Westinghouse Research Laboratories @ 

EM LOCATION SYSTEM PROTOTYPE 

AND 

COMMUNICATION STATION MODIFICATION 

FINAL REPORT 

July l973 

Submitted to: 

Bureau of Mines 

Department of Interior 

under Contract H0232049 

Georesearch Laboratory 

Boulder. Colorado 
LOCATION SYSTEM PROTOTYPE 

AND 

COMMUNICATION STATION MODIFICATION 

Part I LOCATION SYSTEM PROTOTYPE DEVELOPMENT 

AND TEST 

Part I1 COMMUNICATION STATION MODIFICATION AND 

DEMONSTRATION AT BRUCETON EXPERIMENTAL 

MINE 

Part I11 ARMCO #9 LOCATION SYSTEM DEMONSTRATION 

Prepared by 

Westinghouse Georesearch Laboratory 

8401 Baseline Road 

Boulder, Colorado 80303 

DISCLAIMER NOTICE 

The views and conclusions contained in this document 

are those of the authors and should not be interpreted 

as necessarily representing the official policies of the 

Interior Department's Bureau of Mines or the U. S. 

Government . 

For the 

U. S. BUREAU OF MINES 

DEPARTMENT OF THE INTERIOR 

USBM Contract HO 23 2049 
PART I 

LOCATION SYSTEM PROTOTYPE DEVELOPMENT 

AND TEST 

A. J. Farstad, C. Fisher, and R. F. Linfield 

CONTENTS 

1. 0 INTRODUCTION 

2. 0 PERFORMANCE PREDICTIONS 

2. 1 EM Location Concepts 

2, 2 Field Strength Predictions and Measurements 

3.0 INSTRUMENTATION DEVELOPMENT 

3. 1 Transmitter 

3. 2 Receiver 

3. 2. 1 @Manpack Locator Model C842A 

3. 2. 2 @Multichannel Receiver Model C849A 

4. 0 PERFORMANCE TESTING 

4. 1 Laboratory Tests 

4. 2 Field Tests 

4. 2. 1 Description of Field Site 

4, 2. 2 Surface Tests 

4. 2. 3 Helicopter Tests 

4. 2.4 Signal and Noise Recordings 

5. 0 DIFFERENTIAL LOCATION RECEIVER 

5. 1 System Concept 

5. 2 System Implementation 

6.0 CONCLUSIONS & RECOMMENDATIONS 

Page 

1 

2 

2 

6 

18 

18 

22 

22 

25 

36 

36 

38 

38 

43 

56 

58 

68 

68 

69 

74 

7. 0 REFERENCES 76 
SUBJECT INVENTIONS REPORT 

The following Subject Invention was disclosed on Contract H0232049. 

Westinghouse Disclosure No. 73-240 

"Differential Technique for Locating 

Electromagnetic Signal Sources. It 

Date of Disclosure: May 8, 1973 
FOREWORD 

This final report was prepared by Westinghouse Georesearch Laboratory, 

8401 Baseline Road, Boulder, Colorado 80303, under USBM Contract 

No. H0232049. The contract was initiated under the Coal Mine Health 

and Safety Research Program. It was administered under the 

technical direction of the Pittsburgh Mining and Safety Research Center 

with Mr. H. E. Parkinson acting as the technical project officer. 

Mr. G. Honold was the contract administrator for the Bureau of Mines. 

This report is a summary of the work recently completed as part of 

this contract during the period April 1973 to July 1973. This report 

was submitted by the authors on 20 July 1973. 

This technical report has been reviewed and approved. 
ABSTRACT 

A prototype EM Location System was fabricated and tested at the 

Geneva Mine near Dragerton, Utah. This system consisted of six manpack 

transmitters and six manpack receivers covering the frequency band from 

900 Hz to 2900 Hz as well as a six channel receiver designed for use in a 

helicopter. Tests showed that EM signals from lightweight (314 lb) transmitters could be detected both on the surface and from a helicopter through 

-2 

as much as 1650 ft of relatively high conductivity (2 X 10 mhoslm) overburden. Furthermore, the resulting field strength profiles could be used 

to determine source location to within 150 ft of the actual surveyed location 

in the mine. 

Modifications were made to a Monitoring, Locating and Communications 

System being tested at the Bruceton Mine, Pittsburgh, Pennsylvania. These 

modifications have improved the performance and quality of the paging and 

voice telephony mode of the system. Subsurface receiver modifications have 

improved reception of voice downlink signals. 
PREFACE 

This final report covers the work performed for the U. S. Bureau 

of Mines on Contract H0232049 between April and July 1973. 

The report is divided into three parts and covers in detail all 

work accomplished on Tasks I, 11, and 111 under the contract. 

Task I involves the development and test of an electromagnetic 

location system prototype. Based on design concepts developed previously 

on Contract H0 22007 3, six each single frequency manpack transmitters 

and receivers were fabricated, along with one airborne six-f requency 

detection receiver. Subsequently one of these units was demonstrated 

at the Armco #9 coal mine near Charleston, West Virginia, and all were 

field tested at the U. S. Steel Corporation's Geneva Coal Mine near 

Dragerton, Utah. Part I of this report describes the systems fabricated 

and the results of the field tests. Conclusions and recommendations are 

included with the report. 

Task I1 involves modifications and additional tests on the Monitoring, 

Locating and Communications System which was initially developed on 

Contract HO220073. Part 11 of this report covers the modifications made 

to the system and the results obtained after reinstalling the equipment in 

the USBM Experimental Mine near Bruc eton, Pennsylvania. 

Task 111 resulted from a contract modification to furnish the 

engineer services and equipment necessary to conduct an EM location 

system demonstration at a West Virginia coal mine for the Bureau of 

Mines. The demonstration was performed in conjunction with other mine 

emergency operations equipment, including the Seismic Location System, 

the EM Communications equipment, and support equipment normally located 

at the Charleston Staging Facility in West Virginia for emergency use. 

Part 111 of this report summarizes the EM location activities performed at 

this demonstration. 
INTROD UC TION 

A prototype electromagnetic location system consisting of 6 miniature 

transmitters, 6 miniature receivers and one multichannel receiver was 

developed by Westinghouse Georesearch Laboratory and is described in this 

section of the report. The equipment was designed to operate in a deep coal 

mine of relatively high overburden conductivity. One mine having these 

characteristics is the Geneva Coal Mine operated by U. S. Steel Company near 

Dragerton, Utah. Since Westinghouse personnel had previously conducted 

similar tests at this mine (September 1971) and the pertinent parameters such 

as overburden depth and ground conductivity were well known, it was decided 

to return to this mine and conduct field performance tests of the newly developed 

equipment. Because of significant overburden depth and high conductivity, it 

was felt that if the equipment performed satisfactorily at the Geneva Mine it 

could also perform satisfactorily at most coal mines in this country. 

The field tests consisted basically of the detection of magnetic field 

intensities produced by the miniature EM manpaek transmitters deployed in the 

mine and the utilization of the measured field strength patterns to estimate the 

location of the transmitters. Both surface measurements and airborne 

(helicopter) measurements were conducted. Prediction of field strength at a 

variety of frequencies and overburden depths were made prior to the development of the equipment to insure that the transmitters would have sufficient 

transmitting moment to penetrate the deep, conductive overburdens and that 

the receivers would have sufficient sensitivity and selectivity to detect the 

fields produced on the surface as well as above the surface. Comparisons 

between predicted and measured field strengths are given in this report along 

with comparison of location determined by electromagnetic methods and 

conventional land surveying. Representative tape recordings were made from 

each of the receiver outputs and were photographed on playback in the laboratory. 
2. 0 PERFORMANCE PREDICTIONS 

In this section we discuss the basic concept of the EM Location System, 

the limitations on transmitted signals, and indicate the detectable depth of 

different system configurations in terms of the antenna moment, conductivity, 

and the noise environment expected at the Geneva Coal Mine test site. 

