USRM Grant No. GI33023 

THRU-THE-EARTH ELECTROilIAGNETHCS WORKSHOP 

Richard G. Geyer 

USBM CONTRACT FINAL REPORT 

(Contract/Grant No. G133023) 

Date: December 3 1, 1973 

DEPARTIWNT OF THE INTENOR 

BUREAU OF MINES 

WASHINGTON, D. C. 
DISCLAIMER NOTICE 

The views and conclusions contained in this document are those 

of the author(s) and should not be interpreted as necessarily 

representing the official policies of the Interior Department's 

Bureau of Mines or the U. S. Government. 
PROCEEDINGS OF 

THRU-THE-EARTH ELECTROMAGNETICS WORKSHOP 

August 15 - 17, 1973 

Colorado School of Mines 

Sponsored By 

United States Bureau of Mines 

Edited By Richard G. Geyer 
FOREWORD 

This report was prepared by Richard G. Geyer under USBM 

~ontract/Grant No. G133023. The grant 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. A. L. Metheney 

was the contract administrator for the Bureau of Mines. 

This report is a summary of the work recently completed as 

part of this grant during the period April 1, 1973 to December 

31, 1973. This report was submitted by the author on December 

31, 1973. 
Conference Chairman: 

Richard G. Geyer 

Advisory Committee: 

James R. Wait 

Howard E. Parkinson 

James Powell 

Session Chairmen: 

Richard G. Geyer 

James Powell 

David A. Hill 

John Murphy 

James R. Wait 

Research Associate, Geophysics 

Department, Colorado School of 

Mines 

Director , Environmental Research 

Labor ator ies , NOAA; Consultant, 

Institute of Telecommunication 

Sciences 

Supervisory Electrical Research 

Engineer, Industrial Hazards and 

Safety Research Center, U. S. 

Bureau of Mines 

Staff Research Engineer, 

Mining Research Center, U. S. 

Bureau of Mines 

Research Associate, Geophysics 

Department, Colorado School of 

Mines 

Staff Research Engineer, 

Mining Research Center, U. S. 

Bureau of Mines 

Research Scientist, Institute of 

Telecommunication Sciences, 

Boulder, Colorado 

Supervisory Electrical Research 

Engineer, Industrial Hazards 

and Safety Research Center, 

U. S. Bureau of Mines 

Director, Environmental Res earch 

Laboratories, Boulder, Colorado 
Group Chairmen: 

Electromagnetic Noise 

Environment 

Location and DirectionFinding 

Uplink and Downlink 

Communications 

Operational Communications 

John W. Adams, National 

Bureau of Standards 

Arnie J. Far stad, ~esiin~house 

Geores earch Laboratory 

Robert L. Lagace, 

Arthur D. Little, Inc. 

Martyn F. Roetter, 

Arthur D. Little, Inc. 

iii 
TABLE - OF CONTENTS 

Preface ....................... 

Objectives and Constraints of Through-the-Earth 

Electromagnetic Communication Systems 

Howard E. Parkinson .............. 

State of Knowledge of Analytical Techniques for Thru-theEarth Electromagnetic Wave Problems Relevant to Mine 

Rescue 

James R. Wait ................. 

Electromagnetic Field Solutions for mite and Finite 

Cables for Conducting Half -Space Models - - Both Frequencyand Time - Domain 

David A. Hill .................. 

Theory and Experiments Relating to Electromagnetic Fields 

of Buried Sources with Consequences to Communication 

aad Lacation 

Richard G. Geyer ................ 

Electromagnetic Direction Finding Experiments for 

Location of Trapped Miners 

R. G. Olsen and A. J. Farstad .......... 

Theory of Propagation of UHF Radio Waves in Coal 

Mine Tunnels 

Alfred G. Emslie, Robert L. Lagace, and Peter F. 

Strong ...................... 

Page 

viii 

Guided Propagation of Radio Waves 

P. Delogne, L. Deryck, and R. Liegeois ...... 49 

Radio Propagation Measurements in Coal Mines at 

UHF and VLF 

Arthur E. Goddard ................. 54 

Performance of Manpack Electromagnetic Location 

Equipment in Trapped Miner Location Tests 

A. J. Farstad. .................. 62 
Fields of a Magnetic Dipole Fxcited Buried Cylinder 

Allen Q. Howard, Jr. ............. 

The Electromagnetic Response of a Buried Sphere 

for Buried Dipole Excitation 

D. A. Hill and J. R. Wait ........... 81 

The Perturbation of Alternating Electromagnetic Fields 

by Three- Dimensional Bodies 

F. W. Jones. ................. 86 

A Discussion on the Three-Dimens ional Boundary Value 

Problem for Electromagnetic Fields 

David Rankin ................... 92 

Reply to: "A Discussion on the Three-Dimensional 

Boundary Value Problem for Electromagnetic Fields 

(David Rankin) 

F. W. Jones. ................. 

Use of Zoral Hzrmonic Series for Obtaining Numerical 

Solutions to Electromagnetic Boundary Value Problems 

Richard L. Lewis ................ 

Application of Lebedev-Kontorovich Transforms for . 

Describing Electromagnetic Wave Propagation in an 

Inhomogeneous Earth 

Richard G. Geyer ................ 

Subsurface Applications of Periodic ~lectroma~netid 

Video Pulse Signals 

David L. Moffatt ................ 

Electromagnetic Survey Method Applicable to 

Underground Quarries 

R. Gabillard, J. P. Dubus, F. Cherpereel .... 

Feasibility of a Radio Communication in Mine Galleries 

by Means of a Coaxial Cable Having a High Coupling 

Impedance 

J. Fontaine, B. Demoulin, P. Degauque, and 

R. Gabillard .................. 
Page 

Electromagnetic Transmission and Detection at 

Deep Depths 

D. B. Starkey ................. 

Admittance and Effective Height of Buried Antennas 

Giorgio Franceschetti ............. 

Spectrum Measurements of Electromagnetic Noise 

in Coal Mines 

W. D. Benserrra and J. W. Adarns ....... 

Amplitude Statis tics of Electromagnetic Noise in 

Coal Mines 

M. Kanda and J. W. Adarns .......... 

Design of an ELF Noise Processor 

J. E. Evans and A. S. Griffiths ........ 

Ekperimental Comparison of Buried and Elevated 

ELF Transmitting Antennas 

Peter R. Bannister, Frederick J. Williams, 

J. Robert Katan, and John R. Ball ....... 

An Underground Electromagnetic Sounder Experiment 

Lambert Dolphin, Jr. , Robert Bollen, and 

George Oetzel ................... 7 

Summary Report on Electromagnetic Noise 

Measurement Program 

John W. Adams ................ a 

Summary Report of Electromagnetic Location 

Techniques Working Group 

Arnold J. Farstad ............... 

Summary Report of Uplink and Downlink Communications 

Working Group 

Robert L. Lagace ............... 

Summary Report of Operational Communications 

Working Group 

Martyn F. Roetter ............... 

List of Attendees .................. 

vii 
PREFACE 

Over the past several years the United States Bureau of Mines 

has supported various research programs oriented toward the development of basic data applicable to the design of electromagnetic communications sys tems. Such systems would provide rescue communication 

links during emergencies. They might also serve as monitors of the 

mine environment, as well as special communication links for increasing 

the day-to-day efficiency of the mine operations. 

Therefore r es earch programs have been involved with experimental 

and theoretical investigations of wireless signal transmission through 

the earth. Experimental studies revolve about the identification of the 

electrical properties of rocks over coal mines insofar as the propagation 

of communications signals would depend on such properties, on the 

ambient electromagnetic noise fields over and in coal mines insofar 

as such noise would limit the detectability of communications signals, 

and on the effect of mine structures on the transmission of communications signals. Active theoretical work has consisted of the analysis 

of a variety of techniques for thru-the-earth communications and of 

a number of approaches for location of trapped miners. To name 

but a few techniques, either loops or grounded wire may be used for 

the subsurface or surface antenna. Each may be oriented in a variety 

of ways. Each may be excited with an impulsive signal or a continuouswave signal. 

In short, a need arose to assemble individuals .who have been 

active in current research on the topic so that the problem of how 

information may be conveyed by electromagnetic waves propagating 

through rock media without the benefit of lines and cables may be 

better understood. Furthermore, some of the limitations and scope a 

of such schemes needed to be identified. 

Thus a "Thru-the-Earth Electromagnetics Workshop" was sponsored 

by the United States Bureau of Mines. This workshop was held at 

Colorado School of Mines on August 15-17, 1973. The papers which 

follow are representative of the topics discussed at that workshop. 

Richard G. Geyer, General Workshop Chairman 
OBJECTIVES AND CONSTRAINTS OF 

THROUGH-THE-EARTH ELECTROMAGNETIC 

COMMUNICATION SYSTEMS 

Howard E. Parkinson l 

ABSTRACT 

In 1969, new coal mine health and safety legislation was enacted in the United States. 

As a result of thls legislation, the Department of Interior, Bureau of Mines has carried out 

communications research aimed at developing whole mine communications for increased 

safety and efficient operation. The research has advanced and many promising results are 

now being demonstrated to the mining industry. However, there remains a challenge to 

further improve wireless communications within the mine workings, and thro~tgh the 

overburden above the workings. The objectives for future research and development will be 

discussed, together with the physical and operational constraints that new mine communication systems must face. 

INTRODUCTION 

The involvement of the U.S. Bureau of Mines, Department of the Interior, with 

electromagnetic (EM) communications dates back fifty years. Recently, as a result of 

legislation enacted by the Federal Government in 1969 concerning Coal Mine Health and 

Safety, the Bureau has entered into a new and promising era of communications research. 

As part of this effort, the Bureau is endeavoring to exploit the promise of through-the-earth 

communication by means of EM fields. 

The program of the Bureau of Mines is unique. This program is a cooperative effort 

under the direction of the Bureau. Through contracts and grants, thegBureau purchases the 

technical efforts of universities, private communications companies - both manufacturers 

and consultants - and other governmental agencies, and coordinates the participants into a 

team. Through cooperative cost-sharing agreements with mine operators, research is performed in the working mines, where extensive demonstrations are made of new concepts 

under actual operating conditions in the mine. 

The Bureau's philosophy is to develop day-to-day-operational communications systems 

that can adequately meet communications requirements for making mining safer on a 

1. Supervisory Electrical Research Engineer, Industrial Hazards and Communications, Pittsburgh Mining 

and Safety Research Center, U.S. Bureau of Mines. 
day-to-day basis, improving the efficiency of mining operations, and remaining fully operational and ready to provide essential communication needs in an emergency. 

THE OBJECTIVES OF MINE COMMUNICATIONS SYSTEMS 

Fundamentally, the Bureau has three objectives for communications systems: 

(1) To be a silent partner monitoring the mines, watching over the environment 

to ensure that the mine is providing a healthy and safe working environment; 

(2) To provide reliable rescue communication links during emergencies; and 

(3) To ensure efficient day-today mine operations. 

The communications systems designed to achieve these objectives must satisfy the 

following requirements: 

(1) The silent partner monitoring the mine environment must be highly reliable, 

and is required to work even when there is a loss of power and a loss of wire 

channels to transmit signals; 

(2) The rescue communications system must be able to function under the most 

adverse conditions, and be able to: 

Allow live miners to notify the surface of their presence in the mine; 

Supply sufficient data to determine the position of the live miners; , 

Provide communication with the live miners to assist in their rescue. 

(3) The day-to-day operational communication system must be able to report 

automatically the status of major factors of production: additionally, communication must be extended from fixed locations and vehicles to roving 

miners, so that a miner working on the face can troubleshoot equipment 

directly with his surface supervisor without having to leave the immediate 

work site at the face. 

CONSTRAINTS OF THE MINE ENVIRONMENT 

The environment for communications systems in a rnine is a severe one, both under 

normal operating conditions and in emergency situations. This environment, which includes 

the miner himself as well as his surroundings, poses several major constraints and/or 

specifications to which the communications equipment must conform. 
General Environmental Characteristics 

( 1) Through-theearth EM communications systems must operate in the face of the 

conductivity and depth of the earth overburden. We have chosen values of 1 o-~ mhos per 

meter and 1,000 feet, respectively, as nominal design values for these parameters. The vast 

majority of U.S. mines fall within these limits. While higher values do exist. these are 

difficult to cope with while remaining within reasonable limitations of power, weight, and 

size for the communications equipment. 

(2) The effectiveness of communication is a function of the signal-to-noise ratio. 

Although we have not accurately determined what the limits on usable signal-to-noise ratio 

can be, these limits bind us in both the operational and emergency modes. During normal 

operations, the most adverse noise that we have experienced to date in a mine has been in an 

all DC mine using 600 volt haulage and face equipment. During emergencies, through-theearth emergency communications must be received in the face of ambient surface noise of 

both man-made and natural origin. Examples of performance estimates based on early noise 

data are shown in Figures 1-3. 

Emergency Environmental Characteristics 

(3) During a mine disaster tremendous flame pressure fronts are often experienced. The 

communications gear must be rugged enough to remain workable despite the stresses to 

which it is subjected. For example, the Bureau has developed special equipment that has 

endured pressures in excess of 25 psi at temperatures of greater than 1 ,800°F for 30 

seconds. 

(4) During an emergency the mine must be assumed to be gassy, and this imposes 

severe intrinsic safety restrictions2 on the communications equipment. There are three 

major restrictions to consider: 

(a) The minimum ignition current for a particular value of capacitance in the 

output of the communications gear; 

(b) The minimum ignition current for a particular value of inductance in the 

communications gear; 

(c) The minimum ignition current at the voltage being impressed on the circuit 

at a particular instant. 

2. R J. Redding, "Intrinsic Safety,The Safe Use of Electronics in Hazardous Locations, McGraw-Hill, London, 

1971. 
Characteristics of a Miner 

(5) As a source of power for the communications equipment we have placed strong 

emphasis on the miner's cap lamp battery, and what we call the surplus energy, the energy 

available after an &hour shlft, as shown in Figure 4. The cap lamp consumes 1-1.2 amperes, 

and the cap lamp cord is designed for this low current. Excessive loss would occur if we 

were to draw five to six times that current, as might be required by some types of 

emergency EM transmitter. Critical design constraints are therefore placed on the selection 

of components used in EM transmitters, and receivers, as well as circuit configurations, to 

enable the maximum magnetic moment to be developed in the most efficient manner for 

- the desired period of operation. 

(6) All equipment must be practical for day-to-day operational use. This will ensure 

that the miners know how to operate the equipment, and that the equipment is in working 

order should an emergency develop. 

(7) The weight limitations chart (Figure 5) shows the weight of the items now carried 

by miners. We wish to limit the communications gear carried by the miner to no more than 

a 5% incre'ase in the weight of the items that he now carries. The section foreman in 

particular is a key individual in this respect. 

(8) The cost of the communications gear must, of course, remain within reasonable 

limits. The cost chart (Figure 6) shows the selling prices of various equipment often carried 

by miners. We have estimated what we think should be the cost of simple special communications gear; namely, about $65 for a CW transmitter for locating trapped miners, and 

about $130 for a call alertlemergency voice receiver such as that shown in Figure 7. 

CONCLUDING REMARKS 

We have had remarkable success with many of our communications experiments; 

however, our approaches still need to be refined. You will be hearing about many of these 

experiments, both with regard to EM propagation and EM noise m.easurements. This is a 

workshop seminar. Its output is anticipated to be at least two position papers that will 

summarize the present state of the Bureau of Mines through-the-earth wireless communications program and progress as you see it, and then define a short- and long-term research 

effort that you as researchers and equipment developers can participate in to improve these 

wireless communications capabilities. This definition of research efforts should include 

estimates of what the dollar cost might be as well as the time necessary to complete each of 

the suggested projects. 

We are indeed grateful for the efforts of each participant and we sincerely hope this 

seminar will be beneficial to you, as well as to us. We hope that you will go away with a 

clearer vision of our needs, and that the Bureau, through your eyes, will see more clearly 

what next steps should be taken in our continuing efforts to improve mine health and 

safety. 
FIGURE 1 MmNLlNK AVERAGE POWER AND 

RMS ANTENNA CURRENT FOR 12db SIN RATIO OVER THE VOICE BAND 

UI1- r)o Hz for 0 :. 10-2 mharlnuter nd Long Wire Tr-it Antmna 

For Surfkc No~se Cond~t~ons -0-law 

Allen M~ne 1F1g 7 Benrema) (Vertical) NBS R~~~ 10739 

+ 

- - Llnmln Mlne (FI~ 24 Benserna) (Venlollt 

Gunn Ouealy Mlne lWGL Data) (Vertical) Westlnghouse F~eld Rqmrt 

-+ - Representative Low Atmospha~c Nolse (Florlda W~nter) 

-Representatlve ulgn Atmospher~c Nmse (~lorlcia; SrnmrlJ 1968u0r1zmta1 

L~ncoln Laboratory Report TN 1969-18 

Signal Power 

(sm) spot 4 Noly p-r hsitl) 

WRE 2 UPLINK SIGNAL-TO-NOISE RATIO FOR LOOP TRANSMIT ANTENWOVERBURDEN 

mDUCTIVIT 8 - 10.2 mholwwtor,LOOP MOMENT NAI - 1460 AmpnwtaZ 
2 4 6 8 10 12 14 16 

TIME, HOURS 

Figure 4 Voltage Curves for Miner's Cap Lamp Battery 
Figure 5 

TY PICA1 EQUIPMENT WEIGHTS 

For Lor*~~Knrhdr.2.0Ib 'SOUND LEVEL For Rmf Bo(tl Md En-mufh .0.8 lb 

For Ikrh.nic Add OMchnla Kit - 24.0 Ib 

METER 

21 O DUST SAMPLER 

I METHANOMETER 

SOUNDING STICK 

FLAME SAFETY 

LAMP 

MISCELLANEOUS 

CAP LAMP 

SAFETY SHOES 

SELF RESCUER 

HARD HAT 

- BELT 

SAFETY GLASSES 

MINER ONOT ALWAYS FOREMAN 

Figure6 TYPICAL EQUIPMENT COST 

Sofety Glosses 

Sounding Stick Pick 

Hard Hat 

Misc 

Belt 

Earmuffs 

Battery Charger 

Safety Shoes 

Self Rescuer 

CapLamp 

Flame Safety Lamp 

Mechanics Kit 

Anemometer 

Sound Level Meter Cal 

Metha nometer 

Dust Sampler 

Sound L e v e l M e t e r 

.2.25 

-3.25 

-4.25 

-4.50 

-4.60 

8.40 

9.00 

r 18.00 

2 4.75 

I 33.50 

35.00 

. 94.

, 

I- 

, 
FIGURE 7 

OPERATIONAL CALL ALERTIEMERGENCY THROUGH THE EARTH VOICE RECEIVER 

Through the Earth 

Emergency 
two pairs of terminals and the associated transient responses. 
correction as being grossly pessimistic for many cases of interest 

to mine rescue or to communication to a receiving. station above ox 

helow the- source loop. 

by using the correct electrodynamic forms of the magnetic 

dipole fields in an homogeneous conductive medium of infinite exterlt, 

we can obtain a better estimate of the relevant value of Z . For 

example, if we are transmitting between two small coaxiaf"loopss 

we can easily show that 

where r = (1 + i) 6 and Z is the static or D.C. coupling limit. 

0 

Now usually the transmission takes place from the earth's 

surface to a buried receiving terminal. Or the converse situation 
may exist if we are dealing with up-link communication. In 

both cases the air-earth interface must be considered. Here we 

can immediately call attention to the classical formulation of Arnold 

Sommerfeld that dates back to 1909 in its earliest version. This 

was used as the basis of much analysis in later years but the 

interest was mainly for the case where the range exceeded a freespace wavelength. Here we are interested in the near zone where 

the significant distances are small compared with the wavelength 

in the air but such distances (i. e., depth or offset) may be comparable 

with the wavelength in the earth. This is quasi-statics in the 

vernacular of the current workers. To obtain field estimates we 

now have to "do" some integrations. Fortunately it was found that 

identities in Bes sel function theory, recognized by mathematicians 

of the late 19th century were ripe for the picking. Thus some 

closed form field expressions were obtained and published in the 

1950's for a fairly broad class of such problems. Numerical results 

requi red the manipulation of modified Bes s el functions whose arguments 

were complex. Fortunately for cases of interest, the phase angles 

were near 14 radians or 45O so that the tabulated Thomsonts 

functions could be used. These are sometimes called the Kelvin 

functions and denoted ber, bei, kei, etc. (Note William Thomson 

became Lord Kelvin who was also noted for his work on laying the 

first Trans -Atlantic cable. ) 

With the ready availability of high speed computers, the use 

of intricate closed form field formulas is giving way to direct 

numerical integration of the Sommerfeld integrals. ' This is OK provided 

the programmer has some limiting checks or if he can refer back 

to some of the earlier work where the more elegant closed-form 

expressions in terms of special functions are used Also, as has 

been shown quite recently, in the treatment of a fi~ite-length source 

elements the special function representations for the dipole source 

is a convenient starting point. Otherwise double numerical 

integration is needed ! 

As we have indicated above, the air -earth interface problem 

is treated by regarding the overburden as a homogeneous half-space. 

A rather straightforward extension is the stratified half-space wherein 

the- intermediate interfaces are plane and parallel to the air-earth 

interface. Further extensions involve electric dipoles rather than 

magnetic dipoles. These are appropriate when dealing with grounded 

electrodes connected by insulated cables. No new basic difficulties 

are encountered here. 
In the class of problems discussed above, all three of the 

impedance elements Zm, Z1, and Z2 are determinable. Thus, 

in principle, a complete determination or prediction of the system 

performance is possible. This includes down-link and up-link 

communication efficiency - and estimates of source location accuracy 

for the models assumed. 

Traditionally, the source current is as sumed to vary harmonically 

in time. For dealing with transient excitation or in estimating 

signalling rate we need to under stand the time -domain behavior of 

the system. Formally this is obtained by Fourier or Laplace 

transform inversion of the preceding transfer functions. In general, 

this is not a trivial task. However, some rather interesting closedform results can be obtained if the original spatial wavenumber 

integration can be deferred until after the Laplace transforms are 

inverted. 

Now that the homogeneous and stratified half-space models have 

been exhausted it is appropriate to consider more realistic situations. 

For example, the earth's surface is not always flat. One approach 

to allow for this situation was to adapt cylindrical and spherical 

boundaries. In such cases, the local radius of curvature was 

chosen to (more -or -les s) match the local topography. Results of 

calculations from such models indicated that transmission ranges 

were not markedly affected but that source location errors could be 

significant. An even simpler method conceptually was to retain the 

uniform half-space model but to allow the plane interface to be 

tilted at such an angle that the local slope of the terrain was 

matched. Further analytical work on such models is justified since 

much insight can be gained without excessive numerical effort. 

Unfortunately, formally exact solutions for such models are strictly 

limited. One class of boundary that could be treat'ed for line source 

excitation is a parabolic or hyperbolic interface. The three dimensional 

version involving dipoles in the presence of a paraboliodal interface 

is extremely intricate but it may be worth doing also. 

One must clearly recognize that the overburden is not a homogeneous slab whether it be bounded by plane or uniformly curved 

interfaces. Thus, the influence of omni-present inhomogeneities 

needs to be understood. An extreme case is when metallic tracks 

and pipes are in the vicinity of the normal transmission path. To 

treat such configurations, two dimensional models have been used. 

An example is the line source excited buried cylinder of infinite 

length. This is not a trivial problem if one considers the interactions 
between the buried cylindrical inhomogeneity and the air-earth 

interface. Two specific analytical techniques have been used to get 

numerical results for this problem. One is essentially a perturbation 

procedure that involves success ive reflections between the cylinder 

and the plane interface while the other is a sophisticated integral 

equation procedure. The results between those methods agree in 

a common region of validity. The extension to three dimensional 

versions of these problems is nontrivial. Some progress has been 

made, however, in using perturbation procedures. An example is 

the buried sphere in the presence of a surface based dipole source. 

Some work has also been done on cylinders of finite length where 

certain assumptions were made about the nature of the axial induced 

current flow at the ends of the cylinder. Among other things this 

analysis showed that predictions based on infinite cylinders may 

be quite misleading. 

Within the scope of these analytical techniques, we encompass 

both acti,ve and passive location concepts. In the active method, of 

course, the unknown source is energized by the to-be-rescued party. 

On the other hand, in the passive method, the target to be located 

is typically a loop of wire or a similar configuration. Various 

geometries, for such problems, have been considered in both the 

frequency and the time domain. 

The foregoing account of theoretical analyses of electromagnetic 

induction problems is, by no means, claimed to be exhaustive. The 

selection has been based mainly on work that the author is familiar 

with. 

APPENDIX 

A rather brief "guide" to the literature is as .follows: 

E. D. Sunde - Earth Conduction Effects in Transmission Systems - 

Dover Publications, New York, 1968 - (Comprehensive review of 

much of the early work in the 1930-1940 period carried out at Bell 

Telephone Labs. - includes many useful formulations for mutual 

impedances of grounded circuits relevant to coupling between power 

and communication circuits). 

Do B. Large, L. Ball, and A. J. Farstad - Radio Transmission 

from Underground Coal Mines - Theory and measurement trans. IEEE 

Comm. 2 1, 194- 10204, 1973 (good recent account of Westinghouse's 

investigations). 
R. E. Collin and F. J. Zucker (editors) - Antenna Theory Part I1 - 

McGraw-Hill, 1969 - (Chap. 24 by J. R. Wait gives a basic account 

of the theory for sources in conducting media - also an extensive 

bibliography is included). 

L. L. Vanyan - Electromagnetic Depth Soundings (edited and translated 

by G. V. Keller) - Plenum Press, 1967 - (An authorative review 

of Russian work on electromagnetic induction in the earth). 

Also see recent volumes of the Journal Radio Science, IEEE Transactions 

on Antennas and Propagation, Journal of Applied Physics, and 

Geophysics. 
Since the finite cable solution is more complicated than both the infinite 

line source and the short dipole solutions, we explore under what 

conditions a cable appears to be either infinitely long or very short. 

Some special cases, such as. the low frequency limit, permit analytical 

solutions which exhibit the dependence on various parameters, such as 

cable length, quite clearly. 

Frequency Domain Subsurface Fields 

The geometry of a line source of length 24 located on a conducting halfspace with the observer in the half-space is shown in Figure 1. The fields 

*The research reported here was supported by the U. S. Bureau of Ivlines, 

Pittsburgh Mining and Safety Research Center. 
a function of observer position (Hill and Wait, 1973b). 

The transient subsurface fields of a finite cable are more complicated, 

but the frequency domain results of the previous section are useful. The 

necessary inverse Fourier transform can be done in closed form for 

some of the field components, but a numerical inversion is required for 

others. Details of the procedures plus various numerical results are 

available (Hill and Wait, 1973~)~ and related transient problems have 

been treated by Wait (1960). 

Surface Fields of a Buried Cable 

In the uplink application where the transmitting antenna may be in a coal 

mine environment, the cable is not necessarily level. Even if it is level, 
the earth surface may not be. Consequently, the geometry with a 

tilted cable shown in Figure 2 was examined. Results simplify if 

the observer is on the surface (z = O), and the necessary Sommerfeld 

type integrals can again be integrated in closed form if the quasistatic assumption is made. We are primarily interested in the 

magnetic field components since reception at the surface is normally 

with loop antennas. 

Since the additional complication of an arbitrary cable angle o( 

has been included, the source integration along the cable must in 

general be done numerically. The details of the formulation along 

with various numerical results are available (Hill, 1973). Again 

there is a wide range where the cable can be approximated by neither 

the short dipole nor the infinite line source. 

REFER ENC ES 

Banos, A., 1966, Dipole radiation in the presence of a conducting 

half-space: Oxford, Pergamon Press. 

Hill, D. A. , 1973, Electromagnetic surface fields of an inclined 

buried cable of finite length, Preliminary Report to U. S. Eureau 

of Mines. 

Hill, D. A., and Wait, J. R., 1973a, Subsurface electromagnetic 

fields of a grounded cable of finite length, Preliminary Report to 

U. S. Bureau of Mines. 

Hill, D. A., and Wait, J. R., 1973b, Diffusion of electromagnetic 

pulses into the earth from a line source, Preliminary Report to 

U. S. Bureau of Mines. 

Hill, D. A., and Wait, J. R., 1973~~ Subsurface electric fields of 

a grounded cable of finite length for both frequency and time 

domain, Preliminary Report to U. S. Bureau of Mines. 

Sunde, E. C. , 1968, Earth conduction effects in transmission systems: 

New York, Dover Publications, Chapt. V. 

Wait, J. R., 1952, Electromagnetic fields of current-carrying wires 

in a conducting medium, Can. J. Phys., v. 30, p. 512-523. 
Wait, J. R. , 1960, Propagation of electromagnetic pulses in a homogeneous conducting earth, Apple Sci, Res., B., v. 8, p. 213-253. 

Wait, J. R. , 196 1, The electromagnetic fields of horizontal dipole in 

the presence of a conducting half-space, Can. J. Phys. , v. 39, 

p. 1017-1028. 

Wait, J.R., 1971, Transient excitation of the earth by a line source 

of current, Proc. IEEE, v. 59, p. 1287-1288. 

Wait, J. R. and Spies, K. P. , 1971, Subsurface electromagnetic fields 

of a line source on a conducting half-space, Radio Sci., v. 6, 

p. 781-786. 
I Observer (x,y,z) 

Length 21 

FIGURE 2. Tilted cable in a homogeneous half-space. 

FIGURE 1. Finite length cable on a homogeneous half- space. 
THEORY AND EXPERIMENTS RELATING TO ELECTROMAGNETIC 

FIELDS OF BURIED SOURCES WITH CONSEQUENCES TO 

COMMUNICATION AND LOCATION * 

Richard G. Geyer, Consultant 

3973 Baumberger Rd. 

Stow, Ohio 44224 

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. Various communications techniques can be 

considered, including electromagnetic systems, acoustic systems, 

and hybrid systems. The electromagnetic and acoustic systems would 

be independent of existing mine communications systems and set up 

specifically for use in emergencies. Hybrid systems might use a 

short emergency link to existing telephone systems in the mine. 

Many variations are technically possible, and the primary task is 

in evaluating the relative merits of each, so that the most workable 

system can be selected. 

Introduction 

The problems related to the development of an emergency 

communications system are only partly technical in mature; to a large 

part, the choice of an optimum system will depend on human engineering factors which are sometimes difficult to formalize. Basically, 

the problem is that of providing the miner underground with a 

communications system which he may count on usiqg if other means 

of communication are interupted by some mishap or disaster in the 

mine working. 

One important aspect of a communications system is the amount 

of information that can be transmitted over it per unit time. Generally, 

for human voice transmission, data rates as great as 3000 bits per 

second are desired, bur rates as low as 1 bit per second are usable 

for beacon transmission if nothing better is available. A second 

important property of a communications system is whether it is oneway or two-way. One can conceive of simple communications systems 

which would allow the buried miner to make his presence known to 

people on the surface, but which 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 

*The research reported here was supported by the U. S. Bureau of Mines, 

Pittsburgh Mining and Safety Research Center. 

20 
To some extent, the above design parameters may be unique 

for every mine so that for ultimate optimization of emergency communications systems, it would be necessary to have this parameter 

information for every drift of every mine working. This may be 

feasible in the future if conductorle s s communications systems are 

used for routine mine communications, but it is probably not feasible 

for an emergency system. Rather, we must settle for near-optimization based on the idea that similar mines in similar geological 

settings will be characterized by narrow ranges for design parameters, 

and that these ranges may be determined with reasonable statistical 

reliability by making measurements only over selected mines. 

Electrical Properties Studies 

The electrical properties of the earth affect signal strength in 

a way which is determined by the wavenumber for the medium or 
media through which propagation takes place, the wavenumber being 

defined as 

where f is the frequency used, is the magnetic permeability 

of the medium, a is the electrical conductivity, and € is the 

dielectric constant. Only one of these quantities, the frequency, is 

a design parameter. In order to specify values for the wavenumber 

in theoretical studies, we need values for the other three quantities, 

possibly as a function of frequency, if any of the factors should be 

frequency dependent. 

Thus, knowledge of the electrical properties of the geologic 

section overlying a mine working is important for ensuring the best 

possible transmitter -receiver electromagnetic coupling for either 

emergency or ~outine communication purposes. 

Both galvanic and induction surface-bas ed techniques for measur - 

ing the electrical properties of the rock sections overlying a number 

of coal mine workings in various mining provinces were used and 

the results have been reported (Geyer, 1971b; Geyer, 1972b). A 

typical resistivity section observed at the Montour No. 4 Mine, Pa. 

is shown in Figure 1. 

Ambient Electromagnetic Noise Environment Studies 

Studies of ambient electromagnetic noise statis tics are complimentary to investigations of the overburden electrical transmission 

properties and are necessary for proper design of any subsurface- 

.surface electromagnetic communications system. qmbient 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 (Geyer, 197 lb). 

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 aetermined. An example of noise level histograms obtained with 

an electric-field sensor is shown in Figure 2, with the time of day 

noted for each curve. 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 incor - 

porated into an uplink receiver system by setting thresholds for 

signal detection just above the median or maximum percent noise 

level. 

Thus, the noise amplitude density functions presented (Geyer, 

1971b) may be used in conjunction with measured resistivity data 

to design and implement a practical operating beacon electromagnetic 

communications 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 the electrical properties 

data together with electromagnetic noise environment data for 

systems design considerations have been given for a loop-loop 

source-receiver configuration (Geyer, 1971a) and a line-line sourcereceiver configuration (Geyer , 197 lb). 

Field Transmission Tes.ts 

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 (Geyer, 1972~; Geyer, 1972~; 

Geyer, 1973a; Geyer, 1973b). 

The various source-receiver configurations considered for 

downlink pulse communications are shown in Figure .3. Generally, 

for impulsive excitation of a line current source, coupling transients 

for all field components decay less rapidly and positive signal peaks 

for the electric field occur later in time as the horizontal sourcereceiver offset distance becomes larger. Furthermore, all coupling . transient responses increase as the square of the resistivity of the 

ear th. 

The results of numerous uplink C. W. field transmission tests 

(see Figures 4 and 5), on the other hand, which were made over a 

broad range in frequency and at several geographic receiving sites, 

show, in general, good agreement with theoretical considerations 

(see Figure 6) and several features of the produced surface magnetic 

field from a buried vertical-axis transmitting loop could be used 

for beacon location criteria. One criterion would be in the surface 

mapping of the maximum in the vertical magnetic field directly over 

the vertical-axis loop transmitter and the associated null in the 
vertical magnetic field (for extremely low frequencies) at a horizontal 

offset distance which is about 1.4 times the depth of burial of the 

transmitter (see Figure 7). Another 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 the first case, 

the size effect of the transmitter loop must be taken into consideration 

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 both cases care should be 

exercised in ascertaining that measured nulls and maxima are along 

radials through the axis of the 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, although secondary and perhaps undesirable maxima 

and null phenomena (as well as possible penetration problems) would 

occur in many coal mine provinces in the behavior of the surface 

horizontal magnetic field if transmission frequencies as high as 10 

kHz were used. 

Summary 

Field measurements and tests, together with th'eor etical consider - 

ations, enable us, on a practical basis, 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 a through-the-earth beacon electromagnetic 

communications system. Provided the overburden is relatively 

conductive (< 100 ohm-meters) and contact resistance at the current 

electrodes is no problem, it is often more convenient to put current 

directly 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 and for a receiving 

surface electric-field sensor does not seem to provide as convenient 

a means for location as does a loop-loop source-receiver configuration 

(Geyer, 1973a). Thus, received magnetic-field signals in the ELF 

range are less sensitive in general to secondary scattering sources 

in the overburden than are electric-field signals. 
U. S. Bureau of Mines, 51 p. 

