station 1 is 

shown in Figure 57 as a function of frequency from 20 Hertz to 10,000 Hertz. 

At this station the offset distance p was approximately zero so that D A p/h = 0. 

The field data demonstrate close agreement with the theoretical considerations 

of Wait and Spies (1971a) except at 32 Hertz and 330 Hertz, where large noise 

levels were noted. Another obsenration which should be noted from Figures 57-61 
RECEIVER 6 UIL~NX C.W. TRRNSHISSION TEST ,,r U.S. BUREAU OF UTNES EXP 

0.01 mnos/m ,,'. 23 MINE. PA. 

mop TmwsmITTriR 6-1 a = .h r 0.39 

D =,A, IO.0 

STATION 1 

Fig. 57 

RECEIVER U.S. SURER" OF MINES EXP. WINE. PA. UPLINK C.W. TWSHISSION 

TEST a = ah = 0.39 

h = 23 rn =y~ = 0.46 

loop TWBMITTER& .i. 

$ 1.2 STATION 2 D 

t A THEORETICAL 

(lifter Wait and spies, 1971~1 

Fig. 58 
a A A A 

UPLTm C.W. TWSHISSIOLI TEST 

RECEiYER & U.S. BURERU OP MINES EX?. HINE, Pii 

R I a/h C 0.39 

FIELD DATh dc 0.01 mhodrn 1 D = ?,% c 0.9 

h=23m 

STATION 3 

loop TmwsmTTER&-- --1 

A THEORETlCRL +p 4 

l~fter Walt and spies. 1971al 

I I I 

100 1000 10.000 

FREQUENCY (Hz1 

Fig. 59 

UPLTNX C.W. TWSHlSSXON TEST U.5. BUREW OF MINES EXP. MINE. Pli. dzzz-r it=;a/hc0.39 

h = 23 m =,~fi z 1.45 

1.2 PTELD DATA 

LOOP TWVTSMITTE 

(THEORETTC~I NULL POSITION) A TnEaRelrcar. 

(lifter wait and spzes. 1971ai . 

. 

Fig. 60 
1.41 UPLTM( C.W. TEST 

FIELD DATA 

RECEIVER U.S. BUREAU OF HINEB EXP. MINE, 6' 0.01 mhos/m PA. h-23m 

A TxeoRerrcaL 

i~frer: wair and spies. 1971.1 

, 

0.6 

- 

% I 

0.0 I I I I 

10 100 1000 10.000 

PREQrnBCY (HZ1 

Fig. 61 
is that for any given geographic setup the response is essentially constant; 

that is, there is very little attenuation in the vertical magnetic field over 

the 10-10,000 Hertz frequency range when propagating through the overburden at 

the Bruceton Mine. Another way to sa this ;s that the attenuation function Q 

does not decrease with the parameter ~IT~IL~G)~RI as quickly as the function 

e-1 (~~uo~)~~I (which function describes the attenuation dependence for a plane 

wave in a lossy medium). 

At an offset distance equal to about four-tenths of the source antenna 

depth of burial (see Figure 58), good agreement with theoretical considerations 

was also attained, little scatter being present in the data. The mutual coupling 

responses measured at stations 3 and 4 (see Figures 59 and 60), however, show 

considerable scatter from theoretical calculations. I.e., a field null position 

in 1~~1 was detected at D - 1 rather than the theoretical (static) D = 1.4. This 

obsenred scatter may be explained in the following way. Close to stations 3 and 

4 (see Figure 62) were located both surface pipelines and overhead NE-SW powerlines. The effect of the overhead powerlines and/or the surface pipelines may 

have been responsible for shifting the null position in IH~ 1 ; i.e., either the 

powerline or the pipelines or both may have been excited by the buried loop 

antenna which, in turn, acted as secondary scattering sources and led to distortion of the profile of the attenuation function IQI as measurements were 

taken close to them. The reason why such scatter was not present in measurements 

taken at station setup 2 is that the signal/noise ratio was higher (instmnentation used was capable of measuring signal levels which were 10 db less than 

ambient electromagnetic random noise levels). 

Such an explanation seems quite reasonable when the theoretical effects 

of conductive rails and pipes are considered. Wait (1971f) and Howard (1972) 

have considered the effect of cylindrical conductors buried in the earth when 

they are excited by a long uniform line source of current on the surface and 

have shown the shift in the subsurface vertical magnetic field null directly 

under the surface line antenna in a direction away from the cylindrical scattering conductor. Although this model is = that which was present in the 

actual field tests, it does provide an intuitive feel and answer for the behavior 

of electromagnetic fields excited by other sources in the presence of such 

scattering conductors. It perhaps should be emphasized that such scattering 

effects (noise factors aside) can be limitations in schemes which attempt to map 

nulls for location purposes. The scattering problem of conductive bodies under 

excitation by dipolar sources is not at all trivial, and recently some notable 

progress has been made in gaining insight into such distorting effects (Wait 

and Hill, 1973a; Hill and Wait, 1973b; Hill and Wait, 1973~). 