2, 1 EM Location Concepts 

It has been demonstrated theoretically and experimentally that miners 

equipped with a radio transmitter and loop antenna which generates a vertical 

magnetic dipole moment can be located by measuring the electromagnetic fields 

on the surface 111, 181. Directly above the source there is a single null in the 

total horizontal magnetic field and this null provides the location criteria, 

Location accuracy depends on the depth of null which is a function of signal 

level and the background noise. Accuracy also depends on the axis orientation 

of the buried source and the slope of the terrain on the surface, as well as the 

presence of any structure in the area, 

The system designer can select operating frequencies and transmitter 

power, and detection schemes to optimize the signal-to-noise ratio at the 

receiver and thereby increase detection range and location accuracy. Source 

axis tilt is under the control of the transmitter operator. Terrain slope 

corrections can be developed, and when applied, could reduce terrain produced 

location errors. 

Figure 2-1 shows the equivalent circuit of the full-wave transmitter 

developed for location purposes. The symbols represent the following : 

Eb = Battery voltage 

Rb = Internal resistance of battery 

R = Internal resistance of switch 

S 

R = Loss resistance in antenna circuit 

a 

L = Antennainductance 

a 

Ca = Antenna tuning capacitor 
Figure 2- 1. Equivalent Circuit of Location Transmitter used at Geneva. 
Solid state devices are used for the switches and these are operated 

on an intermittent duty cycle using a continuous waveform generating circuit 

whose frequency is controlled by a stable oscillator. When the loop antenna 

inductance is resonated to the operating frequency, the current waveform in 

the antenna circuit is sinusoidal with an rms amplitude given by equation (I), 

( see Reference [ 11). 

I - rms Rb+2R +R ' 

s a 

The antenna moment is given by INA, where I is the rms current, N 

the number of turns, and A the area enclosed by the loop. Usually this 

moment is expressed in the international system of units, i . e . , ampere2 

turn-meters . 

Figure 2- 2 shows the maximum intrinsically safe antenna moment 

which can be achieved as a function of the length of #I0 wire used to form a 

square loop antenna. 

The coal pillars at the Geneva test mine in Utah were found from 

mine maps to be typically 500' (152.4 m) in circumference and usually 

shaped in the form of a parallogram 80 X 170 feet with the acute angle of 

approximately 45 degrees. The antenna moment with this antenna configura2 

tion varied between 1800 and 2600 ampere-turn-meters depending on whether 

#52 or a #9 AWG wire was used. These antenna moment estimates are 

based on measurements made near the WGL laboratory using a similar 

antenna configuration laid out on the surface. In the mine the antenna 

moments could be different because of the different losses in the surrounding 

coal. Antenna currents were not measured in the mine because permissible 

current measuring equipment was not available. 
Wire Length, 1, in feet 

5 
2.2 Fieldstrengthpredictions andMeasurements 

Given the antenna moment and the conductivity of the overburden, 

it is possible to predict the vertical electromagnetic field component, HZ ' 

over the transmitting dipole as a function of depth. For a coaxially oriented 

receiver antenna, H is given by equation (2). (See Reference [2]). z 

where z is the depth and 1 G I is an attenuation factor which is a function of 

the skin depth and varies with frequency and conductivity of the overburden. 

Figure 2-3 shows how H varies with frequency and conductivity for 

Z 

a fixed antenna moment and a depth of 500 meters (1640 feet). An estimate 

of the expected summertime atmospheric noise in a 6 Hz bandwidth is shown 

on the same curve. A comparison of the signal strength and the noise 

indicates that at a depth of 500 meters the optimum signal-to-noise ratio 

occurs at lower frequencies as the earth conductivity increases. 

The effective conductivity of the earth at 460 meters over the Geneva 

Mine had been measured earlier, using an Eltran array on the surface, 

- 2 and found to be 2.7 X 10 mhos/meter. This was based on a two layer 

interpretation of the conductivity profile . Given this value of conductivity 

and the transmission frequency, the expected signal strength can be :rr 

calculated for various depths. The results, obtained by computer, are 

shown in Figures 2-4 through 2-9 for the six different transmitter frequencies 

-2 used at Geneva and assuming a constant conductivity of 2.7 X 10 mhos /meter 

for all depths. 

A comparison of the measured field strengths and these predictions was 

made, and in general, the measured fields were higher than expected. This 

indicates that either the in-mine antenna moment was greater than measured 

INA 

8 

INA 

10 
INA 

11 

INA 

13 
L 0 2 103 

INA 

14 
on the surface or that the actual conductivity was less than originally 

assumed. The conductivity profile was later reexamined and a threelayer interpretation performed to obtain effective conductivities as a 

function of depth. Measured and predicted values of field strength are 

given in Section 4. 2 using these revised conductivity estimates. 

Predictions of the vertical magnetic field, HZ, as a function 

of frequency for a fixed antenna moment can be obtained from Figures 2-4 

through 2-10. This was done for the antenna moment expected for Geneva tests 

2 (INA 2000 ampere-turn-meters ) and for an effective overburden 

conductivity of 2.7 X togL mhos/ meter. Results are shown in Figure 2-11 

with depth as the parameter. 

Also Figure 2-11 shows the expected atmospheric noise in a 

1.5 Hz bandwidth for the spring, summer, and winter seasons. These 

noise curves are based on measured values of the median levels of the 

vertical magnetic noise fields from 35 Hz to 3875 Hz. These measurements 

were obtained by WGL at Boulder, Colorado in the Spring of 1972 [3] and 

extrapolated for the summer and winter seasons. The standard deviation 

of the measured data for a one month period (March, 197 2) was 10 dB 

at 1000 Hz. This deviation accounts for diurnal variations of the noise 

and for day to day variations during the month. The ratio of the rms to 

average value of the noise, vd' was also measured for shorter periods. 

V varied depending upon frequency and time of day, but was usually less d 

than 2 dB when measured in a 6 Hz bandwidth. Thus the atmospheric noise 

envelope has almost a Rayleigh distribution at the frequencies and bandwidths 

used for the Geneva tests. 

By comparing the available signal fields with the expected noise 

fields in Figure 2-11 it is apparent that the maximum signal-to-noise 

ratio is quite broad and occurs at decreasing frequency with increasing 

depth. At 300 meters the maximum signal-to-noise ratio is obtained around 
Frequency in Hertz 

16 
1000 Hz, and at 500 meters it is obtained around 200 Hz. However, 

man-made noise, which contains spectral lines at the harmonics of 

60 Hz, increases substantially at the lower frequencies and leads 

to the selection of the higher operating freque~cies. The 6 transmitter 

frequencies used for the Geneva tests fell between 900 Hz and 3000 Hz. 

Data obtained at Geneva tend to verify the noise model shown 

since field strength measurements as low as 0.03 CIA/meter were 

measurable at 1900 Hz. 

Field strength measurements obtained for four different frequencies 

at a depth of 457 meters (1500 feet) are also shown on Figure 2-11. The 

decreasing signal level as a function of frequency follows the predicted 

value, although the results indicate that the initial conductivity interpretation 

(2. 7 X mhos/meter) is high. 
Much of the developmei~t of the transmitter used iiz the Geneva field 

tests was accomplished on BuMines Contract H0220073. Out of this work 

evolved a transmitting system prototype utilizing a full wave transformerless 

switchi~lg amplifier as shown in the schematic of Figure 3-1. This transmitter is designed specifically to get maximum switched AC, current from a 

low voltage DC source (4 volt cap lamp battery)into a low impedance series tuned 

loop antenna (heavy wire wrapped around a mine pillar). The output transistors 

used were complementary MJ 4032 and MJ 4035 (Motorola PNP and NPN low 

saturation voltage devices). Laboratory performance tests of this transmitter 

powered by a 4 volt miners cap larnp battery to drive a 0.4 i2 resistive load 

indicated an equivalent switch resistance of 0. 2 ohms per transistor. 

The transmitter derives its oscillation frequency from a tuning fork 

oscillator and its tone pulse repetition rate from an RC integrated circuit 

oscj.llator, ,411 of the integrated circuits used in this transmitter are complementary MOS (CMOS) and a. bank of 2N4400/4402 transistors is used to 

interface the CLIOS outputs to the power transistors. A three way manual 

power switch is provided so the operator may either key the transmitter to 

send coded messages to the surface or turn it on and leave it run on a 4070 

duty cycle interrupted tone sequence. The Geneva tests were performed using 

the iriter ruptecl tone sequence switch position with the transmitter unattended. 