Geyer, R. G., 197213, Research on the transmission of electromagnetic 

signals between mine workings and the surface: Annual Summary 

Rpt. for period June 16, 1971 to June 15, 1972.for U. S. Bureau 

of Mines, 49 p. 

Geyer, R. G., 1972c, Research on the transmission of electromagnetic 

signals between mine workings and the surface - Passive detection 

of a buried loop from the surface, Downlink pulse signalling to 

underground mine workings, Downl.ink C. W. signalling to underground 

mine workings: Quart. Tech. Rpt. for the period July 15, 1972 

to Sept. 30, 1972 for the U. S. Bureau of Mines, 35 p. 
Geyer, R. G., 1973a, Research on the transmission of electromagnetic 

signals between mine workings and the surface - uplink field 

transmission tests using a vertical-axis loop source: Quart. 

Tech. Rpt. for the period Oct. 1, 1972 to Dec. 31, 1972 for 

the U, S. Bureau of Mines, 52 p. 

Geyer, R. G., 197313, Uplink field transmission tests using a grounded 

buried horizontal electric line source antenna: Quart. Tech. Rpt. 

for the period Jan. 1, 1973 to April 15, 1973 for the U. S. Bureau 

of Mines, 17 p. 

Wait, J. R., and Spies, K, P., 1971a, Subsurface electromagnetic 

fields of a circular loop of current located above ground: Informal 

Rpt. to U. S. Bureau of Mines (Nov. 15, 1971), 16 p. 
NW - SE RESISTII SECTION AT 

NO. 4 COAL MINE, PA. 

n A"' DIPOLE - DIPOLE ARRAY 

p (ohm -meters) a 

HORIZONTAL SCALE 

Figure 1. 
Fig. 2. 

28 
DOWNLINK SOURCE - RECEIVER SENSOR 

CONFIGURATIONS 

,Line Source 

6 Grounded 

Line 

,Line Source 

Horizontal Loop 

.s ,Line Source 

Vertical Loop 

Figure 3. 
EARTH ' S SURFACE 

LOOP TRANSMITTER 

MINE WORKING 

Fig. 4. vertical-axis loop transmitter antenna 

and induction loop receiving sensors for 

uplink C.W. transmission. 
EARTH 

REC 

,X 

VER 

p LINE CURRENT TRANSMITTER IN 

/ MINE WORKING 

~ig. 5.Three-dimensional view of transmitter antenna 

and receiver sensor under consideration for 

uplink C.W. transmission. 

SENSOR 

f = 310 

Data Poin

1 10 100 

OFFSET DISTANCE FROM BURIED 

TRANSMITTER (meters) 

Figure 7. 
ELECTROMAGNETIC DIRECTION FINDING EXPERIMENTS FOR 

LOCATION OF TRAPPED MINERS 

R. G. Olsen and A. J. Farstad 

Westinghouse Electric Corporation 

Georesearch Laboratory 

8401 Baseline Road 

Boulder, Colorado 80303 

The Coal Mine Health and Safety Act of 1969, passed by Congress 

in the wake of the disaster at Farmington, West Virginia, led to an 

extensive study by the National Academy of Engineering of the problems involved in coal mine safety. ' One of the results of this 

report and subsequent research conducted by the U. S. Bureau of 

Mines has been the investigation of electromagnetic techniques for 

the location of trapped miners after a mine emergency. The 

Westinghouse Georesearch Laboratory has been involved in both the 

2 theoretical and field studies relating to this problem . 

A compact personal beacon transmitter has been developed which 

can be attached to and powered by a miner's lamp battery. A compatible lightweight loop antenna package has also been developed which 

can be easily carried on a miner's belt. This antenna can be quickly 

deployed on the mine tunnel floor. For the purpose of calculating 

its electromagnetic fields on the earth's surface, the loop can be 

considered to be a magnetic dipole buried in the earth. (The earth 

is assumed to be homogeneous and to have a conductivity of tr mhos/m.) 

The surface of the earth is assumed to be uniformily sloping and the 

transmitter dipole oriented as in Figure 1. The fields on the earth's 

surface can be computed by using a superposition of the fields of a 

4 vertical magnetif dipole3 and of a horizontal magnetic dipole in 

proportions which depend on the slope of the terrain. 

In the case of level terrain a null in the horizontal magnetic 5 field occurs on the surface directly above the dipole . This null is 

designated as the apparent source location and is the criterion by 

which the source is located. The criterion was tested at two hardroek 

mines and two coal mines in the fall of 1972. The apparent source 

locations were compared to actual locations determined by conventional 

surveying techniques. It was found that due to nonlevel terrain the 

apparent and actual source locations differed by up to 13 meters. 

This discrepancy can be resolved by an examination of the fields of 

a source beneath uniformly sloping terrain. 

Figure 2 shows the experimental and theoretical results for a 
0 

case where the hill slope is 15 (Hp is the horizontal field, and H z 

is the vertical field. ) The null is displaced downhill by an amount 

which depends on the depth of the source, the ground conductivity, 

and the hill slope. 

These results can be used to improve the location technique. 

If the apparent source location is known along with the ground conductivity, 

hill slope, and approximate source depth, the theoretical null offset 

can be used to obtain the actual source location. 

References 

1. G. Meloy, et a1 (1969 Mine Rescue and Survival, Nat. Acad. Eng. ) 

2. D. B. Large, L. Ball and A. J. Farstad (1973 IEEE Trans. Comm. 

Tech., vol. COM-21, No. 3, p. 194-202). 

3. J. R. Wait (1971 Proc. IEEE, v. 59, n. 6, p. 1033-1035). 

4. A. K. Sinha and P. K. Bhattacharya (1966 Radio Science, v. 1 

(New Series), n. 3, p. 379-395). 

5. J. R. Wait and L. L. Campbell (1953 Jour. Geophysics Res., 

v. 58, n. 2, p. 167-178). 

- 

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4.- 

Q, 

C 

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C 

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1. 

.50 .*a - -30 -20 4 0 0 10 20 

Distance from Center, Meters , 

. , 

Figure 2. Electromagnetic Surface Pat tern from Burled VMD 

IA-lol~fi T'fil2 Sl.onc1 

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I 
THEORY OF THE PROPAGATION OF UHF RADIO WAVES 

IN COAL MINE TUNNELS 

Alfred G. Emslie, 

Robert L. Lagacet2 and Peter F. Strong2 

ABSTRACT r 

This paper is concerned with the theoretical study of UHF radio communication in coal mines, 

with particular reference to the rate of loss of signal strength along a tunnel, and from one tunnel to 

another around a comer. Of prime interest are the nature of the propagation mechanism and the 

prediction of the radio frequency that propagates with the smallest loss. Our theoretical results are 

compared with measurements made by Collins Radio Co. This work was conducted as part of the 

Pittsburgh Mining and Safety Research Center's investigation of new ways to reach and extend 

two-way communications to the key individuals that are highly mobile within the sections and 

haulage ways of coal mines. 

INTRODUCTION 

At frequencies in the range of 2004,000 MHz the rock and coal bounding a coal mine tunnel 

act as relatively low loss dielectrics with dielectric constants in the range 5-10. Under these 

conditions a reasonable hypothesis is that transmission takes the fonn of waveguide propagation in 

a tunnel, since the wavelengths of the UHF waves are smaller than the tunnel dimensions. An 

electromagnetic wave traveling along a rectangular tunnel in a dielectric medium can propagate in 

any one of a number of allowed waveguide modes. All of these modes &e "lossy modes" owing to 

the fact that any part of the wave that impinges on a wall of the tunnel is partially refracted into 

the surrounding dielectric and partially reflected back into the waveguide. The refracted part 

propagates away from the waveguide and represents a power loss. This type of waveguide mode 

differs from the light-pipe modes in glass fibers in which total internal~eflection occurs at the wall 

of the fiber, with zero power loss if the fiber and the matrix in which it is embedded are both 

lossless. It is to be noted that the attenuation rates of the waveguide modes studied in this paper 

depend almost entirely on refraction loss, both for the dominant mode and higher modes excited by 

scattering, rather than on ohmic loss. The effect of ohmic loss due to the small conductivity of the 

surrounding material is found to be negligible at the frequencies of interest here, and will not be 

further discussed. 

The views and conclusions contained in this document are those of the authors and should not be interpreted as 

necessarily representing the official policies of the Interior Department's Bureau of Mines or the U.S. Government. 

This paper was prepared under USBM Contract No. H0122026. 

1. Consultant: Formerly with Arthur D. Little, Inc. (Retired) 

2. Arthur D. Little, I nc., Cambridge, Massachusetts. 
The study reported here is concerned with tunnels of rectangular cross section and the theory 

includes the case where the dielectric constant of the material on the side walls of the tunnel is 

different from that on top and bottom walls. The work extends the earlier theoretical work by 

Marcatili and ~chmeltzer( ' ) and by laser(' ) which applies to waveguides of circular and parallel-plate 

geometry in a medium of uniform dielectric constant. 

In this paper we present the main features of the propagation of UHF waves in tunnels. Details 

of the derivations are contained in Arthur D. Little, Inc. reports.(3 ) 

THE FUNDAMENTAL (1,l) WAVEGUIDE MODES 

The propagation modes with the lowest attenuation rates in a rectangular tunnel in a dielectric 

medium are the two (1,l) modes which have the electric field 8 polarized predominantly in the 

horizontal and vertical directions, respectively. We will refer to these two modes as the E, and E, 

modes. 

The main field components of the Eh mode in the tunnel are 

Ex = Eo cos k, x cos ky y e-ikzz 

H, = (k, /UP, )E, cos & x cos k,, y e-ikzz 

where the symbols have their customary meani~g. The coordinate system is centered in the tunnel 

with x horizontal, y vertical, and z along the tunnel. In addition to these transverse field 

components there are small longitudinal components E, and Hz and a small transverse component 

H,. For the frequencies of interest here k, and k,, are small compared with k, which means that 

the wave propagation is mostly in the zdirection. From a geometrical optics point of view, the ray 

makes small grazing angles with the tunnel walls. 

In the dielectric surrounding the tunnel the wave solution has the form of progressive waves in 

the transverse as well as the longitudinal directions. The propagation conitant k, for the ( 1,l) mode 

is an eigenvalue determined by the boundary conditions of continuity of the tangential components 

of and 2 at the walls of the tunnel. Owing to the simple form of the wave given by (1) and (2) 

these conditions can be satisfied only approximately. However, a good approximation to k, is 

obtained. The imaginary part of +, which arises owing to the leaky nature of the mode, gives the 

attenuation rate of the wave. The loss I+h in dB for the (1,l) Eh mode is given by 

where K1 is the dielectric constant of the side walls and K2 of the roof and floor of the tunnel. 

The corresponding result for the (I, 1 ) E,, mode is 
These results are valid if the wavelength h is small compared with the tunnel dimensions dl 

and d2. The same formulas are also obtained if one adds the attenuations for horizontal and vertical 

slot waveguides with dimensions d2 and dl, and dielectric constants K2 and Kl,respectively. The 

losses calculated by (3) and (4) also agree closely with those calculated by a ray approach. 

Figure 1 shows loss rates in dB/l'00 ft as functions of frequency calculated by (3) and (4) for 

the (1,l) Eh and Ev modes in a tunnel of width 14 ft and height 7 ft, representative of a haulage 

way in a seam of high coal, and for K1 = K2 = 10, corresponding to coal on all the walls of the 

tunnel. It is seen that the loss rate is much greater for the Ev mode. Figure 2 shows the calculated 

Eh loss rate for a tunnel of half the height. The higher loss rate in the low coal tunnel is due to the 

effect of the di tern in (3). 

Two experimental values obtained by Collins Radio CO.(~ ) for horizontal-horizontal antenna 

orientations are also shown in Figure 1. These values agree well with theory for the Eh mode for 

415 MHz, but not so well for 1000 MHz. The departure suggests that some additional loss 

mechanism sets in at higher frequencies. 

It is also significant that the experimental values of the loss rates for dl three orientation 

arrangements of the transmitting and receiving dipole antennas, namely, horizontal-horizontal, 

vertical-horizontal, and vertical-vertical, are surprisingly close to each other. The independence of 

loss rate with respect to polarization is not predicted by the theory discussed so far, as seen in 

Figure 1 for the Eh and E, modes. Indeed, the theory predicts no transmission at all for the VH 

antenna arrangement. 

PROPAGATION MODEL 

The higher observed loss rate at the higher frequencies relative td the calculated Eh mode 

values, and the independence of the loss rate on antenna orientation can both be accounted for if 

one allows for scattering of the dominant (1,l) Eh mode by roughness and tilt of the tunnel walls. 

The scattered radiation goes into many higher modes and can be regarded as a diffuse radiation 

component that accompanies the Eh mode. The diffuse component is in dynamical equilibrium 

with the Eh mode in the sense that its rate of generation by scattering of the Eh mode is balanced 

by its rate of loss by refraction into the surrounding dielectric. Since the diffuse component consists 

of contributions from the (1,l) E, mode and many higher order waveguide modes, all of which have 

much higher refractive loss rates than the fundamental Eh mode, the dynarnical balance point is 

such that the level of the diffuse component is many dB below that of the Eh mode at any point in 

the tunnel. 

Our propagation model, comprising the (1,l) Eh mode plus an equilibrium diffuse component, 

explair~s the discrepancy between theory and experiment in Figure I, since the loss due to scattering 
of the Eh mode is greater at 1000 MHz than at 41 5 MHz owing to the larger effect of wall tilt at the 

higher frequency. The model accounts for the independence of loss rate on antenna orientation, 

since the loss rate is always that of the Eh mode, except for initial and final transition regions, no 

matter what the orientations of the two antennas may be. The transition regions, however, cause 

different insertion losses for the different antenna orientations. 

Further strong support for the theoretical model is provided by the discovery by Collins Radio 

Co. that a large loss in signal strength occurs when the receiving antenna is moved around a comer 

into a cross tunnel; and that the signal strength around the comer is independent of receiving 

antenna orientation. This is exactly what our model predicts since the well collimated Eh mode in 

the main tunnel couples very weakly into the cross tunnel, whereas the uncollimated diffuse 

component couples quite efficiently. Since the diffuse radiation component is likely to be almost 

unpolarized, the observed independence of signal strength on receiving antenna orientation is 

understandable. 

Another experimental result is that the initial attenuation rate in the cross tunnel is much 

higher than the rate in the main tunnel. This is also in accord with the model since the,diffuse 

radiation component has a much larger loss rate than the Eh mode owing to its steeper angles of 

incidence on the tunnel walls. 

THE DIFFUSE RADIATION COMPONENT 

Scattering of the (1,l) Eh mode into other modes to generate the diffuse component occurs by 

two mechanisms: wall rcughness and wall tilt. 

Roughness is here regarded as local variations in the level of the surface relative to the mean 

level of the surface of a wall. For the case of a Gaussian distribution of the'surface level, defined by 

a root mean square roughness h, the loss in dB by the Eh mode is given by the formula 

This is also the gain by the diffuse component due to roughness. 

Long range tilt of the tunnel walls relative to the mean planes which define the dimensions d, 

and d2 of the tunnel causes radiation in the Eh mode to be deflected away from the directions 

defied by the phase condition for the mode. One can calculate the average coupling factor of such 

deflected radiation back into the Eh mode and thereby find the loss rate due to tilt. The result in 

dB is 

where 0 is the root mean square tilt. Eq. (6) also gives the rate at which the diffuse component gains 

power from the Eh mode as a result of the tilt. 
It is noted from (5) and (6) that roughness is most important at low frequencies while tilt is 

most important at high frequencies. 

Figure 3 shows the effect on the (1,l) Eh mode propagation of adding the loss rates due to 

roughness and tilt to the direct refraction loss given in Figure 1. The curves are calculated for a root 

mean square roughness of 4 inches and for various assumed values of 8. It is seen that a value 0 = lo 

gives good agreement with the experimental values of C'ollins Radio Co. The effect of tilt is much 

greater than that of roughness in the frequency range of interest. 

Having determined the value of 8, for the assumed value of h, we can now find the intensity 

ratio of the diffuse component to the Eh mode from the equilibrium balance equation 

b , main I1h, main = Lh d lLd 

where bd is the loss rate from the Eh mode into the diffuse component, and Ld is the loss rate of 

the diffuse component by refraction. To estimate Ld approximately, we take the loss rate to be 

that of an "average ray" of the diffuse component having direction cosines (1 w, 1 v; 1 v). 

Then 

where R, the Fresnei reflectance of the average ray for KI = K2 = 10, has the vzlue 0.'28. Then for 

dl = 14 ft, dl = 7 ft, z = 100 ft, we find that Ld = 1 19 dB11 00 ft. This value has to be corrected 

for the loss of diffuse radiation into cross tunnels which we assume have the same dimensions as the 

main tunnel and occur every 75 ft. From relative area considerations we find that this loss is 

2 dB/ 100 ft. The corrected value is therefore . . 

Ld = 121 dB1100 ft. 

which is independent of frequency. 

The loss rate Lhd is shown in Table I as a function of frequency for the 14 ft x 7 ft tunnel. 

The values are the sum of the roughness and tilt losses calculated by (5) and (6) for h = 4 inches rms 

and 0 = lo rms. The diffuse component level relative to the Eh mode, calculated by (7), is given in 

the fourth column of Table I. The diffuse component is larger at high frequencies owing to the 

increased scattering of the Eh mode by wall tilt. 

PROPAGATION AROUND A CORNER . 

From solid angle considerations one finds that the fraction of the diffuse component in the 

main tunnel that enters the 14 ft x 7 ft aperture of a cross tunnel is 15% or - 8.2 dB. The diffuse 

level just inside the aperture of the cross tunnel, relative to the Eh mode level in the main tunnel is 

therefore obtained by subtracting 8.2 dB from the values in column 4 of Table I. The results are 

shown in column 5 of the table. A dipole antenna with either horizontal or vertical orientation 
The theoretical model also allows us to predict the effect of antenna orientation when the 

transmitting and receiving antennas are far enough apart so that dynarnical equilibrium between the 

Eh mode and the diffuse component is established. We start with both antennas horizontal (HH 

configuration) ar?d consider this as the 0 dB reference. Then if the receiving antenna is rotated to 

the vertical (HV configuration) this antenna is now orthogonal to the Eh mode, and therefore 

. responds only to one half of the diffuse component, so that the loss is 3 dB more than the values in 

Table I, column 4. The result is shown in Table I11 column 2. Now, by the principle of reciprocity, 

the transmission for VH is the same as for HV as shown in column 3 of Table 111. We now rotate the 

receiving antenna to get the configuration W. Again we incur an additional transmission loss df ' 

3 dB more than the values in Table I, column 4. The W values are shown in Table 111, column 4. 

ANTENNA INSERTION LOSS 

Dipole or whip antennas are the most convenient for portable radio communications between 

individuals. However, a considerable loss of signal power occurs at both the transmitter and receiver 

when simple dipole antennas are used because of the inefficient coupling of these antennas to the ~ 

waveguide mode. The insertion loss of each dipole antenna can be calculated by a standard 
microwave circuit technique for computing the mount of power coupled into a waveguide mode 

by a probe, whereby the dipole antenna is represented as a surface current filament having a 

sinusoidal current distribution along its length. The result is 

Z, is the characteristic impedance of the Eh (1,l) mode and R, is the radiation resistance of the 

antenna, which are approximately 377 and 73 ohms, respectively, provided that X is small compared 

with dl and d2. 

Formula (1 1) applies to antennas placed at the center of the tunnel and gives the results shown 

in Table IV, where the insertion loss Li in dB is equal to - 10 log, ,C. It is seen that the insertion 

loss decreases rapidly with increasing wavelength, as one would expect, since the antenna size 

occupies a larger fraction of the width of the waveguide. The overall insertion loss, for both 

antennas, is twice the value given in the table. A considerable reduction in loss would result if high 

gain antenna systems were used. 

OVERALL LOSS IN A STRAIGHT TUNNEL 

The overall loss in signal strength in a straight tunnel is the sum of the propagation loss arid the 

insertion losses of the transmitting and receiving antennas. Table V lists the component loss rates 

for the (1,l) Eh mode due to direct refraction, roughness, and tilt; the total propagation loss rate; 

the insertion loss for two half-wave antennas; and the overall loss for five different distances. The 

overall loss for the HH orientation is also shown in Figure 6, where it is seen that the optimum 

frequency for minimum overall loss is in the range 500-1000 MHz, depending on the desired 

communication distance. 

It is also of interest to combine the results in Table V with those in Table I11 to obtain the 

overall loss versus distance for the HH, HV (or VH), and VV antenna orientations. In order to 

compare the theoretical values with the experimental data of Collins Radio Co., which are expressed 

with reference to isotropic antennas, we add 4.3 dB to the overall loss calculated for half-wave 

dipoles. The theoreiical results for the three different antenna orientations for frequencies of 41 5 

. MHz and 1,000 MHz are compared with the experimental data in Figures 7 and 8. It is seen that the 

theory agrees quite well with the general trend of the data. 

OVERALL LOSS ALONG A PATH WITH ONE CORNER 

Table VI gives the overall Eh mode loss for a path from one tunnel to another, including the 

corner loss involved in re-establishing the Eh mode in the second tunnel. The loss is the sum of the 

comer loss, given in column 3 of Table I1 and repeated in Table VI, and the straight tunnel loss 

given in Table V for various total distances. The results in Table VI are for the case of half-wave 

dipole transmitting and receiving antennas and are valid when neither antenna is within about 100 ft 

of the comer. The overall loss is less than the values in Table VI if the receiving antenna is within 

this distance, owing to the presence of the rapidly attenuating diffuse component that passes 
around the comer. From the principle of reciprocity, the same is true if the transmitting antenna is 

within 100 ft of the comer. 

The results indicate that the optimum frequency lies in the range 400- 1,000 MHz. However, if 

one installs horizontal half-wave resonant scattering dipoles with 4S0 azimuth in the important 

tunnel intersections, in order to guide the Eh mode around the comer, the optimum may shift to 

somewhat lower frequencies since a greater fraction of the incident Eh wave will be deflected by the 

longer low-frequency dipoles. 

' CONCLUSIONS 

The kind of propagation model developed in this paper, involving the (1,I) Eh waveguide 

mode accompanied by a diffuse component in dynarnical equilibrium with it, seems to be necessary 

to account for the many effects observed in the measurements of Collins Radio Company: the 

exponential decay of the wave; the marked polarization effects in a straight tunnel; the independence of decay rate on antenna orientation; the absence of polarization at the beginning of a cross 

tunnel; the two-slope decay characteristic in a cross tunnel; and overall frequency dependence. All 

of these effects are moderately well accounted for by the theoretical model. However, considerable 

refinement of the theory could be made by removing some of the present oversimplifications, such 

as: the assumption of perfectly diffuse scattering both in the main tunnel and immediately around a 

comer in a cross tunnel; the use of the "average ray" approximation; and the description of the 

propagation around a corner in terms of two asymptotes only. 

The last item particularly deserves more attention since we have not included the conversion of 

the diffuse component in the transition region near the beginning of the cross tunnel into the Eh 

mode. For this reason we think that the good fit of the theory to the experimental data in Figures 4 

and 5 may be somewhat fortuitous. More data at greater distances down a cross tunnel would be 

very desirable to settle this question. Data covering a wider frequency range in both main and cross 

tunnels would also allow a more stringent test of the theory. 

REFERENCES • 

.I. E. A. J. Marcatili and R. A. Schmeltzer, "Hollow Metallic and Dielectric Waveguides for Long 

Distance Optical Transmission and Lasers," The Bell System Technical Journal, Vol. 43, 1783, 

1964. 

2. J. I. Glaser, "Attenuation and Guidance of Modes in Hollow Dielectric Waveguides," IEEE 

Transactions on Microwave Theory and Techniques, March 1969, p. 173; and M.I.T. Ph.D. 

Thesis, "Low-Loss Waves in Hollow Dielectric Tubes," February 1967.' 

3. Arthur D. Little, Inc., reports to U.S. Department of the Interior, Bureau of Mines, Pittsburgh, 

Pa. 

4. Collins Radio Company, "Coal Mine Communications Field Test Report," December 29, 1972 

prepared for U.S. Department of the Interior, Bureau of Mines, Pittsburgh, Pa. 
LorIndB 

pr 100 Fat 

REFRACTION LOSS FOR Eh AND €FOES 

IN HlQH COAL 

r 

- 

14 Ft. 

-Thosy 

8HH Exp. Dm (Collinr Rdb) 

FIGURE 2 

REFRACTION LOSS FOR Eh MODE IN LOW COAL 

90- 

amasoa 

LorindB 

pr 100 Fat 

10 

6 

f IQURE 3 

RESULTANT PROPABATION LOSS FOR Eh MODE IN HlQH COAL 

(Rdmcth. Wall Roughmu wd Tilt) 

Conrr Lou 40 

h dB so 

60 

m 

10 

m 

28 

20 

16 

I 1 

0 loo0 2000 3000 

- 

FIQURE 4 

CORNER LOSS IN HIGH COAL 

14 ft. 

- Throcy 

- 

- 

-m 

O HH Exp. Dnr 

(Cdllm Rdio Co.1 

Eh Mod8 

RMSRoqhrr Mtndm 
FIGURE 7 

OVERALL LOSS IN A STRAIGHT TUNNEL IN HlGH COAL 

(For Irotropic Amnrr) 

FIGURE 6 

CORNER LOSS IN HlGH COAL 

-Thorv 

8 Exp. DatdCollins Radio 

Conr Lon 

in dB 

w . i 

Dirtrno Around Corm in Fwt 

FIGURE 6 

TOTAL LOSS FOR VARIOUS DISTANCES ALONG A STRAIGHT TUNNEL 

2so 

OHRll Lon 

in dB 

I 

- 

- 

Onnll Lor 

in dB 

enm=10 

h nm = 113 Ft. 

K, -%- 10 

dl = 14 Ft., d2 = 7 Ft. 

Haltmw Antrnnr, 

&¬ion (HH) 

100 - 

170 - 

Dittlna in Fat 

,@qt- 

,000 '" 

-- 

I I I I I I I 

FIGURE 8 

OVERML LOSS IN A STRAIGHT TUNNEL IN HlGH COAL 

(For lmtropic Antmnrr) 

00 - Fnquw - 1OOO MHz 

Expuimont 

Throry - 

Antrnnr 

(Cgllins Radio) 0ri.nPtion 

140 - 

150 - 

100 - 

170 - 

1 d, Id, 1, i I I I 

DirPrrcr in Fat 
DIFFUSE RADIATION COMPONENT IN MAIN TUNNEL 

AND AT BEGINNING OF CROSS TUNNEL 

TABLE II 

EXCITATION OF E~ MODE IN CROSS~UNNEL 

BY DIFFUSE COMPONENT IN MAIN TUNNEL 

I 1 

# 

.TABLE Ill 

EFFECT OF ANTENNA ORIENTATION 
first of all, when the cable is suspended in the middle of 

the cross section of the gallery, the electromagnetic field 

occupies all the space between the wire and the walls of the 

gallery. When the wire is close to the wall, the electromagnetic field tends to become concentrated between the wire and 

the wall, with the consequence that, with the given monofilar 

mode power, the aerial of a receiver standing somewhere in 

the tunnel will capture a weaker and weaker signal. Secondly, 

owing to the fact that the mode of monofilar propagation uses 

the ground as return conductor, its power of propagation is 

considerably diminished; the closer the wire is to the wall 

the higher the attenuation, for only a very small part of the 

wall is utilized as return conductor. 

To cover great distances, everything else being equal, two or 

more conductors must be placed in the gallery. Lf the distance between two conductors of a cable is much smaller than 

the distance between one or other of the conductors and the 

walls; or again if one of the conductors surrounds the others 

and forms a screening, two sorts of modes appear : on the one 
hand the monofilar mode in which the wires or the screening 

serve as an outward conductor and the ground as a return conductor and, on the other hand, one or several modes in which 

the outward and return are effected solely by wires without 

intervention of the earth. This new type of mode will 

therefore be characterized by a relatively slight attenuation 

of propagation : hence, the ranges will be greater. However, 

the electromagnetic field of these mcdes is concentrated in 

the neighbourhood of the wires or inside the screening and it 

may not influence the antenna of mobile sets; only the monofilar mode possesses this property. For this system to be 

completed, it is therefore necessary to produce energy exchanges between these two types of mode, the monofilar mode 

being used in as small a quantity as possible for the link 

with the antenna, and the other mode for long distance propagation. 

Svstems Based on the Use of a Coaxial Cable 

The guide-cable with the greatest range is that in which the 

attenuation is the slightest : it consists .=>f a coaxial cable. 

For purposes of clearness we shall call the coaxial mode that 

which is propagated betxeen thc inncr conductor azd the 

screening and the sirgle wire mode that which is situated 

outside the cable. The coaxial cable has a second advantage 

over cables such as the bifilar twin lead cable : the coaxial 

mode is entirely protected against the outer environment and 

does not suffer at all from a prolonged stay in a damp dr 

dusty atmosphere or from being placed against the wall of the 

gallery. 

The INIEXiDelogne 'system consists of connecting along a coaxial cable, radiating multi-purpose devices including conver - 

sion of modes. 

Generally speaking, any antenna could perform these functions, 

but there is only one which is simple, small, economic and at 

the same time remarkably efficient : it consists of a cutting 

of the screening of the cable to which concentrated elements 

- coils and condensors - can always be added in order to improve the working. 

Theoretical and Experimental Studv 

To study the INIEX/Delogne radiating devices it was necessary 

to solve the Maxwell equations, utilise the computer and verify the theory on the small-scale model with UHF in the la- 
boratory. The full study has been published in a technical 

review (1); we give here the most important results of this 

study. 

From the point of view of the propagation of the coaxial mode, 

the complete cut of the outer conductor behaves like an impedance inserted in series between the two lips in the gap of 

the outer conductor. This impedance is an electrical resis - 

tance and a capacity put in pazallel (fig. 1). The power 

that should be dissipated by the electrical resistance we 

imagine is exactly the one which in reality goes out of the 

cut of the outer conductor and this power is representative 

of the radiation and of the monofilar mode, and this, of 

course, means that there is a loss of signal for the propagation in the coaxial mode. 

To reduce the impedance of the cut we may add a condensor as 

shunt. 

We may balance the effect of the condensor by inserting in 

series with the inner conductor a resonating spool matched 

with the condensor on the working frequency. 

An interesting characteristic of the radiating waves is that 

they are concentrated in directions situated not more than 

about 10 degrees further away from the axis of the cable. 

While systems based on coaxial cable described by others have 

to take into account a coupling 10s s of the order of 75 to 

105 dB between the coaxial mode and the input of a mobile receiver, the coupling loss is only 25 dB in the INIEX/Delogne 

system. Therefore it allows the greatest ranges one cbuld obtain at present without any intermediate amplification. 

The system is now operating in several mines (fig. 2). A patent has been issued in several cantries and is pending in 

the United States. 

The INIEX/Deryck System 

When a bifilar cable is suspended in an underground gallery, 

two modes of propagation appear : the parallel mode and the 

anti-parallel mode (fig. 3). 

The way it works has been analysed intensively (2); it has been 

possible to provoke a controlled diaphony by inserting in the 

line devices called "mode converters" which are quadripoles 

with a transverse asymetry. Figure 4 shows two models of such 

a mode converter. 

51 
It happens that a bifilar transmission line equipped with the 

mode converters works in an entirely similar manner as the 

coaxial cable equipped with INIEX/ Delogne radiating devices . Ranges are smaller because the attenuation of the bifilar 

line is higher than that of a coaxial cable. A bifilar line 

is less resistant to weathering than a coaxial cable, but it 

is cheaper. 

REFERENCES 

P. DELOGNE & R. LIEGEOLS. - Ann. ~61., France, t. 26, no 3-4, 

pp. 85-100, 1971, mars-avril. 

L. DERYCK. - Etude de la propagation des ondes blectromagnitiques guidges dans les galeries souterraines. Thsse de 

doctorat en Sciences, ~niver sit; de LiZge, Belgique, 1973. 
Fig. 4 

(a) narraw band 

(b) - -- wide band - 

cbrvertiaseurs & mode JZectif (a

et d Zarge ban& (b) 
comnunications for the Bureau of Mines. The measurement and analysis 

effort centers on the propagation characteristics of radio signals in 

a working coal mine environment. The intent of the resulting data is 

to provide basic propagation loss characteristics in a form convenient 

for establishing requirements and evaluating alternate approaches for 

an integrated mine communications system. The goal of such a system 

is to satisfy both operational and emergency cmunication needs in the 

mine. 

Field testing was conducted during the last week of November, 1972, 

at a coal mine operated by Inland Steel Company, near Sesser, IL. The mine 

has a vertical shaft entry with 750 feet of overburden and is located 

* This work was supported by the U. S. Department of Interior, Bureau 

of Mines. 

" The views and conclusions contai~ed in this document are those of the 

authors and should not be interpreted as necessarily representing 

the official policies of the Interior Department's Bureau of Mines 

or the U. S. Government." 
in the Illinois No. 6 seam. At present, the mine is 8,000 feet NorthSouth by 13,000 feet East-West with an ultimate size 3 miles NorthSouth by 7.5 miles East-West. All tunnels and haulage ways are 14 feet 

wide by 7-8 feet high with pillars running 60 feet by 74 feet. Roof 

bolts are 6 to 9 feet in length and secured by expansion achors. The 

roof bolts are placed on 4 foot centers throughout the mine. 

The test area extended along a 4450 foot segment of the main west entry. The tunnel in which measurements were taken was the same as used for 

haulage of men and supplies via battery-operated, rubber tire vehicles. 

This tunnel also contained a 7200 VAC, 3-phase prime power cable suspended 

from the roof. A cross section of the tunnel is shown in figure 1. 

Measurements of corner attenuation were made a1 ong cross-cuts at right 

angles to the main tunnel. A typical corner geometry is shown in figure 

2, 

UHF TESTS 

At UHF, transmission loss along the main tunnel and around corners was 

measured as a function of frequency, polarization and distance. All 

UHF tests utilized a 20-watt source and h/4 ground plane antennas for 

transmission and standard h/2 dipole antennas for reception. At each 

receiverpoint, a series of measurements was made to determine local 

signal strength variation over a range of several wavelengths. Horizontal and vertical polarization measurements were conducted to determine 

the extent of signal depolarization. Measurements were continued a1 ong 

the main tunnel and around corners at appropriate intervals determined 

by the rate of signal attenuation. Measurements were continued outward 

from the transmitter until no further signal could be detected. Transmission loss measurement accuracy is - +2.5 dB based on receiver and antenna 

caT i bration to1 erances , 

The observed signal attenuation along the main tunnel is shown in figures 

3, 4, and 5 for 200, 415, and 1000 MHz respectively. Attenuation is 

plotted as the power transfer ratio between isotropic antennas (basic 

transmission loss) for the indicated polarizations. Transmission loss 

may be combined directly with equipment parameters to establish comnunications performance. 

Significant propagation characteristics are: 

a) Attenuation (in dB) increases nearly linearly with 

increasing di stance. 

b) Horizontal polarization produces significantly lower 

transmission loss at a given distance than does 

vertical polarization. Cross polarization produces 

a loss intermediate between horizontal and vertical. 

c) Transmission loss decreases significantly at a given distance as the frequency is increased from 200 to 

1000 MHz. 
Linear attenuation (in dB) versus distance is a characteristic of waveguide propagation; the tunnel geometry also suggests a guided mode of 

propagation. From the slope of the attenuation curves, attenuation rates 

have been determined as shown in figures 3-5. The values for 200 MHz 

are considered to be very approximate because they are based on a small 

number of data points. For comparison, Fanner and shephardl report a 

value of 12 dB/lOO1 at 160 MHz for straight passageways underground and 

in buildings. 