At an offset distance from the subsurface antenna axis of 60 meters (station setup 6) close agreement with theory again was attained. As predicted by 

theory the vertical magnetic field produced at the surface is at a maximum 

directly over the vertical-axis loop antenna, drops to a null position at an 

offset distance of about 1.5 times the depth of burial of the source antenna, 

builds up to a second maximum (which is about 20 db lower than that directly 

over the source), and then continues t o f a l l off with increasing distance. 

These phenomena are summarized in Figure 63 at four different transmission frequencies (20 Hz, 100 Hz, 1000 Hz, and 7000 Hz). Figure 63 shows that the 

measured field null at these frequencies did not coincide with that expected 
\ CIU.IBRAT-TION CURVE OF aIR-CORE 

REcE~VER COIL AND 60 HZ NOTCH 

FILTER SYSTEN 

I I 

LO 100 iuoo 

Frequency Hertz1 

Fig. 62 

I mhoa,m 

I FlEWl D

,001 I I I I 

0.0 0.5 1.0 1.5 2.0 

NORMALXZED OFFSET peon BURIED T-SMITTER 

Fig. 63 
theoretically, being slightly skewed to smaller offset distances. These data 

also show a shift in the null toward greater offsets as frequency (or "H") 

increases; this shift is similar to that predicted by theory (Wait and Spies, 

1971a). 

The supposition might also be advanced on the basis of field experience 

that it is probably better to base location schemes, not on absolute field 

strength measurements, but on relative field strength behavior. Further, although the data gathered at separations of 65-90 meters don't show too much 

scatter at frequencies less than 1000 Hz, good lock-in on signals in the frequency range 1000-2000 Hz seemed to be attained most quickly. The latter 

statement is demonstrated by field measurements taken at the U.S. Steel Rubina 

No. 4 Mine, Uniontown, Pennsylvania, which are described below. 

Vertical Magnetic Field Measurements - Surface Vertical-Axis Loop Transmitter - 

U.S. Steel Robena No. 4 Coal Mine, Uniontown, Pennsylvania 

A downlink C .W. transmission test was made at the U.S. Steel Robena No. 4 

mine, Uniontown, Pennsylvania, on September 6, 1972. In this transmission test a 

vertical-axis single-turn loop antenna was set up at the surface with an area of 

2200 square meters. An induction loop sensor to measure the amplitude of the 

subsurface vertical magnetic field was set up underground around a mine pillar 

with a separation between the centers of the source antenna and receiving sensor 

of about 194 meters. The source antenna was powered with 5 amperes of current 

and transmissions were made at 20 Hz, 50 Hz, 95 Hz, 160 Hz, 210 Hz, 270 Hz, 330 

Hz, 450 Hz, 600 Hz, 720 Hz, 850 Hz, 1000 Hz, 1400 Hz, 2000 Hz, 5000 Hz, and 7000 

Hz. The loop source antenna was set up in the immediate vicinity of the 3-phase 

INA rectifier system which was used to drive fans for mine ventilation and the 

receiving sensor loop was placed at a depth of 194 meters beneath in a mine working so that the loops were roughly coaxial. 

The immediate presence of the rectifier system presented perhaps the worst 

situation insofar as ambient background electromagnetic noise is concerned. The 

results of this transmission test are portrayed in Figure 64. These mutual 

coupling data show that most scatter occurs below 1000 Hertz, a fact not 

unexpected in light of a priori knowledge of noise sources at the mine. Above 

1000 llertz the coupling decreases with increasing frequency. With the above 

constraints an optimal transmission frequency in this case might be between 1 

and 2 kHz. Time did not allow any surface-based resistivity measurements to 

be taken at the Rubina No. 4 Mine. Therefore, in order to have sane comparison 

of the empirical data with computed theoretical models an average effective 

resistivity of about 100 ohm-meters was assumed. It is perhaps because this 

choice for an average effective overburden resistivity is too high that a more 

rapid fall-off of coupling at frequencies greater than 1 kHz than that predicted 

by theory for a uniform earth having a resistivity of 100 ohm-meters is 

observed. In general, however, the transmission tests are considered quite 

successful so far as demonstrating through-the-earth communication capabilities. 
LOOP TRAbSMITTER -,T & U.5. STEEL RLVINR NO. 

4 MINII. UNIONTOW. Pa. 

h Z 194m 

RECEIVER a = a/h 

D =,A,- 0.0 

1. 

A 

I A THEOWTICAL 

(after wait and Spiee. 1971aj 

FREQUENCY iRe1 

Fig. 64 omlink c.w. transmission rest 
Horizontal Magnetic Field Measurements - Buried Vertical-Axis Loop Transmitter - 

U. S. Bureau of Mines Experimental Coal Research Mine, Bruceton, Pennsylvania 

Measurements of the surface horizontal magnetic field produced by a vertical-axis loop were made in actual field transmission tests at the U.S. Bureau 

of Mines Experimental Mine, as well as the vertical magnetic field (discussed 

above). The source antenna-receiver sensor configuration in this case is also 

illustrated in Figure 46, with the receiving sensor loop axis being horizontal. 