Figure 3-2 is a photograph of the transmitter chassis and leather carrying 

case. This transimitter was tested at the BuMines approval and Testing 

Laboratory in Pittsburgh, Pennsyl.vania and has been rated permissible for 

use in gassy ~nines. A complete list of transmitter specifications is shown 

in Table 3- 2. 
Fig

@ 

WESTINGHOUSE GEORESEARCH LABORATORY 

8101 BASELlNEROAD 

BOULDER,COLORADO 80303 

EM FUL

MODEL 

DRAWING NUM

Spc%(.il'i<-;~ t ions for Westinghol~sc M;jn/>;~c- k '1'1-;I ns~~iitt (.I-, pMoclc.1 C8/1 HA 

Ilow (tr Sour(-cb : 4 volt ~riinc-r's lanip 1)attt.r~ f~ir-rlisl~t~(l wit li c.;~c-t~ rlnit. 

'1'1-;I ris~iitt c. I- F.111 1 W;jvt. - Swit c-tling Moclch. Irit r-irlsic-;11 l y s;~ l'c. with 

'7'y I)<\ : slwc:ific.d ;~ntc:r~n;a. 

Ope. ra t ing 

E' rc~qlicnc-y : 

(1 of the following): 022. 5, 982. 5, 1700, 1()00, 2500, or 2()OO TT7,. 

Frcqucncy 

Accuracy: A 2 Hz. 

Frequency 2 x F max. 

Stability : 

Signal Output Interruptcd c:ontinuous squa re wave into resistive 

Format : load. 

Duty Cycle: On 0. 2 seconds A 10%) 

Off 2, 0 seconds A 1070 

Temperature 0- 120" F 

Range : 

External Battery and antenna connections. 

Connections : 

Minimum 

Output: 

8 amperes peak-to-peak into 0. 4 ohm resistive 

load at 70 F. 

Controls : Single on-off switch. 

Operating Life : Approximately 30 hours, using flllly c:hargc:d 

battery at 70" F. 

Transmitter Less than 50 cubic inches with suitable belt attachPackage : ments, battery and antenna excluded. 

Transmitter Less than 2 lbs. excluding battery and antcnna. 

Weight: 

Antenna 500 feet of # 12 AWG insulated copper wire. 0. 8 ohms, 

Furnished: 300 phenries as single turn loop , and resonated 

to operating frequency with suitable capacitor. 
3. 2 Receiver 

TWO different types of receivers were developed on this contract: 

(1) a portable single channel receiver utilizing an air core loop antenna tuned 

to one particular frequency and (2) a multichannel receiver utilizing a broadband loop antenna and preamp caxbinaticn to receive up to six frequencies 

simultaneously. Six of the portable receivers were fabricated, each one 

tuned to a different frequency while one multichannel receiver capable of 

receiving six frequencies simultaneousl-y was built for use in the helicopter 

search mode. Each of these receivers was tested and evaluated at the Geneva 

Mine in June 1973. Technical descriptions of each receiver and results of 

their field evaluation are given in the following sections. 

3. 2. 1 Westinghouse Manpack Locator Model C842A 

The Westinghouse Manpack Locator Model C842A is designed for single 

frequency detection and source location for an underground transmitter transmitting that particular frequency. A schematic diagram of this receiver is 

shown in Figure 3-3 and a photograph is shown in Figure 3-4. The receiver 

chassis weighs about 2 lbs and attaches to the operation belt. The received 

signal is detected on a. set of headphones; source locations are determined by 

evaluating the surface magnetic. field pattern of the transmitted signal. When 

good signal to noise ratios are present the source location can easily be 

determined by the measured null in the total horizontal field. 

The receiver has six independently tuned stages counting the loop antenna 

tuning. In addition to the parallel resonated loop, the receiver employs four 

RC bandpass amplifiers, each with a Q of 10 and one sharply tuned tuning 

fork filter with a Q of roughly 500. 


The tuned frequencies of operation range from 922. 5 Hz to 2900 HZ, 

with 3 dB bandwidths ranging from 2 Hz to 5 Hz. A typical response curve 

obtained from one of these receivers (2900 Hz) is shown in Figures 3-5a & b. 

With this receiver, out of band interference * 300 Hz away from center frequency is attenuated approximately 70 dB. This receiver was originally 

specified as having a sensitivity of 0. 1 pA/meter; however, the Geneva tests 

showed it capable of receiving signals as low as 0. 03 pA/m. A complete 

list of specifications for the manpack locator is given in Table 3-2. 

3. 2. 2 Westinghouse Multichannel Receiver Model C849A 

A schematic of the Multichannel Receiver C849A which was used in the 

helicopter test is shown in Figure 3-6. An active loop antenna circuit with 

a step-up transformer and a preamp gain of 20 dB was used with this receiver 

as shown in the schematic of Figure 3-7. This antenna preamp combination 

was designed to pass the signals in the operating band from roughly 1 kHz to 

6 kHz with constant gain as shown in the response curve of Figure 3-8. The 

response is given as a transducer transfer function in terms of output voltage 

per amperlmeter applied field strength. Figure 3-9 shows a photograph of the 

multichannel receiver and its associated loop antenna and built in preamp. 

Detailed specifications for the receiver are given in Table 3-3. 

An 8-channel event recorder is included in a separate chassis to monitor 

the sequence of recorded impulses from the receiver and thus determine which 

of the channels are responding to signals in a periodic fashion. Antilogarithmic 

amplifiers, ac coupled to the recorder inputs, are used to extract the signal 

impulses from the background noise. A schematic for the recorder interface 

circuits is shown in Figure 3- 10. 
TABLE 3-2 

Specifications for Westinghouse Manpack Locator Model C842A 

Antenna : 

Antenna Q: 

Receiver Gain: 

(max. ) 

Air core loop, 15-inch diameter, 500 turns 

#22 enameled copper wire, with electrostatic 

shield. 

Approximately 30. 

Approximately 100 dB. 

Receiver 

Bandwidth: 3 Hz * 2 Hz. 

Frequency 

Accuracy: Within * 2 Hz of specified frequency. 

Frequency 

Stability: 2 x " F 

Normal Full 1 pA/meter field provides approximately 1 volt 

Scale Sensitivity: into head phones. 

Minim urn 

Detectable 

Signal : 

Temperature 

Range : 

Power Source : 

Total Weight: 

(Antenna, receiver 

and earphones) 

Receiver Case: 

Two 6-volt, 1 amp-hour rechargeable batteries 

(nominal time between charges - 20 hours. ) 

Less than 10 pounds. 

Approximately 5 X4 X 2 inches with suitable belt 

attachments. 

:: This receiver has exceeded this specification I'n practice. Successful measurements as low as . 03 pAmp/m were obtained. 
TABLE 3-2 (contld. ) 

Specifications for We stinghouse Manpack Locator Model C842A (Continued) 

Operator 

Controls : 

On-off switch and gain adjustment on receiver; 

antenna hand held and manually oriented for optimum 

signal detectability and for null location. 

Frequencies : Each receiver is capable of receiving one of 

the six following frequencies: 922. 5, 985. 5, 1700, 1900, 

2500, or 2900 Hz. 
1000 2000 3000 

Frequency, Hz 

Figure 3-5a Frequency Response of Manpack Locator Model C842A 

TABLE 3-3 

S~ecifications for Westinghouse Multichannel Receiver Model C849A 

Operating 922. 5, 982. 5, 1700, 1900, 2500, and 2900 Hz. 

Frequency (all 

of the following): 

Frequency 

Accuracy: 

Frequency 

Stability : 

Temperature 

Range : 

Minimum 

Detectable 

Signal: 

Bandwidth: 

Output : 

Pane 1 

Controls : 

Pane 1 

Display: 

Package Size : 

Antenna Configuration: 

Weight: 

* 2, 0 Hz. 

Crystal Earphones with jack selectable to any 

one or all of the six channels. 

Channel Selector, individual channel gain adjust, 

on-off switch. 

Miniature Edgewide Meters for each channel. 

Less than 240 cu. inches. 

Air Core Loop (640 turns) with built-in wideband 

preamplifier at the end of a 35 ft. tethering cable. 

Less than 20 lbs. with batteries, but excluding 

antenna. 

pz 

v 

IS" W 

w Lr. 