With the main tunnel measurements as a reference, data were also obtained 

around corners. Observed corner attenuation is shown in figures 6, 7, 

and 8 for 200, 415, and 1000 MHz, respectively. Corner attenuation 

is plotted in dB relative to the horizontally polarized signal level 

observed in the center of the main tunnel. Significant propagation characteristics are: 

a) Signal attenuation imnediately around a corner is 

consi derabl e at a1 1 three frequencies. 

b) Complete signal depolari zation is observed around 

the corner. 

Because of the high attenuation of a single corner, propagation around 

mu1 tiple corners is expected to be even more severely attenuated. Consequently, the signal existing at any point can be reasonably assumed 

to have followed the path including the least number of corners. The 

transmission loss at any point along 2 cross tunnel can then be estimated 

by adding the attenuation from the appropriate curve in figure 6, 7, or 8 

to the transmission loss corresponding to the distance along the main tunnel 

back to the transmitter. 

One possible UHF communications system consists of a 20-watt base station 

and 1-watt walkie-talkie. As shown in Table 1-A, the base to walkietalkie 1 ink has a "range" of 156 dB wherein satisfactory comnunications 

can be obtained. Comnunications coverage can thus be determined by comparing the predicted transmission loss to the 156 dB limit. 

VLF TESTS 

At VLF, the transmission loss and field strength were measured as 

a function of frequency, distance, and antenna orientation. All VLF 

tests utilized a 7-watt source with roof bolt (line source) antennas 

for transmission and standard loop or roof bolt antennas for reception. The line source antenna was established by clamping a pair of wire leads 

to two roof bolt heads separated by 52 feet. At each receive point, signal strength readings were made for two orthogonal positions of the 

roof bolt antenna and three orthogonal positions for the loop. The entire 

measurement sequence was then repeated for a second orientation of the 

transmit antenna. Measurements were conducted at three points spaced 

at intervals determined by the rate of signal attenuation. Transmission 

loss measurement accuracy is - +2 dB based on receiver calibration tolerance. 
Observed signal attenuation. for intra-mine roof bol t-to-roof bol t antennas 

is shown in figure 9. Attenuation is plotted as the power transfer ratio 

between the roof bolt antenna terminals. Transmission loss may be added 

directly to transmitter power to determine the received signal level (available power into matched load). Significant propagation characteri stics 

are: 

a) Minimum transmission loss occurs for the end-to-end 

antenna orientation. 

b) Transmission loss is relatively flat versus frequency. 

cc) The attenuation rate is approximately 5 dB/100' 

averaged over a1 1 frequencies and antenna orientations. 

The available measurements do not fully characterize the propagation 

behavior. In particular, measurements over a greater range of distance, 

frequency and roof bolt spacing are desirable. However, the range 

of a roof bolt voice radio system can be estimated from measured data. 

For a 25-watt transmitter and other parameters as shown in Table lB, 

the "range," of a typical system is 158 dB. Assuming that the curves 

of figure 9 extrapolate at 5 dB/100, 158 dB transmission loss then occurs 

at a distance of 1200 to 1400 feet, depending on antenna orientation. 

Observed field strength for intra-mi ne roof b o l t - t o - l o o p antennas 

is shown in figure 10. Field strength is shown as the magnetic field 

intensity in dB/,uA/m. This quantity is chosen rather than trar,snission 

loss to facil i tate trade-off studies i nvolvi ng 1 oop parameters such 

as size and weight. Field strength measurement accuracy is - +2 dB based 

on receiver and loop cal i bration tolerances. 

Significant propagation characteris ti cs are: 

a) Maximum field strength occurs with the vertical 

magnetic dipole oriented along the axis of the 

roof bol t antenna. 

b) Field strength is relatively flat veisus frequency. 

c') The attenuation rate is approximately 4 dB/lOO1 

averaged over a1 1 frequencies and antenna orientations. 

The range of a roof b o l t - t o - l o o p v o i c e bandwidth radio system can be 

estimated from measured data. Assuming the equipment parameters listed 

in table 1B and a 30-inch diameter, 20-turn loop for receiving, the 

required field strength is -14dB/'A/m at 50 kHz and +20 dB/~A/rn at 1 kHz. 

Further assuming that the curves of figure 10 extrapolate at 4 dB/10O1. 

the minimum useful field strengths are reached at approximately 1500 feet 

at 50 kHz and 800 feet at 1 kHz when the loop is oriented as a VMD. 
ACKNOWLEDGEMENT 

The cooperation and assistance of management and engineering personnel . 

at the Inland Steel Co. coal mine is gratefully acknowledged. 

REFERENCES 

1. Farmer, R. A., and No He Shephard, "Guided Radiation. . .The Key 

to Tunnel Tal king," IEEE Transactions on Vehicular Communications, 

pp 93-102, March, 1965. 

TABLE 1 COMMUNICATION LINK CALCULATIONS 

(A) UHF, 2-WAY FM RADIO 

Transmit Power, Pt = +13 dBW 

Transmi t Antenna Gain, Gt = 0 dB1 

Receive Antenna Gain, G, = 0 dDi 

ReceiveSensitivity*, Pr = -143dBW 

System "Range" = Pt + Gt + Gr - Pr 

= 156 dB 

*0.5uV for 20 dB quieting 

Transmit Power, 

External Noise, 

Receiver Noise, . 

Total Noise 

Requ i red SNR* 

System "Range" = Pt - N, - SNRo C 

= 158 dB 

Articulation index = 0.3 
(NOT TO SCALE) 

- 597 

LIMESTONE - - - 606 

COAL 

SOFT SHALE a608 

- - fl 619 

LIMESTONE 4 - 624 

SANDSTONE - 637 

SANDY SHALE WITH SANDSTONE BANDS .. 650 

SANDY SHALE 

FIRE CLAY # 

- - 737 

LIMESTONE 

I 

- # 757 

FIG. 1 - TUNNEL CROSS SECTION 

-I 

0 = MEAS

SCA

FIG. 2 - TYPICAL CORNER GEO
FIG. 3 

1000 1500 2000 

DISTANCE, FEET 

- - - 

0 500 1000 1500 2000 2500 3000 

DISTANCE, FEET 

FIG. 4 

UHF TRANSMISSION LOSS ALONG STRAIGHT TUNNEL 

1 C" 

0 500 1000 

DIST

FIG. 5 

0 0 

- 10 - 10 

m -20 n 

0 . -30 

m Z 

-40 n 

, -10 z 5 -50 

2 -20 t- ; -60 

4 

,' -30 % -70 

W 

t- - 80 2 -40 - 90 

-50 

0 20 40 60 80 100 0 20 40 60 80 100 120 140 0 20 40 

DISTANCE, FEET DISTANCE, FEET DI ST

FIG. 6 FIG. 7 FIG. 8 

UHF ATTENUATION OF SINGLE CORNER 
DISTANCE, FEET 

ORIENTATION 

(TOP VIEW) 

FIG. 9 - VLF TRANSMISSION LOSS FOR 

ROOF-BOLT ANTENNAS 

FIG. 10 - VLF FIELD STRENGT

BOLT ANTENNA 
PERFORMANCE OF MANPACK ELECTROMAGNETIC LOCATION 

EQUIPMENT IN TRAPPED MINER LOCATION TESTS 

by 

- \ 

A. J. Farstad 

Westinghouse Georesearch Laboratory 

8401 Baseline Road 

Boulder, Colorado 80303 

I. 0 Introduction 

The use of radio signals for underground communications was 

considered as early as the mid-1920's [ 11 - [ 41 . However, early experiments did not produce promising results with the type of equipment that was 

available then [ 51 . With advancements in solid state technology many of 

the schemes considered impractical in the early days suddenly became 

practical. 

The properties of electromagnetic wave behavior in conducting 

media have been intensively studied by Wait [ 61 and others and can be 

utilized not only in communication with trapped miners, but also as a means 

of locating their position. This report deals with the development of 

portable electromagnetic equipment, powered by the miner's conventional 

lamp battery, which can be used to alert surface rescue personnel of the 

trapped miner Is location. The Westinghouse Georesearch Laboratcry (WGL) 

under BUMINES sponsors hip initially approached the trapped miner location 

problem using seismic techniques. It was learned from this effort that 

two serious shortcomings of seismic techniques are (1) the extremely weak 

signals available on the surface from miner generated hammer blows, 

requiring long processing times at the surface receiver, and (2) the relatively 

slow procedure of deploying geophone arrays to sense the seismic uplink 

signals. By contrast, the electromagnetic techniques have proven much 

more practical from a deployment time standpoint and in most cases have 

also proven to be more accurate in determining location. ' Furthermore, 

these techniquzs have been successfully tested using a receiver in a helicopter for reconnaissance purposes. However, before any electromagnetic 

location technique can become fully operational, the miners themselves 

must be equipped with emergency transmitters to be carried with them 

on each work shift. Consequently, much emphasis has been placed on the 

development of extremely lightweight transmitting equipment. The 

performance of some of the electromagnetic location equipment developed 

by Westinghouse will be described in the following sections. 

2.0 Basic Concepts 

Theoretical concepts for locating magnetic dipoles in the earth have 

been treated by Wait [ 71 and Olsen [ 81 . A generalized curve for the 

attenuation of magnetic fields in a conducting half-space is derived from 
Wait's work [7] and is shown in Figure 1. This curveapplies only to fields 

directly above or directly beneath a transmitting source. Experimental 

verification of the validity of the theory is shown in Figure 2 which compares 

actual field measurements obtained over the Imperial Mine near Boulder, 

Colorado with the theoretical curve based on Wait's theory. The value of 

conductivity used in determining the theoretical curve was obtained from a 

dipole-dipole conductivity sounding taken over the Imperial Mine. 

While the vertical magnetic field over vertical magnetic dipole source 

reaches a maximum directly over the source, the corresponding horizontal 

magnetic field experiences a null as shown by the example in Figure 3. 

This curve was obtained at the Rainbow No. 7 Mine in Rock Springs, Wyoming 

and depicts close agreement between the measured and calculated field profiles for both the vertical and horizontal fields. This was a relatively shallow 

(136 feet) mine, and the resulting signal-to-noise ratios at the surface were, 

in most cases, greater than 40 dB. However, the preliminary equipment 

used to obtain this data was not sufficient to obtain usable location data at 

the deeper mines of 500 feet and greater. Consequently, Westinghouse 

Georesearch Laboratory, under BuMines sponsorship, embarked on a development program aimed at producing portable equipment which would be operable 

at mine depths as great 1500 feet. Since geometric spreading accounts 

for an (inverse distance) roll-off of field strength and increased losses in 

the earth at greater depths could account for up to 40 dB additional attenuation, 

we were faced with the problem of dealing with sigr~al strengths as much 

as 100 dB down from those observed in the shallow mine tests. The equipment 

development and the results of the deep mine field tests are described in the 

following sections. 

3.0 Description of Equipment 

3.1 Operating Frequency 

It is well known that low frequencies penekate conductive material 

better than high frequencies. However, there are also other factors to 

consider in choosing the best frequency for an uplink electromagnetic 

location system, among which are (1) natural and man-made background 

noise, (2) availability of circuit components, (3) suitability of circuit 

miniaturization, and (4) permissibility re strictions in coal mines. Figure 

4 shows a family of curves of expected field strength parametric in mine 

depth and superimposed on expected atmospheric noise levels for different 

seasons of the year. Based on signal-to-noise ratio alone, the optimum 

frequency for a 1500 foot (457 meter) mine would be on the order of 350 Hz. 

However, in most mining districts, there are strong harmonic interference 

fields from power lines which make frequencies this low difficult to use in 

a practical system. C onsequently, to avoid the power line interference, 

our choice of frequencies was considerably higher, covering the band from 

900 Hz to 2900 Hz. 
. . 

Figure 1. Loop Antenna and Horizontal Coupling Relationships 64 , 
Frequency (Hz) 

Figure 2. Comparison of Measured Magnetic Field With Theoretical 

Prediction for Beacon (Uplink) System 
Figure 3 Electromagnetic Surface Pattern from Buried VMD 

(North-South Profile) 
1, , , ' iT I -+ - 

1x1 , I\' I II\lI i 1 I I I\' I Ill 

-- ---I. -: -p- -- + - -L -- t-2-f- \ . 1 - -- - -. Ad=t 2\+-!--\-?! - .- ti ! , kt\ .--tzt---tz-tt-i i 1 j f 

Frequency in Hertz 
of the tuned frequency. It has a phone jack output which can be used for 

hook up to earphones for null detection purposes or a portable oscilloscope 

for measuring field strength profiles. 

3. 4 Multichannel Receiver 

The multichannel receiver consists of six narrowband receiver circuits 

tuned to different frequencies and packaged in a common enclosure. The 

outputs of each channel are selectively summed together i11 a summing 

amplifier to drive a set of earphones. Also, each channel is individually 

metered on the front panel. This receiver was designed primarily for 

helicopter reconnaissance work but also is useful for ground search operations. 

The antenna used with the multichannel receiver is a 24" square loop with 

600 turns of No. 28 wire. A built-in battery operated preamp is included in 

the antenna enclosure to drive the 35 foot connecting cable between the antenna, 

towed beneath the helicopter, and the receiver in the helicopter cabin. 

4. 0 Test Results 

4. i Robena No. 4 Mine, Waynesburg, Pennsylvania 

68 
Table 1 outlines the results of the tests conducted at.the Robena Mine 

No. 4 near Waynesburg, Pennsylvania. The overburden at this mine ranged 

from 725 feet to 990 feet at the location chosen for the tests. Comparison 

of surveyed locations with locations determined by the electromagnetic null 

were mostly in agreement within less than 50 feet. This is considered sufficient 

location accuracy for practical mine rescue applications. Helicopter tests 

were also conducted here and demonstrated a horizontal detection range of 

about 1000 feet while flying at an elevation of I00 feet over a location with 

725 feet of overburden. 

4. 2 Geneva Mine, Dragerton, Utah 

Tests at the Gkeva Mine represented the severest test to the use 

of the electromagnetic location system of any coal mine visited to date. Not 

only was the overburden depth considerably greater than the mines in the 

Eastern coal region, but also the conductivity was higher ( a 2 X 1om2 mhos 

per meter), and the terrain more rugged. The feasibility -of using intrinsically safe manpack transmitters in the mine with signals detectable from a 

helicopter over 1500 feet of overburden was clearly demonstrated. Oscilloscope photographs of the receiver output, recorded in the helicopter, are 

shown in Figure 5. Location accuracies at this mine were not as good as anticii)ated bet \were ir. most cases within 1 or 2 coal blocks (250-300 ft) of the actual 

source. Some ambiguities were uncovered in the null technique when 

operating in extremely rugged terrain such as this. These ambiguities, 

which consist of secondary nulls, have been verified by comparison with 

theoretical profiles for the Geneva Mine conditions . 5.0 Conclusions 

Results of field tests conducted under I1worst caset1 conditions have 

demonstrated the feasibility of using thr ough-the-earth electromagnetic 

signals for detection and location of trapped miners equipped with manpack 

transmitters. Furthermore, the prototype receiving equipment is lightweight and easy to operate suggesting that unskilled personnel could be used 

in rescue search operations. Helicopter reconnaissance using a multichannel 

receiver and a towed loop and preamplifier provides a rapid means of determining the existence and general location of signals emanating from the mine. 

Signal reception at the mine portal provides another quick method of detecting 

the existence of signals. Precise location of EM sources over hilly terrain 

is sometimes hampered by profile ambiguities and secondary nulls. - Further 

research is needed to develop techniques for resolving these ambiguities 

in the field. 
TABLE 1 

Results of Robella Mine Tests 

Location 

Uncorrec

Transmitting * Location 

f 

- Antenna Overburden Slope Field strength-' Error 

2070 #10 Wire 800 ft. 16" - 5dBI pA/m 18 ft. 

2010 4 Manpack Antennas 725 ft. 0 -. 13 ' - i4dBl yA/m 27 ft. 

3030 , #10Wire 725 ft. 0 - 13' -3d~/yA/m 50 ft. 

1050 4 Manpack Antennas 990 ft. 16" -26dB/ @/m 8 ft. 

2070 #10 Wire 990 ft, 16" - 15dB/ yA/m 17 ft. 

* Represents relative field strength indication on the output meter. For pulsed 

meter indication is about 20dB low. 

Undetectable Signals Comments 

20.10 Single Manpack 800 ft, 16" Power Line within 200 ft. 

Antenna 

3030 11 I I tt 800 ft. 16" - fi 

1050 11 11 11 725 ft. 13' Power Cables and metal Pip

Running up both Sides of E
d z 1500 feet 

a Alt. t 100 feet 

Figure 5. Received Signals from Multichannel Receiver in Flight 
[5] L. C. nsley, H. B. Freeman, and D. N. ~ellers, "Experiments 

in underground communication through earth strata, " U. S. Bureau 

of Mines, T. P. 433, 1928. 

[6] J. R. Wait,' Electromagnetic Waves in Stratified Media, 

Pergamon Press, 1962. 

[7] J. R. Wait, "Criteria for Locating an Oscillating Magnetic 

Dipole Buried in the Earth, Proc. IEEE, Vol. 59, No. 6, 

pp. 1033-1035, June, 1971. 

[8] R. G. Olsen and A. J. Farstad, "Electromagnetic Direction 

Finding Experiments for Locating Trapped Miners , " to be 

published in "IEEE Transactions on Geoscience Electronics, 

October, 1973. 
FIELDS OF A MAGNETIC DIPOLE EXCITED BURIED CYLINDER 

Allen Q. Howard, Jr. 

U. S. Dept. of Commerce 

Institute for Telecommunication Sciences 

Office of Telecommunications 

Boulder, Colorado 80302 

Abstract 

An approximate solution to the electromagnetic boundary value 

problem consisting of a horizontal cylindrical conductor buried in 

a lossy half-space and excited by an arbitrarily oriented magnetic 

dipole is found using an iterative perturbation technique in a double 

Fourier transform space. This model is used to gain insight into 

the anomalous fields due to strong scatterers such as pipes or tracks 

which would be in close proximity to an EM mine rescue operation. 

The novel, three-dimensional aspect of the problem (i. e. , the source) 

imposes the complexity that the current in the cylinder is not uniform. 

The field expressions are ideally suited to evaluation using FFT 

algorithm. 

The electromagnetic response of an inhomogeneous half-space 

continues to be a subject of high interest because of its direct 

application to EM prospecting techniques, mine rescue operations, 

and location of buried gas or water pipes. By in large, with the 

notable exceptions d D'Yakonov (1959), and Hill and Wait (1973), 

the theory applied to this class of problems has been restricted to 

two-dimensional time harmonic analyses. For example, the February 

'71 issue of Geophysics was devoted entirely to numerical solutions 

of this type. The comparatively simple sub-case consisting of an 

idealized buried cylindrical inhomogeneity has also received considerable 

attention (D' Yakonov (1959), Wait (1972), and Howard (1972)). We 

comment that D'Yakonov published no numerical results; the approximate 

iterated perturbation method due to Wait, which accounted for the 

interaction of the air -earth interface and the induced axial monopole 

current is readily evaluated. Numerical results based on Wait's 

method have been shown to be in complete agreement with the integral 

equation solution of Howard (1972). All three of these solutions are 

two-dimensional since the primary excitation in each case is taken 

to be a uniform line current parallel to the cylinder. 

Herein, we consider a three-dimensional extension of these 

solutions. That the extension is non-trivial is attested by the effort 

involved in considering such problems in the absence of the air-earth 
interface (Wait, 1960). Thus, the uniform line source is replaced 

by a more realistic arbitrarily oriented magnetic dipole in the 

earth. This introduces several complications. The most serious 

are that we now need a two-dimensional transform representation 

and the problem is now intrinsically vector. The vector nature 

requires either the introduction of both electric and magnetic potentials 

(Wait (1 958), Weaver (1 970)) or the appropriate Green's function dyadic 

(Tai (1 97 1)). For an arbitrary cylinder, the relevant two-dimensional 

Fourier integral dyadic can be used with a vector integral equation 

formalism. It is, under certain conditions, permissible and much 

simpler to by-pass the integral equation technique and use a perturbation analysis. 

Thus, to keep the problem tractable, we will assume that the 

cylinder is perfectly conducting and electrically small so that only 

an axial surface current density has appreciable excitation. This 

allows us to perform a perturbation analysis similar to that of Hill 

(1970) and Wait (1972) to obtain an iterated approximant of the axial 

surface current. Note that in so doing, the features of an air-earth 

interface and an arbitrarily oriented magnetic dipole are not compromised. 

Hence, the problem remains three -dimensional; however, the simplifying as sumptions divide the problem into three almost completely 

independent parts - each one of which is a well-defi~ed boundary 

value problem. 

As an overview, we give in section two a two-dimensional 

Fourier integral representation of the magnetic loop .in a half - space. 

With an eye to the application of this "incident" field, we represent 

the resulting interface fields in terms of magnetic and electric vector 

potentials parallel to the cylinder axis which is introduced in the following step. In the third. section, we obtain the zeroth order surface 

current on the cylinder which includes the "over and down" mode 

coupling. This is done for one spectral component of the incident 

field - i. e., an arbitrarily polarized plane wave constituent. 

Then, in the fourth section, we obtain the interface dependent 

potentials for a given axial surface current. It then becomes a straightforward matter to iterate the correction to the surface current starting 

with the zeroth order current obtained in the previous section. The 

anomalous fields are, of course, written in terms of the potentials 

derived in section four. 

I I. Two-Dimensional Fourier Integral Incident Field 
- - 

by way of the potentials F and A , i. e., 
- 

For an elementary magnetic dipole, M is given by 

n 

Now for a homogeneous space A = 0 and 

This, in the representation required, is also written 

- - S dZk LK*(~-- 

F = ik, ~.rn - e ~4 

0 1 k, = (K:- kt - K; yL 8n" -.. K; 

r* (K*\ >, 0 

where 'A'K =AK~AK, 

To account for the interface at z 

where 

zrd 

z='d 

d, we define 

( xs 

The pertinent solution is 

ZLK, h 

- i e 

A=- 

kL 

where 
111. Zeroth Order Surface Current 

We now consider the interaction of a plane-wave constituent 

of the previous section with the cylinder surface current. Basically, 

we require a transverse vector cylindrical wave expansion of a 

 lane wave constituent of the "incident" field. 

The M and N functions are orthogonal; it is easy to solve (3. 1) 

for an and bn. Now again we match boundary fields, this time at 

the cylinder surface. Thus let 

and one finds 

The surface current in amperes/meter on a perfectly conducting 

body is proportional to the total magnetic intensity H , i. e., 

This condition yields the zeroth order transformed axial surface current 

density - 

o -.LEx(K x E'\I.:, 

Kx (Kx? K = Y kL P T H:" (kp 4 

where 

Hence (3.5) becomes 
Define 

Note that Kxl is the interface independent zeroth order axial 

surface current transform. The interface term Kx2 (Kx) is 

unfortunately a numerical integral. 

- - . . ".. ..*... , . - . - . . 

IV. Iterated Current - and Anomalous -- Fields 

We now temporarily assume the current is known; i. e. , 

The excitation potential for this section is thus 

Now, substituting (4. l), and G as given by (2. 5) into (4.2), and 

Q carrying out the spatial integrations gives 

- - 

-0 A = $ eiK- x -0 - 2 - $ 

a (K) d K , ao = 1 iPQ4 O 

x k (KJJ-,,CK~~\ (4.3) 2 K; 

Again, we now introduce potentials A*, Fk , match fields across 

the interface and obtain in the quasi- static approximation 

Thus, the air-earth interfzce to first order modifies the potential 
it can be shown that the remaining term is then identical to the 

exact expression in the absence of the interface. Using the key result 

(4. 7), the anomalous magnetic field H in the lower half-space is 

given in terms of the potentials AO, A, and F- . 

where H0 depends on the interface only through the current (it 

corresponds to AO with iterated current (4.7)). In particular 

where 00 

The H: term is given by Am, F- through (2.2). For completeness 

these potential coefficients are determined to be 
involving spheroidal, spherical and cylindrical bodies, J. of Res. , 

v. 64B, n. 1, p. 15-32. 

Hill, D. A., and Wait, J. R., 1972, Electromagnetic scattering of 

a small spherical obstacle near the ground, Can. J. Phys. , v. 

50, n. 3, p. 237-243. . Hill, D. A. , 1970, Electromagnetic scattering concepts applied to the 

detection of targets near the ground: Ph. D. dissertation, Ohio 

State University, Columbus, Ohio. 

Weaver, J. T., 1970, The general theory of electromagnetic induction 

in a conducting half-space: Geophysics. J. R. Astr. Soc., v. 22, 

p. 83-100. 

Tai, C. T. , 1971, Dyadic Green's functions in electromagnetic theory: 

Intext Educational publishers, San Francisco, California. 

Acknowledgment 

This research was sponsored in part by the U. S. Bureau of Mines. 

The author is indebted to Dr. J. R. Wait for his help in carrying out 

this research. 
THE ELECTROMAGNETIC RESPONSE OF A BURIED 

SPHERE FOR BURIED DIPOLE EXCITATION* 

D. A. Hill and J. R. Wait 

U. S. Department of Commerce 

Office of Telecommunications 

Institute for Telecommunication Sciences 

Boulder, Colorado 80302 

Introduction 

The feasibility of locating a buried vertical magnetic dipole source 

(horizontal loop) from surface measurements of the vertical and horizontal magnetic field components has been investigated by Wait (1971). , 

For sufficiently low frequencies , the magnetic fields have a s imple 

static-like behavior, and a single observation of the ratio and relative 

phases of the vertical and horizontal field components is sufficient for 

location when thc carth is homogeneous. However, when inhomogenieties 

are present, the surface fields will be modified, and source location may 

be more difficult. 

In order to obtain a quantitative idea of the surface field modifications, 

we consider a spherical conducting zone as a perturbation to the homogeneous half-space. A rigorous solution for the buried sphere problem 

has been formulated by D'Yakanov (1959). Unfortunately, his solution 

is restricted to azimuthally symmetric excitation, and even then the 

solution is not in a convenient computational form. However, if the 

sphere is electrically small and is located at a sufficient distance from 

both the dipole source and the interface, the scattered fields can be 

identified as the secondary fields of induced dipole moments. The latter 

are equal to the product of the incident fields and the polarizabilities of the 

sphere. The details of the approach are given by Hill and Wait (1973). 

Wait (1968) has used this induced dipole moment approach for 

scattering by a small sphere above a conducting half-space. The 

method has the advantage that it is easily generalized to scatterers of 

other shapes for which both the electric and magnetic polarizabilities 

are known, such as spheroids (Van de Hulst, 1957). This concept has 

*The research reported here was supported by the U.S. Bureau of 

Mines, Pittsburgh Mining and Safety Research Center. .- 
also been considered by Ward (1967) in the context of electromagnetic 

detection of massive sulfide ore bodies from airborne platforms .. 

Induced Dipole Moments 

The geometry of the situation is shown in Figure 1. The buried sphere 

has radius a, conductivity Cs, permeability ps, and permittivity E 

Although the distances involved are much less than a free-space wavelength, they are not necessarily small compared with a wavelength in 

the lower half-space. Consequently, the induced electric dipole moment - 

must be considered as well as the induced magnetic dipole moment. The 

unperturbed magnetic field has both horizontal and vertical components at 

0 0 the sphere center, Hx and H and the unperturbed electric field has Y d only a horizontal component, Ey . The components can be obtained 

from Wait (1951; l971), Banos (1966), or Ward (1967). The induced 

magnetic dipole moments, mx and m are given by the product of Y' the magnetic polarizability and the unperturbed magnetic field (Wait, 

1960; 1968). The induced electric dipole moment, P is given by the Y' 

product of 'the electric polarizability and the unperturbed electric dipole * 

moment (Van de Hulst, 1957; Wait, 1960). 

In order that higher order multipoles are not important, it is necessary 

that the sphere radius is small compared to both the wavelength in the 

lower half-space (Stratton, 1941) and the geometric mean of the source 

and observer distances (Wait, 1960). The sphere radius must also be 

small compared to the depth so that interactions between the sphere and 

the interface are not important. Actually, interface interaction terms 

have been computed for a buried cylinder (Wait, 1972), but the interaction terms are more complicated for a sphere because of coupling 

between the electric and magnetic dipole modes (Hill, 1970). In the 

static limit, the interaction has shown to be unimportant for a sphereinterface separation of at least two sphere radii (Hill apd Wait, 1972). 

Surface Fields 

The total magnetic field at the surface is the sum of the vertical magnetic 

dipole source field and the reradiated fields of the dipole moments 

induced in the sphere. The fields of buried vertical and horizontal 

magnetic dipoles are given by Wait (1971; 1972), and the fields of a 

buried horizontal electric dipole are given by Wait (196 1) and Banos (1966). 
The actual calculations involve the evaluation of numerous Sommerfeld 

type integrals which must be done numerically. However, all of the 

integrals can be evaluated in closed form in the zero-frequency limit. 

If these limiting forms are subtracted, the convergence of the remaining 

integrals is significantly improved. These static limits may even be of 

direct interest if the frequency is sufficiently low. In the static limit 

the contribution from the el ectric dipole moment goes to zero, but the 

contribution from the magnetic dipole moment goes to zero only for a 

non-magnetic ( pS = ro ) sphere (Ward, 1959). 

Discus sion and Conclusions 

A computer program was written to calculate the magnitude and phase 

of the ratio of the'vertical and horizontal magnetic field components at 

the earth's surface. These are the measurable quantities which Wait 

(1971) has suggested for location of the vertical magnetic dipole source. 

For spheres of radius less than .2 times the source depth, the change 

in surface fields .are found to be insignificant. For a sphere of radius 

.3 times the source depth located at half the source depth, some 

noticeable changes in the surface fields begin to occur. However, 

resultant errors in source location should still be small. 

Larger errors can be expected when the sphere is either larger oz 

closer to the source or interface, but the simplified theory presented 

here is not valid under such conditions. The study of such cases is a 

worthwhile extens ion. 

R EF ER ENC ES 

Banos, A. , 1966, Dipole radiation in the presence of a conducting 

half-space: Oxford, Pergamon Press. 

Dl Yakonov, B. P. , 1959, The diffraction of electromagnetic waves by 

a sphere located in a half-space: Bull. Acad. Sci. USSR, 

Geophys. Ser., v. 11, p. 1120-1125. 

Hill, Do A. , 1970, Electromagnetic scattering concepts applied to the 

detection of targets near the ground: Ph. D. dissertation, Ohio 

State University, Columbus, Ohio. 

Hill, D.A. and Wait, J.R., 1972, Electromagnetic scattering of a 

small spherical obstacle near the ground: Can. J. Phys. , v. 50, 

No. 3, p. 237-243. 
Wait, J. R. , 1972, The effect of a buried conductor on the subsurface 

fields for line source excitation: Radio Sci. , v. 7, No. 5, 

p. 587-591. 

. Ward, S. H. , 1959, Unique determination of conductivity, susceptibility, 

size, and depth in multifrequency electromagnetic exploration: 

Geophysics, v. 24, p. 531-546. 

Ward, S. H. , 1967, Electromagnetic theory for geophysical applications: 

Chapt. 11, Pt. A in Mining Geophysics, v. 2 (ed. by D.A. Hansen, 

et al): Tulsa, Oklahoma, Society of Exploration Geophysicists. 

THE PERTURBATION OF ALTERNATING ELECTROMAGNETIC FIELDS 

BY THREE-DIMENSIONAL BODIES 

F. W. Jones 

Dept. of Physics and the Institute of Earth 

and Planetary Physics 

Univer s ity of Alberta 

Edmonton, Alberta, Canada 

Abstract 

The perturbation of alternating electromagnetic fields by threedimensional structures is considered. The general model is that of 

a semi-infinite conducting half -space which consists of regions of 

different conductivities. A numerical method is used to obtain the 

solution for the equations and boundary conditions. The effects on 

the fields at the surface of the half-space due to the conductivity 

discontinuities are shown by three-dimensional amplitude and phase 

plots. 

General Method 

During the last year or so a numerical method has been 

developed for studying three -dimensional electromagnetic induction 

problems. The conductive structure of interest is defined over a 

three-dimensional mesh of grid points and a solution for the equations 

and boundary conditions is obtained by a finite iterative technique. 

By means of this method, the effects of buried anomalies have been 

studied (Jones and Pascoe, 1972) as well as islands and coastlines 

(Lines and Jones, 1973a, b) and cubic conductors (Jones, 1973) for 

uniform inducing fields. 

The general method is described in detail by Lines (1972). If 

we neglect displacement currents as is normal for the slowly varying 

fields we wish to consider, and assume a time variation exp(iw t), 

we obtain from Maxwell's equations (in e. m. u. ): 

where 

Equation (2) must be solved in all regions. Also, the usual 

boundary conditions must be satisfied. This equation may be written 

as three scalar equations in Cartesian coordinates: 
of a double mesh to accomodate all possible occurences of double 

valued functions for the arbitrary models we wish to. consider is 

prohibitive in computer time and &cost. Therefore, at discontinuities 

the average of the values of the E component normal to the boundary 

on either side of the discontinuity is used to represent the value at 

the boundary. When this is done, and the boundary conditions applied a 

to E , ?I2 in the difference equations representing '(3), (4), and (5) 

is replaced by 7 , the average of 37' for all the regions surrounding 

the point being considered. This implies that the conductivity discontinuity 

is represented by a transition zone from one conductivity region to 

the other. This is a reasonable approximation for geophysical situations. 

If the boundary is a tr3nsition zone from one conductivity region to A 

the other, then J (= 6 E) must be continuous, and if u varies continuously through the transition, E will vary as we have assumed. 

After the electric field components are approximated by the GaussSeidel iterative technique, the magnetic field components are calculated 

by taking the curl of the electric field, i. e. , 

Fig. 1. The general three-dimensional model 

(from Jones and Pascoe, 1972) 
PHRSE OF EZ AMPLITUDE OF EZ 

Fig. 2. Electric field component amplitudes and phases for the 

model as in Fig. 1, for u', 7 Pa. (From Jones and Pascoe, 1972). 
PHRSE OF HX 

'PHRSE OF HY 

RMPLITUDE OF HX 

RMPLITUDE OF HY 

PHRSE OF HZ RMPLITUDE OF HZ 

Fig. 3. Magnetic field component amplitudes and phases for the model 

as in Fig. 1. for u,, b'. . (From Jones and Pascoe, 1972). 
A DISCUSSION ON THE THREE-DIMENSIONAL BOUNDARY VALUE 

PROBLEM FOR ELECTROMAGNETIC FIELDS 

David Rankin 

Institute of Earth and Planetary Physics 

Dept. of Physics 

Univer s ity of Alberta 

Edmonton , Alberta 

Three -dimensional boundary value problems are difficult to 

solve. Indeed, while the separation of the scalar wave equation 

can be effected in 11 different coordinate systems, an analytic solution 

requires that the boundaries, both external and internal, possess 

the same symmetry as the coordinate system. Numerical methods 

are thus of great importance for the solution of such problems; 

however, despite the availability of high-speed, large memory digital 

computer s , the solution to a significant three -dimensional problem 

is by no means trivial. It is unfortunate that the results reported 

here by Jones are invalid. 