In the above discussion the attenuation function M for the horizontal (radial) 

magnetic field at the surface was given by the expression: 

where the integrand factor JL(Ax)/(Ax/2) takes into account field behavior 

departures from a dipolar source condition when the source antenna loop becanes 

large relative to separation distances from the receiving sensor. 

Several features of the behavior of the attenuation function M (or the 

horizontal magnetic field Hp) at the surface are noted. First, directly over 

the vertical-axis buried loop transmitter there is a sharp null present. Then 

the value IN[ builds up to a maximum value at very low frequencies at a horizontal offset distance of about 112 the depth of burial of the source antenna 

after which the amplitude smoothly falls off with increasing distance from the 

source. At an offset distance of twice the source depth of burial the horizontal magnetic field has fallen by about 20 db from its maximum. 

Thus two phenomena in the horizontal magnetic field behavior could be used 

as location criteria. One of these would involve the detection of the null in 

the horizontal magnetic field directly over the buried transmitter. The other 

would involve the determination of the maximum in the horizontal magnetic field 

at a surface offset distance equal to half the source depth of burial. A cautionary note should probably be given here. If one maps just the amplitude l~pl 

at the surface, he must be certain that what is measured as a relative null is 

in fact a null which is on a radial line through the axis of the transmitter 

loop. Therefore one of two procedures is required: (1) systematic grid mapping 

is performed or (2) the horizontal-axis receiving loop sensor is rotated 360 

degrees around a vertical to the earth's surface! 

As in the case for vertical magnetic field null measurements, the size of 

the loop relative to the overburden thickness can change the position of the 

maximum in the horizontal magnetic field, although the size of the loop transmitter never affects the presence of the null in the horizontal magnetic field 

directly over the transmitting loop. As the effective radius of the source loop 

becomes larger the maximum in M or l~pl occurs at a greater offset distance 

(>D = p/h = 0.5). The percent change in position of this maximum for very low 

frequencies is shown in Figure 65 as a function of size of the loop transmitter. 

Comparison of Figures 55 and 65 shows that the position of the maximum in l~pl 

changes much more rapidly with increase in the size of the transmitter loop 

than the position of the null in H although in both cases the position of 
sonrv\~rar:o aao~us OF L~OP mmsnrrrrn. a = ah 

Fig. 65 
the maximum and null, respectively, occur at greater offset distances with 

increasing A. Similarly, the magnitude of the maximum value of decreases 

as A increases (see Figure 66). 

The effect of markedly increasing frequency to the order of magnitude of 

10 kHz leads to the introduction of secondary surface nulls and maximums in 

1 (see Figure 66, which, for the sake of simplicity, should probably be 

avoided in the field. As a general rule, these secondary surface nulls and 

maximums only occur when the depth of burial h (meters) of the source transmitter is greater than the factor 750/&, o being the conductivity of the 

overburden in mhos/m and f being the frequency in Hertz; i.e., 

A typical overburden conductivity in a coal mining province might be 

mhoslm so that if a transmission beacon frequency on the order of 100 Hz were 

chosen, the overburden thickness would have to be at least 750 meters thick 

in order for the secondary surface nulls and maximums in /Hpl to be present. 

On the other hand, if a transmission frequency of 10 kl!z were chosen, the overburden thickness would have to be greater than or equal to 75 meters for the 

above secondary phenomena to be present. 

Unlike the maximum in the vertical magnetic field directly over the buried 

loop transmitter, the presence of the null in the horizontal magnetic field is 

; 

not affected by a change in conductivity of the overburden provided that the 

sources are present. The influence of the presence of a scattering object 

(such as a pipeline on the surface or an overhead power line) on the fields of 

a radiating buried dipole antenna has not been analyzed but is being investigated (Wait, 1973), and as mentioned earlier, is far from being a trivial problem. 

The results of several uplink C.14. transmissions involving the measurement of the surface horizontal magnetic field produced by a buried verticalaxis loop transmitter are shown in Figures 67-71 over a suite of frequencies 

in the audio range and for horizontal offsets which range from zero to about 

three times the transmitter depth of burial. These field data are reduced 

and presented in a form analogous to that used by Wait and Spies (1971a) so 

that a direct comparison may be made with their theoretical results. 

These field data generally illustrate close agreement with theoretical 

considerations. A null in [MI, for instance, was observed at station 1 (see 

Figure 67) where D = p/h = 0 and a maximum detected at station 2 (see Figure 

68) where D = 0.5. As in measurements of IQ I, a fair amount of scatter was 

present in the data at station 3 (see Figure 691, particularly at frequencies 

less than 100 Hz, although at station 4 (D = p/h = 1.45) very little scatter 

was observed at frequencies above 500 Hz (see Figure 70). 