J 

Llr 3 --.-- -1 

-- I 

-- - --- + 27 0 0 

b 

0 

z= \% r Y 

0 b! 

0 r( - 

4 1 11 .I 

n > b 

d 03 I 

Cc, 

4- 

cc a, 

A 

M 

. d 

4 b ~r 

* 

- 

rJ -. 

a3 20 

s?," 

LL. 

3 

3 

P -3 

Y 

m - 

0 vc 

0 

.d 

C, 

d 

L) 

0 

4 

k 

; 

5 

&I . 

a, 

.d +I 

.d 

4 

d 

a, 

2 

T3 

c 

d 

a 

0 

O 

45 

k * 

a, 5 '1' 

:g 5 Zr ts: s:s 

). 

a 

0 

C 

4 

a 

0 

rn 

4 

A 

s 

U Q: 0 m 4 cr, 

w 0 

V) 00 

w 0 

QU 0 ESz 

Ow a 

ZZ 5 =A 0 owu 

sm - O4P: 

f*" C3 +-A 

V)o3 

0 

3=3= 

b 

---------

Figure 3-9 WESTINGHO

gppy 

desirable to include in the receiver a variable trimmer cc3ctrol to precisely align 

each receiver before going in the field. 

Preliminary performance measurements me re xade ia a field adjacent 

to the Georesearch Laboratory to insure that the receivers were all 

operating satisfactorily and that the transmitted signals co:~ld be detected at 

least 1500 ft. away from the source using a ~mor~?ent comparable to that anticipated 

for the actual mine tests. Each receiver was caliSrat.ed for total gain in the 

full scale position. The results of the receiver calibrations are given in 

Table 4-1. These receiver gains were used to reduce the field data from output 

volts to equivalent pAmps/m of field strength. 
TABLE 4-1 

Gain Calibration of Manpack Receivers 

Current 

Frequency (Hz) Ser. No. (Amps) d H z (PAI~) v 

out 

Gain 

V/Amp/m 

1.3 X 10 6 

1.3 X 10 6 

7.1 X 10 6 

3.67 X 10 6 

4.5 X 10 6 

4.5 X 10 6 
4. 2 Field Tests 

The Geneva Coal Mine, operated by U. S. Steel Corp. near Dragerton, 

Utah was chosen for the test mine because of its high conductivity and deep 

overburden. The combination of these two characteristics make this mine 

an ideal test bed for determining performance limitations of the system. 

4. 2. 1 Description of Field Site 

The Geneva Mine is located at the north edge of Emery County, in 

east-central Utah, about eight miles southwest of Dragerton, Utah. The mine 

is developed in the extensive Book Cliffs coal field, in which bituminous coal 

occurs in the Mesaverde group of Upper Cretaceous Age. The Book Cliffs 

are a prominent physiographic feature in central and eastern Utah, and extend 

into western Colorado. In the local area of this report, the cliffs form a 

southwest-facing escarpment with about 2000 feet of topographic relief. The 

rocks at the base of the escarpment are Mancos shale, an even-bedded marine 

shale which erodes faster than the relatively more resistant rocks of the 

overlying Me saverde group and the younger (Tertiary) rocks overlying the 

Mesaverde. The coal occurs in the Blackhawk formation, the basal unit of the 

Me saverde. The Blackhawk is divided in this locality into a basal marine 

sandstone, and overlying mixed marine and continental units which contain 

the coal seams. The continental sandstones in the Blackhawk and the overlying 

Price River formation (upper part of Mesaverde group) are discontinuous 

channel sands characteristic of fluviatile deposits [4, 5 1. The major coal seams 

are the Upper and Lower Sunnyside seams. The unit being mined at the Geneva 

Mine is the Lower Sunnyside seam, which has a thickness at this location of 

about 14 feet [ 61. 

The portal of the Geneva Mine is at an elevation of 6400 feet in Horse 

Canyon. The tests performed by the Westinghouse Georesearch Laboratory were 

conducted within an area of about 5000 feet by 7000 feet, mostly in s%-3- T~~S-RI~E 
.I. 

(see Figure 4-l)".. The workings extend to the east and southeast under a portion 

of the Book Cliffs known locally as Lila Point. The relatively flat portion of Lila 

Point has a surface elevation of 7500 feet. The mine workings slope downward to 

the north and east at about 500 feet per mile, following the regional dip of the 

Blackhawk formation. The strike of the beds is about N2 1 " W, according to the 

U. S. Steel mine maps. The terrain is extremely rugged with sharp mountain 

peaks, shear cliffs and deep canyons. A typical terrain profile is given in 

Figure 4-2 showing the unexaggerated elevation change with horizontal distance 

for a straight line cross section proceeding 21" NE from the exhaust fan in 

Lila Canyon. The average slope of this profile is about 30 " . 

The overburden depth where this terrain profile intersects the road is 

about 1100 feet. In general the overburden depths for the transmitter-receiver 

locations used in the Geneva Mine tests varied from 1100 ft. to 1650 ft. 

Extensive ground conductivity surveys were run over this mine during 

the previous visit by Westinghouse personnel and were not repeated during these 

tests . The conductivity sounding used to obtain the initial average conductivity 

-2 

estimate of 2. 7 X 10 mhoslm is shown in Figure 4-3. A layer interpretation 

was made on the conductivity sounding which shows the high conductivity 

material to be near the surface with a much lower conductivity material down 

at the mine level. This is to be expected since coal seams generally exhibit 

low conductivity. The effective horizontal layer interpretation indicates a 

- 1 surface layer about 7 meters thick with a conductivity of 1. 1 X 10 mhoslm, 

an underlying layer approximately 150 meters thick with a conductivity of 

- 2 

3. 5 X 10 mhos/m and a basement layer having a conductivity of approximately 

7 X mhos/m. In terms of average overburden conductivity, for the shallow 

- 2 

overburdens of 1100 ft. , the average conductivity is about 2. 1 X 10 mhos/m 

and for the deeper overburden of 1650 ft., the average conductivity is 

- 2 

approximately 1. 7 X 10 mhoslm. 

* Transmitters were installed at locations (A1, A2, BiY B2, C1, Dl) with 

antennas deployed around the coal pillars shown in black on Figure 4- 1. 
Figure 4 -1. Map of Geneva Mine Transmitting Locations 

40 


4. 2. 2 Surface Tests 

The surface tests conducted at the Geneva Mine included: (1) magnetic 

field profile measurements using manpack locator with portable oscilloscope 

(Tektronix 21 I), (2) magnetic tape recordings of manpack locator and multichannel receiver outputs for laboratory signal analysis and (3) EM location 

determination by null technique and comparison with surveyed location. The 

trapped miner location problem consists of two phases: (1) detection of airborne and surface signals from the underground transmitters, and (2) analysis 

of the resulting field patterns to determine most likely location of source. The 

results of the Geneva Mine tests were encouraging from the standpoint of detection 

range and capability; however the null locations were not as accurate as those 

obtained previously in shallower mines. 

Magnetic Field Profiles 

Figures 4-4 and 4-5 show orthogonal field profiles of vertical and horizontal magnetic field strengths measured about the estimated horizontal field 

null point for a 2500 Hz transmitter installed at A-1. It is immediately apparent 

in these figures that the horizontal field null is much deeper in the NW-SE line 

(Figure 4-4) than it is in the NE-SW line (Figure 4-5). This is partly attributed 

to the fact that the transmitting antennas were deployed around large quadrilateral 

pillars of dimensions roughly 70 feet by 180 feet. The deeper null in the horizontal 

field was observed for that profile (NW-SE) cutting across the smaller dimension 

of the rectangle. The average hill slope of this locationwas 22" in the NE/SW 

direction. The uncorrected null point at this location was about 200 feet downhill from the true location. This was a larger offset than was experienced at the 

shallower and lower conductivity mines previously visited in the east. Part of 

this can be attributed to the fact that conductivity anomalies have a much greater 

influence on the EM field readings at Geneva than at the eastern coal mines 

because the propagation path in terms of skin depth is much greater. Figure 

4-6 shows skin depth penetrations of from 5 to 8 at Geneva while most of the 


10 11 12 13 14 

Penetration into Medium (z/ 6) in Skin Depths 

Figure 4-6 Loop Antenna and Horizontal Coupling Relationships 4 6 
eastern mines had equivalent overburden depths of less than 1 skin depth. 