Jones discusses a model in which a three-dimensional island 

lies off a linear coastline where all the interfaces lie in the coordinate 

planes in a Cartesian system. A downward plane em wave polarized 

with the electric field parallel to the linear coastline, ar,d two of the 

islands coast, is incident downward. This, of course, requires that 

the incident electric field is perpendicular to the other faces of the 

idealized Cartesian island. 

The significant equation used by Jones for his finite difference 

calculations is 

where a = rp . is the conductivity, p =p, is the permeability 

of the appropriate medium, and w is the angular frequency of a 

Fourier component of the incident field. We see that the displacement 

current term is omitted, which is an excellent approximation for the 

parameters of interest in this model. In the similar equation for the 

y- component of the diffusion equation, the same considerations which 

we are about to discuss are equally valid. 

It can be seen from the equation that the field components are 

related and thus any errors introduced will be propagated into all 

three components. The difficulty arises when Ex or E is perpendicular Y to one or other of the surfaces of discontinuity in 6. The continuity 

of current at the air-earth interface (upper surface), e. g., jy, requires 
that Ey is discontinuous; i. e., jY = pYl = =$Ey2. 

Jones 

uses the average value of E at the interface in the be ief that Y continuity of E is necessary in order to calculate b~~/ b y in the Y first equation above. The calculations are made in the finite difference 

approximation. The result is that all the field components and the 

derivatives are grossly distorted in the region about the boundary 

and this distortion will be propagated outward into the surrounding 

regions. Since the magnetic field components are deduced from the 

electric through Maxwell's equations, they will also suffer a distortion. 

Self-consistency required that the correct electric field components 

are themselves deducible from the magnetic, clearly impossible in this 

case. 

Furthermore, it is also necessary that if E is forced to be 

continuous, then Y 

j Ymust be discontinuous. For the values used 

by Jones 3 

jyl/jy2 - S1/Q2 = 4 x 10 . This would be in clear 

violation of the continuity condition on j which is used explicitly by 

Jones in his work and would in turn require that 

which requires a by no means negligible displacement current. 

The anti-skin effect and the nonzero values of E which Jones 

z obtains at the air-earth interface may be due to a computer artifact 

as well as the incorrect computational procedures. In any event, the 

incorrect procedures could have been avoided since good numerical 

approximations can be obtained by taking one-sided derivatives of the 

discontinuous functions near the interfaces. The averaging of these 

derivatives would, of course, be a permissible procedure. 

a 

For a complete discussion of the nature of the discontinuity in 

the electric field in the two-dimensional case, see D'Erceville and 

Kunetz (1962). The relevance to the three-dimensional case is quite 

clear. The subject matter discussed here by Jones appeals in the 

publications Jones and Pascoe (1972), Lines and Jones (1973a, 1973b), 

and Lines (1972). 

References 

1. DIErceville, I., and Kunetz, G., 1962, The effect of a fault on 

the Earth's natural electromagnetic field: Geophysics, v. 27, p. 65 1 - 

665. 

2. Jones, F. W., and Pascoe, L. J., 1972, The perturbation of 

alternating geomagnetic fields by three-dimensional conductivity 

inhomogeneities: Geophys. J. R. Astr. Soc., v. 27, pa 479-485. 
3. Lines, L. R., and Jones, F. W., 1973, The perturbation of 

alternating geomagnetic fields by three-dimensional island structure s: 

Geophys. J. R. Astr. Soc., v. 32, p. 133-154. 

4. Lines, L. R., and Jones, F. W., 1973, The perturbation of the 

alternating geomagnetic fields by an island near a coastline: Can. 

J. Earth Sci., v. 10, n. 4, p. 510-518. 

5. Lines, L. R., 1973, A numerical study of the perturbation of 

alternating geomagnetic fields near island and coastline structures: 

M. Sci. Thesis, University of Alberta. 
REPLY TO: "A DISCUSSION ON THE THREE-DIMENSIONAL 

BOUNDARY VALUE PROBLEM FOR ELECTROMAGNETIC 

FIELDStt BY D. RANKIN 

F. W. Jones 

Dept. of Physics 

Institute of Earth and Planetary Physics 

University of Alberta 

Edmonton, Alberta, Canada 

I do not regard the criticism by Rankin of my work as valid 

or even meaningful. I strongly reject the assertion by Rankin that 

my results are invalid. 

As stated in my paper, the equation to solve (in e. m.u. ) is: 

where n = 4 TT tr w . This may be rewritten as three scalar 

equations in Cartesian coordinates, one of which is 

The equation to be solved is the diffusion equation, and it is 

not appropriate to consider waves in this context. 

In the method a point-wise solution to the problem is approximated. 

As discussed in my paper the approach taken at dis~ontinuities in 

conductivity is to choose the normal component of E at the discontinuity 

as the average of the normal components on either side of the discontinuity. This is similar to the well-known assuhption made in 

obtaining Fourier series representations for piecewise continuous 

functions (Pipes, 1958; p. 51). In the geophysical problem this means 

that the discontinuity is approximated by a transition zone, which is 

a very good approximation for such cases. By using this approach, 

both the values of the function and its derivatives give a good estimate 

for those associated with the physical situation. It is not apparent 

and Rankin has not shown that "all the field components and the 

derivatives are grossly distorted in the region about the boundary" for 

geophysically realistic situations. He has given no evidence to support 

his assertation that the results are "invalid". 

Although Rankin declares that one cannot calculate the electric 

components from the magnetic, in his discussion he has not stated 

why this is so. After once calculating the magnetic components the 

electric components may be calculated, but one must insure that the 

initial solution is accurate enough to prevent significant round-off errors. 
REFERENCES 

DIErceville, I. and Kunetz, G. , 1962. The effect of a fault on the Earth's 

natural electromagnetic field. Geophysics - 27 , p. 65 1-665. 

Jones, F. W. , 1973. Reply to the discussion by D. Rankin of the paper: 

" The perturbation of alternating geomagnetic fields by an 

island near a coastline" by L. R. Lines and F. W. Jones (Can. 

J. Earth Sci. 10, - p. 510-518). Can. J. Earth Sci. , in press. 

Jones, F. W. and Pascoe, L. J. , 1972. The perturbation of alternating 

geomagnetic fields by three -dimensional conductivity inhomo - 

geneities. Geophys. J. R. astr. Soc. - 27,p. 479-485. 

Jones, F. W. and Price, A. To , 1970. The perturbations of alternating 

geomagnetic fields by conductivity anomalies. Geophys. J. R. 

astr. Soc. - 20,p. 317-334. 

Lines, L. R., 1972. A numerical study of the perturbation of alternating 

geomagnetic fields near island and coastline structures. 

M. Sc. Thesis, Dept. of Physics, Univ. of Alberta. 

Lines, L. R. and Jones, F. W. , 1973a. The perturbation of alternating 

geomagnetic fields by three-dimensional island structures. 

Geophys. J. R. astr. SOC. - 32, p. 133-154. 

Lines, L. R. and Jones, F. W. , 1973b. The perturbation of alternating 

geomagnetic fields by an island near a coastline. Can. J. 

Earth Sci. - 10, p. 510-518. 

Lines, L. R. and Jones, F. W., 1973~. Reply to the discussion by E. 

Nyland of the paper "The prturbation of alternating geomagnetic fields by an island near a coastline" by L. R. Lines 

and F. W. Jones (Can. J. Earth Sci. - 10, p, 510-518, 1973). 

Can. J. Earth Sci. , in press. 

Nyland, E., 1973. The perturbation of alternating geomagnetic fields by 

and island near a coastline. Discussion. Can. J. Earth Sci. , 

in press, 

Pipes, L. A., 1958. Applied Mathematics for Engineers and Physicists. 

McGraw-Hill Book Co. Inc. 
USE OF THE ZONAL HARMONIC SERIES FOR OBTAINING NUMERICAL SOLUTIONS TO ELECTROMAGNETIC 

BOUNDARY VALUE PROBLEMS 

Richard L. Lewis 

Office of Telecommunications 

U. S. Department of Commetce 

Institute for Tclecon~munication Sciences 

Boulder, Colorado 80302 

This is a discussion of the application of the zonal series solution. One interesting analytical step that is 

harmonic series to problems involving dipole sources utilized by both the contour integral formulation as well 

in the vicinity of spherical boundaries. The applica- as zonal harmonics is to expand the denominator for 

tion of a formal zonal-harmonic series solution to each component of the series in a geometric series and 

directly obtain numerical results involves two distinct then interchange the order of s~mmation.~ This has the 

problems: namely, the convergence of the series soh- advantage for mode theory that the ionosphere term is 

tion and the accurate calculation of the special func- removed from the residue calculation, while for zonalti~~in~~lvedinthe~erie~ solution. Someinteresting harmonicstheconvergence ofthe seriesisgreatly 

techniques will be introduced which help to improve improved. In each case, the outer series has the 

the convergence properties of the series solution. physical interpretation of wave-guide hops for which the 

Introduction series converges rapidly for VLF frequencies on up. 

The numerical techniques involved will be discussed In the discussion of these two applications, the conm light of two distinct problems for which numerical vergence of the series has been alluded to numerous 

gummation of the zonal-harmonic series is especially 

applicable. In the first problem, we are interested in 

calculating the magnetic field wave-tilt on the surface 

of a spherical hill due to a buried magnetic dipole 

orientated in a vertical direction off of the axis of 

symmetry, as illustrated in figure 1. This problem 

has application to locating the position of a buried 

magnetic dipole source in hilly terrain from surface 

measurements of the field. The major distinction of 

.this problem is that the spherical Bessel functions involved in the formal series solution all have complex 

~rguments, of which more will be said later. The 

Porrnal solution or' this problem proceeds, incidentally, 

by summing the zonal harmonic series solution for an 

inclined dipole source and then performing a rotation 

of coordinates to allign the dipole axis with the vertical. This leads to the interesting problem of relating 

the spherical coordhate d ' of the rotated sphere to 

the horizontal distance behveen two surface points that 

grow further apart (the distance X in figure 1) such 

that the bearing $' from the source to the observer 

times. Generally speaking, adequate convergence of 

the amplitude of the zonal harmonic seri is attained 

by summing 10 X k a terms, where k = is the 

wave number and a is the radius of the spherical 

boundary.3 Adequate convergence of the phase usually 

requires a 50% increase over that number of terms. A 

few techniques have been utilized to make minor im - 

provements in the speed of convergence, such as noting that the Legendre polynomial is a cyclic function 

of the summation index and consequently selective 

groupkg of the terms produces partial sums which can 

be averaged to give sorne iimprovemcnt. However, the 

major technical problem is the calculation of the special functions, using recursion formula. The Lejiendre 

polynomial is well behaved, but not so the spherical 

Bessel functions. One spherical Bessel function increases asymptotically with the summation index, 

while the other decreases asymptotically. Consequently the functions must always be paired to avoid ex-onential underflow or overflow problems on the machine. 

This can involve some additional problems in arrangremains constant. Another problem of note is to ob- kg the Bessel functions as components in each term of 

tain the magnitude and phase of fie maximum compon- the formal series solution. For the case of the Bessel 

ent of the field in the horizonal plane when the two iunction which decrea6es asympotically with the index 

component fields are not necessarily in phase. 

The second problem of interest is the application of 

the zonal harmonic series to the calculation of the 

fields in the zarth-ionosphere waveglride due to a dipole source on the earth's surface. In the preceeding 

problem, the frequency range is restricted to low 

enough values so that the zonal harmonic series was a 

natural consideratian, and so it is also in the case of 

the earth-ionosphere waveguide for frequencies in the 

ELF or lower part of the VLF range. Nevertheless, 

it is frequently felt by many that the mode series solution is preferable for physical interpretation as well as 

for iirn?licity of conlputatlon. Tilc ;r,:mer point cannot 

be argued, but the lnttcr point is only valid when ~t is 

possible to oversimplify the problem, say by assuming 

of summation, backward rather than forward recursion 

must be utilized. How far backwards one must recurse 

is always a problem, but for the case of Bessel functions of complex argument it is of significant practical 

interest to note that tlie starting point need not be at an 

order which exceeds the magnitude of the argument, as 

is necessary for Bes sel functions of real argument. ' 

This is of particular importance for calculating the 

fields below the ssrface, since there the wave number 

may be much greater than the free space wave number, 

with the consequence that the argument of the spherical Be ssel function can be quite large. Fortunately, 

tllc calculation of the Bessel functions by recursion is 

not only fast but produces very accurate results since, 

for both forward and backward recursion, the numerical process converges naturally to the desired function. 

3 ion~sphc re reflection cocfficicnt is nearly unity. Buried Magnetic Dipole Model hen more realistic assumptions about the upper 

boundary are made, a mode theory solution requires a ~~~i~~ thus introduced the rmge of our investigation, 

search for modes by some sort of iterative process1, we can proceed with introducing explicit expressions 

which can be more time ccnsuming than summing the for a buried magnetic dipole in a spherical hill. 
/ i.lgur. I - PROBLEM CONFICUUTION JHOYLVG VERTICAL MAGNETIC DLPOLI 

Y SOURCE UNDER THE SURFACE OF A CONDUCTLYC HEMISPHERE 

. In figure 1 the z ' axis /I 

forms a part of a rotated (x , y , z ') coordinate 

system. The x' axis subtends an angle 0 with the 

jC axis, where 8 is the dipole tilt angle. The y' and 

y axes are coincident. The corresponding spherical 

coordinates in the rotated coordinate system are R, 

Q', and 13' , where the latter angle lies in a plane 

perpendicular to the z' axis. The formal series 

solution for the magnetic field components can easily 

be expressed in the rotated coordinate system. For 

convenience we introduce the abbreviations 

Thus, those terms multiplied by a denote the contribution to the fields due to the source component 

orientated along the z' hxis, v.-bile 'those terms 

multiplied by denote the contribution from Ft 

source component orientated parallel to the x axis. 

The series solution for the component fields at the 

surface of the spherical hill is given by 

In the above, $,(x) is the spherical Bessel function of 

the first kind and 5k2)(:)(1) is the spherical Hankel function of the second kind, the primes on the spherical 

Bessel functions denote differentiation with respect to 

the argument in parenthesis, kl is the complex wave number for propagation in the media inside the spherical hill, 

and ka is the free space wave-number exterior to the 

spherical hill. The dipole magnetic current normalization is Jmda = -jwQoR0A, where A is the surface 

area of the sphere. Those terms in equations (1) containing an and bn constitute contributions from the 

scalar magnetic potential, while those terms containing cn are contributions from the scalar electric 

potential. 

The summations indicated in (1) receive their maximum contribution from term s of the order of ma 1 kl R I . For orders of n much larger than this, we have 

Ro n Dn -- -2n and qn(k1 Ro) / ln[kl R) - ( T) . Consequently, we can expect the magnitude of subsequent . 

terms to decrease very rapidly. Moreover, if terms 

of order very much less than ( kl~ I are removed 

from the summation there is negligible effect on the 

summation value. This, in conjunction with the fact 

that k2R is quite small in most problems of interest, . 

leads to the validity of the approximation in (5). Moreover, the contribution from the scalar electric potential term (3) can be expected to be small, so that to a 

first approximation the only term involving frequency 

in the expressions for the field is klR. Consequently, 

it can be anticipated that the ratio of the source depth 

to skin depth. H =J'= X d, where d is the depth 

below point B to the source, is a significant parameter of the problem. 

We can introduce the following asymptotic formula for 

the Legendre function, 

aR which provides us .with a suggestive means for grouping H~,=~ ~(zn+l) ~A(cos 8')an un terms of the summatiqn in order to improve the con2n+l d P~(COS e')P~OS B')~~] vergence properties of the ~eries.~ Viewed as a -B COS+*Z- n(n+l) [ doo bn+ sins' function of the summation $I.-dex n, equation (6) is -- nearly periodic of period . Consequently, termin2 n+l P;(COS 0') 8~: (COS 8') ating successive partial sums when the Legendre func - 

H#~~S~~CZ~)[ sins' bn+ cnl ' . tion changes sign and averaging only even numbers of 

where R is the radius of the spherical hill (c.f. 

figure I), Ro is the radial distance to the dipole 

such partial sums is equivalent to integrating (6) with, 

respect to n over the period. - 

source, the index of summation is n, Pn(X) is the 

Legendre polynomial of order n, Fe associated The practical question to be answered is how many 

Legendre function is 3) = 5 pn(cos 31, and terms are required for graphical accuracy. This is 

obviouslv a function of the ratio Rn / R as well as the we have value ofd IklRI. Assumingthat R~/R isnearlymity 

)Oln(klRo) $IF (klRo) (2) and that the elevation anglc of the observation point 5' 

an = )Oln( k,R /On bn'klR-xun/Dn 8 

*,(kl R ) lies well away from the estreme positions zero or pi, 

. - then the summation can bc truncated at n = 10 (kl~ I 

qn(k,Ro (k R 1 with negligible effect on the value of the summation 

Cn8 x(~,R)~ s (3) from the neglected terms. It should be pointed out, 

un- id[$n( kin! however, that for very small angles 3 ' the number 

1 of terrns required for convergence increases drastiwhere cally (unless the ratio Ro / R is much less than unity). Dnsun-k,R lnt[$,( k,~)] ( 4) 
The calculation of the special functions involved in tl~e 

eurnmation proceeds by invoking the recursion formufor each special function required. Thus the series 

,urnmation and the special function calculation are 

able to proceed rapidly together. The recursion 

formula for the Legendre polynomial is 

Po(x) = 1 and Pl(x) =x ( 7) 

This recursion process, as well as related formulas 

for the derivatives of the Legendre function, absolutely converges to the required functional values. % The 

calculation of the logarithmic derivative of the spherical Hankel function is also readily accom?lished using 

foward recursion. ' The spherical Bessel function of 

the first kind, however, requires use of a backwards 

recursion formula and careful attention to the prob- 

'lem of computer underflow. The backwards recursion 

relation for the logarithmic derivative is 

The starting value of the index ns need only be 

slightly larger than the value of the index n required 

in the series summation when n 2 / z \ . For n significantly le s s than / z 1 , where the a gument of z is 

large, then a starting %a.lue ns G - '2 , say, woul 

sufficient5 to gene rate functional values when n 2 5 , 'zl 5 1000. On the other hand, if z is real, then a 

;tartirig vahe ns z is required at all times. The 

initial value of the logarithmic derivative is ar'uitrarily chosen to be zero for the backward recursion pro- . cess. 

Using the value of the logarithmic derivative, the 

ratios of two spherical Bcssel functions of the first 

kind with different arguments can be obtained from the 

relations + '(XI n +n-I(~) "+- 

+,,(XI x +n(X) 

(9) 

and 

x *,.,(XI q (XI $"-,(Y) -a,- - . (10) 

+, (y) x Y +n(Y) 

The starting value fo;. (10) is given by 

- q0(x) sinx =- y 5'" Y 

Thus, by restricting our calculations to the ratios of 

the Bessel functions, we have avoided the numerical 

underflow problem. 

The wave tilt of the magnetic-field at an observation 

point on the surface of the spherical hill is defined as 

the ratio of the maximum horizontal field component 

to the vertical field componcnt, where the vertical 

direction coincides with the direction of the magnetic 

dipole current :noment. Two complex components in 

'he horizontal planc combine to form tke maximum 

aorizontal component, which lies at some unknown 

angle. Y in the horizontai Y - 7 plane. The unknown 

angle can be found by setting to zero the derivative 

with respect to Y oi 

resulting in 

From (1.1) and the trigometric half angle formulas, we 

are able to obtain an expression for Tan Y which enables us to determine the angle Y within IT radians. 

Finally, artifically interchanging H, and K, in X (13) whenever the former is smaller in rnagnitudeYthan 

the latter enables us, after some ado, to keep track of 

the sign ambiguity in (12) and so resolve it. 

Explicit e-upressions for the field components H , H- , and % , orientated along the axes of the Zrte&an coordinate system of figure 1, are obtained by 

applying well-known coordinate transformation formulas to the field components given by equations (1). The 

resulting formulation is lengthy and will not be re - 

produced here. 

The magnetic field components (1) can be calculated 

independently of both 3 and V' , this being the slowest computer operation, and then combined in a subsequent program to produce plots of the wave tilteversus the horizontal distance X (figure 1). Since the 

component calcuiation routine must have specified 

sequential values of 3 ', and since the spherical coordinate transformation requires both 2' and G' to 

locate a point in the u-arotated (R, 8 , V ) coordinate 

system, we need to determine a relatianship which 

will enable us to obtain Q' as a function of 8' , 3,and 

Jf if we are to hold constant the dipole tilt angle 8 and 

the azimuthal hearing angle $ when plotting wave tilt 

versus X. From simple geometrical relationships in 

figure (I) we have the law of cosines, 

and the law of sines, . 

from which we obtain 

she 'COW' = bOsYJ( I -Sin2.Sin21)Sin2B '-& -'0.8 ')2sin2~Sin2 y 

In utilizpg (15) it is necessary to avoid - specifying an 

angle 8 too small to meet the arc OB. The minimum angle 901 that can be used is obtained from 

sine: = stn $- jl + 1 

T~~~YCOS~(~-~BIR) . (1 6, TT Also, for angles 5 > - , for X to increase monotonicly the a&e 3' must decrease from it's maximum 

-Afi- -1 value to JO and then start to increase. It is at 

this crossover point that the sign of the square root in 

(1 5) changes. Frequently 30' is less than the minimum angle 3 ' for which it is practical to sum the 

series to convergence, due to the extreme increase in 

the number of terms required when 3' is small. In 

16, we have - 

AB=, - Ro 

R (F sin". 

As previously mentioned, thc ratio H of source 

depth to skin depth is an important problem parameter. Equally irnpo;tant is the ratio $ *, where 
g

Figure 3 shows a plot of the amplitude of the wave tilt 

as a function of X / d, parametric in sphere radius 

(curve numbering refers to the increasing values of 

R) . but with H held nearly constant (R-R, = 100 m in 

MAGNETIC FIELD STRENGTH RATIOS 

'"5~~~"~~"~"~~""'~~ 

2 

. lo-' 

- 0 1 2 3 4 

HORlZONTAL DISTANCE TO DEPTH RATIO U 5 05 5 21 5 36 5 51 5 565.59 ('45 ,=120 Figure 3. n roo 700 loo0 24pg sooo 9500 

for j = 1, 2, . . . ,[%I. The inithl value for the re- L. cursion process is 

- I 

, iil 

W I 

Fig, 4 Stratified ionosphere model of earthionosphere waveguide. 
additional technique that is particularly useful in improving the series convergence properties is to subtract, term by term, the series expansion of the quasistatic fields of a dipole source near a perfectly cbnducting sphere from the summation (1 7). The asymptotic expansion of each term in (If), for n large, 

can be shown to equal the corresponding term of the 

quasi-static series expansion, with the result that the 

term-bv-term subtraction introduces a convergence 1 factor of - into the series summation. The quasistatic fierds can be added back in at the end as a 

closed form expression. One note of caution regarding 

the truncation of the infinite series applies to the calculation of the phase correction term. This is defined 

as (3= = - qr - kld # (22) 

where Or = arg {E,] and d = a9. Since (22) is the 

difference of two very nearly equal terms (Q is negative), practical experience has dictated that Ihe accur - 

ate calculation of the phase correction may require 

roughly twice as many terms as is required to calculate the function amplitude. 

In addition to the normal difficulties experienced in the 

-alculation of spherical Bessel functions regarding 

.derflow and overflow, there arc also some particular 

problems associated with the formulation (17). Specifically, for the case b = a, we note that, for large n, 

( 2, , assuming that the surface impedance of 

the earth is large. This machine under- fa., 

An example Of the capability of the formulation is depicted in figure 6, where we show the complicated field ... : ,. - ..- .,--,.--,-..,-- $ ....r. ---> -..- a,., .. . , 

" 5!: I 

Fig. 6. Two layer ionosphere with low conductivity 

bottom layer. 
struiture produced by a two layer ionosphere in which 

the upper layer has a very high conductivity while the 

lower layer is a moderately lossey dielectric. Thus, 

"he two layer "ionosphere" functions as a dielectric 

oated conducter which guides a surface wave completely 

around the spherical wave -guide, leading to the 

standing-wave pattern shown. The upper trace is the e 

Er field, while below that are the Hg and E0 fields. 

The lowest layer, at a height of 70 km, was taken to 

be 11 km thick, while the excitation frequency is 100 Hz. 

-0 w'w'lrIn figure 7, we show the effect of increasing the conductivity of the lowe st layer by an order of magnitude. 

Here, the standing wave structure is gmatly enhanced. 

Finally, in figure 8, we have decreased the conducti- h 

vity of the lowest layer by an order of magnitude from 

the value used in figure 6. Figure 8, incidentally, is 

identical with the result obtained for a single layer or 

sharply bounded ionosphere commonly used when the 

conductivity of lower layers is simply ignored. 

Fig. 8. Two layer ionosphere with extremely' 

low conductivity bottom layer. 

l~ichard L. Lewis, "T5e wave-hop fields for an inclined dipole over a spherical earth with an anisotropic ionosphere", Telecommunication Research Report 

OT/ ITS 5, U. S. Dept. of Commerce, Oct. 1970. 

a J. Ralph Johler , "Spherical wave theory for MF, LF , =d VLF prqsgztion", Radio Science 5, 12, sp. 1429- 

1443, Dec. 1970. 

'J. Ralph Johler and Richard L. Lewis, "Extra lowfrequency terrestrial radio-wave field calculations 

with the zonal-harmonic series", Journal of Geophysical Research 74, 10, pp. 2459-2470, May 15, 1969. 

4~alter Gautschi, "Computational Aspects of Three - 

Term Recurrence Kelations, " SLAM Review - 9, 1, 

pp. 24-82, January 1967. 

5~ichard L. Lewis, "Recursion Formula for the 

Logarithmic Derivatives of Spherical Be ssel Functions in the Complex Plane", ESSA Technical 

Fig. 7. Two layer ionosphere with moderate Report ERL 151 -1TS299, U. S. Dept. of Commerce, 

conductivity bottom layer. January 1970. 

'J. R. Wait, "Electromagnetic Sources in Lossy 

In conclusion, it might be worthwhile to contrast the 

zonal-harmonic series summation with the more convential mode theory approach. Although either for - 

mulation is equally valid, it must be noted that the 

latter approach requires a time consuming search for 

modes, possibly involving a very complicated modal 

equation. Especially this would be true for the treatment of a spherically layered model. Furthermore, 

it should be pointed out that more than one mode may 

be requi red to fully explain a complicated field structure such as is shown in figure 6, even at the low frequency of 100 Hz. Consequently, although the determination of the field strength can proceed extremely 

rapidly once the modes are known, I submit that the 

Iecessary mode determination could be a detering factor from considering a mode theory approach. The 

zonal-harmonics summation, on the other hand, can 

be extremely rapid, particularly when 1 kl a1 is not 

too large, although some physical insight may be 

sacrified by it's use. 

Media, " Chapter 24 in ~ntenna Theory, pt. 11, - 

Ed. by Collin & Zucker, McGraw Hill 1969, 

cf. also: J. R. Wait, "Influence of earth' s curvature on the subsurface fields of a line source, " 

Electronics Letters 1, 23, pp. 697-699, Nov. 1971. 

J. R. Wait, Electromagnetic Waves in Stratified 

Media, Pergamon Press 1970, 

c.f. also: J. Ralph Johler, "Zonal harmonics in 

low frequency ter re stria1 radio wave propagation, 

NBS Tech note 335, U. S. Dept. of Commerce, 

April 13, 1966. 

J . Ralph J ohle r , "Concerning limitations and 

further corrections to geometric-optical theory for 

LF, VLF propagation between the ionosphere and the 

ground. ", Radio Science 68D, 1, pp. 67-78, 

January 1964. 

~ohler, J. R., and J. C. Morgenstern, ItPro?agatim of the ground wave electromagnetic signal with 

particular reference to a pulse of nuclear origin", 

Proc. IEEE - 53, 12, pp. 2043-2053, 1965. ,-. 
Theorv 

A general analytical solution of the effect of an inclined contact 

separating two media of differing electrical properties on a normally 

incident plane electromagnetic field has been given (Geyer, 1972). 

These solutions for the electromagnetic response at any observation 

point P(r, (P ) are given in the form of inverse Lebedev--Kontorovich 

transforrns. Two modes of excitation are considered: (1) case where 

current density flows across strike of the contact separating the media 

of differing electrical properties (only Er , EV , and Hx differ 

from zero) and (2) case where current density flows along-strike 

(only Hr , HQ , and Ex differ from zero). The electromagnetic 

fields in the frequency range of interest and for the rock conductivities 

under consideration are quasi-static in nature so that to a first-order 

approximation the horizontal magnetic field at the surface of the earth 

is constant and continuous acros s the discrete discontinuity in resistivity 

for case (1) above, and so that the tangential derivative of the 

horizontal electric field at the surface of the earth is constant and 

continuous across the discrete discontinuity for case (2) above. Of 

course, it is well known that where large horizontal gradients in 

conductivity exist, there also exists large anomalous components of 

the vertical magnetic field (Weaver, 1963). The recognition of such 

anomalous components and their magnitudes due to the presence of 

lateral resistivity changes in the overburden becomes very important 

in any wavetilt type measurement which may be used to locate a 

miner. 

When the lateral change in resistivity becomes ,quite large, the 

integral solutions (expressed as Lebedev-Kontorovich transforms) 

simplify considerably and allow ready numerical evaluation. Examples 

of the amplitude and phase responses of the radial electric, horizontal 

electric, and vertical magnetic field in the vicinity .of the contrast 

in resistivity are shown in Figures 2, 3, and 4. Of particular significance is the fact that the anomalous vertical magnetic field is 

three orders of magnitude larger near the contact than far from the 

contact. This fact implies that where large resistivity contrasts do 

occur in the rocks overlying mine workings, that significant errors 

may be present in wavetilt measurements as used above for location 

criteria! Another observation insofar as the anomalous vertical magnetic 

field is concerned is the linear variation of phase as one proceeds 

away from strike in induction number (increased distance or frequency). 

For a pulse -excited communications sys tem this phenomena would 

have a bearing on where the zero crossings would occur (i.e., 'pulse 

breadth') in the transient coupling between any given source-receiver 

configuration and at any given receiver site. 
Summary 

Application of Lebedev-Kontorovich trans forms together with 

the imposition of the quasi-static condition on the electromagnetic 

field at the air-earth interface have led to analytic integral solutions 

for the electric and magnetic fields inside two regions separated by 

a sloping interface where the normally incident fields were plane 

waves. In the derivation of the solutions the assumed boundary conditions were that the horizontal magnetic field (H polarization) and 

vertical derivative of the horizontal electric field (E polarization) 

are continuous and constant acros s, the interface. Therefore the 

results are correct to O( LUG/ un)z . These solutions may be used 

to compute a correction term to the field in the air above the 

lateral resistivity contrast (and thus along the surface) which, in turn, 

might be used to evaluate field solutions on either side of the dipping 

contact to O( w t/ ) . This type of iteration may be continued 

indefinitely with eacfi step improving ,the order of accuracy of the 

solutions in the quantity ( w E / n)4 . 

The integral solutions obtained simplify considerably where 

large resistivity contrasts occur. By use of saddle-point techniques 

it is possible to evaluate the fields for f /4 4 6 4 3~ 14 and 

at the same time to estimate the error involved in the asymptotic 

expansions in terms of the proximity to the interface separating the 

two regions of contrasting resistivity. 

The numerical results indicate that one must exercise caution 

in wavetilt schemes for the location of a miner, esp'ecially in those 

regions where the presence of lateral resistivity contrasts in the 

rock overlying mine workings may affect the positioning of maxima 

and null phenomena in the surface magnetic fields produced by a buried 

loop antenna. • 

References 

Clernrnow, P. C., 1953, Plane wave spectrum representation of 

electromagnetic fields. 

Geyer, R. G., 1971, Research on the transmission of electromagnetic 

signals between mine workings and the surface: Quart. Tech. Rpt. 

for period Oct. 1, 1971 to Dec. 30, 1971 for U.S. Bureau of Mines, 

106 p. 

Geyer, R. G., 1972, The effect of a dipping contact on the behavior 

of the electromagnetic field: Geophysics, v. 37, n. 2, p. 337-350. 
MEDIUM 2 

' Fig. 1. Geometry of model where discrete contrasts 

in electrical properties occur. 
NQRMRLIZEO RMPLITU

NORMRLIZED RADIRL ELECTRIC FIELD [H-POLRRIZRTI
CIA 2 

'dm 

NORMRLIZED RMPLIT

m ,$ NORMRLZZED HORIZQNTRL ELECTRIC FIELD [E-POLRRI
Fig. 4. Normalized anomalous vertical magnetic field over poorly 

conducting region for several dip angles (E-polarization). 
SUBSURFACE APPLICATIONS OF PERIODIC 

ELECTROMAGNETIC VIDEO PULSE SIGNALS 

David L. Moffatt 

The ElectroScience Laboratory 

Department of Electrical Engineering The Ohio State University Col urnbus, Ohio 4321 2 

Introduction 

Considerable current interest is evident[l,2] in the use of transient electromagnetic methods for subsurface remote sensing. A transient 

method is discussed which uses a periodic video-type pulse containing a 

wide frequency range. A video pulse is used as an interrogating signal 

both for "target" identification and detection for re1 atively short depth 

(c 20 meters) applications in soil and soft rock media. If the medium is 

favorable, e.g., hard rock, greater depth applications appear to be feasible. The same procedures would be applicable for deep geological explora- tion but would require a substantial reduction in frequency and increase 

in target size. It should be noted that video pulse generators with 35 kv 

peak voltage and nanoseconds base widths are now within the state-of-theart, thus significant power over broad spectral ranges is possible. 

In the design of a video pulse sounding system, 5 main problem 

areas can be identified; 1 ) selection of the pulse shape or spectral content of the video pulse, 2) design of a radiating structure which effectively couples such a pulse into the medium, 3) isolation of the radiating and receiving mechanisms, 4) real istical ly accounting for the dispersion and attentuation of the medium and the air-medium interface, and 5) 

interpretation or processing of the received signal waveform. Each of 

these problem areas wi 11 be discussed and i 11 ustrated wi-th experimental and/or theoretical resul ts . 

The Video Pulse 

Selection of the proper video pulse signal is dictated by 3 considerations, the maximum desired depth of penetration, the attenuation of the 

medium and the required resolution and size of the target. For a lossy medium, penetration and resolution are clearly at cross purposes and a 

compromise is necessary. The exact nature of the compromise is dictated 

by the application, but the spectrum of the video pulse should not contain 

any frequencies higher than the predicted cut-off, i .e., that frequency 

which is attenuated to the threshold level of the receiver. As a practical 

matter, it may be necessary to select 1 or 2 video pulses reasonably suited 
for a variety of applications. For example, the experimental data presented here were obtained using 2 video pulse signals - a 1,000 vol t 

peak picoseconds base width with a spectrum essentially flat to the GHz 

range and a 50 volt peak nanoseconds base with significant spectrum to 

roughly 60 MHz. With current technology, the video pulse can be taylored 

to an individual application. It is stressed however that for a diagnostic capability, a1 1 frequencies which can real istical ly be observed 

must be included in the pulse spectrum. Geological exploration would 

involve larger targets at greater depths but pulses with lower frequency 

content would be used. 

The Antenna 

Required is a structure which when oriented on or above a given 

medium effectively couples energy into the medium over a broad range of 

frequencies. The broadband requi rement precl udes achieving signif i cant 

gain. For the video pulse sounding system, a dipole antenna is used with 

each dipole arm made up of two linear conductors in a V arrangement. 