Figure 72 shows the general field behavior of IMI as a function of horizontal surface offset from the underground beacon transmitter at frequencies 

of 20 Hz and 1000 Hz. Both of these graphs show good agreement with theory in 

that the null and maximum in \MI could be used conjunctively for location of 

the transmitting beacon. If the depth h to the mine working, for instance, were 
UPLINK C.W. TRANSMISSION RECEIVBR 4 U.S. BURFA" OP HINL 1LXI'. IIINli. 

,,'I:-.'? ;O.O;mhos/q-' P& . 

(FIELD &UD THEOnETICiiL 

h - 2J A = ah. 0.39 

NULL! TRIYISMITT& ----I D = fh 0 

STATION I 

0.5 FP * 

d,TMEORCTICAL 

(~frer wait and spies. 1971a) 

m 

,? 

m 

0.2- 

- 

L 

- 

0.1- 

0.0 

10 

. . . . . . . - . . .. , . 100 L. 1000 - 1v. PO0 I 

FREWENCY (Hz! 

fig. 67 
I UPLINK C.W. TWSMISSION 

TEST 

RECE1vER 4 U.S. BUREAU OP MINES CXP. MINE, 

PA. 

h=>3m 

. FIELD Dm* a = 6 0.33 

LOOP TRWSNITTP o -ph E 0.44 

STATION 2 

0.5 

&P-4 

A TneoRerrcaL 

(After Walt and spies. 19~1~) 

. 

0.0 I 

10 I 

100 I 

1000 ii' I1DU 

FKEQUENCY (Hi) 

Fig. b8 

! UPLiNK C.W. TRIWSMTSSION TEST RECEIVER 4 ,,, ".S-BUREAU OF MINI.:8 CIF. MIIVE. PA 

A = ah 10.39 0' 0.01 mhos,. 1 0.6 D =Ih ' 0.90 h -21m STIITION 3 

. FIEWI DaTA 

i-P -i 1 A mrnm~c~~ 

liifrer wait and spies. 1471c) - 

0.0 . 1 10 I 100 1000 I 10.0~10 

FREQUEUCY iHz1 

Fig. 69 
UPLINK C.W. TRIWSMISSION TEST ReCElVeR 4 U.S. BVREIU OF MINES EX?. 

6 cZZ~- MINE. PA. 

= 23 A = ah t 0.39 

0 -?,ll G 1.65 

TaoP TWFILlTTER & '-- STATTON 4 

. FIELO DATA &P 4 

(After wart an* spies. 19,la) 

Fig. 70 

UPLINK C.W. TWWSHISSION RECEIVER 4. TEST U.S. BUREUI OP HTWES EXP. NINE 

61 a- PA. " = 23 ir ; a,,, c 0.19 loop TWSMIPTER 4- . ,/h . ,64 

!-P + STRTlON 6 

Fig. 71 

- 

0 

4 

3.4- 

- 

\ 0.3- - 

- 0" 

.c 

" 0.2- 

4 THEO%eTlCaL 

(After Walt and Spres. 1971n) 

A 

* A 

A . . . .*. 
known, the horizontal offset could be determined by the approximate relation 

p = 0.5 h. However, if only measurements of the spatial variation of IMI are 

made, it is firmly recommended that both the null and maximum in [MI be determined either by means of a systematic grid or by rotation of the horizontalaxis induction receiving coil through 360 degrees. In principal, only one 

profile of IN/ would need to be made; whether or not this profile is on a radial 

through the axis of the transmitting loop could then be determined by analyzing 

the rate-of -falloff or horizontal gradient of \MI for normalized offset distances greater than D = 0.5 and for radials not through the axis of the source 

loop. However, not always do these gradients coincide with theoretical predictions (gradients presented in Figure 72 for offsets greater than half the 

source depth of burial are greater than those predicted by theory); therefore 

discretion would probably dictate the use of a grid. 
RLCCIVGR U.S.BUREX4 0s FMlhES 

EXP. MINE. P*. 

- 23 n, A = ./h 1 0.39 

LWP TRINSMITrEK 6 -AI I I I I I 0.5 1.0 1.5 2." 2 NORMALIZED OFFSET FRON BURIED TRANSMITTER WOP.,O/h 

Fig. 72 
SUMMARY 

Electrical properties measurements made in several coal mining regions 

(Gary No. 14 mine, West Virginia; Peabody No. 10 mine, Illinois; Imperial and 

Eagle mines, Colorado; Montour No. 4 mine, Pennsylvania; U.S. Bureau of Mines 

Experimental Research Mine, Pennsylvania) enable one to characterize these 

mining regions by overburden conductivity distributions. Such characterization 

provides the basis of applicability for various theoretical geoelectric models 

being examined which predict throuth-the-earth electromagnetic propagation 

phenomena and also are useful in ensuring the best possible transmitterreceiver coupling for uplink, downlink, or intramine communication. For example, if geoelectric layering is present, the electrical conductivity will be 

greater parallel to bedding planes (at) than across bedding planes (at). The 

propagated through-the-earth electromagnetic fields of a loop source antenna 

depend on the maximum conductivity at; thus, one has maximum coupling through 

the earth when the axis of the loop transmitting antenna is perpendicular to 

geoelectric planes of maximum conductivity. In many of the mining regions 

investigated where electrical layering of the transmission properties was 

observed, it was such that the planes of maximum conductivity lay in a horizontal plane; in the Gary No. 14 region, however, this layering was tilted. 