Figures 4- 7 and 4-8 show the corresponding field profiles obtained at 

the same location, except the 2500 Hz transmitter was replaced by a 982. 5 Hz 

transmitter. Again the deep null is observed in the NW/SE direction consistent 

with transecting the short dimension of the source antenna. The measured field 

strength at 982. 5 Hz is 4 to 5 dB higher than at 2500 Hz as expected. The location 

determined by the 982. 5 Hz transmission was about 25 meters closer to the actual 

location than the 2500 Hz as slope correction theory indicates. 

Figures 4-9 and 4-10 show field profiles obtained at 1900 Hz at location 

B with an overburden depth of 1500 ft (457 meters). Calculated profiles, shown 1 

as dashed lines on these figures, compare favorably with the measured data for 

the horizontal field (H ) in Figure 4-9, and are generally low by 5 to 8 dB for 

P 

the other field components. Most of these discrepancies, however, can be 

attributed to the unsuitability of the continuous uniform hill slope model for the 

irregular terrain of the Geneva Mine. The data obtained for the horizontal 

field in the vicinity of the null point is contaminated by background noise since 

the signal levels were relatively weak for this overburden depth. These measurements did show that the receiver was capable of detecting signals well below 

the specified sensitivity of 0. 1 pA/m (-20 dB re1 to 1 pA/m). 

Extended field profiles out to a distance of 550 meters in the southwest 

direction were also made at B for frequencies of 1900 Hz and 982. 5 Hz. (See 1 

Figures 4-11 and 4-12). It is interesting to note the similarity of the shape of 

these profiles and in particular the vertical field (H ) nulls occurring for 

z 

both frequencies at a distance of 200 meters. According to Wait [ 71, 

a null in H is to be expected. In the limiting case, under static field conz 

ditions, this null should be quite pronounced and should occur at a horizontal 

distance of 1.4 mine depths away from the source location. However, at progressively higher frequencies, this null should become less pronounced and its 

distance from the source location also decreases. Based on Wait's curves, for the 

present conditions the null should occur at a distance of 274 meters for f = 

982. 5 Hz and 229 meters for f = 1900 Hz. However his curves also indicate that 






the nulls should be almost indistinguishable for these frequencies and overburden conditions. This does not agree with our observations in that the 

measured nulls were very well defined. Note also that in Figure 4-12 there 

is a secondary null in the horizontal field at a location of 450 meters from 

the source. This coupled with a corresponding secondary peak in the vertical 

field could result in determining a false location at this point. Further 

investigations should be made to better define the field profile behavior 

at greater distances from the source and under rough terrain conditions. 

Location Accuracv 

The locations determined from magnetic field profiles at the Geneva 

Mine were not as accurate as those obtained earlier at shallower and less 

conductive eastern coal mines. Part of the increased offsets observed can be 

attributed to a general increase in scale brought on by a larger antenna dimension 

and an increased overburden depth over any used in previous mine tests. Also, 

the increased conductivity of this mine overburden produced a situation 

where the field strength became much more sensitive to local conductivity 

anomalies than had been experienced at eastern mines. The resulting location accuracies determined from the Geneva Mine tests are shown in 

Table 4-2. For signals that were easily detectable, the locations were accurate 

to within three hundred feet. At locations C and D where the signals were 1 1 

somewhat marginal, the locations were difficult to determine because of poor 

signal to noise ratios. No location was obtained at C even though signals at fre- 1 

quencies as high as 1700 Hz could be detected there, At D an attempt was made 1 

to determine its location even though the signal was marginal; this location proved 

to be 700 feet off the surveyed point. Hill slope corrections were not applied but 

would have brought the locations closer since all of the offsets were down hill 

from the surveyed points. Since the transmitting antennas were wound around 

pillars of dimensions 70 ft. X 180 ft., the location accuracies obtained were 

mostly within 1 pillar length away from the actual transmitter locations and as 

such do provide useful information from a mine rescue standpoint. 
TABLE 4-2 

Location Accuracy 

L 

Map De signation 

A1 

*1 

A2 

B1 

B1 

B2 

Hill Slope 

22" 

22" 

Frequency 

2500 Hz 

982. 5 Hz 

922. 5 Hz 

1900 Hz 

982. 5 Hz 

2900 Hz 

1700 Hz 

1900 Hz 

- 

Uncorrected Location 

Error 

200 ft. 

130 ft. 

350 ft. 

130 ft. 

130 ft. 

130 ft. 

No location 

700 ft. 
Field Strength Comparisons 

Table 4-3 shows the comparison between computed and measured field 

strength for the surface fields measured above the Geneva Mine. The average 

conductivities llsed in the computations are based on the three layer conductivity 

interpretation described in Section 4. 1. These conductivities vary from 

-2 -2 

1. 68 X 10 mhos/m to 2. 1 X 10 mhos/m depending on the total overburden 

depth. The resulting field strength computations were obtained using the 

following expression: 

H = INA ]GI 

Z 2 -rr z 

3 

and the attenuation curve given in Figure 4-6. The maximum difference between 

measured field strength and computed field strength occurred at location C 1 

where the measured value was 6.8 dB less than the computed value. 

As mentioned before, local variations in conductivity can play an important part in distorting field patterns and lowering (or raising) field strengths. 

It is likely that the conductivity at the C location was somewhat higher than the 1 

value used in the computation. For the most part the agreement between predicted and measured field strengths is quite good, and illustrates again the 

suitability of the theoretical model used in the computations. 

4. 2. 3 Helicopter Tests 

On two of the days of testing at the Geneva Mine a Bell Jet Ranger 206B 

helicopter was leased to determine the performance characteristics of the 

multichannel receiver in helicopter reconnaissance operations. Based on the 

results of previous tests at the Robrna Mine iil January 1973 [8], the antenna/ 

preamp package was lowered beneath the helicopter by about 35 feet to minimize 

the effect of electrical noise from the engines. The results of the helicopter 
TABLE 4-3 

Measured and Computed Field Strengths, Geneva Mine 

Location 

A1 

A2 

B1 

Dl 

Depth 

(Ft. ) 

1150 

1400 

1500 

1450 

16 5 0 

HZ (measured) H (computed) z Frequency 

( Hz) 

982. 5 

1900 

2500 

922.5 

982. 5 

1700 

1900 

2900 

2500 

1900 

dB difference 

(dB) 

+ .5 

- 1. 5 

+ 2. 3 

- 3. 26 

+ 4. 3 

- 2. 3 

+ 5.4 

+ 1.0 

t 6. 8 

t 2. 8 

(dB relative 

1. 6 

- 4,3 

- 4. 9 

- 9.06 

- 4. 8 

-15.9 

-10.3 

-23.0 

-26 

-22 

I 

to 1 pAmp/m) 

1. 1 

- 2. 8 

- 7. 2 

- 5. 9 

- 9. 1 

-13. 6 

-15.7 

-24.0 

-19.2 

-19.2 
tests were mostly qualitative but did show the utility of using a helicopter for 

quick scans over the mine area to determine the presence or absence of transmitting signals in the mine and their approximate location. At the shallowest 

overburden location (A ) of 1150 ft. , the horizontal detection range observed with 1 

the multichannel receiver at an altitude of 100 ft. was about 4000 ft. Detection 

of signals was also made at overburden depths up to 1500 ft. but the horizontal 

detection range was limited to about 1000 ft, for the deeper overburden locations. 

The signal was also detectable above the A location at helicopter altitudes as 1 

high as 700 ft. Photographs of the receiver output for these tests are shown in 

Section 4. 2. 4. 

One problem area which soon became apparent in these tests was the 

electrostatic potential difference developed between the antenna shield (lying on 

the ground but cabled to the receiver chassis in the helicopter) and the helicopter 

frame. This potential difference caused electrical shock to the operator at 

times when the helicopter was in the air while the loop was on the ground. To 

remedy this problem the receiver chassis was firmly grounded to the helicopter 

chassis. 

4, 2.4 Signal and Noise Recordings 

Uplink signals were recorded at the outputs of both the manpack locator 

and the multichannel receivers at several receiving locations on magnetic tape 

using a Wollensak Model 4300 cassette tape recorder. These signals were 

later played back onto a storage oscilloscope and were photographed as shown 

in Figures 4-13 through 4-19. 