The angle of the V is adjusted to control, in situ, the characteristic 

impedance of the dipole at the feed point. In the time domain the 

objective is to minimize sequentually and individually each reflection 

along the feed system and antenna. This is most simply done experiniental ly using a fast rise time pulse with a very long period and viewing the 

feed system and antenna as a transmission line with changing characteristic 

impedance along its length. Assuming a well selected feed system, the 

major sources of reflection are the antenna feed point and the ends of 

the dipole arms. Feed point reflections are minimized by adjusting the angle of the V arms. End reflections are controlled by 3 different 

mechanisms. In soil media, adjustable depth grounding rods at the end of 

each dipole arm element effectively controls the reflection. Often, if 

the soil has a vegetative cover, the attenuation of the vegetation is 

sufficient for control. On rock media, the large dipole (28 feed) has 

8 foot long solid metal sheets at the end of each arm. For the small 

dipole, a thin layer of absorber material (Hairflex) is inserted between 

the antenna and the medium. When the 1 ocal geometry permits, the most 

effective method of end reflection control is simply to extend the arms 

such that end reflections are beyond the "time window," i .e., would 

return to the feed point at times later than those from the target. 

Successful end ref 1 ecti on control usi ng these mechanisms have been 

documented wi th experimental measurements [3,4]. 

With video pulse systems, the receiver is usually a sampling 

oscilloscope. In a direct or monostatic mode where a single antenna is 

used for both transmission and reception the antenna can be switched 

between generator and sampler or alternatively a limiter to protect the 

sampler is provided and the entire time history is viewed. This direct 
mode of operation has the advantage of using same polarization transmission 

and reception. A major disadvantage is that a strong reflection from the 

air-medium interface can be anticipated. In addition, the ringing 

associated with horizontal ly stratified media would always be present 

which may or may not be desirable depending upon the application. The 

video pulse sounder ecan be operated in the direct mode (with limiter) but 

measurements indicate a need for faster diodes in the limiter for high 

power operation. Is01 ation via bistatic mode operation with two antennas 

(one for transmission one for reception) arrayed side by side with some 

horizontal separation is also possible. With dipole antennas on rock and 

soil media it was found that a surface wave component bound to the lower 

impedance medi um resulted in a strong direct coupl ing which would be 

difficult to control . 

The video pulse sounder is operated principally in an orthogonal 

mode using again two dipoles on the medium surface but oriented orthogonally with the feed points in very close proximity. In this case the 

direct coupl ing i s a substantial ly reduced rep1 ica of the pulse del ivered 

to the input feed point and therefore has a very brief time duration therefore permitting a clean time window. Orthogonal mode operation has the 

disadvantage (assuming only partial depol arization) of using the depolarized component of the scattered field. Note however that in this 

mode the sounder does not respond to targets which are symmetric to a 

vertical axis through the feed points. Thus the air-medium interface 

and other horizontal stratifications are not seen. 

Attenuation, Dispersion, Interface 

To properly process video pulse soundings it is desirable to remove 

from the received transient signal spectrum the effects of the antennas 

in situ, the feed system, and the medium. The effects of the feed 

system can be accurately measured. While certain locations and media 

offer geometries whereby the antenna in situ and medium effects can be 

measured, a theoretical capabi 1 i ty is highly desirable. 

An ana?ysi s and computer programs have been developed employing 

piecewise-si nusoidal expansion functions and Galerkin 's method to formulate a solution for an arbitrary thin-wire antenna configuration in an 

infinite, homogeneous, isotropic, conducting mediurn[5]. The solution 

determines the current distribution, impedance, radiation efficiency, 

gain and both near zone fields and far-field patterns. As a first order 

approximation to account for the half-space, 3 possible modifications 

have been added. Note that the goal is a simple modification of an 

existing state-of-the-art computer program for an lnfini te medi utll which 

yields reasonable results in the vicinity of broadside for the same 

antenna on a half-space. One modification is an image-type using a plane 

wave reflection coefficient most appropriate if the antenna were imnersed 
in the half-space. A second modification takes the wavenumber for the 

current flowing on the dipole to be the geometric mean of the wavenumbers 

for air and the half-space. A third modification combines the first 2. 

An example of the results is shown in Fig. 1 , where the result of each 

modification is shown and compared to the result for an infinite medium. 

The antenna is a 6 foot V dipole with an included V angle of 300. The 

frequency and half-space parametersare shown in the figure. Similar 

results for this antenna and ha1 f-space spanning 100 KHz to 100 MHz are 

given in reference 4. At 100 KHz (Fig. 1 ), none of the modifications 

a1 ter the 0-dependence of the fields. At higher frequencies 141, the edependence does change. In the broadside region (e = oO) and at low 

frequencies , both the image and current modi fi cations a1 one are roughly 

compatible with results given by Wait[6] for an infinitesimal dipole 

under the quasi-static approximation. At this point, no claim is made as 

to the appropriateness of these modifications but the versatility of the 

avai 1 able programs di ctate an attempt at simp1 e corrections. That only 

simple corrections may be needed near broadside is illustrated in Fig. 2 

where a comparison of measured and cal cul ated pul ses transmi tted through 

20 feet of 1 imestone is shown. The calculated pulse was obtained without 

any modification of the infinite medium program. The agreement between 

calculated and measured pulse shape is excellent. The magnitude error is 

partially accounted for by the fact that in the measurements, the thin 

absorber layer described previously was used on both the transmitting and 

receiving dipoles. The absorber was not taken into account in the 

cal cul ations . 

Attenuation and dispersion may also be calculated using the program. 

In Fig. 3, calculated received pulses are shown for transmission between 2 

identical dipoles (Fig. 1) over 2 different path lengths. The antennas 

are broadside to each other, the transmitted signal is a 45 volt peak 

50 nanosecond base width pulse, and the electrical properties of the 

medium have the assumed 1 inear frequency dependence given in Fig. 3. A 

linear frequency dependence may not be realistic, but because the 

c a l c u l ated pul ses are obtained vi a transforms of frequency domai n c a l c u l ations, any frequency dependence can be used. 

Processing 

The processing scheme presently being utilized is to obtain from 

the measured orthogonal mode signal the band-limited impulse response of 

the target. In this approach, a given signal in the "time window" is 

isolated in time by gating and then transformed to the frequency domain. 

This spectrum is then normalized by the spectra of the interrogating 

pulse, the feed system, the antennas and the medium. The resultant 

spectrum is then inverse transformed to the time domain. In Fig. 4, the 

band-1 imi ted impulse response of a 1 arge cyl i ndri cal void (road tunnel ) 

in limestone at a depth of 20 feet is shown. The specular-type impulse 
response in Fig. 4 is reasonable considering the size (% 25 foot diameter) 

of the tunnel. 

Concl usions 

In applications where either the medium or required penetration 

depth make the use of video pulse signals feasible the electromagnetic 

video pulse sounder offers a new capability. Such applications as 

plastic and metal pipe detection in soi 1, overburden profil ing, hazard 

detection in hard rock tunneling, route selections for subsurface 

instal 1 ations, vol umetri c mapping of abandoned and ha~ardous drift coal 

mines, backfill analysis of pipe installations in permafrost, etc., 

would appear to be within the capabilities of the present system. Again, 

deep probing would require much different pulses and 1 arger targets. 

The present system has also been used to experimentally record the 

responses of geologi cal features such as joints, faul ts- and a l i tho1 ogi c 

contrast in a dolomite medium. 
References 

1. Wait, J.R., "On the Theory of Electromagnetic Sounding Over a Stratified Earth," Canadian Journal of Physics, Vol. 50, No. 11, 1972. 

2. Vanyan, L. L., Electromagneti c Depth Soundings , Trans1 ated from the 

Russian by G.V. Keller, Consultant Bureau, 1967. 

3. L. Peters, Jr., and D.L. Moffatt, "Electromagnetic Pulse Sounding for 

Geological Surveying with Appl ication in Rock Mechanics and Rapid 

Excavation Program, I' Report 3190-2, September 1972, El ectroscience 

Laboratory, Department of Electrical Engineering, The Ohio State 

University ; prepared under Contract No. H0210042 for Bureau of Mines, 

Department of Interior (ARPA), Denver, Colorado. 

4. Moffatt, D.L., R.J. Puskar, and L. Peters, Jr., "Electromagnetic Pulse 

Sounding for Geological Surveying with Appl ication in Rock Mechanics 

and the Rapid Excavation Program," Report 3408-2 (i n preparation) , ElectroScience Laboratory, .Department of Electrical Engineering, The 

Ohio State University; prepared under Contract No. H0230009 for Bureau 

of Mine's, Department of Interior (ARPA) , Denver, Colorado. 

5. Richmond, J.H., "Radiation and Scattering by Thin-Wire Structures in 

the Complex Frequency Domain, " Report 2902-1 0, July 1973, El ectroScience Laboratory, Department of Electrical Engi neeri ng, The Ohio 

State University; prepared under Grant ?lo. NGL 36-008-138 for 

National Aeronautics and Space Administration, Washington, D.C. 

6. Wait, J .R., "Theory of Wave Propagation Along a Thin Wire Para1 lel 

to an Interface," Radio Science, Vol. 7, No. 6, June 1972. 
Er= 10 

CT = 0.005 

0,000 1 r- 5' 

t - 105 . 

I VOLT INPUT 

0 ( DEGREES ) 

(0) 

Fig. 1. Subsurface fields 

(a) x-componen t 

- INFINITE SP

------- PLANEWAVE 

- A PFRO.X I M

--- CURBENT MO

IN FINITE S

-- CUR9ENT h"

PLANEWA'J E 

9 

0 10 20 30 40 50 

8 (DEGREES

(b) f 6 foot dipole, 100 KHz. 

) z-componen t. 
Comparison of measured and cal cvlated pul sa 

transmission waveforzs - Fig. 2 

i ,: 20 TO 10 

Cr : 0.005 TO 0,015 

LINEAR VARIATION OVER THE 

FREQUENCY RANGE FROM 

-30 2 TO 40MHf 

50- P 

. 25' PATH 

-5+7?l 8 100 2CO 300 400 1 500 

t ( nsec ) 

(a) 

Fig. 3 Effect of dispersion on pulse propagation. 

(a) time domain waveform 

(b) amplitude spectra 
100 i 50 

t ( nsec 1 

(a) 

-800 

30 160 240 310 400 

FREQUENCY ( MHz) 

(b) 

Fig. 4 Processed measurements over the tunnel. 

(a) time domain waveform 

(b) amplitude spectra. 
ELECTROMAGNETIC SURVEY METHOD APPLICABLE TO UNDERGROUND 

QUARRIES 

R. Gabillard, J. P. Dubus, F. Cherpereel 

Electronics Department 

University of Lille - 59650 

Villeneuve dlAs cq 

France 

Abstract 

We present an electromagnetic method taken from the well known 

radiogoniometer that allows us to locate at the surface of the ground 

the vertical axis of a magnetic antenna transmitter buried at a depth 

h in the earth. 

The method and the equipment achieved are called R. E. P. E. 

(Relev'e Electromagnetique de Points Enterre's). They are daily used 

to accurately survey old quarries by using triangulation instruments 

at the surface of the earth rather than in galeries where the setting 

difficulties considerably reduce their accuracy and ircrease the work 

length. 

The results of several surveys allow us to give the optimum 

working conditions of the instruments and show that, when the transmitter is at a 20 meters depth, its position is known at the surface 

with an error lower than 1 centimeter. The power transmitter is of 

about 8 watts and we can find its location with an error smaller than 

1 meter when buried at a depth of about 100 meters. 

This equipment may be successfully used for location of subsurface 

miners. The space surrounding the transmitter must then be free 

from ferromagnetic bodies. I 

Principle -- of the Method 

The principle of the method is illustrated in Fig. 1. We put on 

the ground (for example, in a mine galery) a small emitter; this emitter 

has an antenna that we consider as a vertical magnetic dipole. Lf the 

signal frequency is low enough, the discontinuities of the mine aleries 

and the surface of the ground do not deform the magnetic field 711, (2) 

produced by the emitter. Under these conditions the magnetic field 

is symmetrically distributed around the vertical axis of the antenna. 

For determining the position of the vertical axis of the antenna, it 

is sufficient, on two different stations at the surface of the ground, 

to practice - with a magnetic antenna receiver - two sightings in the 

direction of the horizontal component of the magnetic field. The 
position of the vertical axis of the emitter antenna is then at the 

intersection of the two sightings. 

Description -- of the Equipment 

The equipment consists of: 

- an emitter set in the mine galeries 

- a magnetic receiver operating at the surface of the ground 

- and a pair of radiophones allowing to the workers to communicate 

through the earth. 

The Emitter 

The -block diagram of the emitter is presented in Fig. 2. The 

signal frequency radiated is about 1 kHz and the power produced by 

the emitter is of about 8 watts. 
oscillator Q 3966Hz 

* 

niodulation 

signal 

i 

I coil I ferrite 

The antenna is constituted by a coil wound around a small bar 

of ferrite half a meter long. Figure 3 shows the photo of the emitter. 

The apparatus is mounted on the embase of a theodolite Wild to 

maintain the verticality of the coil axis. 

The - Receiver 

The receiver, as shown in Fig. 4, is made of two perpendicular 

coils and of electronic.amp1ifier and filters. This receiver can operate 

either as an amplitude detector or as an angle detector. 

Amplitude Detector Operation 

When the receiver is connected (Fig. 5) as an amplitude detector, 

we only use the coil A. The signal appearing at the ends of the coil 

A is amplified and simultaneously heard by a headphone and read on 

the dial of a voltmeter. 

To determine the direction of the horizontal component of the 

magnetic fields, one turns the axis of the coil A in a horizontal plane 

until one finds a minimum deviation on the dial. Then the direction 

of the horizontal field component corresponds to the coil B axis. 

* 

t- 

- Detector I 

Amplifier i 

Filters 

Annle Detector Operation 

In this case the coils A and B are both used and the receiver 

acts as shown in Fig. 6. It detects the phase difference between 

the signals issued from the coils A and B. _____ ____ _. _ - . - . -- - - - - 

i 

.L- 1 

r f 

. filter and Phase 

Detector 

amplifier 

As is shown in Fig. 7, when the phase difference is /2, the 

coil A or the coil B are in the emitter direction. When one of them 

is in the emitter direction a system of control allows us to verify 

the coil A is found in the right direction. The use of the receiver 

as an angle detector permits good accuracy when the signallnoise 

ratio is high. 
These radiophones allow communication between the surface of 

the ground and a station buried at a depth of about 25 meters. Figure 

8 shows a radiophone set on the ground. 

Fig. 8. 
Performances of - R. E. P. E. 

This system has been used for three years to survey old quarries 

near Lille in the North of France. The experience acquired with 

this apparatus allows us to study the different kinds of errors made 

during the survey and the average of errors taking account of the 

working conditions. 

We can distinguish four sorts of errors: 

- the error due to anomalies of conductivity included in the surrounding 

ground of the emitter 

- the error due to the verticality of the antenna emitter axis 

- the error due to the horizontality of the receiver coil axis 

- the error due to the noise signal acting near the receiver. 

We have particularly studied the first three of them. 

Anomalies - of Conductivity 

The magnetic lines of force are theoretically distorted by 

conductivity and magnetic perlxleability anomalies. We havc chosen 

the signal frequency of the emitter in order to verify the relation: 

where = conductivity of the anomaly 

,f a = dimension of the anomaly 

In these conditions we can deduce from(2) that the magnetic lines of 

force are not modified by the finite conductivity and the geometry of 

possible anomalies. 

In our case where the main anomalies are cavities, the experiment 

shows that the magnetic field is not disturbed when 'the frequency is 

about 1 kHz and when the depth of the emitter is about 12 meters. 

For larger depths it could be necessary to reduce the frequency until 

we verify the above equation where 1 represents the emitter depth. 

However, the magnetic lines of force are distorted by ferromagnetic 

bodies whatever frequency is used and, in general, it is impossible 

to survey near iron pipes or metallic buildings. We can consider the 

survey must bc made according to the R.E. P. E. method for distances 

greater than 16 meters from iron pipes and 8 meters from metallic 

buildings. 

Verticality of the Emitter Antenna Axis 

The error of verticality of the antenna axis is mainly due to the 

reading of the air-level which equips the emitter. Several measure- 
ments showed that this error is about 2.4 mm when the emitter is 

at a depth of 12 meters. 

Horizontality Error of the Receiver __C 

When the receiver is used as an amplitude detector, the principle 

of the determination of the emitter direction consists of the evaluation 

of the amplitude of the horizontal magnetic field component produced 

by the emitter. If- the receiver coil axis does not turn in a horizontal 

plane, the emitter direction does not correspond to the direction of 

the minimum signal issued from the receiver. 

It is shown in another publication(6) that the angle error in the 

determination of the emitter direction is given by the formula: 

tg s = tg V sin 2% 

where: V = tilt angle of the magnetic field vector with the 

horizontal plane 

6 = accuracy of the air level of the receiver 

S = horizontal angle error of the emitter direction 

The accuracy of the air level of the receiver is about 1'5; consequently, 

- when the emitter is set at a depth of about 12 meters and the receiver 

at a horizontal distance of 1 meter from the vertical axis emitter, 

the theoretical error of the vertical position of the emitter is 0.5 cm. 

6 error 

fig.9. 

This curve shows that there is an optimum distance D which gives 

a minimum error. The distance is about 1.5 m. 

Conclusions 

The experience acquired during these last three years of surveys 

allows us to say that this method is a fair extension of the traditional 

survey with a theodolite and permits the reduction in price of the 

survey by about 20%. But this method is not a substitute for the 
traditional survey; in particular, when the ground contains magnetic 

materials or iron pipes, or when the survey is made under buildings, 

the theodolite must be used alone. Practically, the survey must be 

made according to the R. E. P. E. method for distances greater than 

16 meters from iron pipes, 8 meters from metallic buildings and 

3 meters from any car. However, under certain working conditions, 

the R. E. P. E. method is the only available, particularly when the 

galleries are tortuous and their sizes small or when the ceiling is 

very near the floor. 

Acknowledgements 

We would like to thank MM. P. Mangez and J. Baudet for 

their contribution during the realization of these apparatus. 

References 

(1) Sinha, , A. K. , and Bhattacharya, P. K. , 1966, Vertical magnetic 

dipole buried inside a homogeneous earth: Radio Science, v. 1, 

n. 3. 

(2)' Wait, J. R. , 1971, Electromagnetic induction technique for locating 

buried source: I.E. E. E. Trans. Geosci. Electr., v. GE-9, n. 2. 

(3) Wait, J. R., and Spies, K. P., Electromagnetic fields of a small 

loop buried in a stratified earth: I.E. E. E. Trans. Antennas and 

Propagation, v. AP-19, n. 5. 

(4) Wait, J. R., 1971 Criteria for locating an oscillating magnetic 

dipole buried in the earth: Proc. I. E. E. E. (letters), v. 59, n. 6. 

(5) Gabillard, R., ~ubus, J. P., and Cauterman, Ma, 1971, Cam~agne 

de detection de carrigres souterraines dans la rdgion lilloise par 

une mzthode de prospection el&ctrique: Bull. liaison lab0 P. et C., 

v. 54. 

(6) Gabillard, R., Dubus, J. P., Cherpereel, F., Nouveau ~rocgdg 

de levg de plans applicable aux carri%res souterraines: Bull. liaison 

labo P. et C. (2 para?tre). 
FEASIBIL7TY OF A UADI0COMMUNICATION IN MINE GALLERIES BY MEANS OF A COAXIAL 

CABLE HAVING A HIM COUPLING IMPEDANCE. 

By J. FONTAINE, B. DEMOULIN, P. DEGAUQUE and R. GABILLARD 

Electronics Department, University of LILLE - 59650 - Villeneuve d'~scq - FRANCE 

To heaeize a tcadiowave wmmunication dong mine g&d~ a dew kieometm long WitholLt upeateh, we show .that it it possible to u6e a coaxial cable beatLing 

an hpohtant smdace .&am duz .impedance like a ~mdsion fine wLth an inductive 

cou&ng & the thema md .the treceiva. We d~~e .the deothicd and 

geomettLicat coa.ta& 06 fie cable and the optimal dhequencq. 

To realize a radiowave communication along mine galleries a few 

kilometers long, it is possible to use different kinds of transmission lines 

with an inductive coupling to the transmitter and the receiver. 

The communication could be established without line amplifier 

with a two-wires line for frequencies lower than ]MHz. But such a solution 

requieres a very good laying which is not always possible in all the galleries. 

An other solution consists of a coaxial cable with spaced tuned slots. 

This process has been developped by Professor DELOGNE in Belgium iines in the 

5-!OMHz frequency range. This method gives very good results but the cable is 

Quite expensive. 

With the classical slotted coaxial cables it is necessary to use 

very high frequencies to obtain a good radiation. But the losses are important, 

and the distances between the repeaters has to be smaller than a few hundred 

meters. 

In this paper, we show that it is possible to use a coaxial cable 

bearing an important ,surface transfer impedance. By using the coupled transmission 

line theory, we determine the electrical and geometrical constants of the cable, 

and the optimal frequency range. 

The transmission line is made of an insulated coaxial cable connected 

to both ends to its characteristic impedance. It is set in a parallel direction to 

the gallery at a distance h of the wall (fig.1.). 

We have two coupled lines. The first consists of a single wire which is 

made of the. braid outer conductor of the coaxial and ofl the earth-conductor. This 

line is adapted and coupled to magnetic transmitter and receiver set near the 

cable at points A and B. The electrical constants of this first line are : 

Zcl = characteristic impedance 

y1=al+j~ = propagation constant where a is the attenuation factor. 1 1 
The second line is the cmxial itself with a characteristic impedance Zc2 and 

a propagation constant y 2 

The coaxial braid has a coupling impedance Zt per unit length. Zt has the 

following form : ZT = jL a where LT is the transfer inductance of the braid. T 

A part of the transmitted energy on the line 1 can excite the coaxial line by 

means of the coupling impedance. The other part quickly vanishes by propagating 

on the single wire line presenting important losses. 

The energy propagating in the coaxial has a much lower attenuation 

for a2 << al and can re-induce a current on the line.]. If we call A1 and A2 

the coupling attenuation between line 1 and 2 and reciprocally, expressed in db, 

~a and Ag2 the attenuations along the first and second lines, expressed in 

db)m, we can find a length of transmission L such as : 

If so, the communication will get improved by the coupling between the both 

lines. 

On figure.2. we have represented the signal level on the receiver as a 

function of the distance AB = x. A particular point of this curve is.the 

point C which corresponds to the minimum distance &in to verify the 

inequality (1). If the receiver sensibility is lower than the level obtained 

at 'min we see on figure.2. that the communication distance is increased. 

' TUEORUTCAL ANALYSlS 

I] 1121 

We study this problem by means of the theory of coupled lines . The following equations of telegraphy could be used as starting point : 

I1 and I2 are the currents along the two lines 

Y1 and Y are the admittances per length unit of lines I and 2. 2 

This system solution is : 

rlx -r x r x 2 -r x 

I1 = kl(Ale + Ble ) - kVl(A2e + B2e 

2 

) 

r, x r2X 

+ B2e 

-r2x 

I2 =-k (A e 2 1 ) 

with : kl = -2 Y t 1 
The constants A1, B1, A2, B2 could be determined by using the boundary 

conditions on the ends of lines. 

The boundary conditions are obtained in the form of a four linear complex 

equations system. The system may be solved by means of a computer. 

2 r12 and r2 are the eigenvalues of the system (2) : 

If we suppose that ly, 1 >> ly,land that the transfer impedance is much L smaller than Zcl and ' Zc2, the expression (4) could be simplified and 

we finally obtain : 

The previous conditions are always valid for the low coupled lines that we 

consider. 

DETERMINATION OF COAXIAL CABLE CHARACTERISTICS AND OPTIMAL FREQUENCY 

We study the effect of each parameter of the transmission line 

on the receiving level. It is sufficient to determine the current I 1 

on the line 1 as a function of x, (distance AB between the two transmitters) 

the receiving level being a linear function of I1. 

We have represented on fig.3. the distribution current Il (x) 

for a given frequency f = 9MHz and for 3 values of ZT. The other parameters 

being constant, we observe that the level I corresponding to the minimum 

2 cl distance Lmin is a linear function of ZT . 

On fig.4. we have studied IC1 as a function of the frequency for 

different values of the permittivity cr2 on the coaxial cable. In this study 

we suppose that the parameters al, a2, Zcl, Zc2 are constant. In reality 

it is not quite exact, particularly for a and a which vary respectively 1 2 

according to the frequency and the square root of the frequency. But we 

can see on the next figures.5. and .6. that if al and a2 vary, I is not 

c 1 

affected if al is much larger than a Thus we observe that the current on 2' 

line 1 is all greater as cr2 is near 1. For classical values of cr2 for 

braided coaxial cable we observe an optimal frequency of about 7MHz. 
On figures.5. and .6. we have represented I (x) for different values of dl and a 

I keeps constant and we notice a region wiere they are beats between the 2 

wgves propagated on both lines. 

Figure.6. shows that we must choose a ratio al/a2 > 5 to use the coupling effect. 

To build a coaxial which has good performances for this type of communication 

we study the current distribution I,(x) on line 1 for different values of LT 

at frequency f = 7MHz (see fig.7 .). 

For large values of L we take into account an important coupling between both T lines. On the curves In fig.7. we can see that there is an optimal transfer 

inductance LT =40.10-9H/m for a communication of one kilometer long. 

It is quite evident that it is necessary to determine the geometrical and 

electrical characteristics of cable to obtain a minimum attenuation factor a2 

in spite of the important value of LT (for a classical TV coaxial, LT is of 

about 0.2.10-9~/m) . Figure.8. shows the structure of a braided shielding. The transfer inductance 

of such a braid is given by ~S~ell3 1 : *- . 

where $J is the angle of braid wire with cable axis 

a is the dispersion factor 

0 = 1 -K2 where k is the coupling factor of the two half-braids, which behave 

like two coils winded in the opposite directi0n.a is much smaller than 1. 

It is very difficult to give an anilytical expression for 0. The actual theory 

of leak transformer is insufficient to take into account all the parameters 

of the braid in the calculation of a. 

Kriigel has shown that a is an increasing function of $ and a decreasing 

function of the coverage.\ratio for $ > 45O. 

R is given by : 

actual area of braid R = =2F-F 2 

total surf ace area 

where F is the fill factor, i.e., the ratio 6f the actual width of one pick 

to the width of one pick for 100% coverage : 

F = -- a- CNd - <1 b 2r(D+2d)cos$ 

C : number of carriers 

N : number of braid wires per carrier 

d : braid wire (or strand) diameter 

D : diameter over cable core 

The attenuation factor a of a braided coaxial cable for copper is given by : 2 

a = 2.6 . loe9 I/E;;. 

2 D log - 

Di 
where Di is the inner conductor diameter 

K is a factor larger than 1 if the core wire is made of strands (for example s 

K = 1.06 for 7 strands) S 

. 141 Kb is the braiding factor. It has the following form . 

1 - 2 .rr(D+2d) 

= 2 F.cos $ CNd cos$ 

According to equations (6) until (10) we see that for a given value of LT 

and J, it is possible to increase C, N and d to obtain an attenuation factor a2 

not too high. 

To determine the best ratio D/D; we have represented on figure.9. the current I 

as a function of Z c l 

c2' 

We observe a maximum value of I for Zc2 = 75Q. If we build a cable having a 

c l 

classical value for the outer diameter of the braid D+2d = 7,5mm, a characteristic impedance Zc2 = 75Q with r2 = 1.5, N = 4, C = 8, d = 0.25mm, some 

-9 measurements have shown us by extrapolating that we can obtain LT=40. 10 ~/m 

for an angle of braid $ = 60". -6 

Then the attenuation factor a has the following form a = 8.10 . db/m. 2 2 

With this cable we have made the theoretical study of the communication along 

galleries of mine between the two magnetic transmitters. On figure.10. we have 

represented I (x) for three values of the frequency : f = 3,7 and 2OE1Hz. 1 The attenuation factor of line 1 is evaluated by measurements in coal mine 

in the South of France. For a single wire line situated at 30cm of the gallery 

wall, we have : 

The levels R corresponding to the receiver sensibility have been represented S 

on figure. 10. R is proportional to f2. We observe on the 3 curves that the 

S communication distance is increased when we use the coaxial cable with a 

coupling effect. Indeed xi represents the maximum distance of communication 

using a single wire line having an attenuation factor a]. xri is the 

maximum distance when we use the coaxial cable. 

For these three frequencies, we have xri > xi but we notice that 7 MHz is 

an optimal frequency. x2 = 550111 and xr2=1200m. There is a ratio 2 between 

these two distances. 

With this coaxial cable we have studied, we have the possibility to 

increase the maximum distance of communication in a ratio greater than 2. 

An other advantage of this system is that the line could be set careless. 

Indeed the attenuation factor a1 of the single wire line made of the cable 

and the gallery could be important without influence on the communication. 
Thanks are due to C.E.R.C.H.A.R. (Centre d'~tudes et de Recherches 

des Charbonnages de France) who suggested the investigation. 

REFERENCES 

(')P .I. KUZNETSOV and R.L. STRATONOVICH, The propagation of electromagnetic 

waves in multiconductor transmission lines, PERGAMON PRESS, 1964 

(2)~. MIJRATA, S. INAO and H. HONDO, Electromagnetic interference with 

video transmission cable, 20th International wire and cable symposium 

proceedings, Atlantic City, N.J., Dec. 2, 1971 

(3)~on L. KRUGEL, Abschirmwirkung von Aussenleitern flexiber ~oaxialkabel, 

Telefunken-Zeitung, Jg 29 (Dezember 56), Heft 114 

(4) J* SPERGEL, Computer aided design of braid parameters for coaxial cable, 

20th Internation wire and cable symposium proceedings, Atlantic City, 

N.J., Dec.2, 1971 
Maximun communication Mdxirnun communication 

distance without couDlins effect distance with 

coupling 'effect 
'I- 

, X a W 200 UO 4 500 $00 700 400 000 1000 

Fig .6 
\ 

'\ 

\ ' . 

, , x' 

100 200 300 400 500 600 700 800 goo - 1&0 

ELECTROMAGNETIC TRANSMISSION AND DETECTION AT DEEP DEPTHS 

Dm B. Starkey 

Acceptance Technology Division 8344 

Sandia Laboratories Livermore 

Introduction 

Investigation into the expressions of the field produced by a loop over 

a conducting half space or in an infinite conducting half space are well 

documented. Reasonably complex descriptions of the effects of stratification 

also are reported. However, a model which must contain all of these complexities because of transmission to depths of 15,000 feet and also include 

the pertibation of the field by a verticle cylindrical steel casing loses its 

facility in the field or for quick, accurate approximation to field magnitudes 

and phase. 

This paper describes the results of two field tests to determine the 

attenuation and phase versus depth and frequency of an electromagnetic wave 

(induction field). A simple two-layer model is postulated to predict the 

magnitude and phase of the induced field which does not require sophisticated 

computing tools. Correlation of the model with experimental results show 

agreement of most amplitude measurements to within 3 db and most phase measurements to within five degrees. The tests were conducted in a 7400 foot deep 

well in Nevada and an 11,000 foot deep well in Wyoming which had conductivities 

ranging from .O 25 mhos/m to .25 mhos/m. The surface transmitting dipole had 

an area of Q 10 ! feet squared and constant frequency signals of 1.5 Hz to 20 Hz 

were transmitted through the earth utilizing less than 65 watts of power. 

Model Description 

The magnetic field intensity components generated by a magnetic dipole 

immersed in an infinite homogeneous medium can be expressed in spherical 

coordinates as: 3 

NI (naL) 

He = 3 [l+y+yrl [e -yr~ sine 

Z = verticle depth between coaxial loops 

2 where N, I, and (na ) are the transmitting antenna turns, current, and effective area, respectively, and r is the range from the loops circumference to 

the measurement point. The case of interest for these experiments, however, 

is the special case of coaxial dipoles, i.e., 8 = 180'. For this case 

He= 0 and H reduces to: 

r n 
wavelengths involved, many thin layers of different conductivity can be 

ignored because only small phase delays are encountered in a layer, resulting 

approximately in cancellation of the two reflections. 

The resulting two-layer solution is shown below: 

where 2 112 Y = radius at the interface (z12 + a ) 

2 1/2 X - radius below the interface (Z + a ) 

Hr (Y) = magnetic field intensity at interface from Equation 3 

y2 = propagation constant for the second layer 

Z1 = verticle depth from transmitting loop to interface 

Z2 = verticle depth from interface to receiving loop 

Well Casing Attenuation 

Assuming that the resultant field from the above calculations is that 

which is present at any depth, the attenuation of the magnetic field intensity 
by the well casing is calculated by the following relationship described by 

Shenfeld for infinite long cylinders whose radius is much greater than the 

cylinder thickness : 

Ho = magnetic field intensity outside the cylinder 

- 

Hi = magnetic field intensity inside the cylinder 

- 

y = propagation constant for the wall material 

6 = skin depth in the wall material 

y = wall material permeability (200 vo) 

r = cylinder radius 

d = cylinder wall thickness 

The above expression is said to be valid for the following cases: 

1. High or low frequencies (converges to the skin depth solution at 

high frequencies) 

2. Magnetic or non-magnetic wall material 

3. Orientation of H is unimportant 0 

Although this expression diverges somewhat from the concept of developing a 

. simple model, it is required at the frequencies of interest (below 20 Hz). 

Figure 1 is a plot of the attenuation from Shenfeldrs relationship and plane 

wave attenuation from .1 to 10 Hz for a typical casing. It can be seen that 

even at 10 Hz the plane wave attenuation is % 6 db pessimistic relative to 

Shenfeld's formulation. Multiple casings' encountered in some of the testing 

are handled as a single cylinder whose thickness is equal to the sum of all 

the casings' thickness. 

Receiving Antenna 

rn 

The receiving antenna is a ferrite loop stick in construction. Assuming 

that the core "captures" all of flux within the casing and that this flux is 

uniformly distributed across the internal cross section, the open circuit 

voltage from the antenna is described as follows: 

*eff = effective area of the receiving antenna 

w = angular frequency 

= free space permeability 

Since Ti for a given depth and media decreases with frequency and the 

voltage output has frequency as a multiplier, an optimum frequency is expected. 

The model (using the Wyoming data below) shows an optimum at % 2 Hz at 6000 

feet and % 1 Hz at 12,000 feet. 
6 2 Two transmitting loops were used, each with an area of Q 10 feet . One was 

offset approximately 1200 feet from the well center. The transmitted power 

(I~R losses) was varied from 4 to 13 watts with signal-to-noise ratios at 

5 Hz varying from 35 to 12 db at 1000 and 7410 feet respectively. The offset 

antenna was used among other techniques to give assurance that the signal was 

not propagated down the cable or well casing--since the phase would be different for this case versus through the earth, particularly at the shallow 

depths. Figures 4, 5, and 6 show the relative strengths of the measured 

fields as compared to the model predictions. 

Wyoming Tests 

Figure 7 is a graphic view of the Wyoming test configuration. A triple 

casing extended to 437 feet. From 437 feet to 8255 feet a double casing 

existed and below 8255 to 11,000 feet only a single casing was present. The 

model parameters used were: 

Interface Depth: 8100 feet 

Layer One: a = .05 mhos/m 

Layer Two: a = .25 mhos/m 
6 2 One transmitting loop was used with an area of 4 x 10 feet . The transmitter 

power was varied from 4 to 65 watts with signal-to-noise ratios at 5 Hz varying from 20 db to 2 db at 3500 feet and 11,000 feet respectively. Figures 8, 

9, and 10 show the relative strengths of the measured fields, 

1 Discussion of Results 

The correlation between calculated and experimental values indicates that 

the model is extremely accurate. Although the phase plots have been omitted 

because of space limitations, the two-layer model is also capable of high 

accuracy in this prediction at all depths also. Only at the shallow depths 

is the phase prediction high by approximately five degrees. The use of the 

infinite homogeneous model appears to balance any effects in the field caused 

by the steel casing, 

The amplitude plots show a discontinuity at the location of a change in 

the number of casings. This fact, plus the use of the offset antenna at NTS, 

show that the signal is propagating in the medium and not down the casing. 