The electromagnetic fields propagated from a grounded line antenna, on 

the other hand, depend instead on the geometric average =measured along 

and across the bedding in a laminated sequence of rocks. Thus the fields 

radiated by a line source antenna will be more sensitive to gross inhomogeneities present in the overburden. 

Resistivity measurements in the above coal mining provinces were made 

both by galvanic and induction techniques; all resistivity values show, in 

general, for any given region a unimodal character. The spread, however, in 

the probability density plots of the resistivity values collected in any given 

area varies. For example, the resistivity probability density plot for the 

Peabody No. 10 mine appears quite similar to that for the Imperial and Eagle 

mines. This canparison implies that electromagnetic transmissions through 

the earth, ambient noise and logistic factors being equal, would be less distorted by the resistivity distribution apparent at the Imperial and Eagle 

mines or at the Peabody No. 10 mine than by the resistivity distribution apparent at either the Montour No. 4 mine or the Gary No. 14 mine. Consequently, 

theoretical models of a beacon source antenna imersed in an electrically and 

magnetically homogeneous halfspace would be applicable in mining provinces 

which have geoelectric sections similar to those measured at the Imperial and 

Eagle mines or the Peabody No. 10 mine and should yield insight into the 

behavior of electromagnetic transmissions. An average resistivity of the 

overburdens overlying coal mine workings from the data collected and presented 

here would be about 100 ohmmeters; loss tangents, on the other hand, are 

generally vanishingly small in the E.L.F. and V.L.F. ranges. 

Studies of ambient electromagnetic noise statistics are complimentary to 

investigations of the overburden electrical transmission properties and are 

necessary for proper design of any subsurface-surface electromagnetic communications system. Ambient electromagnetic noise levels from 20 Hz to 10 kHz have 
been characterized in a number of mining provinces as a function of time of day 

by amplitude histograms. 

In so doing, the signal from either an electric field sensor or magnetic 

field sensor is first passed through a filter which strongly rejects below some 

lower frequency limit and above some upper frequency limit. Then, within 

these frequency bands, the number of times ambient noise events occur which 

exceed specified levels is determined. The advantage of this approach is that 

it provides information on the design of a high-sensitivity receiver system 

not readily available in the more conventional spectral approach. For example, 

optimum noise rejection could be incorporated into an uplink receiver system 

by setting thresholds for signal detection just above the median or maximum 

percent noise levels. 

Statistical histogram analyses of ambient background electromagnetic 

noise data above working faces at the surface show that in most mining provinces 

the background noise is: (1) independent of time of day while coal mine operations are in progress, and (2) primarily due to artificial, man-made sources as 

distinct from atmospheric noise in the frequency range 20 Hz - 2 kHz. The magnetic field and electric field noise amplitude density functions presented in 

this report may be used in conjunction with the measured resistivity data to 

design and implement a practical operating beacon electromagnetic cormnunications 

system in which an adequate source strength is determined which will yield a 

specified signal/noise ratio at the surface. Actual examples of the use of 

electrical properties data together with electromagnetic surface noise environment data for systems design considerations have been given for a loop-loop 

source-receiver configuration and a line-line source-receiver configuration. 

Studies of the ambient electromagnetic noise environment in most coal mine 

provinces may also be used in analyzing the feasibility of a passive loop detection scheme for location of a subsurface miner. It is deemed that a passive loop 

detection scheme for location of a subsurface miner would best be employed only 

as a backup to an active loop detection scheme, because secondary electromagnetic 

field strengths that are radiated from a buried passive loop excited by an active 

loop antenna at the surface are too small compared with the ambient noise. 

Numerous transmission tests, both uplink and downlink, and both of the 

C.W. type and pulsed type, were performed with a grounded line transmitter and a 

vertical-axis loop transmitter and either electric or magnetic field receiving 

sensors. The feasibility of a pulse electromagnetic communication system is 

shown both theoretically and by actual field tests for small overburden thicknesses with the qualification that if one wishes to examine the late-time 

transient coupling response in an operating coal mine environment with transmitter antenna power output levels between 10 and 50 watts, he may have to 

I, process (i.e., stack") the received signals to enhance signal-to-noise ratios. 