Figure 4-13 shows signals received at the A location, 1150 ft. above 1 

the 2500 Hz transmitter. The signal to noise ratio is higher for the manpack 

locator than it is for the multichannel receiver; rough signal to noise estimates 

obtained for the twc receivers from these photographs are: 

Manpack Locator - 24 dB 

Multichannel Receiver - 14 dB 
Multichannel 

Receiver 

Output 

Figure 4-13. Received Surface Signals from 2500 Hz Transmitter, 

1150 feet Underground 
H = 0.305 p.A/m z 

Multichannel Receiver Output 

Figure 4-14 Received Surface Signal from 1900 Hz Transmitter, 

1500 feet Underground 
H = 0.27 pA/m z 

at 1900 Hz 

H t 0.102 pA/m z 

at 2500 Hz 

I 

b) Mine Portal 

Figure 4-15 Received Signal on Multichannel Receiver 

61 
Figure 4- 16 Combined Surface Signals from 4 Underground 

Transmitters Measured at Location A with 

Multichannel Receiver 1 
Transmitter Depth = 1150 feet 

Figure 4-17 Received Signals from Multichannel Receiver in 

Flight (f = 982. 5 Hz) 

Flying over A 

Location at 

1 

Speed of 2 0 mph 

and altitude of 300 

feet. 
d 

Alt. 

Figure 4-18 Received Signals fl-91-1 1Ar1itichannel Receiver in Flight 

6 1 

1500 feet 

100 feet 
Figure 4-19 Expanded Signal Pulses from Multichannel Receiver in ~ii~ht 

6 5 
Signals of the above quality are not difficult to detect either audibly or 

with a visual output such as a meter or an oscilloscope. The received signal 

of the multichannel receiver is shown in Figure 4-14 at location B 1500 it. I ' 

above the 1900 Hz transmitter. Here the estimated signal to noise ratio is 

also about 14 dB. 

Figure 4-15 shows the combined signals from two transmitters (1900 Hz 

and 2500 Hz) received on the multichannel receiver both at the surface location, 

A and at the mine portal. It is apparent that signal to noise ratio is 

1 

degraded at the mine portal; however the very fact that signals are even 

detectable at the mine portal is quite encouraging since in this case the horizontal 

distances represented are 5500 ft. and 7500 ft. from the 2500 Hz and 1900 Hz 

transmitters respectively. In order to receive the signals at the mine portal, 

care had to be taken with the orientation of the receiving ioop to null out the 

major sources of mine interference. It is apparent that signal enhancement due 

to coupling in the rails and or cables leading out to the mine portal is taking place. 

Otherwise, the signals would have been completely lost in the noise. The 

estimated signal to noise ratio for these conditions are 16. 5 dB at the surface 

location A and 6 dB at the mine portal. Being able to detect these signals at 

1 

the mine portal could play an important part in preliminary mine rescue 

efforts to determine whether any transmitters are transmitting and pos s ibly 

to aid in communications with trapped miners throughout the mine. Also, the 

long propagation ranges observed for these transmissions would make this 

system concept ideally suited to a mine paging application in an operating mine. 

Figure 4-16 shows composite signals from 4 underground transmitters 

measured at location A with the multichannel receiver. In this figure it is 1 

apparent that the background noise increases with the number of channels being 

added to the composite output. The signals w e r e a l s o m e a s u r e d a t this location 

using the calibrated manpack locators and the equivalent field strengths are 

shown on the figure for the different transmitting frequencies. 
Figures 4-17, 18 and 19 show received signals from the multichannel 

receiver output while it was being used in a simulated helicopter reconnaissance 

flight. In Figure 4-17, it is apparent that signal to noise ratios improve 

considerably when the helicopter is allowed to hover or fly slowly over the area. 

Some of the noise obtained is likely due to vibrations of the loop antenna, 

tethered 3 5 feet below the helicopter, interacting with the earth1 s magnetic 

field. However, even at speeds up to 20 mph, the signals were distinctly 

audible and the periodic pulse repetition rates were distinguishable. Also, the 

helicopter could increase its altitude to about 700 feet above the A location 1 

before the signal became difficult to detect. This represents a total transmitter 

to receiver separation of 1850 feet at this location. For these tests, the helicopter's generator was turned off as soon as it became airborne. This was 

necessary because the generator interference was of an intolerable level for 

detection of these extremely weak fields. 

Figures 4-18 and 4-19 show signals received from the helicopter in 

flight during the second day of helicopter tests after the transmitters had been 

relocated. Figure 4-18 shows the received signals as the helicopter flew past 

each of 4 different transmitters operating from depths from 1150 feet to 1500 

feet. With the exception of an occasional uncorrelated noise burst as shown 

in the photographs, the signals were clearly distinguishable and would not 

have been difficult to detect with the present equipment in an actual emergency. 

However, there were two locations where the signals were not detectable by the 

helicopter in flight. These were C and D representing overburden depths 1 1 

of 1500 feet and 1650 feet respectively. These signals were detectable earlier 

using the portable manpack locators and helicopter receiver on the surface. 

However, different transmitter frequencies were being used at these locations 

at that time and it is not known whether they would have been detected on the 

helicopter receiver in flight since the helicopter was not available at the time. 

In general, it can be stated that the lower frequency transmitters had the 

greater probability of success at the Geneva Mine due to (1) their increased 

signal propagation characteristics through the deep conductive overburdens, and 

(2) the virtual absence of man-made noise on the surface above the Geneva Mine. 
5.0 DIFFERENTIAL LOCATION RECEIVER 

One subtask performed under Task I on this contract was to conduct 

a laboratory investigation of differential techniques for improving signal 

detectability and null re solution. If the laboratory test results were 

favorable then the concept should be tested in the field. The following 

paragraphs describe the differential concept, system design. and test 

results. 

5. 1 System Concept 

The differential receiver concept involves measuring the gradient 

of the horizontal field over the region of the null of the absolute field. This is 

accomplished by using two loops for receiving antennas. These loops are 

mounted in a coaxial configuration separated by a distance, r? x, which is typically 

6 to 10 feet. For small loops the spacing reduces the mutual inductance so 

that they may be tuned with a single capacitor. Resonating the loops in this 

manner increases the receiver's noise figure. 

When both loops are conilected in series with the turns series aiding, 

the receiver responds to a distant source just as a single loop and can 

therefore be used to measure horizontal (H ) or vertical (H ) components P z 

of the fields from that source. 

When the terminals of one loop are reversed and the two loops are 

connected in series bucking, then the signals sensed by each loop are 

subtracted. In this configuration and with the loop plane s perpendicular 

to the plane of the transmitting loop, then the profile observed over a buried 

source is the gradient AH^ . In the region of null of a nearby signal source 

Ax 

the gradient is a maximum. External noise from distance sources is 

minimized by the subtraction of the nearly equal fields sensed by each loop. 

Two important factors to consider when designing such a system are: 

1) to make the antennas identical in order to increase noise cancellation 

and 2) to achieve a low receiver noise figure to permit detection of small 
signal gradients. 

5. 2 System Implementation 

A breadboard of the differential sensing system was constructed 

and tested in the laboratory. Two 15-inch air core loops consisting of 

500 turns of #ZZ AWG wire were installed in a coaxial configuration. Each 

loop inductance was about 200 millihenries and the two loops in series 

aiding or bucking were resonated to 1900 Hz with a 0. 015 p fd capacitor. 

Bandwidth measurements indicated an antenna system Q of approximately 45. 

A schematic diagram of this antenna system and the receiver block diagram 

are shown in Figure 1-1. The receiver bandwidth was determined by a 

1900 Hz tuning fork. Overall system bandwidth was 4 Hz at the 3 dB points. 

A magnetic dipole source was simulated in the laboratory using a 

small ferrite core loop (approximately 1 inch in diameter) excited with a 

1900 Hz signal generator. Tests were conducted by moving this source 

perpendicular to the planes of the receiver loops. Profiles were measured 

for the H component (series aiding) and the field gradient (series bucking). P 

Results are shown in Figure 1-2 which indicates that the system is capable 

of measuring the field gradient. A 10 dB reduction in the noise was also 

observed when the loops were connected in series bucking. The amount 

of noise concellation was not as great as expected. This was attributed 

to an imperfect loop orientation and because nearby man-made noise sources 

were not equally coupled to both loops. 