Additional calculations yielded a maximum attenuatioc rate of only .9 db/1000 

feet presuming the signals was coupled down the armored cable used to lower 

the receiving package. Since the actual attenuation rates were much greater, 

this was also eliminated as a possible coupling link, 

1 Acknowledgements 

I This work was supported by the United States Atomic Energy Commission 

. Division of Peaceful Nuclear Explosives, Contract Number AT- (29-1)-789. 

The author would also like to thank El Paso Natural Gas Company for 

providing the test well and their capable assistance in the conduction of the 

Wyoming experiment. 

References 

1. S. Shenf eld, "shielding of Cylindrical Tubes" , IEEE T-ransactions on 

Electromagnetic Compatibility, Volume EMC-10, No. 1, March 1968 

2. James Re Wait, "Electromagnetic Fields of Sources in Lossy Media", 

Antenna Theory: Part 11, McGraw-Hill Book Company, 1969 
WELL CAS I NG ATTENUATION 

FREQUENCY (HZ) 

Figure 1 

TRANS 1 

MON 

TRANSMITTER LEVEL 

COHERENT 

RnFa GAIN 

EXPERIMENTAL HARDWARE 

Figure 2 

145 
-- 

z1 - 8100 feet 

Figure 7 

146 
PAHUTE #1, NTS 

OCTOBER, 1972 

0 MEASURED 

W 

z I-- 4 

-180 - 

1 2 3 4 5 6 7 1 

DEPTH (THOUSANDS OF FEET) 

-.I 

*I 'I IGNALS REUT

IGNALS RELATIVE TO 1 VOLT RMS AT THE RECElVlNG ANTENNA FOR 

AMP RMS IN THE TRANSMITTING LOOP, AMP RMS IN 

MEASURED ATTENU

MEASURED ATTENUATION RATE: -4.4 o~/1000 FEET (UST 3500 FEET) 

5 Hz SIGNAL ATTENUATION 

Figure 4 
DEPTH (THOUSANDS OF FEET) 

IGNALS RELATIVE TO 1 VOLT RMS AT THE RECEIVING ANTENNA FOR 

F 

IGNALS REL

AMP RMS I

U RI. IN THE TRANSMITTING LOOP. 

MEASURED ATTENUATION RATE:-7,l ~~/1000 FEET (DOUBLE CASING) MEASURED ATT

5 Hz SIGNAL ATTENUATION 

Figure 8 
p

approximation 1 is almost always appropriate. Computations were 

carried out for bare antennas, the extension to insulated ones being, 

however, straightforward. 

Although a rigorous theory of the antenna perfbrmance in a 

realistic semi-infinite conductive medium has not been developed, the 

results quoted should also be applicable when antennas are a few skin 

depths below the earth's surface. In particular, for the evaluation 

of radio-links under up-over -and-down operating conditions, the 

antennas can be replaced by dipoles of length equal to the effective 

height. 

Since the antenna's parameters are given in analytical form, it 

is also possible, by using Laplace transform techniques, to compute 

the transient response of the antenna. The transient is described 

by pole (short-time behavior) and branch-cut contributions (large time 

behavior). For these last terms, an asymptotic evaluation is possible, 

SPECTRUM MEASUREMENTS OF ELECTROMAGNETIC-NOISE IN COAL MINES* 

W. D. Bensema and J. W. Adams 

National Bureau of Standards 

Abstract 

A portable, multichannel battery-operated mine-permissible 

measurement system was developed to measure the rms magnetic-field 

noise spectrum in the frequency range from 100 Hz to 375 kHz. 

During each measurement, the entire spectrum is measured simultaneously through the use of time-domain recordings which are later 

analyzed by Fast Fourier Transform processing. Dynamic ranges of 60 dB 

in a 125 Hz bandwidth are obtained for spectra covering the range from 100 Hz 

to 100 kHz. The method also allows a three-dimensional display of the way 

spectrum occupancy changes with time. Calibration and correction procedures 

allow absolute field strength measurement with an uncertainty of not more than 

3 dB. We feel that further analysis of the system errors will allow this 

uncertainty to be reduced tu less than 1 dB. 

Ambient magnetic field noise spectra covering frequencies from 100 Hz 

- to 100 kHz are given for several underground coal mine locations. Examples 

are also given of magnetic field noise on the surface above the mine, noise 

in the mine face area, noise radiated by specific equipment, the voltage 

spectrum found on a 600 V dc trolley wire, and noise picked up simultaneously 

on loops and on roof support bolts. 

I. Introduction 

Measurements of the ambient electromagnetic noise present at the 

receiving terminal must be made in order to design a communication system 

usable for routine and emergency communications, either operating through 

the earth or utilizing wire or wireless links within the mine. 

Constraints on the experiment made it necessary to rapidly measure 

electromagnetic noise over most of the spectrum below 300. kHz. Therefore, 

broadband, analog, real-time recording for later processing was chosen 

as the means of study, rather than the more usual but slower, narrow band 

swept-frequency techniques. The chosen method rapidly and continuously 

records the entire spectrum. The entire spectrum of an event lasting a few 

seconds, such as an electric locomotive passing by, can be captured quickly 

on magnetic tape, and later analyzed in great detail. 

*Sponsored by U. S. Bureau of Mines 
II. Measurement Instrumentation 

The system block diagram is shown in Figure 1. Figure la 

shows the field-portable portion of the system. Three data channels 

were needed to obtain three orthogonal field components at each 

site. An analog 7-track tape recorder running at 30 inches per second 

(ips) and weighing 14 kg was used for recording the time series data. 

Figure lb shows the laboratory transcription process used to reduce 

the data bandwidth and transcribe the data to a la boratory-quality 

analog tape recorder with tape-controlled servo capability. The servo 

capability accurately controls the tape recorder speed from a stable 

reference signal recorded on the tape (on a separate channel) at the 

same time the noise signals were recorded. Figure lc shows the 

laboratory digitizing process. During this step, the data bandwidth 

was further reduced; the tape recorder was run in the tape-controlled 

servo mode. The digitized data was then processed by a digital 

computer. The available digitizing instrumentation was not capable 

of accepting the full bandwidth of the data as recorded. It was 

therefore necessary to reduce the effective bandwidth of the data by 

using a slower speed on playback than was used on recording. This 

process retained .all of the original information but traded bandwidth 

requirements for processing time. A Fast Fourier Transform (FFT) 

algorithm was used to change the time series data into a frequency 

series presentation in spectral form. The processed data in its 

initial form comes from the microfilm plotter and printer, Figure Id. 

The FFT routines were developed primarily by L. D. Lewis of the 

Space Environment Laboratories, National Oceanic and Atmospheric 

Administration, Boulder, Colorado. Lewis based his algorithm 

primarily on methods outlined by Welch (1). 

III. Typical -- Coal Mine Spectra 

Data were taken in Robena No. 4 Mine, wayn;sburg, Pennsylvania. 

Figure 2 shows a spectrum measured in the face area, about 10 

meters behind a continuous mining machine in full operation. The 

machine was powered by 600 volts dc. For the curve shown, the 

antenna sensitive axis was oriented for a vertical moment. The 

- field strengths measured were about 39 db above one micro-ampere 

per meter (39 db pA/m) at 10 kHz and about 0 db pA/m at 100 kHz. 

The lower curve shows system noise with the antenna terminals shorted. 

In addition, the two horizontal antennas recorded mine noise spectra 

(not shown) that were lower in amplitude by about 10 db at 100 kHz 

and about 35 db at 10 kHz. Spectra taken in haulageways in the mine 

tended to show magnetic field strengths typically 60 to 70 db pA/m 

out to a few kilohertz, which then dropped off sharply above 8 to 12 

kHz. One exception was a spectrum taken near a dc motor-driven 

hydraulic pump (car pull). This spectrum peaked at 78 db p A/m at 

1000 Hz, dropped to 47 dbpA/m at 10 kHz, and was down to 25 dbpA/m 

at 30 kHz. 
Measurements of voltage between roof support bolts showed the same 

high spectrum occupancy below approximately 10 kHz. Simultaneous 

measurements of roof bolt voltage and magnetic field showed that 

noise events were not necessarily correlated. In one case, a 

noise burst received on the roof bolts was not received on the 

loop antennas. This problem needs more investigation. 

Simultaneous recordings were made of noise underground and on 

the surface 30 meters away from a 180 meter deep shaft used 

for ventilation and power distribution. These measurements 

showed a coherence between surface and underground signals greater 

than 0.85 over the frequency range from 250 Hz to 6.8 kHz, with 

rapidly decreasing coherence above 6.8 kHz. 

An approximate cross-check of the rms magnetic field noise at 

10 kHz was made using data from the system described in the 

companion paper "Amplitude Statistics of Electromagnetic Noise 

in Coal Mines" by M. Kanda and J. Adams. Kanda's system uses a 

different measurement technique and hence gives an independent 

value for the rms field level. The cross-check is only approximate in that the two measurements were made with about a 

3-hour time separation and a 30 meter spatial separation in the 

same,mine. Operating conditions in the mine were essentially 

the same. With these differences, agreement was within 10 dB. 

IV. Conclusion 

Two significant results are reported. First, a powerful measurement technique was developed which clearly shows the simultaneous 

time and frequency variations of complicated signals. The method 

of making the spectrum measurements combines the convenience of 

lightweight, portable instrumentation with the power of a large 

computer FFT package. A singular advantage of the FFT is the unity 

probability-of -intercept of signals occurring at any time and at 

any frequency within the bandwidth covered. One consequence of 

this is the ability to generate 3-D plots showing how spectrum 

occupancy changes with time. 

The second significant result is a set of electromagnetic spectrum 

measurements made underground in coal mines. These spectra cover 

the frequency range from 100 Hz to 320 kHz, and along with those 

reported by Bensema (2), are the first extensive set of spectral 

measurements made in an underground coal mine environment. 
V. References 

1. P. D. Welch, (1967 IEEE Trans. on Audio and Electroacoustics, 

vol. AU-15, No. 2, pp. 70-73). 

2. W. D. Bensema, (1972, Coal Mine ELF Electromagnetic Noise 

Measurements, NBS Report 10739). 

Another reference that may be of interest is: 

3. R. B. Blackman and J. W. Tukey (1968 The Measurement of Power 

Spectra, Dover Publications, Inc. , New York). 
I DIGITIZER I 

(TAPE SERVO BIOEI 

+ 

AF1PLlf IERS 

AND 

Figure Ice 

Laboratory dlgitizTng 

FIELD -' 

PORTABLE 

TAPE 

. 

* 

FILTERS 

LOOP 

Figure lb. 

Laboratory transcription 

. RECORDEE 

(30 IPS) 

ltlCROFlLEf 

PRINTER 

LABORATORY 

TAPE 

RECORDER 

(60 IPS) 

J A A1JTEi;;Ii'iS 

Fj.~3ire l:* 

F5eld portable equJpnent 

FIELD 

PORTABLE TAPE 16 CHA':~:ELS 

Figure Id. 

Spectral computati on, plotting, 

aqd pr?nt-ing 

RECORDER 

(15 IPS) , 

' Ff gure 1. System block dingram sho7i ng data process; ng flow. 

OF DATA 

F+ gure 2. Hametic fteld no1 sc measured in Robena Coal 1-Xne face 

area, 10 meters behlnd an operating 6CO V dc continuous ininer. 

Antcnna r;lolxent vert'cal. Do not scale absolute field strengths 

above 100 Wz. The lower curve shows receiver noise (antenna 

shorted). 
AMPLITUDE STATIST1 CS OF ELECTROMAGNET1 C 

NOISE IN COAL MINES* 

M. Kanda and J.W. Adams 

Electromagnetics Division 

National Bureau of Standards 

Boulder, Colorado 

Abstract 

A system for measuring amplitude probability distributions (APDfs) 

of electromagnetic noise in coal mines is described and typical 

APDts from an underground coal mine are presented. The APD is 

a basic statistic required for the design and analysis of communication systems, especially those intended for use in noisy 

environments, and where neither overdesign nor underdesign is 

acceptable. The rms and average field strengths are obtained by 

integration of the APD, and examples are shown at several frequencies. All field strength levels are given in absolute units. 

Selected frequencies cover the range from 10 kHz to 32 MHz. 

I. Introduction 

Ambient electromagnetic (EM) noise often limits radio communications in mines. Therefore, it is necessary to know the detailed 

statistical characteristics of the interfering noise in order to 

design optimum performance receivers and effective errorcorrecting coding schemes. The cumulative amplitude probability 

distribution [APD) of the received noise envelope is one of the 

most useful statistical descriptions of the noise process for 

the design and evaluation of a telecommunication system operating 

in a noisy environment [1,2]. 

This paper describes the system used to measure magnetic field 

noise in a coal mine. The system is an extension of one designed by Matheson [3]. Typical APDfs are presented. Figures 

on-~ayleigh graph paper show the fraction of the time that this 

noise exceeds various levels. This particular Rayleigh graph 

paper has scales chosen so that Gaussian noise (e. g. , thermal 

noise) plots as a straight line with a slope of -4. Noise with 

rapid large changes in amplitude (e.g., impulsive noise) will 

have a much steeper slope, typically -4 or -5, depending on the 

receiver bandwidth. The APDfs are then integrated to give rms 

and average values of the field strength, according to the 

equations 

*Sponsored by U.S. Bureau of Mines, United States Department of 

the Interior. 
portable recorder response band was from 100 Hz to 56 kHz at the 

2 dB points in the direct recording mode. The dynamic range of 

the recorder was 48 dB. The tape was then transcribed through 

another tape recorder whose servo system could take out the 

flutter and wow introduced by the portable recorder. The data 

processing system consisted principally of the analog magnetic 

tape recorder for a playback unit and an instrument which provided 

a direct digital display of the percentage of the time each of 

15 levels, 6 dB apart, were exceeded. 

The bandwidth of the whole system was primarily determined by the 

data processing system and was found to be about 2 kHz. The 

dynamic range of the whole system, including the recording and 

the data processing systems, was primarily limited by the magnetic 

tape recorder to about 45 dB. The system used for recording and 

data processing is shown in Figure 1. 

The calibration of the whole system, including the loop antennas, 

field strength meters, mixers, magnetic tape recorders, the 

impedance transforming amplifiers, and the digital counter was 

performed by immersing the receiving loop antennas in a known 

fi d, generated at the NBS field calibration site. Thus all 

levels of field strength are given in absolute units. 
times. For instance, at 10 kHz underground, magnetic. fields 

50 dB above one microampere/meter will be exceeded only .0001 

percent of the time, while fields 5 dB above one microampere/meter 

will be exceeded 99 percent of the time. An rms value of 24 dB 

above one microampere/meter will be exceeded (in this case) about 

36 percent of the time. The surface data show a corresponding 

rms value of 3 dB above one microampere/meter. 

References 

1. W.Q. Crichlow, et al. (1960 NBS Monograph 23). 

2. W.I. Thompson, I11 (1971 DOT Rep. DOT-UMTA-71-3). 

3. R.J. Matheson (1970 IEEE Trans. on EMC, EMC-12, p. 151). 
Field Strength Meter C A R s--v-- A .--= --- --.. .---=--.sp I 

l~re~ucnc~ Cove rage I 10 kHz-250 kHz 150 kHz-3

IF 1 Bandwidth (Impulsive) Narrowband , 110 Hz , 5.0 kt

Broadband I 2.7 kllz I 

bynamic Range 1 67 d~ / 60 d

Spurious Response 

Rejection 

(Image, IF, etc.) 

TABLE I CHARACTERISTICS OF THE 

FIELD INTENSITY RECEIVERS 

COLLAPSIBLE 

ANTENNA 

L 

ANTE 

- . . . . . , . . - - 

CHANNEL 

FIELD METER 

BW=5kHz PORTABLE 

TAPE RECOR

f =I 50kHz-32MHz I DIRECT 

- 

a) FIELD RECORDING f =100Hz-50k
FREQUENCY 

20 :][I I 

RMS - 

Fig. 4 VARIATION OF FIELD STRENGTH, H (UNDERGROUND) 

- 

F 0 

5 

m 

z -20 

Z 

-40 

-60 

0 AVERAGE 

A MINIMUM (99%) - 

- 

I I1 111111 I I 

1 OkHr 100 1 MHz 10 100 

11111, 1 , lljl, il ,: 
DESIGN OF AN ELF NOISE PROCESSOR* 

J. E. Evans and A. S. Griffiths 

MIT Lincoln Laboratory,Lexington, Mass. 

. -.- -.--.---. -- Abstract 

Noise processing experiments with ELF (3 to 300 Hz) atmospheric 

noise and signals in the 40 to 80 Hz range are described. The primary 

pu.rpose of the experiments was to record and analyze wideband ELF 

noise in order to design the noise processing portion of a receiver 

which minimizes the required transmitter power. The application of 

appropriate theory and extensive simulations led to a noise processor 

which consists of the following functions: (1) a compensating (or whitening) 

filter, (2) a prenotch filter clipper, (3) notch filters at frequencies of 

manmade interference (e. g., power lines), (4) a post-notch filter clipper, 

and (5) a phase-coherent linear matched filter. 

The nonlinear processing provides considerable gain relative to a 

linear receiver (i. e. , a receiver consisting only of a matched filter 

and appropriate whitening filters). It is convenient to measure sys tem 

performance in terms of an "effective noise level" which is equal to 

twice the received signal energy divided by the signal to noise ratio at 

the matched filter output. For example, the highest effective noise 

levels observed at 45 Hz with the nonlinear processor were about -134, 

-131 and -137 dB wrt 1 amp/m using Saipan, Malta and Norway 

data, respectively, compared to 1 percent exceedance levels of effective 

noise for the linear receiver of about -115, 115 and -130 dB wrt 1 amp/m 

93 respectively. 

*This work was sponsored by the Department of the Navy. 
EXPERIMENTAL COMPARISON OF BURIED AND ELEVATED ELF 

TRANSMITTING ANTENNAS 

Peter R. Bannister, F. J. Williams, J. R. Katan, 

and J. R. Ball 

! Electromagnetic Systems Department 

Naval Underwater Systems Center 

New London Laboratory 

New London, Connecticut 06320 

Abstract 

During the last week in March and the first week in April, 

1973, the Naval Underwater Systems Center measured the 76 Hz 

magnetic field strength (in Wisconsin and Texas) produced by both 

the elevated and (newly installed) buried Wisconsin Test Facility 

(WTF) north-south (NS) antennas. The principle result obtained 

from these measurements is that there is no measurable difference 

in performance between the WTF buried and elevated NS antennas. 

Introduction 

The U. S. Navy Project SANGUINE Wisconsin Test Facility is 

located in the Cheqcamegon National Forest in north-central Wisconsin 

about 8 km south of the village of Clam Lake, This particular 

location was chosen because of its low electrical conductivity and 

sparce population density. This facility was designed to test and 

demonstrate interference mitigation techniques. It has also been 

used for ecological studies and as the source for ELF propagation 

rnea sur ements. 

The WTF consists of two 14 mile NS antennas (one elevated, 

the other buried at a depth of approximately one mkter) and one 14 

mile elevated EW antenna - with the transmitting station at the intersection near the midpoints of the antennas. Each antenna is grounded 

at both ends. The average direction is 19' E of N for the NS antenna, 

and 109' E of N for the EW antenna. Although the Wisconsin Test 

Facility was designed for a maxin~um current of 540 amps into each 

antenna, it has been operated to date at only 300 amps, which is 

about one-third of its power output capacity. 

During August of 1972 we measured the effective earth conductivity 

(6,) beneath both elevated antennas of the WTF at 45 and 75 Hz (1). 

The H/I method was utilized with each WTF antenna alternately 

employed as the source. (In the H/I induction method, a long insulated wire grounded at both ends is energized by an AC generator at 

the frequency of interest. The mag~itudes of the magnetic fields are 
then measi~red at various distances and angles from the wire (2). ) 

These measurements were performed mainly at distances of 45 to 

75 km from the transmitter, in line with and broadside to each WTF 

antenna. The principal. result obtained from these measurements is 

that the effective conductivity under the EW antenna is greater than 

that under the NS antenna. 

During the last week in March and the first week in ~~rii of 

1973, we measured the 76 Hz magnetic field strength (in Wisconsin 

and Texas) produced by both the elevated and (newly installed) 

buried WTF NS antennas. The main purpose of these measurements 

was: (a) to compare the buried and elevated NS antennas; and (b) to 

ascertain the repeatability of the August, 1972 (1) measurements. 

Wisconsin Results 

During this latest test, measurements were performed at distances 

(p ) of 10-70 km from the WTF transmitter -- approximately broadside to the NS antennas. Presented in Figure 1 is a plot of the Hp 

component versus range for the elevated NS antenna. These Hp I 

values are normalized to a transmitter current (I) of 300 amperes 

and azimuth angle (6 ) of 90* (i. e. , directly broadside to the antenna). 

The solid line in Figure 1 was derived (1, 3) by assuming an effective 

conductivity under the NS antenna of 2.2 x loB4 mhos/m. This is 

the value that was measured during the August, 1972 test (1). From 

this curve it is observed that the theoretical and experimental values 

of Hp are in excellent agreement for measurement distances greater 

than 26 km. Thus, the repeatability of the August, 1972 measurements 

is excellent. 

It has been shown (4) that for the homogeneous isotropic earth 

case, the measurement distance must be greater than seven skin 

depths ( b 500/.Ifa , where f is the frequency in Hertz and o 

is the uniform earth conductivity) in order for the Hp component to 

be inversely proportional to . Referring to Figure 1, we see 

that the measurements taken at ranges less than 26 km are grossly 

different east and west of the NS antenna. Since 26 km is approximately 

seven effective earth skin depths (beN 500/-, meters) at 76 Hz, 

it appears that the 7 6 criterion is also valid for the geoelectrically 

complex WTF area. (It should be noted that Wait (5) has recently 

shown that -- for certain two-layered earth conditions -- even the 

7 &, measurement distance is too close for determining re). 

Presented in Figure 2 is a plot of the Hp component versus 

range for the buried NS antenna at 76 Hz. The H values are P 
normalized to I = 300 amperes and # = 90°. These measurements 

were taken at distances of 36-71 km from the WTF transmitter, 

approximately broadside to the buried NS antenna. They were repeated 

at each of the 3S6sites. The solid line in Figure 2 was derived 

(1, 3) by assuming an effective conductivity under the buried NS 

antenna of 2.2 x mhos/m (i. e. , the same ce as measured under 

the elevated NS antenna). From this curve it is observed that the 

theoretical and experimental values are in excellent agreement. Thus, 

within the experimental accuracy of the measurements (to. - 1 db), 

there is no difference (in produced field strength) between the buried 

and elevated WTF NS antennas. There is also no difference in the 

pattern of these two antennas (t - lo ). 

Texas Measurements 

During the period extending from mid-March through early April, 

1973, the Naval Underwater Sys tems Center performed a conductivity 

survey of the Llano Uplift area of Texas using the wave impedance 

measurement technique (with the WTF NS axtenna as the transmission 

source), An additional task, the far field comparison of the WTF 

elevated and buried NS antennas, was performed during the first. week 

in April in Kingsland, Texas. This far field site (p - 1. 85 Mm) is 

approximately in line (i. e. , # = 0' ) with the WTF NS antennas. The 

measurement period was divided into two segments (1030 - 1430 GMT 

and 1530 - 2330 GMT) during which the elevated and buried NS antennas 

were utilized. (The first two hours (1030 - 1230 GMT) were not used 

for the buried versus elevated comparison as this period coincided 

with Texas sunrise. ) At 1430 GMT each day, the transmitter was 

switched from the buried to elevated antenna (or vice-versa). 

The measured daily averages for the H) magnetic field strength 

component are presented in Figure 3. It should be' noted that some 

of the atmospheric noise estimates taken during this period were 

upper bounds to the noise (i. e., these noise estimates are suspected 

to be contaminated with 60 Hz). Therefore some of the 8070 confidence 

intervals (computed for each of the daily averages) may be too large. 

Receiver integration times of 30 minutes per sample were employed 

for the majority of these measurements resulting in 36 samples for 

the buried NS antenna and 34 for the elevated. Referring to Figure 3, 

we see that the six day H# average is -142.6 t 0.5 dBAm for the 

buried NS antenna and -143.2 t 0.6 dBAm for the e:evated NS 

antenna. Since these confidence intervals overlap, there is no 

discernible difference between the buried and elevated NS antennas. 
(The buried antenna appears to be slightly better although not enough 

data exists to make it statistically significant. ) 

Conclusions 

The principal results obtained from these two different range 

measurements are: 

1. There is no difference in performance between the WTF 

buried and elevated NS antennas (within the experimental 

accuracy of the .measurements). 

2. The NUSC August, 1972 WTF effective earth conductivity 

measurements are repeatible, and 

3. The measurement distance must be greater than seven effective skin depths in order for the H component to be inversely 

proportional to fie . P 

It should be noted that Lincoln Laboratory personnel measured 

in Norway during the same time period. Their results (6) also 

indicate that there is no measurable difference in performance between 

the WTF buried and elevated NS antennas. 

References 

1. Bannister, P. R., and F. J. Williams, "Results of the August, 1972 

Wisconsin Test Facility Effective Earth Conductivity Measurements, I' 

(to be published in Journal of Geophysical Research). 

2. F. C. Frischknecht, I1Electrical Methods in ~ed~h~s ical Prospecting, " 

Pergamon Press, Oxford, 1966, Chapters IV and VI. 

3. Bannister, P. R., "The Image Theory Quasi-Static Magnetic Fields 

of a Finite Length Horizontal Electric Antenna Located Near the 

Earth's Surface, 'I WSC Tech. Report 1011, 4 June, 1969. 

4. Bannister, P. R., "Quasi-Static Fields of Dipole Antennas at the 

Earth's Surface," Radio Science, vol. 1 (New Series), no. 11, p. 

1321 - 1330, November, 1966. 

5. Wait, J. R., "Note on the Surface Impedance and Wave Tilt for a 

Line Source - Excited Two-Layer Earth, I' (Private Communication, 

April, 1973). 

97.5 

2 0 2 5 30 35 40 5 0 60 70 

p (km) 

Figure 3. TEXAS HO daily averages ' 

---_____ - --- 

o ELEVATED ANTENNA BURIED ANTENNA A SIX-DAY AVERAGE 

-1461 I I I I I J 

APRIL 1 2 3 4 5 6 

DATE - -.- - 
AN UNDERGROUND ELECTROMAGNETIC SOUNDER EXPERIMENT 

Lambert Dolphin, Jr. , Robert Bollen, and George Oetzel 

Radio Physics Laboratory, Stanford Research Institute 

Menlo Park, California 

Abstract 

An electromagnetic sounder developed for an archaeological application in Egypt has been successfully tested in a California dolomite 

mine. Chambers in the mine 100 to 130 feet from the surface gave 

intense, well-defined echoes cons istent with an average attenuation 

coefficient of 0.6 dB/m and a relative dielectric constant of 11. By 

moving the transmitter and receiver units on the hillside above the 

underground chambers, various characteristics of the propagation could 

be observed such as dispersion, cha~nber aspect sensitivity and cross 

section. The transmitted pulse was one to one-and-a-half radio-frequency 

cycles long at a peak power of O* 2 MW. Frequencies employed were 16 to 

50 MHz. The light weight, highly portable, battery-powered equipment 

is potentially suited to other underground sounding applications. 
visited and establish uncertainty bounds for these predictions, and (3) 

establish techniques for predicting noise in mines that have not been 

visited (but that use similar equipment to those visited) and also 

establish uncertainty bounds for these predictions. 

Specific factors or formats to be presented include: 

(A) For underground broadband measurements: 

1. 300 Hz to 100 k~z spectral plots 

2. 300 Hz to 384 kHz spectral plots 

3. 300 Hz to 192 kHz spectral plots 

4. 300 Hz to 6kHz (subject to change to 3 or 12 kHz) spectral 

plots 

(B) For surface broadband measurements: 

1. 300 Hz to 6 kHz (or 12 kHz) cspectral plots 

2. 300 Hz to 3 kHz spectral plots 

(C) For underground narrowband measurements : 

1. APD plots at 12 frequencies from 10 kHz to 32 mHz of 

magnetic field strength 

2. R. M.S. and average values will be calculated based on 

time averaging for these plots 

(D) Sample spectral measurements of: 

1. Roof-bolt voltages 

2. Trolley wire-to-rail voltages 

3. Phone line currents 
Future efforts will be decided after data processing is completed. 

Possible tasks are: 

(1) Measurements in an all-AC mine 

(2) Measurements in some typical hard-rock mines 

(3) spectral measurements below 300 Hz (requested by Arnie 

Far stad) 

(4) Time statistical presentations from existing data 

(5) Development of noise prediction techniques and uncertainty 

bounds 

(6) Determine technique for using copies of analog noise tapes 

for system tests. 

1.0 INTRODUCTION 

The Location Group conversed in the Petroleum Room of the 

Green Center with the purpose of addressing the following list of 

topics in light of the Bureau of Mines' requirements for detecting 

and locating trapped miners by electromagnetic methods. These topics 

include: Noise, Sources, Antennas (transmit and receive), the Propagation 

Medium, Signal Processing, Optimum Frequency, Receivers, Pulse 

vs. CW Techniques, Passive Location Techniques, and Obstacles. 

Practical constraints such as equipment size, weight and power requirements, cost and intrinsic safety were kept in mind during these 

discussions. It was emphasized that in-mine equipment nlus t be 

inexpensive, rugged, simple and lightweight. Otherwise, the mine 

operators probably would not buy it nor would the miners be able to 

effectively use it. 

Pr es ent techniques for experimentally detecting and locating trapped 

miners were discussed, along with techniques to improve their location 

accuracy. Analytical, numerical, and scale modelling methods were 

considered as a means of giving further insight into measurement. 

anomalies caused by terrain variations, conductivity contrasts , and 

obstacles. 

After discussion of these topics, conclusions were drawn and 

existing problem areas were identified. These problem areas were 

address ed in developing guidelines for future efforts. Following the 

discussion period, the Chairman gave a summary of the Location Group 

report and comments were solicited from the entire group. 

GROUP DISCUSSION HIGHLIGHTS 

2.1 Noise 

W. Bensema asked the group whether the noise data provided by 

NBS was adequate. It was determined that for location systems analysis, 

the most important form of noise data was the frequency spectra of 

vertical and horizontal magnetic fields on the surface, and the answer 

to his question was affirmative. Drs. Wait and Frischknecht pointed 

out the difficulty in accurately measuring the true vertical magnetic 

field since these measurements are heavily influenced by the loop 

setting and the local geology. Nevertheles s, these measurements are 

important since the same perturbations would also affect a receiving 

loop oriented to receive the vertical magnetic field. Dr. Rankin pointed 
out that the magnetic noise fields pass through a null somewhere 

between 0.2 Hz and 5 Hz, that this frequency range- may have merit 

in deep mine detection problems. 

2,2 Sources 

Sources that have been used most extensively and successfully to 

date in mine location problems are vertical magnetic dipoles (VMD) 

consisting of single turn and multiple turn loops of wire powered by 

a source of AC current. The advantages and disadvantages of using 

a horizontal magnetic dipole (HMD) as an alternate to the vertical 

dipole were discussed. The main advantage in using the HMD is that 

the null field is the vertical magnetic field (HZ) and that a greater 

resolution of the null plane may be realizable due to a lower HZ 

background noise. However, the main disadvantages in using a horizontal 

magnetic dipole source is that only one null plane in HZ is produced 

by a single HMD source and that additional information is needed to 

resolve the null point location. Furthermore, there are practical 

constraints on how large an area can be realized in an HMD configuration 

and how difficult it would be for a miner to implcmcnt in an actual 

emergency. 

2,3 Antennas 

Transmitting antennas that have been succes sful in location exercises 

for the Bureau of Mines have consisted of single turn and multiple 

turn loops deployed on the mine floor usually in intersections or 

around coal pillars. These antennas have produced adequate signals 

for detection and location at mine depths as great as 1650 feet. However, 

it was pointed out that these antenna configurations are not possible 

in most metal mines because of the random mine tunnel construction 

(following the ore veins) and the relatively narrow kidth of the walls. 

Consequently, it may become necessary to revert to long wire antennas 

tied to roof bolts or large area loops if closed paths are available. 

Since metal mines are normally deeper than coal mines, the increased 

moments oi these large antennas may be required anyway to produce 

detectable signals on the surface. 

The pos sibility of using ferrite cor c! transmitting antennas was 

also discussed. In view of Dr. Gabillard's success with these antennas, 

it was decided that they warrant further investigation for location 

purposes. 
The relative advantages of using air core and ferrite core receiving 

antennas were also discussed. Ferrite core antennas are more compact 

but are not as stable as air core loops. Since much of the experimental work for the Bureau of Mines entails the measurement of absolute field strength profiles, it was concluded that for the present 

time, air core loops are more desirable. 

Other forms of receiving antennas that were discussed were a 

dual coil system for field gradient measurements and a broadband 

gradiometer for ultra low frequencies. Some preliminary work has 

been done by Westinghouse on the dual coil system but it is yet too 

early to evaluate the results. 

2.4 Propagation Medium 

In dis cussing the propagation medium, the general cons ensus 

among the members of this group was that the half spacelplane earth 

was well -understood. Simple extensions of the plane earth theory, 

such as hill slope corrections, have proven useful in refining the 

location accuracy of the measured results. Cylindrical and spherical 

models have also been developed to account for terrain anomalies but 

have not been fully evaluated with respect to experimental results. 

The request was made by Drs. Wait and Lewis that careful documentation 

of actual field environments including anomalies in the propagation 

media be made and forwarded to those investigators developing analytical 

models to further guide them in producing realistic models. Dr. 

Geyer suggested that analysis of dipping bed conductivity contrasts be 

investigated to determine their effect on location accuracy. 

Analysis of the effects of arbitrary terrain variations in the media 

were also discussed. . Dr. Frischknecht proposed a three-dimensional 

scale modelling approach to the terrain problem wh5le Dr. Greenfield 

suggested a two-dimensional numerical analysis technique. It was 

felt that both of these techniques would enhance our understanding of 

the effects of terrain on location accuracy although Dr. Wait expressed 

reservations on the two-dimensional analysis. 

Techniques for experimentally determining the apparent conductivity 

of the medium are well understood and measurement results can be 

used directly in predicting field strengths when one knows the transmitted 

moment and the overburden depth, 

2.5 Signal Processing/Receiving 

In existing EM location receivers, signal processing is straight- 
forward and consists merely of narrowband amplification of an AC 

signal transmitted from an underground dipole. It was pointed out 

that for mines appreciably deeper than 1500 feet, it will probably be 

necessary to use some sort of synchrorious detection scheme with 

expanded integration time to obtain signal to noise ratios suitable for 

detection and location from existing levels of transmitted moment. 

Dick Myers suggested the possible use of ambient 60 Hz fields as a 

common synchronizing source for both transmitter and receiver. 

2.6 Optimum Frequency 

Although it has been demonstrated that frequencies between 900- 

3000 Hz are ideally suited to uplink detection and location in most 

coal mines, the optimum frequency will no doubt be shifted downward 

when considering their use in deeper and more conductive metal mines. 