Several location criteria were also deduced from the uplink C.W. transmission tests. One could, for example, map the surface null along the polar 

and equatorial axes of a buried line source transmitter when the wavelengths of 

transmission within the earth are considerably larger than all other distances 

involved (ELF range). 
Another location criterion for positioning a transmitting vertical-axis 

loop involved the detection of the null in the vertical magnetic field at an 

offset distance from the buried source of about 1.4 times its depth of burial 

(low frequency criterion). If only a single profile of surface amplitude 

measurements were made which was not necessarily along a radial through the axis 

of the transmitting loop antenna, all that could safely be determined would be a 

minimum depth of burial of the source antenna in a homogeneous earth. For 

example, suppose a straight-line profile were made not on a radial from the 

transmitting loop axis but rather on a radial from a point which was displaced 

from the antenna axis by a distance equal to the depth of burial of the transmitter. In this perhaps sanewhat extreme case, a single profile of amplitude 

measurements of Hz and detection of a relative null in 1~~1 might lead to estimates of the depth of burial of the transmitting antenna which would be as much 

as 30% too low. This field procedure, which makes use of the behaviorial nature 

of the amplitude of the surface vertical magnetic field, need not detract from 

an estimate of the true depth of burial or the true horizontal offset provided 

one of the following were performed: (1) the maximum in the vertical magnetic 

field over the transmitting loop were ascertained (as well as the offset null), 

(2) phase measurements of the vertical magnetic field relative to the horizontal 

magnetic field were made (as well as amplitude measurements), (3) horizontal 

gradient measurements of the vertical magnetic field were made (since the 

horizontal gradient in Hz is dependent on whether or not the measurements are 

made along a radial through the transmitter axis), or (4) either the depth to 

the mine working and not necessarily the surface offset distance or perhaps the 

surface offset distance, but not necessarily the depth to the working where the 

source antenna is located were known. 

Ascertaining the maximum in the vertical magnetic field would involve a 

systematic grid mapping measurement routine. The second alternative, on the 

other hand, is particularly attractive since instrumentally it would involve 

only a highly tuned (perhaps phaselocking) crossed loop ratiometer sensor 

(commonly used in geophysical mining exploration) and would, with little additional expense, combine information present in both the surface horizontal and 

vertical magnetic field components from the buried vertical-axis transmitter 

loop. Gradient field measurements of just IH,~ offer a third alternative and 

would, in principle, allow the determination of position relative to the axis 

of the transmitting antenna. However, gradient measurements seem to have an 

apparent disadvantage in that two measuring coaxial induction coils spaced a 

finite distance apart (say, in this case a practical distance of one meter) 

would have to be used and, in a noisy electromagnetic environment, would tend 

to accentuate noise in the data. On the other hand, the gradient in l~~lcould 

be obtained after measurement and smoothing of IH~( and compared with graphical 

nomograms for positioning. Commonly the depth to working levels in mines 

where men are located during emergency situations is very well known, since 

there are few multi-level working mines in the United States (Parkinson, 1971). 

Thus, use of known constraints would yield a large reduction in the scope of 

the location problem. 

Still another criterion for targeting a buried vertical-axis loop beacon 

is mapping the null conjuctively with the offset maximum in the attenuation 

function for the horizontal magnetic field, /MI. Again the same care would 

have to be taken in field measurements to ensure a null over the transmitter 
were mapped, and the most practical approach seems to lie either in systematically making a series of grid measurements along several profiles at the surface 

or in rotating the sensor coil 360 degrees horizontally. 

It should be noted that the field behavior of a finite loop antenna, as 

opposed to that of a dipole loop antenna, is useful and necessary in analyzing 

coupling data from shallow mines. In most coal mines, for instance, the depth 

of burial of the source loop antenna (for uplink communications) is on the 

order of 100 meters. A practical buried loop antenna might consist of a single 

turn of 1/14 gauge wire wound around a pillar within a coal mine (such as the 

10 m by 24 m loop used in uplink transmission in the U.S. Bureau of Mines 

Experimental Mine) and having an effective circular radius on the order of 8-10 

meters. For shallow coal mines (such as the Experimental Research Coal Mine 

in Bruceton, Pennsylvania) this effective source loop radius is about 113 the 

depth of burial, and finite-size loop behavior should be taken into account. 

For the same size loop in the visited U.S. Steel Rubina No. 4 Coal Mine in 

Uniontown, Pennsylvania, however, the above effective source loop radius would 

only be 1125th the depth of burial and the limiting case of dipole field behavior 

would suffice. 

As a general rule, it may be said that for the frequency range under consideration, one need only consider dipole field behavior for systems design as 

long as the separation distances are 10 times larger than the source loop antenna 

radius. For smaller separations relative to the source loop radius, however, 

the "size effect" of the transmitting loop can and should be taken into account, 

for it could affect an accurate location determination scheme, particularly if 

measurements just of the vertical magnetic field are made. In the analyses 

given in this report, the size effect of the transmitter loop was shown and 

taken into consideration for both vertical and horizontal magnetic field measurements. When feasible, it is desirable to make the effective radius of the 

transmitter loop as large as possible since the source strength of the transmitter is accordingly increased, even if size effects of the transmitter need to 

be accounted for. 

In sunnnary, analysis of the C.W. uplink transmission experiments which 

made use of a vertical-axis loop transmitter demonstrates close agreement with 

theoretical considerations. Generally, noise was less of a problem above 1000 

Hertz, and attenuation was not as severe as that which would be predicted by 

plane-wave theory. 