Subsequently a new antenna system and receiver were fabricated for 

field use. Two identical ferrite core loops using 2000 turns of #26 AWG 

wire were constructed and mounted in each end of a 6 foot long, 2 inch 

diameter, plastic tube with an external electrostatic shield. A battery 

operated receiver with low noise fron,t-end, gain control, and antenna 

tuning and reversing switch was designed and packaged in a portable case. 


'l'he 3 dB bandwidth of this receiver was measured to be 3 Hz, and the 

maximum sensitivity was less than 0. 1 p~lmeter. Tests were conducted 

near the laboratory using a 1900 Hz full-wave transmitter and an 8 foot 

square loop transmitting antenna consisting of 10 turns of 61 6 AKG in a 

vertical configuration. The transmitter antenna moment achieved with 

2, this setup was 96 ampere-turn-meters . This produced a field strength 

of about 10 pA/meter at a distance of 100 meters. Measurements were 

made using ttc difff~ rtlntial receiver with the antennas connected both 

aiding and bucking. It was apparent that the field gradient signal peaked near 

the null. As expected, the observed gradient was considerably less than 

the H component observed with the antennas aiding and coaxially oriented with 

z 

the transmitting antenna. It was not possible to obtain a noticeable degree 

of noise reduction when measuring this field gradient because the receiver was 

front-end noise limited. The front-end noise circuitry was revised and 

filtering added to the output stages to improve the noise figure. These 

revisions improved the receiver sensitivity to about 0.01 p~lrneter. Subsequent measurements, however, indicated that only the peaks of the 

atmospheric noise were exceeding this reduced front-end noise level. 

The differential system was then taken to the Geneva test site 

where field tests were already in progress. At Geneva the differential 

receiver was operated using the 1900 Hz transmitter installed at Location 

A-1 in the mine at a depth of 1150 feet. The vertical magnetic field 

strength was measured and found to be 0. 6 pA/meter. The differential 

receivers performance was similar to the manpack receiver Is performance 

with the antennas connected in series aiding. The null location, however, 

was not well defined and the field gradient was, therefore, not measurable. 

Again, the atmospheric noise at 1900 Hz in the 3 Hz bandwith was 

below observable limits of the receiver. No signal-to-noise improvement 

was obtainable because the field gradient was not great enough to measure. 
Many of the potential advantages of the differential concept 

could not be demonstrated at Geneva. It was apparent that a much 

greater antenna spacing would be required to measure field gradients. 

'lahis would decrease the operational capabilities of the system. Also, 

a major improvement in the receiver noise figure requires further development work and probably more sophistication in the receiver :s front-end 

and in the antenna system. 

Further tests should be made with the existing receiver, but 

at shallower mines and where external noise levels limit performance. The 

basic concept could then be evaluated and the decision made at that time 

whether further development is warranted. 
CONCLUSIONS AND RECOMMENDATIONS 

The results of the Geneva Mine experiments show the feasibility of 

detecting uplink signals, both surface and airborne, from intrinsically safe 

manpack transmitters in a mine with overburden depths up to 16.50 feet and 

-2 

overburden conductivities as high as 2. 1 X 10 mhos /m. Location information 

obtained from the magnetic field pattern behavior was not as accurate as 

anticipated but did nevertheless provide useful data on the general location of 

the transmitters. For the most part, the locations determined by the electromagnetic method were within one or two mine pillars (150-300 feet) from the 

actual transmitting location. Location discrepancies occurring at this mine 

are attributed to (1) terrain irregularities and (2) local conductivity changes 

which strongly affect the field behavior when overburden conductivities and 

overburden thicknesses are high. 

Optimum operating frequencies for a mine such as Geneva tend to be 

lower than the optimum frequency for shallower less conductive eastern mines. 

Based on the results of these tests, and performance predictions given in 

Section 2, the optimum frequency range for uplink signals at Geneva would 

probably fall between 500 and 1000 Hz. 

The equipment developed for this project performed according to 

specifications in all cases, and the manpack locator Model C842A 

exceeded specifications in its capability of receiving signals down to as low 

as 0. 03 p.A/m. The multichannel receiver designed for use in the helicopter 

is also suitable for use on the surface and provides the added flexibility of 

monitoring more than one frequency simultaneously. 111 marginal signal 

situations, a visual indicator such as a meter or an oscilloscope is required 

on the receiver to give added discrimination needed for a proper null determination. It is recommended that on future manpack receivers, a metering 

device be added to the receiver to cover such situations, 
Also, in view of the extremely low background noise observed at 

Geneva, it is recommended that more work be done on the receiver antenna 

and front end to lower its first circuit noise level and thus be able to take full 

advantage of extremely quiet electromagnetic conditions when they do occur. 

The horizontal detection range of the signals transmitted from the 

Geneva Mine varied from about 1000 feet to over 4000 feet depending on 

overburden depth. Furthermore, detection ranges of 8500 feet were obtained 

with the receiver located near the mine portal and the transmitters located 

throughout the mine. This latter phenomenon exists because of the electromagnetic coupling between the transmitter and receiver via the mine rails and 

cables. Computations of uplink field strengths were generally in good agreement with measured values, once again demonstrating the validity of the 

theoretical model used in the computations. 

The feasibility of multichannel reception from a helicopter was established 

using the 6 channel receiver built for that purpose. The signals received in 

the helicopter were sufficiently clean also for obtaining chart records, had a 

dc chart recorder been available for these tests, 

To summarize the present system's capability, we can say.that in over- 

-2 

burden conductivities of 2 X 10 or less, the system has a horizontal detection 

range of at least 4000 ft at a depth of 1000 ft and at least 2500 ft at a depth of 

1500 ft. Location accuracies are generally within 1070 of the overburden depth. 

The transmitter weighs 0. 75 pounds and will operate for a period of 72 hours 

on a fully charged 4 volt cap lamp battery driving a 500 ft long antenna of #12 wire, 

It is recommended that more tests be conducted in deep metal mines 

where intrinsic safety limitations do not exist to determine whether increased 

overburden depths and higher conductivities can be compensated for by increased 

transmitting moments within practical limitations in the mine. Also many 

metal mines do not have the conventional room and pillar entries of most coal 

mines and tests should be conducted to determine the feasibility of using other 

antenna configurations such as long wire antennas for location purposes in these 

mines. 
7. 0 REFERENCES 

[ 1 ] Performance Tests and Evaluation of Full Wave Location Transmitter, 

Westinghouse Georesearch Laboratory, Summary Report on Task I, 

USBM Contract H0220073 (Mod. #3), May 1973. 

[2] Mine Emergency Operations, Vol. I, Data Collection and Field Testing, 

(Section 111), Westinghouse Electric Corporation, Summary Report 

Task I, BuMines Contract H0210063, June 1972. 

[ 3 ] Surface Measurements of Electromagnetic Noise Components, A. J. 

Farstad. Appendix 111-B of Vol. I, Summary Report on Task I, 

BuMines Contract ~0210063, June 1972. 

[4] Brodsky, Harold (1960), The Mesa Verde Group at Sunnyside, Utah, 

USGS Open File Report No. 595. 

[5] Young, Robert G. (1966),"Stratigraphy of Coal-Bearing Rocks of 

Book Cliffs, Utah-Colorado, I' in Central Utah Coals, Utah Geol. & 

Mineralogical Survey Bulletin #8 0. 

[6] (1971) Keystone Coal Industry Manual, McGraw-Hill. 

[ 71 Wait, J. R. , "Criteria for Locating an Oscillating Magnetic Dipole in 

the Earth, I' Proc. IEEE, Vol. 59, June 1971. 