When discussing the range of lower frequencies available, it was 

suggested by Dr. Frischknecht that we look into the use of magnetometers and broadband gradiometers as field sensors. Dr. Rankin 

suggested that if ultra low frequencies are used, we should take advantage 

of the low background noise region between 0.2 Hz and 5 Hz. 

2-7 -- Pulse vs. - CW 

Source location by waveshape analysis of underground transmitted 

pulses has been proposed by Dr. Wait. Analytically, these techniques 

contain diagnostic information not found in simple CW transmissions. 

However, in light of intrinsic safety requirements, the technique was 

not considered practical for use in coal mines. Consequently, the 

Bureau of Mines has pursued the CW approach and with a relative 

measure of swcess. However, this is not to say that the pulse technique 

will not be feasible in hardrock mines where the intrinsic safety 

limitation no longer exists. Pending the outcor~e 0'3 future CW tests 

in hardrock mines, the question of pulse vs. CW should be temporarily 

deferred. 

2.8 Passive Location 

The passive location technique (also proposed by Dr. Wait) is one 

which enables a miner to modulate a secondary source in the mine 

by opening and closing a large coil of wire and by doing so, have a 

recognizable effect on the impedance of the transmitting loop. Initial 

investigations into the practicality of this scheme have been somewhat 

pessimistic in light of realistic parameters of noise, conductivity, 
and overburden depth. Hovrever, it has been suggested that the potential 

use of this scheme be investigated more thor~ughly to decide once and 

for all whether it is practical. The advantages of such a scheme 

are quite obvious in that no power is required on the part of the 

trapped miner. 

2. 9 Obstacles 

It has been shown that obstacles appearing in the mine overburden 

have a distorting effect on uplink field patterns especially if they occur 

near the observer location. Dr. Wait suggested that obstacles of 

primary importance are cylinders and longitudinal conductors. These 

geometric figures can be used to model buried pipelines and cables 

and analytical results are available to study their effects. Dr. Wait 

pointed out that someone should conduct a study to identify what information would be most useful in improving overall location accuracy. 

3.0 IMAJOF, CONCLUSIONS 

The uplink detection problem has been solved for thc vast majority 

of coal mines in the United States. 

Location techniques (uncorrected) are generally accurate to better 

than 1070 of overburden depth. 

The half space/plane earth theory is well understood; however, 

more work is needed on analyzing terrain anomalies and near surface 

conductivity anomalies to obtain more refined correction factors for 

the data. 

The detection problem has not been solved for extremely deep 

hardrock mines; however, removal of intrinsic safety limitations will 

facilitate this problem. Also it may be necessary to lower frequency, 

use long wire a~tennas or coherent receivers to get satisfa-ctory results 

in these mines. 

Noise data of primary importance to location problems are the 

spectral information on the three component magnetic fields at the 

surface. 

4.0 RECOMMENDATIOhTS FOR FUTURE WORK 

Based on the workshop discussions, the following review session 
and a later informal session of the location group, the following 

r~commendations for future work are given in descending order of 

priority. The primary emphasis is on signal detection above deep 

mines and the secondary emphasis is upon refining location accuracy 

obtained using existing techniques. 

The relative level of effort and the time required to achieve the 

desired goals are roughly estimated and are indicated by the following: 

Level of Effort Time Duration 

High (HI 

Medium (M) 

Low (L) 

Short Term (ST) 

Long Term (LT) 

1. Optimize frequency for propagation in deep mines and for 

minimum influence from anomalies. (H, ST) 

2. Investigate alternate detection techniques such as magnetometer, 

gradiometer, multiple coil systems and coherent detection for 

use in deep mines. (M, LT) 

3. Increase understanding of anomalous location results, i. e. , 

arbitrary terrain effects (sczle modelling) (L, ST) 

arbitrary terrain effects (numerical techniques) (M, LT) 

effects of long cylinders and longitudinal conductors (rapid 

analytical methods ). (M, ST) 

geometric terrain effects (spherical/cylindrical). (M, ST) 

4. Investigate alternate transmitter techniques: (L, LT) 

Ferrite loop source vs. air core loop. 

Pulse type transmitter for metal mines. 

Hand crank power source. 

5. Predict performance of passive detection technique. (L, ST) 
SUMMARY REPORT - OF UPLINK - AND DOWNLINK COMMUNICATIONS 

WORKING GROUP 

Robert L. Lagace, Group Chairman 

Arthur D. Little Inc. 
TABLE OF CONTENTS 

I. OVEKVIEW m 

PAGE 

TI. BRIEF DESCRIPTION OF THE FOUR PROMISING SYSTEMS 

A. Uplink-Data 

B. Downlink-Voice 

C. Sidelink-Call Alert Coded Page 

D. Sidelink-Roof Bolt Voice Page 

111. PRESENT STATUS AND RECOMMENDED FUTURE WORK 

A. Uplink Data System 

1. Overview 

a. Nominal Mines 

b. Deep Mines 

c. Equipment 

2. The Channel-Transmission Loss 

a. Loops 

b. Parasitic Structures 

c. Grounded Wires 

3. The Channel-Noise 

a. Past Data 

b. NBS Mine Noise Measurements 

c. Whistler and Geomagnetic Data 

d. Data Utilization 

4. The Source-Message, Coding, Modulation, Operating Frequency 

5. The Receiver-Sensor, ~emodulation/~ecoding, Special Processing 

B. Downlink Voice System 

1. Overview 

a. Experience to Date 

b. Future Developments 

c. Deep Mines 

2. The Channel-Transmission Loss 

a. Long Wire Antennas 

b. Parasitic Structures 

3. The Channel-Noise 

4. The Source-Message, Coding, Modulation, Operating Frequency 

5. The Receiver-Sensor, ~emodulation/Decoding, Special Processing 

a. Downlink. . b. Uplink 

C. Sidelink Call Alert Coded Page System 

1. Overview 

2. The Channel-Transmission Loss 

a. Loops 

b. Parasitic Structures 

c. Roof Bolts 

3. The Channel-Noise 

4,5. The Source and Receiver 

D. Sidelink Roof Bolt Voice Page System 

1. Overview 

2. The Channel-Transmission Loss 

a. Finite Wire Antenna Terminated by Roof Bolts 

b. Parasitic Structures 

3. The Channel-Noise 

4.5. The Source and Receiver 
I. OVERVIEW 

The attention of this group was focussed on four through-the-earth communication systems that are presently of high interest to the U.S. Bureau of Mines; 

four systems for providing operational/emergency communications on the working 

sections of coal mines, indeed up to the very face of the section. The systems 

are: uplink-data, downlink-voice, sidelink-call alert coded page, sidelink-roof 

bolt voice page. Each of these systems makes use of the mine overburden as the 

signal transmission medium, as opposed to the guiding wires, cables, and tunnels 

treated by the operational communications working group. Each of these systems 

satisfies one or more of the Bureau's objectives for mine communications systems; 

namely 

reliable links for monitoring the mine environment under both 

operational and emergency conditions. 

e reliable links for communicating with miners during emergencies. 

e special links for increasing the efficiency of day-to-day operations 

of the mine. 

Each of these systems has been successfully demonstrated on a limited experimental 

basis, and prototypes of all these systems are installed and operating in the 

USBM experimental mine in Bruceton, Pa. Each of these systems must now be optimized regarding its performance, and engineered for practical routine application 

to the working sections of actual operating coal mines, particularly those of the 

room and pillar type. 

This optimization and engineering ~ustake place subject to the principal 

constraints listed by Howard E. Parkinson in his Workshop paper entitled, "Ob jectives and Constraints of Through-the-Earth Electromagnetic Communications Systems'' 

and enumerated below. 

e Depth of Mine Overburden 

e Overburden Conductivity 

Electromagnetic Noise In and Above Mines 

Limited In-Mine Electrical Energy (Stationary or Zhn Carried) During 

an Emergency 

Intrinsic Safety for In-Mine Equipment 

Practical and Rugged Equipment for Use Under Both operational/ 

Emergency conditions 

Severe Weight Limitations for Man Carried Equipment 

Reasonably Low Costs Especially for Man Carried Equipment 

Part I1 of this paper provides a brief description of each system, while Part 111 

summarizes the present status of developments related to these systems and some 

recommendations for future work needed to advance these systems to the practical 

application stage. 
I. BRIEF DESCRIPTION OF THE FOUR PROMISING SYSTEMS 

A. Uplink-Data 

This is a vertical through-the-earth narrow band data channel for monitoring 

important parameters of the mine environment under operational and emergency conditions, and for receiving coded messages or replies from miners during an -2 

emergency. Operating ranges compatible with 1,000 foot deep mines with a = 10 

mho/m overburden are required. The in-mine transmitter would be located at a key 

place on the section, such as the loading point, where the present mine pager 

phone is also terminated. The surface receiver would have to be located in the 

vicinity of the point directly above the in-mine transmitter, primarily because 

of the inherent power limitations imposed on an in-mine transmitter during an 

emergency. This location requirement for the surface receiver may pose a difficulty for some mines with regard to surface access rights over advancing sections, 

and therefore may restrain such uplink communications to emergency situations 

during which mobile equipment can be temporarily installed over the known location 

of the in-mine transmitter. During normal mine operations, the mine environmental 

data could be monitored by means of a carrier channel over the mine pager phone 

line. 

The limited in-mine transmitter power available during an emergency and the 

electromagnetic noise levels present on the surface have led to the conclusion 

that uplink transmission of baseband voice is not a practical goal. Therefore 

it has been deleted as a requirement until practical, voice bandwidth compression 

techniques or other types of signal processing becotue available to change this 

conclusion. 

This is a vertical through-the-earth voice channel for transmitting messages, 

during a mine emergency, to miners carrying a sniall emergency voice receiver, 

preferably built into their helmets. As in the case of the uplink receiver, 

difficulties regarding surface access rights over advancing sections may require 

a mobile surface transmitter installation that is temporarily installed only 

during emergencies. However, since the transmitter power available on the surface is much greater than that underground during an emergency, the downlink 

allows greater operational flexibility in communicating with moving miners, and 

may reduce the surface access rights problem somewhat, because of the potentially greater coverage area of each surface transmitter. As in the uplink case, 

operating ranges compatible with 1,000 foot deep mines with o = mho/m 

overburden are required. 

C. Sidelink-Call Alert Coded Paae 

This is a horizontal through-the-earth narrow band channel for transmitting 

a call alert paging signal to key individuals roving on a working section during 

normal mine operations, to notify them that they are wanted on the mine pager 

phone. The transmitter, activated by a signal sent over the mine phone line, 

would be somewhat centrally located near the section loading point as in tile 

uplink Yysten, and conceivably could be integrated with the uplink equipment if 

desired. 'L't~e receivers would be carried by the miner, preferably in his helmot 

as in the case of the emergency voice receiver for the downlink system. In fact 

the Bureau's prescnt desire is to have this emergency voice receiver serve a 

dual role, for key supervisory and maintenance people, by also operating as a 
narrowband call alert receiver under normal operating conditions. Such a call 

alert system would extend mine phone paging to roving individuals right up to 

the working face, thereby increasing both safety and operational efficiency . This would require operating ranges on the order of 400 to 800 feet in overburdens of o = mho/m in order to cover a typical 600 by 600 foot section, 

depending on the location of the transmitter. 

D. Sidelink-Roof Bolt Voice Page 

This is a horizontal through-the-earth voice channel for transmitting a more 

comprehensive voice message or page, as opposed to a simple call alert, to key 

individuals roving on a working section during normal operating conditions. As 

in the call alert system the transmitter could also be located at the section 

loading point, thereby requiring the same operating range as the call alert 

system. However it most likely would not share equipment with an uplink system 

as a call alert system might. Being a voice bandwidth system for use under 

operational conditions when electromagnetic noise levels are high, particularly 

in the audio band, a significantly higher operating frequency than that possible 

for a narrowband call alert or uplink system is favored. The receiver for the 

roof bolt paging system is presently conceived as a pocket-sized unit, but 

other packages such as a helmet mounted unit are not excluded. 

111. PRESENT STATUS AND RECOMMENDED FUTURE WORK 

To arrive at a design that is at least acceptable, if not optimum, 

regarding performance and practicality for any of the above communication systems, one usually must first determine how each of the major elements comprising 

the system infiuence its performance, and then use them so as to get the desired 

results. An of ten indispensible aid to this process, particularly when design 

information for one or more of the major system elements is missing, is to put 

together a breadboard system based on existing related hardware and try it out. 

Several of the above through-the-earth systems have evolved, with beneficial 

results, from the latter approach. Concurrently, some of the previously 

missing design information on transmission loss and noise has been accumulated. 

Therefore, it will now be possible to better optimize each of these systems by 

quantitative analysis and comparison of alternative designs. 

No papers evaluating or-describing any of the above th~ough-the-earth systems were presented during the Workshop. However, several papers treating two 

major system elements, channel transmission loss and noise, were given. These 

and some past work by the attendees of this working group provided the basis 

for the group's findings and recommendations. We grouped the system elements as 

follows : 

o The Source: its message, modulation or coding, transmitter, and 

operating frequency. 

o The Channel: its transmission loss for each antenna type, and its 

noise characteristics. 

o The Receiver: its pick up sensor, demodulation/decoding, and 

special processing for signal to noise improvement. 
atory (WGL) support this under 5 kHz conclusion for mho/m overburdens, while 

also identifying the frequency band between 500 Hz-to-3 as a distinctly ,, 

optimum one for narrowband systems. The WGL analyses were based on Wait/WGL 

transmission loss curves for loop transmitters and broadband atmospheric noise 

data (under 10 kHz) taken by WGL in Colorado. Signal to noise analyses performed by Arthur D. Little, Inc. (ADL) reach a similar under 5 HZ overall 

conclusion, but do not reveal the presence of an optimum frequency band as distinct as the one by WGL. The ADL analyses were based on the same transmission 

loss curves of wait/WGL, but different broadband noise data, namely surface atmospheric noise data (under 300 Hz) taken by MIT Lincoln Laborato (LL) in Florida 

and early WGL and National Bureau of Standards (NBS) surface no 7 se data (under 

10 k~~) taken over four Western coal mines. The differences in the results of 

the two analyses, regarding the presence o absence of a clearly optimum fre- -5 quency band between 500-3000 Hz for 0 = 10 mho/m overburdens (based on broadband noise levels) should be easily resolved when the large amount of noise data 

recently taken over coal mines by NBS soon becomes available. However, WGL and 

NBS field experiences have revealed a potentially more serious noise problem 

that may tend to favor use of frequencies between 1-5 kHz over coal mines, 

namely the extremely strong harmonics of 60 Hz and 360 Hz caused by the mine 

power conversion equipment, harmonic levels that are high enough in some cases 

to interfere with even narrowband systems operating between the harmonics. 
A multichannel uplink data system of practical design suited for installation and test in an operating coal mine with up to 1,000 feet of overburden 

will soon be needed. The basic monitoring requirements of the in-mine station 

are now being formulated, so that an overall uplink data communication system 

can then be designed and optimized for the operating conditions of this mine. 

2. The Channel-Transmission Loss 

a. Loops 

Uplink communications to date have primarily utilized loop source antennas 

of vertically oriented magnetic moment. These have consisted typically of one 

turn loops (up to 500 foot periphery) wrapped around one or two coal pillars; 

and less frequently a smaller one turn loop (up to 100 foot periphery) placed 

in an entry. Such loops have been preferred over long wire antennas for inmine installations because of their lower input resistance, fixed impedance 

characteristics over time, and convenience of installation and maintenance in 

the adverse mine environment. The primarily vertical magnetic fields produced 

by vertical axis loops Carl also offer a signal to noise advantage on the surface in some cases, depending on the sources of the noise, i.e. natural or manmade. 
The theoretical results of Wait and ITS, regarding the field strengths 

expected on the surface from infinitesimal loops of moment NIA placed in homogenous and layered conducting overburdens, are well established and have been 

found to be in good agreement with experimental data obtained by WGL and Colorado School of Mines (CSM) at several mine sites. For the large overburden 

depths of interest and sizes of corresponding loops required, the simplifying 

infinitesimal loop assumptions apply. Furthermore it has been shown by Wait 

and ITS that typical conducting obstacles, such as pipes, and inhomogeneities 

found in the transmission path to the surface should produce only small effects 

on the resultant magnetic field seen at the surface for the under 5 kHz band 

of interest. WGL and ITS have also shown that the effects of surface topography on the resultant surface field are also small. Consequently these 

effects can largely be ignored for communications applications, as opposed to 

location applications where some of the effects can take on greater importance 

in some cases. 

Therefore it was concluded that no new theoretical derivations were 

required on uplink transmission loss for loop transmitters; but that appropriate 

curves, tables, nomographs, etc. , based on the available theoretical results 

should be prepared, as an aid to uplink systems designers who desire to apply 

the theory to typical mine overburdens. Included in these design aids should 

be curves that show the additional amount of signal loss suffered as the horizontal displacement between surface and in-mine loops is increased. This will 

help determine the surface coverage obtainable from a single in-mine loop. 

b. Parasitic Structures 

All of the above results apply for cases in which no large closed loops 

of wire, cable, or steel rocf mesh are close enough to thrfinite, relatively 

large, in-mine transmitter loops to allow significant currents to be induced 

in these parasitic structures, which in turn might reduce $he effective strength 

or field of the transmit loops. The likelihood of encountering parasitic structures 

on working sections is high, but the degree to which they could adversely 

affect system performance, has not been ascertained. Since this may be a potential problem to both the uplink data system and the call alert page system to 

be discussed below, the practical influence of such structures needs to be 

assessed. However, until that is done, uplink or call aleri system transmit 

loops should be installed away from such structures as steel roof mesh, trolley 

lines, and probably power cables, since the effects of their presence will decrease with increasing separation. 

c. Grounded Wires 

Lastly, should there be a renewed interest in comparing the performance 

of a loop source uplink system with that for a grounded finite straight wire 

source that utilizes a wire terminated by a roof bolt ground rod at each end, 

ITS has derived expressions and curves for the magnetic field produced on the 

surface by such a buried finite wire source. The results apply to the case 

of the wire inclined at an arbitrary tilt angle to the horizontal in a homogeneous overburden. They show that small tilt angles made by the wire with a 

flat or hilly surface do not influence the rcagnitude of the surface field. 
mine with diesel haulage. The measurments encompass operating and quiet conditions for different machines, locations, power centers and boreholes, in working sections, haulageways and on the surface. Some of these noise data have 

already Seen processed and made available, with the remainder to become available within the next six months. 

In-mine measurements have included wideband recordings from 100 Hz 

to 300 Hz of three magnetic field components, and of voltages on telephone lines, 

trolley lines, and roof bolts; from which noise power spectra are being generated. 

Tn addition, narrowband (2kHz) spot frequency recordings were made at eight frequencies covering the 10 kHz to 32 MHz band, of three magnetic field components; 

from which noise amplitude probability distributions (APD's) are being generated. 

On the surface, only the components of magnetic field are required, but over a 

more restricted frequency range, because of the lower frequencies required for 

uplink system. The surface wideband recordings for generating spectra cover 

100 Hz to 10 kHz, while the narrowband spot frequency recordings' for generating 

APD's cover four frequencies in the 10 kHz to 150 kHz band. 

The preliminary results now available from these NBS noise measurements 

indicate that high levels of discrete frequency noise at harmonics of 60 Hz 

and 360 Hz ?redoninate over broadband spectrum levels below about 10 kHz, both in 
orderly manner. Therefore, it was concluded that no new noise measurements 

over and above that already planned by NBS were required at this time. 

In the under 5 kHz frequency band presently of interest to uplink data 

systems noise, power spectra and dubs of selected NBS tape recordings of the 

surface noise will be made available to system designers. Surface data up to 

10 kHz will also be available if needed. The uplink system designers will 

need data on the levels of both discrete frequency and broadband noise components: broadband spectrum levels (and amplitude statistics if possible) for 

optimizing the coding, modulation, and receiver processing for narrowband data 

uplink; and discrete component levels for estimating likely levels of out-of-band 

interference, and ways to combat them by choice of operating frequencies and/or 

receiver signal processing techniques. 

To better estimate these noise levels, particularly the broadband noise 

levels between discrete harmonic components, it was recommended that NBS provide expanded frequency scale spectra, covering only the 0-5 kHz band per 

spectrum plot, as opposed to the more compressed plots presently being prepared. Spectra for vertical and horizontal magnetic field components on the 

surface under both operational and Ifquieter" emergency conditions will be required. Note: these 0-5 kHz expanded spectra will be required not only for 
reports. The specific results obtained with these systems should be reviewed 

as an aid to future designs. 

As mentioned earlier, the data requirements and subsequent systems design 

have not yet been formulated for the uplink data system that will soon be developed for installation in an operating mine. This system design should benefit from 

the additional noise data and field experience now available. 

5. The Receiver - Sensor, Demodulation/Decoding, Special Processing 

As for the source, firm overall conclusions could not be redched, but 

several suggestions were made. An electrostatically shielded and balanced air 

core loop was reconmended as a sensor. Notch filters were suggested to reduce 

interference from strong harmonics adjacent to the channel frequency. 

The present WGL location receiver utilizes several stages of bandpass 

filtering to obtain a resultant bandwidth of 6 Hz. Notches as described above 

apparantly have not yet been required at the mine sites visited to date. The 

present WGL uplink data system utilizes a phase-locked-loop FSK detector prior 

to decoding. Neither system was discussed. 
La: tly , MIT Lincoln Laboratory (LL) has done extensive signal design 

and non-linear receiver processing work almed at optimizing ELF secure narrowband data communications (for the Navy Sanguine program) in the face of highly 

impulsive ELF atmospheric noise, and occasional discrete power line components. 

Reductions in required signal power of 10-20 dB have been reported, depending 

on the level of mzn-made discrete frequency interference, which apparently makes 

the techniques less effective. LL has been cooperative in the past by making 

its noise data and *instrumentation information available to the Bureau and its 

contractors ; and by recently offering suggestions regarding computer simulation 

of receives design configurations for testing performance in the presence of 

environmental noise. This work should be reviewed to see if it can be applied 

to the mine problem,in a practical and economic manner, particularly in those 

cases where transmitter power is at a premium and the noise environment severe. 

This work has recently been reported in the open literature, and more extensively in LL Technical Reports which are available from LL. 

B. Downlink Voice System 

1. Overview 

a. Experience to Date 

The objective of a downlink emergency voice system is to provide coverage of as large an area as possible to mobile miners during an emergency, in 

nines with nominal overburden characteristics of conductivity o = 10-2 mho/m 

and depths to 1,000 feet; aad to do this with as few antennas on the surface 

as possibie wichin practical limits. %his being the case, the overburden trans-. 

mission loss, the "quiet-mine1' noise data available to date, and the greater 

space and power available on the surface, have favored direct transmission of 

500-3,000 Hz baseband voice signals by means of grounded long wire antennas on 

the surface. Under relatively "quiet" emergency conditions, the present sys tem 

designed by WGL with a transmitter capacity of 200 watts, has successfully transmitted intelligible voice messages to miners carrying simple manpack receivers 

to depths of about 1,000 feet. Mine overburdens of low conductivity will of 

course extend this usable range, while deeper or more conductive overburdens 

will quickly deteriorate per.formance or require significantly more power. . The success experienced under emergency conditions led to speculation 

that such a downlink voice system could have some beneficial operational applications as well, if the surface transmitter power and manpack receiver processing 

demands did not become excessive. However sys tem performance was discovered 

to be even more dramatically affected by the in-mine operational noise environment than by the depth and conductivity. Namely, the noise levels severely deteriorated message intelligibility and usable range, demanding greatly increased power to maintain performance. This behavior is predicted by system analyses 

by boih iUjL and WGL, using early NBS and WGL in-mine nulse data, and has been 

confirmed several times in operating mines by WGL. The deterioration occurs 

mainly because of the high levels of 60 Hz and 360 Hz harmonics produced by the 

mine macilinery 2nd DC power conversion equipncnt ,and less often by the broa2band 

impulsive noise near arcing trolleys, levels that can vary over a dynamic range 

in excess of 60 dB depending on location and machinery ogerating cycles. By 

applying simple corrective measures such as varying the orientation of the inanpack receiver antenna for minimum noise pickup, which can be an operational incon- 
venience, and by severely attenuating the large 360 Hz harmonic component by 

filtering in the manpack receiver, WGL was able to obtain some improvement 

in performance; but nGt enough to make it a dependable and practical system 

under mine operational conditions. 

b . Future Developments 

The above experience under operational conditions, when combined with the 

problems associated with gaining surface access rights over advancing coal mine 

sections (for the installation of long wire antennas, perhaps several thousand 

feet in length, or smaller loops, which have to be moved more often) make it 

very unlikely that the permanent surface installations required for operational 

applications will be a practical possibility in the near future. Thus in the 

near term, downlink voice will remain an emergency condition mobile system; 

thereby keeping the communication problem closer to the one originally treated 

by WGL, but with some added features. 

The emphasis for future efforts on this system should probably be in the 

development of a reliable, compact, dual purpose receiver to be carried by a 

miner, preferably integrated into his helmet and operated from the cap lamp 

battery, as described by H. Parkinson in his paper. It will function as a 

downlink baseband voice receiver under emergency conditions, and as a call alert 

page receiver under operational conditions for key mining personnel, as mentioned in Section 11. Though the present surface transmitter is apparently adequate 

to handle several emergency conditions, it too will probably need to be redesigned and optimized: for truly mobile utilization in the sense of being easily 

transportable by backpack or helicopter to the desired spots above the mine; for 

compatibility with the new dual purpose miner carried receivers to be Geveleped; 

and for the mine emergency noise conditions likely to be prevailing. Since the 

downlink voice transmitter will have to be transported to and installed at selected locations above the mine after an emergency has occurred, the mine or 

section of interest will most likely be in a nearly power-down noise condition 

with only essential ,lower-powered ,electrical equipment such as pumps, fans , etc. 

in operation. 

c, Deep Mines 

The above applies to the nominal coal mine conditions specified. The 

deep hardrock mine situation is a vastly more difficult one as described in 

the uplink section, requiring frequencies down to 500 Hz and possibly 100 Hz 

and below even for narrowband data applications. Therefore, unless practical 

and econo~ical tecl~niques for dramatically compressing the bandwidth required 

for intelligible voice transmission become available, downlink voice to deep 

mines on the order of 10,000 feet will not be practically feasible under any 

noise conditions. 
2. The Channel-Transmission Loss 

a. Long Wire Antennas 

Downlink cominunications to date have prinarily utilized grounded, horizontal long wire source antennas on the surface, which reportedly give good 

coverage in the mine to a strip of width about equal to the depth of the mine 

under the wire, and sometimes wider, depending on the depth and available transmitter power. Antenna lengths have typically ranged from a low of about 1,500 

feet up to greater than a mile, WGL field experience indicating that a length 

greater than about three times the mine depth being adequate to assume infinitely long wire behavior for the transmission loss. In the mine, the wire's magnetic field is primarily horizontal, with a gradually increasing vertical component as one moves away from the wire in a perpendicular direction; as opposed 

to the field of a surface loop which has primarily a vertical component. In 

those cases where the position of the miner or communication station is relatively well known and fixed, a large one turn loop, like that for the uplink, 

placed over the miner's position can offer a performance advantage as well as 

one of convenience, depending on the orientation of the miner's receive loop 

and the direction of the maximum noise component in the mine. 

Grounding of the long-wire antenna has been accomplished by means of 

four or more ground rods at each end of the wire, with special care being taken 

to ensure good connections by the use of mud and copious amo~mts of rock salt. 

In this manner, total resistance values between about 50 and 100 ohms can be 

achieved for the long wire antenna; however, maintaining these values over long 

periods of time can sometimes be a problem. 

To establish values of average overburden conductivity for estimating 

system operating ranges at different mine sites, the well established dipoledipole measurement technique has been used extensively, and found to give results that agree reasonably well with system test results in most cases over 

coal mines. 

The theoretical results of Wait and ITS, regarding the magnetic fields 

expected underground from infinitely long, insulated wire sources, placed on 

the surface of homogeneous and layered conducting overburdens, are well established and have been found to be generally in good agrgement with experimental data obtained by WGL and CSM at several mine sites. Similar results 

have been obtained for surface loops. However WGL field experiences have also 

revealed a somewhat greater tendency for occasional experimental deviations, 

from predicted field strength values for the long wire antennas. A possible 

cause cited for this behavior was the presence of long conductors such as 

power cables, trolley wires, or rails in the mine, or large inhomogenieties 

in the overburden. This bears some further investigation. 

A constant current assumption is used throughaut these derivations. 

This has been shown to be valid for the frequencies of interest provided the 

conductors are insulated, which they are in practical applications of interest. 

The Navy experience in particular, with the huge Sanguine transmit antenna, 

provides good testimony to the validity of this constant current assumption. 
ITS and Wait have also derived expressions and curves for the underground . 

magnetic znd electric fields produced by finite, grounded, insulated wire antennas on the surface of a homogeneous, unlayered half space, and for the converse 

situation of the surface fields from buried finite grounded wires. These cases 

are more closely related to actual field installations. Both analyses reveal 

that a finite grounded wire can be treated as an infinitesimal dipcle when the 

observaticn distance, or depth, is more than about twice the cable lzngth; and 

treated as an infinitely long wire when the observation distance is less than 

about one quarter of the cable length, this latter behavior having been experimentally observed at several mines by WGL. In between these depths neither 

approximation is good, the exact curves or analytical formulation being required. 

Examinati~n of these curves also indicates only slight departure from infinitely 

long wire field values for distances up to half the length of the wire, and a 

reduction from infinite wire field values of only about 3 to 6 dB up to distances, or depths, equal to the length of the wire. The degree to which this 

behavior changes as the observation point moves toward and beyond the end of 

the wire was not discussed, but should be obtainable from the analysis. These 

results will be quite useful for determining minimum practical lengths for surface and underground antenna installations, and as a means for understanding 

observed experimental behavior. 

It was concluded that no new theoretical derivations were needed for 

loops or infinitely long wire sources on the surface for downlink transmission 

loss, but as in the uplink case, appropriate practical curves, tables, nomographs ,etc. be prepared based on the above results for homogeneous and layered 

overburdens. Similar curves, nomographs, etc. are needed of the magnetic and 

electric fields for the finite wire cases treated by ITS. !s in the uplink 

case, curves should be included that depict the increase in signal loss with 

horizontal in-mine movement away from the long wire, finite wire, and loop 

positions on the surface, in order to determine in-mine coverage areas. 

The effects of a layered overburden on the fields of finite grounded 

wires have not been treated yet. If it is concluded that layering is likely 

to influence the down]-ink field behavior in a significant manner, this case 

should also be treated by analysis and corresponding practical application 

curves produced. A summary assessment of the importance of layering to the 

fields produced by the other sources would also be desirable. Lastly, new 

and better ways of quickly making good, and long lasting, ground terminations 

in different ground covers should receive some attention. 

b . Parasitic Structures 

Long wire and loop antennas deployed on the surface. are not as likely 

as in-mine installations to encounter parasitic structures in their immediate 

vicinity, unless they have to be deployed in and across the streets of a town, 

or perhaps directly over a gas pipe or under power lines i.n rural areas. In 

, the first case the complexity of the parasitic structure configuration will 

probably defy analytical treatment, and what is perhaps more needed is a 

practical strategy for choice of antenna type and its deployment, based on 

present knowledge. In the second instance, Wait and ITS have examined cases 

of long wire sources parallel to buried conducting non-insulated cylinders. 
These results should be examined for their potential application to the gas 

pipe structure. However, since the effects of such structures generally 

decrease with increasing distance and orientation angle, perhaps a practical 

solution to this potential problem is again a deployment strategy for minimum 

effect, when the presence of this conductor is known and flexibility in 

antenna deployment is available. 

In the section and haulage ways at the receiving end of the downlink, 

metal structures in the vicinity of the man-carried receiving antennas may 

play a more important role in altering or providing a shielding effect to 

underground fields, and, may account for some of the lower than predicted 

levels experienced by WGL in a few instances. Prime suspects for these infrequently reported anomalies could be closed loops made by two or more vehicle 

trolley poles across the trolley-track transmission line, say in the vicinity 

of the section loading point, or steel mesh used for roof support in the 

entries of some mines. The effects of these structures should be estimated 

using approximate methods, to see if they, as opposed to large unknown conducting anomalies in the overburden, could account for the significantly reduced 

horizontal field strength levels observed. 

3. The Channel-Noise 

As concluded during the workshop and discussed in the downlirik overview 

section, it can be assumed for the purpose of system design and optimization 

that the mine or mine sections will be in a non-operational, power-down, condition during the operation of the downlink emergency voice system. All major 

mining and haulage equipment will be turned off, only minor equipment such as 

pumps, fans, etc., my be left on. 

The in-mine wideband noise recordings made by NBS should provide a more 

than adequate data base from which to optimize the design of the downlink 

baseband voice sys tem. Expanded frequency-s cale power spectra covering the 

0-5 kHz band, and depicting discrete frequency and broadband noise levels of 

both horizontal and vertical components of the magnetic field intensity will 

be needed. Dubs of selected noise tape recordings are also desired for 

testing receiver processing techniques and overall sys tem performance in the 

laboratory. Of particular interest will be data during quiet times and locations on the sections and haulageways, that characterize the emergency power 

down conditions. Consultation with Bureau of Mines and NBS staff will no 

doubt be helpful if not necessary in the selection of measurement conditions 

and data that typify this condition. 

4. The Source-Messaee. Codine. Modulation. O~eratin~ Freauencv 

The source topic was given only brief treatment by the group. It was 

noted that performance calculations by ADL using early NBS and WGL mine noise 

data indicate that intelligible downlink baseband voice reception is possible 

to 1,000 feet in mho/m overburdens, with under 50 watts of average power 

under low noise mine conditions. This kind of performance is supported by WGL 

experience in the field. Indeed, even as little as 5 watts may be required 

under some highly favorable non-operational conditions. (These reasonable 

average power requirements can climb to prohibitive levels above 10 kilowatts 

under operational conditions in DC mines .) 
The emphasis fur the downlizk voice system has remai~ed on the direct 

. transmbsion of baseband voice signals through-the-earth, particularly under 

the relatively favorable cnsrgency power-down noise condition. Under this 

condition, the high-level harmonics of 60 Hz, and particularly those of 

360 Hz, will be greatly reduced. Updated performance and overall systems 

analysis calculations based on the more comprehensive mine noise data recently 

taken by NBS will help to verify (or deny) the desirability of this frequency 

band of operation, and better establish the required power levels. These 

noise data should also help identify transmitter signal conditioning techniques 

and receiver signal and/or noise processing techniques that can be used to 

reduce the power, size, and weight required for the mobile, emergency surface 

transmitter . 

The use of pre-emphasized and/or clipped speech upon transmission were 

suggested for consideration as ways to reduce the peak power requirements of 

the transmitter, while sacrificing only little intelligibility for the same 

average speech power transmitted. Means of significantly reducing the bandwidth needed (by more than an order of magnitude) to transmit voice intelligibly have been claimed in the literature. Since such reductions would correspondingly reduce transmitter power, these reported methods should also be 

investigated. 

An alternative method for improving mine communications was also recommended by the group as perhaps a long-term goal for the mining equipment 

suppliers. Namely, the effectjve suppression of electrical noise at its source 

in the equipmerlt whenever practically possible, by means of improved designs 

and/or addition of special noise suppression equipment. 