It is perhaps appropriate at this point to make some qualified remarks on 

which source-receiver configuration might be best to use under a given set of 

circumstances or under a given set of constraints defining the objectives of and 

limitations on a through-the-earth beacon electromagnetic communications system. 

For example, is it desired simply to communicate by induction techniques or is 

the objective to use the source-receiver configuration which in coal mine 

provinces (based on measured electrical properties of the overburden in coal 

mine provinces) provides a more useful field for location purposes? In addition, ease and practicality of deployment of the source antenna within a mine 

must also be considered, particularly in an emergency situation. 
Provided the overburden is relatively conductive (less than several 

hundred ohm-meters) and contact resistance at the current electrodes is no 

problem, it is of ten more convenient to directly put current into the ground 

by a line source. This type of transmitting source, although yielding a very 

adequate means for communication, is more sensitive to conductivity inhomogeneities (as noted in the field test results above) and for a receiving surface 

electric-field sensor does not, in our opinion, provide as convenient a means 

for location as does a loop-loop source-receiver configuration which has been 

discussed. Thus, received magnetic-field signals in the ELF range are less 

sensitive in general to secondary scattering sources in the overburden than are 

electric-£ ield signals. 

On the other hand, for general communication purposes, sme of the secondary nulls in the produced surface magnetic field from a buried vertical-axis 

loop antenna may make a horizontal grounded insulated electric-current line a 

more desirable transmitting source. Of course, the electrical properties of the 

overburden must always be taken into account, for in the case of a highly 

resistive overburden (which, although not found usually over coal mines, is 

frequently found over hard-rock mines), it may not be practically feasible to 

use an electric line source for communication or location purposes at all, 

simply because of the difficulty in putting current into the ground. Thus, for 

the case where a resistive overburden is present, it may necessarily be advantageous to use a loop source antenna. 
APPENDICES 

AMBIENT SURFACE ELECTEOMAGNETIC NOISE MEASUREMEWS 
APPENDIX A 

Gary No. 14 Coal Mine, W. Virginia 

Smoothed Histograms of: 

Time Derivative of Vertical Magnetic Field Noise (A-1 - A-8) 

NE-SW Horizontal Electric Field Noise (A-9 - A-16) 
VERTICRL'~~RGNETIC FIELO NOISE w GRRY M. I4 WRL NlNE U. VR. VERTICAL MRGNETIC FIELD NOISE w GRRI ND. 19 CORL nlNE U. VR. - . - 

FREQUENCY 8RNDUIOTn; 80-160 CPS 
(T1:E 2:RIIIZTim) 

VERTICRL nRGNETiC FIELD NDLSE m GRRl NO. 14 LOfiL RINE U. VR. 

E! 

FREWDICI BFiNOUIOlH; 160-320 CPS 

NUnaER OF SRIIPLES TIIKtN = IOOOO 

[TI..Z DZ! Ix:;'::l,T) 

VERT lCRL nRGNEI 1: F~ELD iiOlSE w 

0. 

GORY NG. I4 CORL nlNE U. VR. 

-F FREQUENCY ~lOUIOlI1; 320-640 CPS I 
( I I - . . 7 \ . - ,>A:*L '...A >-, 

VERTICRL MAGNETIC FIELO NOISE 

n cam NO. I4 WGi nlNE Y. VR. 

'2b FREUENCY nwwmlorns 840-1280 IPS 

[TPZ a~?I"ATI~, 

VERTICRL MRGNETIC FIELO NOISE 
(1.-2 " '7PIrn, 

VERTICRL MRGNETIC FIELD NOISE c. GRRY NO. 14 CG3L nlNE M. VR. 

FREmY BRNWLOtHl 25W-SZDO CPS 

NUnsER OF sm5 Trn - loo00 

VERTICRL MRGNETIC FIELO NOISE c. GIIRY NO. 14 WRL ~INE U. vu. 
NE-SW HORIZONTAL ELECTRIC FIELD NOISE e4 GRRY w. 14 ma n1N U. VP. 

P 

FWCI BROULDTHI LLLM WS 

mff SPPLEs1PlEn- lwm 

NE-SU HORIZONTRL ELECTRIC Flu NO

P( WXV ED. 14 CORL nlN Y. VR. 

P 

FREE*MCI BR~OUID~. ~IM ws 

NUYE8 OF SmS TW - IMM 
FIE-SW HORIZONTRL ELECTRIC FIELD NOISE N 69111 M. I4 CDIIL n1E U. VO. 

9 

FRFAEVCY BRwYIOm~ lmm rn 

NJm ff 6-3 lO(M - lOOW 

K-SY HORIZONTAL ELECTRIC FIELD NOISE c. GORY M. 14 WRL nlK u. VO. 
NE-SW HORIZONTRL ELECTRIC FIELD NOISE N GRRI M. 14 CML ~IK U. VR. 

NE-SW HORIZONTRL ELECTRIC FIELD NOISE m GW W. I4 UX(L n1H U. VR. 

fllLLIVOLTS/HIMETCJI 

A-I 
NE-SW HORIZONTRL ELECTRIC FIELD NOISE N w8n* Pa I* CML nlPz Y. YO. 

9 ~nmrnrl euwulnin~ PS~D-PI)~ CTS 

HIWRff -5 TM - IODM 

PIE-SU HOillZONTRL ELECTRIC FIELD NOISE w GDAV M. 14 t(ll~ nM Y. vn. 

9 

newwn amwinm~ urnlaan os 

WMC~ a sms TN~ - loom 1 
APPENDIX B 

Peabody No. 10 Coal Mine, Illinois 

Smoothed Histograms of: 

Time Derivative of Vertical Magnetic Field Noise (B-1 - B-8) 

E-W Horizontal Electric Field Noise (B-9 - B-16) 
2zaI..,:...7, lL.A .~~ ., 

VERTICRL MRGNETLC FIELD NOISE n PERBDDl tin. 10 CORL nINE Iu. 

- 

FREWMCT BIINOUIOTII: 20-80 CPS 

E NUMBER OF SRALES TIWN = IOOW I 

(TDE DERIVPTIW) 

VERTICRL MRGNETIC FIELD NOISE e. PECBOOY NO. 10 UIR nlNE ILL. 

D 

FRmUENCY 8RNOUIL)TII; 80-160 CPS 

NUmER OF SPJFES TRKM = IOOM 
('CL.L :,z, ?,..T..lj 

VERTICRL PIRGNETIC FIELD NOISE 

N PERSMY HO. 10 COX MlHE ILL. 

FRE&!jMCI aRMIULOTI1; 160-320 CPS 
(TI.2 DEal;*TllliJ 

VERTICRL NRGNETIC FIELD NOISE ,% PEW1 NO. I0 CO% MINE ILL. 

z 

fRErnCV BRNDUIOTH: 640-1280 CFS 

NUnSER OF SRWS TaI(EN = lOOW 

(TI T JERIYATIVE) 

VERTICRL PIRGNETIC FIELD NOISE w PEWV NO. 10 COPL nINC IU. 

z 

FREOUENCV WIOTW 1280-2560 

NUVEER OF SRIPLES TOHEM r 1
LT*..Z DI3Tg,*T:y7, 

VERTICRL flRGNET!C FIELD NOISE 

w 

0 

PER~OO? NO. 10 COK nlVE ILL. 

- FREOUOnl BWULOTH: 2560-5200 CPS 

1 \ . A . , . , ,. . . , VERTICX WGNETIC FIELD NOISE c" PER8007 NO. I0 C%L nlNE ILL. 
E-U HORIZONTRL ELECTRIC FIELD NOISE c. m PEW NO. 10 UXI nlw ru. 

FREDUOlCl MUIOTH; 20-00 CPS 

HUm c6 SWLES TMEN = loow 
E-W HORIZONTAL ELECTRIC FIELD NOISE N PEW NO. 10 U1UL nINE ILL. 

zc 

FREOURlCl BWOUIOTH: 150-320 CPS 

E-W HORIZONTAL ELECTRLC FIELD NOI

N PEABWY NO. LO COUL nlNE ILL. 

0 

t NUll8ER OF SWUS TPXEN - IO
E-W HORIZONTAL ELECTRIC FIELD NOISE a PERB00Y NO. LO COAL MINE ILL. 

2 

FREOUDlCI BRNUIOTH; 640-1280 CPS 

MIWEER W SAIIRES TRKM = LOOOD 

E-U HORIZONTAL ELECTRIC FIELD NOIS

w 

Q PEABODI NO. LO UIR n1NE ILL. 
E-W HORIZONTRL ELECTRIC FIELO NOISE w PERBODY NO. LO COAL nlNE ILL. 

0, FRmWCY BFMULOM: 2560-5203 (PS 

twnm DF WLES TRKM - LOOM 
APPENDIX C 

Eagle Coal Mine, Colorado 

Smoothed Histograms of: 

Time Derivative of Vertical Magnetic Field Noise (C-1 - C-8) 

N-S Horizontal Electric Field Noise (C-9 - C16) 



(713 ~FiIPiI:-.z, 

VERTICRL MRGNETIC FIELD NOISE 

(TI:C 3ZPiPITIVE) 

VERTICRL nRGNETIC FIELD NOISE - ERIE wtx me wm 

2 FREQUENCY IYKOUIom~ 5200-10400 06 



N-S HORIZONTRL ELECTRIC FIELO NOISE N ERIE UIR nltic COLOR^ 

Zk 

FREWWCY wwuIoTn8 25sszoo ws I 

N-S HORIZONTRL ELECTRIC FIELD NOISE w ERIE nlM WLDRROD 

0 

FREPbENCI 89NDULOlH: SZOD-10P00 C

WOER OF SRTLES TWN - IOO
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