5. The Receiver-Sensor, ~emodulation~~ecoding, Special Processing 

a. Downlink 

As for the source, only brief consideration was given to this topic, A 

helmet mounted loop antenna design is desired, together with a similarly 

mounted compact dual-purpose receiver, as mentioned in the sys tern description 

section. The call-alert function of the receiver will be discussed later. 

Use of notch .filtering to reduce the interfering effects of high-level 

harmonics of 60 Hz and 360 Hz, thereby reducing required transmitter power, was 

the principal suggestion. Such filters have been successfully applied in 

France, and reference material on these applications will be forwarded to the 

Bureau of Mines by representatives of the University of Lille. Dramatic 

improvetcents in voice recepticn in the face of harmonic interference have also 

been demonstrated by ADL in the laboratory, with a breadboard design of a 

simple electronic commutator-type filter that is particularly suited to 

rejecting harmonic signals. I'he French and ADL reported results should be 

reviewed, together with other reported notch filter work. They should be 

reviewed for their effectiveness against the mine emergency condition harmonic - 

interference; and for their practical application to a compact helmet mounted 

receiver, should the measured harmonic levels warrant the use of notch filterltlg under eruergency power-down conditions. In regard to this latter point, 

it may be necessary to evaluate the effect of purr or complex "tones" of noise, 
such as those created by harmonics of 60 Hz and 360 Hz, on the intelligibility 

of received speech. The effect of direct audio noise in the mine environment 

(which wili probably be low under emergency conditions) should also receive 

brief consideration along with that of the overall speech sound level to be 

delivered to the miner. 

Performance calculations, similar to those for the downlink discussed 

above have also been performed by ADL for a baseband voice uplink using a 

multiturn 100-foot periphery loop transmit antenna instead of a long wire. The 

results indicate that, except under the most favorable conditions of depth 

(300 feet) and the quietest of surface noise conditions, the levels of transmitter power, voltage, and current required are well in excess of those 

demanded by intrinsic safety, long operating life, and practical size and 

weight for an in-mine emergency unit. These uplink transmitter requirements 

for voice should be reconfirmed, along with those for the downlink voice 

system, in light of the more comprehensive NBS mine noise data now available 

and the larger in-mine loop antennas being considered. However, it appears 

that an uplink voice system that can operate from available emergency power 

will continue to remain impractical until an economic and practical way is 

found to significantly reduce the bandwidth required to transmit intelligible 

speech. 

C. Sidelink Call Alert Coded Page Svstem 

1. Overview 

The call alert system is a recent by-product of the success experienced with the experimental electromagnetic CW transmitter developed by WGL for 

locating miners trapped beneath overburdens of mho/m conductivity to 

depths of 1,000 feet. As mentioned previously, the location transmitter is 

intrinsically safe, operates from a miner's 4-volt cap lamp battery into a oneturn loop placed in an entry or wrapped around a coal pillar, and generates a 

periodically interrupted CW signal in the 1-3 Hz band. This signal is detectable on the surface above the miner and is suitable for locating the miner's 

horizontal position. The development of this location transmitter has now 

progressed to where an improved preproduction prototype unit. is being manufactured by Collins Radio Co. in limited quantities for testing in some operating 

mines. 

Since the horizontal ranges desired for the call alert system 

described in section I1 are commensurate with the vertical ranges obtained for 

miner location, the Bureau is presently giving high priority to the development 

of a call alert page system centered around an adapted version of the WGL location transmitter. As presently conceived, this paging transmitter will be 

activated by means of a carrier signal sent over the mine phone line from the 

surface to the desired section. The call alert transmitter will be connected 

to a loop wrapped around a coal pillar, perhaps at the section loading point, 

and will transmit a single frequency tone (perhaps simply coded) to a compact, 

dual-purpose, helmet-mounted receiver worn by the individual being paged to the 

section's mine phone. Under low noise emergency conditions, this receiver will 
be operable in a baseband voice mode for downlink message reception. Under 

high noise operational conditions, it will operate in a narrowband c a l l a l e r t 

mode, receiving a prearranged CW paging signal spaced bekeen the strong 60 Hz 

harmonics usually present under these conditions. 

A first generation experimental call alert system has been built 

from existing hardware by WGL, and installed in the Bureau's experimental mine 

to demonstrate concept feasibility in a non-operational environment. Though 

the operating frequencies of the present experimental unit are in the 1-3 kHz 

band, to take advantage of the frequencies available with the present location 

transmitter, an overall system analysis for an operational noise environment 

may reveal a more effective operating frequency. 

Overall sys tern requirements are presently being formulated by the 

Bureau. These will form the basis for subsequent systems analysis and optimization of designs that can be converted into practical, intrinsically-safe 

hardware for day-to-day use in operating mines. 

2. The Channel-Transmission Loss 

a. Loops 

In-mine call alert paging is a sidelink application utilizing two 

esserltially coplanar loops, while miner location is an uplink application 

utilizing two essentially coaxial loops. Examination of Wait's theore tical 

coupling curves for infinitesimal loops buried in homogeneous overburdens 

reveals that the operating range for a horizontal coplanar geometry is reduced 

by only about 207; over the range for a vertical coaxial geometry, for ranges 

in the vicinity of three skin depths. This operating range is reduced even 

less at greater distances. Vertical ranges in excess of 1,000 feet have been 

obtained with the location transmitter. At 2 kHz (the center of the 1-3 kHz 

operating band of the location transmitter), the three skin depth range is 

1,100 feet, which gives rise to a potential sidelink operating range of 

900 feet. This 900 foot range is in excess of the 400 to 800 foot range 

needed for c a l l a l e r t coverage of the typical 600 by 600 foot section mentioned in section 11. The above range conclusions are, of course, based on 

equal noise conditions for each case. Since the noise environment will likely 

be more severe for the in-mine, operational, call alert application, its 

effect on operating range and transmitter/receiver design has to be determined. 

At the under 5 kHz frequencies of present interest for the call 

alert system, the theoretical work of Wait/ITS has shown that the effects of 

layering (such as that found above and below coal seams) and air-filled 

cavities (such as tunnels in coal seams) should not be significant for loops, and 

therefore can largely be ignored for comunications applications. Similarly, 

the infinitesimal loop theoretical results should be adequate for making pertormance predictions, particularly at the desircd range limits, for mine sections free of parasitic influences. At ranges close to the loop installation, 

the infinitesimal loop results will tend to overestimate signal strengths 

somewhat. This discrepancy will become important primarily-when treating 

potential coupling to nearby parasitic structures, as discussed below. Therefore, curves, tables, nomographs should be prepared for the vertical magnetic 
field component in the plane of the transmit loop, based on the available theoretical results for coplanar infinitesimal loops. These can be used temporarily for making preliminary performance predictions until more information 

is forthcoming on the effects of the conductors prevalent in mine sections. 

b. Parasitic Structures 

Mine sections typically contain many conductors, such as trolley 

wires and rails, fixed and trailing power cables, roof bolts, and sometimes 

steel roof-supported mesh, that can affect the strength and orientation of 

magnetic fields. Therefore, a series of limited signal strength measurements 

should be conducted in operational mine sections, and in other mine locations 

that are relatively conductor-free. These simple experiments are needed; to 

verify whether the homogeneous-overburden coplanar loop results can be applied 

with confidence to operational sections, and to formulate practical design 

guides for operational sections. Preliminary results from field measurements 

taken by ADL in the Bureau's experimental mine indicate that significant 

departures from the theoretical results can in fact occur. 

In parallel with the above field measurements, corresponding theoretical analyses are needed to predict the degree to which the direction and 

strength of the magnetic fields produced in the tunnels, by finite loops 

wrapped around coal pillars, will be affected by the above mentioned conductors in working sections of the mines. Of particular interest will be the 

effects caused by trailing and fixed power cables, roof bolts, and trolley 

wire/rail structures, which appear manageable analytically. The potential 

problems caused by heavy metal mesh occasionally used for roof support were 

acknowledged, but assigned a lower priority for analytical treatment, because 

of the infrequent use of this mesh and perceived analytical difficulties. 

ITS has done some investigation of the currents induced in a 

thin, infinitely long, cylindrical conductor by a nearby infinitesimal loop 

transmitter. This work reportedly is easily extendable; to include the 

effects of the magnetic field produced by this induced current, and to include 

the effects produced by a finite loop source. The utility of this approach 

should be investigated, and pursued if found applicable. 

ITS has also examined the influence of buried splferical and prolate 

spheroidal conducting objects on the fields produced by infinitesimal loop 

sources. Though originally done in connection with the miner location problem, the results can be applied to the call alert application, to estimate 

the likely field effects produced by machinery and shuttle cars. For the 

frequencies, sizes, geometries, and distances of interest, these objects will 

not significantly alter the magnitude of the fields, but mainly their direction somewhat in the immediate vicinity of the objects. No further investigations of this area were recommended. 

c. Roof Bolts 

If a finite wire terminated by roof bolts is shown to be a favorable 

transmit antenna for the roof bolt paging system, a suggestion was made that 

it ;llso bc considered for use in the call alert system. 
suited to a compact, helmet-mounted receiver need to be determined. 

As mentioned in section 11, this system could conceivably share 

equipment with an uplink data station also located on the mine section. 

D. Sidelink Roof Bolt Voice Page 

1. Overview 

The roof bolt voice paging system is a system conceived and 

recently developed by the Bureau 'for transmitting voice messages to key 

individuals carrying small pocket pagers on working sections under operational conditions. A prototype, using readily available commercial equipnienL, is presently installed in thc Bureau's experimental mine tn demonstrate 

its feasibility in a non-operational environment. The system concept developed as a result of some successful in-mine experiments performed by the 

Pittsburgh Mining and Safety Research Center; whereby a 20-watt trolley 

phone 88 kHz FX transmitter was connected to two roof bolts approximately 

50 to 100 feet apart in an operating mine, and its voice transmission then 

received at distnnces vp to about 600 feet away with a small pocket pager 

uti lizlng a territe loop stick antenna. 
Limited field experience to date indicates that operating ranges 

commensurate with the 400 to 600 feet required to provide section coverage 

may be achievable, under operational conditions, with an operating frequency 

in the vicinity of 100 kHz. At this point in time a more quantitative 

understanding of the transmission loss and what affects it is needed; in 

order to deternine the most favorable operating frequency, to develop practical guidelines for tailoring installations and estimating performance in 

different mines, and to eventually develop an improved system. 

2. The Channel-Transmission Loss 

a, Finite Wire Antennas Terminated by Roof Bolts 

The finite wire antenna in this case is an insulated wire that 

runs along the roof of a tunnel and is terminated at each end by attachment to a roof bolt, Field experience to date has found the total termination impedance for such roof bolt pairs, separated by 50 to 200 feet, to 

fall in the range of 120 to 50 ohms resistive. Theoretical curves and 

supporting experimental data are needed, to adequately describe the behavior of the magnetic fields produced in the tunnels throughout a section 

in which such a finite wire transmitter is located. 

The theoretical work of Wait and ITS on finite wire antennas 

buried in homogeneous overburdens, described in sections IIA2c and IIB2a,' 

should be particularly useful in this regard and for estimating system 

coverage areas in mine sections. Though the present results are for the 

electric and magnetic fields produced on the surface from such buried 

antennas, ITS maintains that the desired field strengths in the coal seam 

tunnels can be easily obtained from its present buried-finite-wire analysis. 

This case of interest corresponds to receiver locations below, but in the 

immediate vicinity of, the plane of the finite wire, 

Frequencies presently being investigated for this voice page application range from 10 kHz to 300 kHz, with present experimental systems 

operating around 100 kHz. Although the frequencies in the upper part of 

this band are higher than originally anticipated for buried finite wire 

applications, ITS believes that its present analysis should apply, 

Therefore, it was concluded that the ITS theoretical analysis of 

the fields from buried finite wires should be used to determine the desired 

magnetic fields in the cod seam, in the 1-300 kHz band; and to prepare 

appropriate practical curves, tables, etc. for we by system designers. In 

addition, since the overburden is usually layered above and below coal seams, 

and since layers of varying conductivity can potentially influence the fields 

from finite wire antennas more than those from finite loops, a theoretical 

analysis to determine the effects of a simple, representative, layered model 

should be performed. 

The non-conducting volumes created by the grid of tunnels in the 

coal seam were considered too difficu1.t for exact analytical treatment at 

this stage; and in fact may not create significant effects on the magnetic 

fields in the tunnels, because the tunnels are relatively narrow and the 

currents can still flow without much alteration through the wider coal 

pi l lal-s. 
3. The Channel-Noise 

Since the roof bolt system is a voice paging system for use under 

operational conditions, its operating frequency will most likely be above 

about 10 kHz, where the operational noise levels decrease to. more tolerable 

levels. Present consideration is being focussed on the 10 kHz to 300 kHz 

band. The present experimental system operates at 88 and 100 kHz, but 

since these frequencies are already utilized by mine trolley wire carrier 

systems, alternative non-interfering frequencies are also desirable. 

As for the other systems, the recently obtained NBS in-mine noise 

data should serve as a more than adequate data base for systems analysis 

and optimization in the 10 kHz to 300 kHz band. From the wideband tape 

'recordings, power spectra for horizontal and vertical magnetic field components will be available, depicting discrete and broadband noise levels 

over the frequency range from 0-100 kHz and 0-300 kHz. In addition, noise 

amplitude probability distributions and rms levels will be available from 

the narrowband (2 k~z) spot frequency noise recordings, at eight frequencies 

over the 10 kHz to 32 mHz band, four of which fall below 300 kHz. Appropriate dubs of selected tape recordings for both types of noise measurement 
should also be available. Detailed reports docuinenting these measurements 

and data will soon be published by NBS. 

The Source and Receiver 

The present experimental system is designed around a commercially 

available 20 watt, mine trolley wire phone transmitter that employs conventional FM modulation and an industrial pocket pager F'M receiver that operate 

at a carrier frequency of either 88 or 100 kHz. Lack of time prevented 

discussion of the overall system by the group, but it was concluded that the 

degree to which this system can or should be optimized or otherwise improved 

with regard to performance, practicality, intrinsic safety, etc. will eventually be determined by the system requirements and a subsequent system 

analysis. 
SUMMARY REPORT - OF OPERATIONAL COMMUNICATIONS 

WORKING GROUP 

Martyn F. Roetter, Group Chairman 

Arthur D. Little, Inc. 
The work of the operational communication systems' group dealt with a 

range of communication needs and functions within mines, primarily along 

haulageways, to and within sections up to the working face itself, and in 

mine shafts. A mixture of communication techniques and hardware is needed 

to satisfy this variety of communication needs within the differing environments encountered in U.S. mines. Substantial progress, both experimental 

and theoretical, has been made in recent years towards developing alternative 

communication systems suitable for use in mines which are based on "guided" 

waves, including wire-less (waveguide-like propagation in mine tunnels) and 

wire-based systems (leaky coaxial cables or wires) . Major priorities identif ied for further work needed to confirm (or deny) the applicability, and 

/ refine the operational specifications of promising communication systems 

for mine use include: 

Short-Term Projects 

o Cost/performance analyses of promising leaky coaxial cable and UHF 

radio communication systems which require further data from: 

- Experimental investigation under U.S. mine conditions of the 

performance of potentially applicable leaky coaxial cable coinmunication systems developed in Europe (France, Belgium, and 

the U.K). 

- Cost estimates on these coaxial cable communication systems. 

- Measurements of UHF radio propagation in low-coal mines. 

- Investigation of the influence of obstacles (e.g. shuttle cars 

and section machinery) in the entries on UHF radio propagation in 

mines. 

o Investigation of the problems of transmitter and receiver coupling 

and termination matching associated with the two-way propagation of Icw 

frequency radio waves in hoist shafts. 

203 
Longer-Term . . -- Projects 

i.. of lesser urgency or where less information is currently available. 

o Investigation of techniques for coupling UHF radio to leaky coaxial 

cable communication systems. 

o Delineation of the role of, and needed interfaces for,operational 

communication capability related to emergency, paging and monitoring functions. 

11. OPERATIONAL COMMUNICATIONS FUNCTIONS 

The communication functions under discussion in this report are primarily: 

o Two-way communication along main haulageways up to 4-5 miles long, 

to vehicles and to maintenance personnel. 

o Two-way communication in sections up to working faces; all entries 

negr the w~rkine o flea ---- shn1llJ -&---- be ccverec!, ?I=+, pcssibly anly z lixitec! 2rcpzrtion of those at or near main haulageways. Communication with roving personnel at up to 3,000 feet away from main haulageways must be established. 

More than one kind of working face ~ust be dealt with, i.e. room and pillar 

(predominant in the U.S .) and longwall. 

o Two-way communication in mine hoist shafts (on the order of 10,000 . feet long). 

Two distinctive categories of comunication are involved, the first 

depending upon a base station, and the' other dealing with direct mobile-tomobile communication. 

Although not discussed in detail, it is recognized that the communication systems designed to fulfil these purposes may interface with other 

communication systems such as trolley phone,as well as play a role in 

assuring some emergency, paging (call alert),and one-way monitoring communication functions. 
111. ENVIRONMENTAL -. CONSTRAINTS 

The primary constraints recognized as affecting the communication systems 

under consideration are: 

o Daily utility of equipment 

o Intrinsic safety 

o Ruggedness and resistance to harsh mine environment 

o Available power 

o Weight and size limitations 

o Cost limitations 

The electromagnetic noise environment at the frequencies typically proposed or employed (a few MHz up to 1 GHz) appears not to be a significant factor 

in determining communication system performance. A possible exception to this 

rule or slightly less clear-cut situation may prevail in the case of low frequency (LF) radio propagation proposed for commurlfcation in mine shafts (at 

frequencies of a few tens of kilohertz), 

OPERATIONAL COMMUNICATIONS SYSTEMS 

Several alternative communication systems are in principle technically 

capable of satisfying the communication needs just described. A tentative 

conclusion in the light of our present state of knowledge is that communication along haulageways may be efficiently provided by one or more of the 

proposed leaky coaxial cable systems discussed later. However, leaky coaxial 

cable sys tems seem incapable of providing communications capability at nore 

than 10 to 20 meters lateral distance from the cable. Thus in order to provide 

wide area communications coverage within the network of tunnels in a coal 

mine section, it would be necessary to string cables along most of them. The 

cost and practical obstacles to stringing all this cable in a continually 

changing section geography favor the application of UHF radio for wide area 

communications within a section. There is both theoretical and experimental 

evidence to indicate that UHF radio is capable of providing this function. 
Coaxial cable, radio, and low frequency TEM radio wave transmissiorl 

in the shaft are all potential candidates for providing communication in 

a mine shaft. No single one of these communication techniques has yet been 

identified as being especially advantageous in this application. 

The communication techniques discussed fall under the general description of "guided" waves and comprise: 

o Wire-less (UHF frequencies) 

- Coaxial cable with periodic radiative structures 

- Coaxial cable with high surface transfer impedance (braid 

outer conductor) 

- Coaxial cable with repeaters 

- Wire pairs 

- Single wire (including LP radio propagzticn in nice 5cist 

shafts) 

It is also recognized that power-line carrier communication techniques 

are potentially attractive for some of the communication applications ulder 

consideration; it is worthwhile to investigate power line carrier systc~s 

further, however, no serious evaluation of them was made in this workshop. . Power line carrier systerrs are already used along the trolleyway in sow? 

mines. 

,' 

In the following,promising commu;lication systems are identified arid 

their current state of development described. Problems and. areas where 

additional data or further thevretical understanding are needed are listed, 

and priorities for future development work are suggested. 
A. UHF Radio (U.S.) 

1. State-of-the-Art 

Marked progress has recently been made in understanding the characteristics and capabilities of UIIF radio wave propagation along coal mine tunnels. 

Measurements taken in mines by Collins Radio indicate that effective communication can be provided throughout most of a typical U.S. coal mine section by 

UHF radio. A theoretical analysis carried out by Arthur D. Little, Inc. (DL) 

staff based upon the hypothesis of waveguide propagation is in agreement with 

the Collins measurements in several important respects. The theoretical model 

is believed to reflect the basic structure of UHF radio wave propagation in 

coal mine tunnels, although it is presently not intended to give accurate 

sl gnal l.oss estimates around corners when either the transmitter or receiver 

is near the corner (less than 50-100 feet). In those cases the model's loss 

assymtotes will over estimate the loss. 

During the workshop an apparent violation of the reciprocity theorem 

was discovered in an application of the ADL theory to extend Collins Radjo's 

data for a determination of the extent of coverage provided by UHF communication within a section. This apparent violation is believed to result from 

an application of the ADL model in a region where it is invalid, namely in 

the corner loss situation just mentioned. The reciprocity theorem must be 

respected, and a refinement of the model is needed to predict transmission 

loss around a corner when-elther the transmitter or receiver is nearby the . corner. Collins Radio has re-evaluated the sectlon coverage predicted for 

UHF radio which can be deduced from their data and extrapolated by the ADL 

theory, assuming the reciprocity theorem holds. The results of this computation are attached; they are very encouraging. 

Leaky coaxial cable communication systems operating between 2-20 MHz 

appear incapable of providing communication along cross-cuts in which they 

are not strung,and hence appear both costly and unlikely of implementation 

for com~unication in the grid of many tunnels which constitute a section. 

UHF radio is likely to he more effective in this situation of areal rather 

?llarl essentially linear or tubular communication coverage. In summary, 
both theoretical and experimental results obtained to date warrent further 

development of UHF radio techniques for providing practical communications 

in coal mine sections. 

2. Future Development Programs 

(a) Short-Term 

No measurements have yet been taken of UIIF radio wave propagation 

in low-coal mines, which constitute a significant fraction of U.S. coal 

mining activity. These measurements are needed to determine if the different geometry of low-coal as against high-coal mine tunnels permits practical 

communication of UHF. 

Additionally information is needed on the influence of obstacles 

in entries and tunnels on UHF radio wave propagation. In a coal mine "obstacles" such as section machinery and shuttle cars are inherently present. 

Some of these obstacles can block the major portion of an entry and may wipe 

out effective communication to various areas of the mine section as they 

move around. Multipath propagation effects may help in overcoming this 

problem; at any rate,data are urgently needed. 

Less urgently, it would be revealing to obtain UHF propagation data 

of higher frequencies (above 1 GHz) where critical tests of the ADL theory, 

including the selection of the optimum operating frequenoy , would be possible. 

In practical terms these measurements are not, as already mentioned, of the 

highest urgency, as the use of a frequency above 1 GHz for mobile UHZ radio 

is improbable since it would entail significantly more expensive (beca!ise 

non-standard equipment) . Standard UHF frequencies for mobile comnunication 

are in the 450 MHz band, and the 960 MHz band soon to be opened by the FCC, 

It may additionally be noted that the FCC may not in any case approve nonstandard UHF frequencies for underground mobile commw.j.catiola, even though 

in principle, use of non-standard frequencies is acceptable for ~nderground 

use as long as no leakage to the surface occurs, The basis fcz t3li.s attitude 

may be explained by the ease with 1~hic11 nobile, as sgainst fixed communications 

gear, niay be taken out of the mine for :>ersonal use. 
The Bureau of Mines should also delineate clearly the alternatives and 

practical considerations associated with the placing of the UHF transmitter 

(and possibly repeaters) to provide the best communication coverage within 

a section, taking account of its continually changing features. 

(b) Long-Term 

A future scenario may be envisaged in which a leaky coaxial cable communication system is in use along mine haulageways, whereas UHF radio provides 

communication up to working faces. In this situation the effective exploitation of all the advantages of these two communication techniques would be 

enhanced by the ability to couple them together. The techniques, costs, 

and performance of methods practicable to accomplish this coupling should 

be investigated. 

B. Leaky Coaxial Cable Communication Systems (Europe) 

Three major classes of coaxial cable communication systems designed for 

asc fn Arx hz-;2 be23 zep~rted ELS being in various stages of development in 

Europe. 

(a) INI~~/Delogne system (Belgium) employing regularly spaced radiating 

devices . 

Much experimental and theoretical investigation of this system has been 

performed including trials at the Bruceton, Pz. experimental mine of the USBM. 

The optimum operational frequency is believed to fall in the range of 2-20 MHz. 

Prototype installations are on order in Belgium, at a price of about $2500/km. 

Firm production sales prices are not yet available. The INIEX/Delogne scheme 

appears potentially suitable for application in U.S. mines, although several 

uncertainties regarding performance/cost trade-offs in typical U.S. mine environments s till have to be resolved, as discussed below. These uncertainties 

are connected in particular with the restraint in U.S. mines, in contrast to 

Europe, of having to install. the cable close to the rib with consequent increases in attenuation, over a more central location in the tunnel, and with 

the influences on performance of dirt and water on the cable and on the radiative devi C~S . 
(b) Coaxial cable with high surface transfer impedance -- specially 

designed "leaky" braid outer conductor (France). 

Theoretical investigations carried out at the University of Lille 

in France indicate that effective communication along several miles of mine 

haulageway may be achieved by use of a coaxial cable whose braid outer conductor is designed for "optimum" leakage of radiaticn. Experinental irives tigations of this scheme in a French mine are planned to be carried out in a 

few months' time. The optimum operational frequency is believed to be 

between 5-10 MHz. 

Similar uncertainties exist with regard to the effects of dirt, 

water, and proximity to the walls of the tunnel on the performance of the 

proposed French scheme in U.S. mine environmeats,as were mentioned in the 

context of the Belgian cable system. 

(c) Coaxial Cable with Repeaters (U.K.) 

It has been reported that coaxial cable comunication systems incorporating repeaters are being tested experimentally in the U.K. At this 

workshop little information on the cost and performance of this system was 

available. Additional uncertainties in the performance and cost evaluation 

of this system are introduced by questions associated with the reliability 

and maintainability of the repeaters that can realistically be expected in 

a mine environment. 

2. --- Future Develo~ment Pro_grams ,, --- ---- ----. 

(a) Short-Term 

Progress achieved in Europe in the de-~elopment of the coaxial cable 

communication systems mentioned above should be carefully and continually 

monitored and evaluated. In particular,cost estimates and further operating 

performance data should be obtained as soon as possible. 
Nevertheless, European results, while valuable and to date encouraging, cannot be directly applied to the different environment of U.S. mines. 

In particular it appears impossible to install communication cables in U.S. 

mines in the locations recommended by European researchers. Specifically 

cables will have to be installed close tc the ribs or walls of tunnels. 

Accordingly different attenuation rates, and correspondingly different 

optimum operating frequencies or trade-offs between the rate of "leakage" 

of power and total communication system length may prevail than in the 

European situation. Experimental investigations in U.S. mines with the 

proposed European coaxial cable systems are required before their applicability in this country can be definitively confirmed or denied, and if 

confirmed, operational specifications written (frequency, design of radiative 

structure or "leaky" outer conductor, and so forth). 

(b) Long-Term 

As was discussed in the section on UHF radio, techniques for coupling 

coaxial cable sys tems to UHF radio communication should be investigated. 

Simple Wire Systems (~urope) 

Wire Pairs 

The technical feasibility of communication via waves propagated along 

wire pairs is well established, and the coupling between and characteristics 

of the unbalanced and balanced modes of propagation are well understood. However the sensitivity and lack of resistance of simple wire pairs to the deleterious effects of the mine environment (dirt, water, rough handling) tend to 

rule them out as practical implementations of in-mine communication sys tems. 

- Single Wire -- 

A single wire communication system is impractical as a solution to a 

mines' operational co~nmunication needs along haulageways or in sections, although a similar type of communication system operating in the low frequency 

(LF) range holds promise for use in mine shafts. 
Dm Low Frequency Radio in a Hoist Shaft (U .S .) 

Theoretical investigations at ADL have analyzed the propagation of LF 

radio waves in deep (10,000 feet) hoist shafts in which the hoist cable is 

the only metal conductor present. Propagatf on losses of approximately 2dB 

over 10,000 feet at frequencies near 50 kHz have been ccimputed. This is 

a very encouraging result. 

Future Development Programs 

(a) Short-Term 

Two difficulties with respect to LF propagatton in hoist shafts 

have been identified. Firstly, the large penetration of the wave into the 

rock outer conductor may present a problem wit31 regard to coupling the transmitter or receiver to the transmission line with a minimum of insertion loss. 

The amount of the inserti~c lcss that czc be tclcrstc2 h3s not yet bcec specified; it may be quite large, in view of the remarkably low transmission losses 

calculated. The coupling problem merits attention to determine, for example, 

how closely to the theoretical distribution of the vertical component of current density,in the fundamental propagation mode, should the aetual driving 

current be distributed. 

Secondly, in order to minimize reflections , both ends of the hoist 

cable-shaft transmission sys tem must be terminated in approximately the characteristic impedance of the transinission line. Further work is needed to 

resolve the question of how, how well, and how expe~lsively matching terminations may be provided. 

LIST OF ATTENDEES 

Adams, John W. 

NBS, 272 55 NBS 

Boulder, Colo. 80302 

Allen, J. W. 

Westinghouse Georesearch 

8401 Baseline Rd. 

Boulder, Colo. 80302 

Anderson, Dean T. 

Collins Radio 

5200 C. Ave., N. E. 

Cedar Rapids, Iowa 52406 

Badgley, Peter C. 

Office of Naval. Research 

Code 410 

Arlington, Virginia 222 17 

Ball, Larry 

Westinghouse Georesearch Lab. 

8401 Baseline Rd. 

Bolzlder, Colo. 80302 

Bannister, Peter R. 

Naval Underwater Systems Center 

New London, Conn. 06320 

Bensema, W. D. 

Nat. Bureau of Standards 

Boulder, Colo. 80302 

Binger, Kenneth B. 

Collins Radio Co. 

M.S. 107 153 4 

Cedar Rapids, Iowa 52406 

Bollen, Robert 

Stanford Research Inst. 

333 Ravenswood Ave. 

Menlo, California 94025 

Burr, John F. 

Lee Eng. Consol. 

200 Hidden Valley Rd. 

McMurray, Pa. 15317 

Caffey, Thurlow 

Sandia Lab. 

Box 5800 

Albuquerque, New Mexico 871 15 

Degauque, Pierre 

Electronics Dept. 

University of LILLE - 59650 

Villeneuve dtAs cq, France 

Dobroski, Harry 

U. S. Bureau of Mines 

4800 Forbes Ave. 

Pittsburgh, Pa. 15213 

Dolphin, Lambert 

Stanford Research Inst. 

333 Ravenscvood Ave. 

Menlo Park, California 94025 

Donaldson, Paul 

Colo. School of Mines 

40 Prospector Village 

Golden, Colo. 80401 

Dowling, Forrest L. 

Office of Naval Research 

536 S. Clark St. 

Chicago, Illinois 60605 

Emslie, A. G. 

Arthur D. ~ittle, Inc. 

15 Acorn Park 

Cambridge, Mass. 02140 

Farstad, Arnold J. 

Westinghouse Geor esearch Lab. 

8401 Baseline Rd. 

Boulder, Colo. 80302 

Fisher, Carl, Jr. 

Wes tinghouse Georesearch Lab. 

8401 Baseline Rd. 

Boulder, Colo. 80302 
Frischknecht, Frank C. Hooker, Vering E. 

U. S. Geological Survey U.S. Bureau of Mines 

Bldg. 25, Denver Federal Center Bldg. 20, Denver Federal Center 

Denver, Colo. 80215 Denver, Colo. 80215 

Gabillard, Robert Howard, Allen Q., Jr. 

Electronics Dept. U. S. Dept. of Commerce 

University of LILLE - 59650 Inst. of Telecommunication Sciences 

Villeneuve d' As cq, France Boulder, Colo. 80302 

Galbraith, Lee Humphreys, Larry D. 

Sandia Laboratory Sandia Laboratory 

Box 5800 Box 969 

Albuquerque, New Mexico 87 11 5 Livermore, California 94550 

Gcycr, Richard G. Jones, F. W. 

Consultant Univ. of Alberta 

3973 Baumberger Rd. Dept. of Physics 

Stow, Ohio 44224 Edmonton 7, Alberta 

Canada 

Gianzero, Stan 

Schlumberger Research Kanda, M. 

Ridgefield, Conn. U. S. Dept. of Commerce 

National Bureau af Standards 

Goddard, Arthur E. 27260 NBS 

Collins Radio Co, Boulder, Colo, 80302 

M.S. 108 185 

Cedar Rapids, Iowa 52406 Lagace, Robert L. 

Arthur D. Little Inc. 

Greenfield, Roy 15 Acorn Park 

Penn State University Cambridge, Mass. 02140 

University Park, Pa. . 

Lewis, Richsrd L. 

Halford, Donald University of Colorado 

National Bureau of Standards 6390 Jay Rd. 

NBS 272.60 .' Boulder, Colorado 80302: 

Boulder, Colo. 80302 

Liegeois, Robert M. 

Havener, Robert E. INIEX 

Mine Safety Applialrces Co. Rue Du Chcra 

Pittsburgh, Pa. 15213 B 4000, Liege 

Belgium 

Hill, David A. 

Office of Telecommunications Linfield, Robert F. 

Institute of Telecommunication Sciences Westinghouse Georesearch Lab. 

Boulder, Colo. 80302 8401 Baseline Rd. 

Boulder, Colo. 80302 
Moffatt, David L. 

ElectroScience Laboratory 

Dept. of Electrical Engineering 

Ohio State University 

Coiumbus, Ohio 432 12 

Morgan, Robert R. 

7805 W. Hampden Ave. 

Denver, Colo. 80215 

Murphy, John N. 

U. S. Bureau of Mines 

4800 Forbes Ave. 

Pittsburgh, Pa. 15213 

Murray, John C. 

Colo. School of Mines 

Rural Route Box 721A 

Golden, Colo. 80401 

Myers, Richard 

U. S. Bureau of Mines 

Bldg. 20, Denver Federa! Center 

Denver, Colo. 80215 

Nasiatka, Thomas M. 

U. S. Bureau of Mines 

9472 Shouse Dr. 

Vienna, Virginia 22180 

Olsen, Robert G. 

Westinghouse Georesearch Lab. 

8401 Baseline Rd. 

Boulder, Colo. 80302 

Parra, Jorge 0. 

Colo. School of Mines 

66 R-ospector Village 

Golden, Colorado 80401 

Powell, James 

U. S. Bureau of Mines 

4800 Forbes Ave. 

Pittsburgh, Pa. 15213 

Rankin, David 

University of Alberta 

Dept. of Physics 

Edmonton 7, Alberta 

Canada 

Roetter, Martyn F. 

Arthur D. Little Inc. 

15 Acorn Park 

Cambridge, Mass. 02140 

Ruskey, Frank 

U.S. Bureau of Mines 

Bldg. 20, Denver Federal Center 

Denver, Colo. 80215 

Sacks, Ken 

U.S. Bureau of Mines 

4800 Forbes Ave. 

Pittsburgh, Pa. 15213 

Smith, Robert L. 

Develco 

530 Logue Ave. 

Mountain View, California 94040 

Starkey, Donald B. 

Sandia Laboratory 

P. -0. Box 969 

Livermore, California 94550 

Stout, Willi am N. 

Collins Radio Co. 

5225 Cave N. E. 

Cedar Rapids, Iowa 52406 

Tibbetts, Benton 

U.S. Bureau of Mines 

Bldg. 20, Denver Federal Center 

Denver, Colo, 80215 

Tockey, Robert 

Sandia Laboratory 

Box 969 

Livermor e, California 94550 
Utter, Stephen 

U.S. Bureau of MI lnes 

Bldg. 20, Denver Federal Center 

Denver, Colo. 80215 

Wait, James R. 

U.S. Dept. of Commerce 

NOAA 

Inst. of Telecommunication Sciences 

Boulder, Colo. 80302 

Aldridge, M. Dayne 

West Virginia University 

Dept. of Electrical Eng. 

Morgantown, West Virginia