[8] Farstad, A. J., C. Fisher, Jr., R. F. Linfield and R. O..Maes, 

Development and Testing of a C W Electromagnetic Location System, 

Westinghouse Electric Corp. Technical Report 73-8H1-MINER-R2, 

USBM Contract H0220073, April 1, 1973. 
PART 11 

COMMUNICATION SYSTEM MODIFICATION AND DEMONSTRATION 

AT BRUC ETON EXPERIMENTAL MINE 

BY 

J. W. Allen 

CONTENTS 

1.0 INTRODUCTION 

Page 

1 

2.0 SYSTEM MODIFICATIONS 2 

2.1 TestlControl Fixture for Subsurface Stations 3 

2. Z Subsurface Station Modifications 3 

2. 3 Surface Station Modifications 7 

2.4 Laboratory Tests 

3.0 BRUCETON TESTS AND DEMONSTRATION 

3. 1 Surface Receiving Antenna Deployment 

3. 2 Installation and Checkout of Subsurface Stations 

3. 3 Demonstration of System to USBM 

4.0 CONCLUSIONS, RECOMMENDATIONS, AND FUTURE 

APPLICATIONS 

4.1 Conclusions 

4. 2 Recommendations 

4.3 Future Applications 

5.0 REF ERENC ES 
1.. 0 INTRODUCTION 

An engineering model of a Monitoring, Locating and Communication 

System was developed on USBM Contract HO220073 and installed in the 

USBM experimental mine at Bruceton, Pennsylvania to demonstrate the 

feasibility of a system concept. This engineering model, which consisted 

of three subsurface stations and one surface station, included multiple 

function capabilities and a variety of features for evaluation. Not all of 

the characteristics incorporated in the engineering model would be 

necessary or even desirable in a system designed for a working mine. 

Like most engineering models of developmental systems, some 

problems became apparent when the system was installed and tested in the 

USBM Experimental Mine at Bruceton. In order to satisfy USBM performance 

requirements, some modifications in design concepts and hardware implementation were deemed necessary. Task I1 of Contract HO 23 2049 provided 

for the modification of the hardware, reinstallation and test at Bruceton, 

and demonstration of system operation to USBM personnel. 
2.. 0 SYSTEM MODIFICATIONS 

Subtask 1 of the contract called for specific system rnodificatiolls 

and additions. These were determined to be necessary or desirable as a 

result of the tests performed during the tests at Bruceton cr, the previous 

contract. They are listed below: 

a) Fabricate one (1) Test/Control Fixture for use with 

subsurface electronics module s . 

b) Replace type CD4009AE and type CD4010AE integrated 

circuits with types CD4049AE and CD4050AE, respectively. 

c) Install additional regulators and filters in subsurface 

units to eliminate feedback and noise on voice uplinks. 

d) Modify power distribution in subsurface transmitters 

and eliminate resistive divider logic level shifters. 

e) Add test jacks, test function switches, and guide rails 

in subsurface units to facilitate maintenznce. 

f) Engrave and fill nomenclature on panels of subsurface 

electronics modules. 

g) Modify phone coupling circuits to incorporate coupling 

transformers and balanced shielded line to all stations. 

h) Modify audible alarm circuits to permit volume control 

of alarm signals. 

i) Extend rangeofSQUELCHcontrolinallvoice receivers 

j) Incorporate preamplifiers in surface ferrite antennas. 

k) Modify subsurface transmitter antenna tuning unit to 

facilitate tuning in the mine and to improve antenna matching at the voice uplink carrier frequency. 

1) Incorporate sensor excitation silpply in th.e subsurface units. 

m) Perform laboratory tests to define the performance 

characteristics of the modified system and deliver a set 

of updated drawings. 
The above listed modifications, together with others deemed 

necessary as a result of system tests, were incorporated in the system 

and are discussed in more detail in the following paragraphs. 

2.1 Test/Control Fixture for Subsurface Stations 

A Test/Control Fixture was designed and fabricated to 

facilitate testing and adjustment of the electronics modules from the 

subsurface stations. This fixture permits testing of the subsurface 

station circuits in the laboratory without removing the cabinet, battery, 

sensor, antenna and associated cables from the mine. It includes basically 

the same battery, speaker, indicator lights, switches, and internal 

connectors normally provided by the subsurface station cabinet. In 

addition, it provides dummy antenna loads f or the subsurface transmitter, 

test jacks for signal inputs and outputs, and a simulated sensor output 

voltage supplied from a 10-turn potentiometer. 

A separate dummy receiving antenna can be connected to the 

RECEIVER TEST input of the TestlControl Fixture to permit testing of 

the EM direct audio voice downlink receiver. 

Although no handset is provided on the Test/Control Fixture, 

a MIC AMPLIFIER input jack, an EARPHONE AUDIO output jack, and 

phone line terminals permit testing of the paging and voice telephony 

circuits. 

2. 2 Subsurface Station Modifications 

The majority of the modifications to the system were made on 

the subsurface stations. Two modifications, which were purely mechanical, 

were made to enhance the appearance of the units and to facilitate testing 

and maintenance. They were the engraving and filling of nomenclature 

on the panels and the incorporation of guide rails for the electronics 

modules. The guide rails made removal and insertion of the electronics 
modules much easier with less chance of damaging components on the 

circuit boards. 

A three position Test Switch was installed between the DATA/ 

REPLY shift register and the DATA/REPLY Voltage Controlled Oscillator 

(VCO) to facilitate measurement and adjustment of VCO frequency. In 

the MINUS position it couples a logic ZERO level into the VCO which then 

generates the lower shift frequency. In this position, Trimmer R-3 can be 

adjusted to set the lower shift. In the PLUS position it feeds a logic ONE 

level into the VCO, which then generates the upper shift frequency. In 

this position, R-2 can be adjusted to set the correct upper shift frequency. 

In the center or NORMAL position, it couples the shift register output 

into the VCO, which then generates a frequency shift-keyed (FSK) signal 

corresponding to the selected DATA or REPLY code. 

All type CD4009AE and type CD4010 AE integrated circuits 

were replaced with types CD4049~E and 4050AE, respectively. According 

to the manufacturer (RCA) these types are more rugged and reliable than 

the previously used types which had exhibited a high rate of unexplained 

failures. 

The power distribution in the subsurface transmitters was modified 

by the addition of filters, transistor switches and voltage regulators. Twelve 

volts DC from the battery was supplied directly to the transmitter output 

power transistors and driver transistors. Filtered 12 volts was supplied 

to the regulator inputs through a lowpass filter consisting of a 15 millihenry 

inductor and a 22 pf capacitor. A Fairchild type F-7808, three terminal 

regulator provided regulated 8 volts for the logic, while two Fairchild type 

F-7806, 6-volt regulators supplied the DATA/ REPLY VCO and the FM 

VOICE VCO. A transistor switch in series with the input to each 6-volt 

regulator turned the power on and off as required by the transmission mode 

selected. 
A lowpass filter consisting of a 10 Hy inductor and a 22 pf capacitor 

were installed on the inside of the cabinet door to filter the bias voltage for the 

handset carbon microphone. The use of the regulated 8-volt logic supply 

permitted elimination of the resistive divider logic level shifters formerly 

used in the transmitters. 

The phone coupling circuit was redesigned to incorporate a coupling 

transformer and a balanced line for paging and voice telephony. DATAIREPLY 

and FM VOICE carriers were coupled onto the 2-wire phone line as a 

common mode voltage to ground. This approach greatly reduces interaction 

between the voice signals and the carrier signals. 

A lowpass filter was incorporated between the phone line coupler 

and the voice uplink VCO to eliminate oscillations caused by feedback at the 

7 kHz voice carrier frequency. 

The electromechanical paging relay was replaced by a solid state 

circuit comprised of a full-wave rectifier, lowpass filter, threshold detector, 

logic level converter and bilateral analog switch. This circuit, which is 

activated by a paging voltage of either polarity in the range of 5-30 volts, 

activates the loudspeaker amplifier for paging and the 7 kHz FM transmitter 

for through-the -earth voice uplink. 

The single pole PRESS TO PAGE switch which previously connected 

one side of the phone line to circuit common (ground) was replaced with a 

double pole switch which simultaneously connects one side of the phone line 

to circuit common and the other side to the plus 12 volt supply through an 

isolation inductor. 

The audible alarm circuit was modified from the old configuration 

which employed the audio power amplifier as a feedback oscillator to the 

new configuration which employs a separate alarm oscillator (an NE 555 Timer) 

and permits volume control of the alarm signals. The front panel VOLUME 
control now controls volume of both alarm signals and EM voice downlink 

signals. An internal trimmer sets the volume of voice paging signals. 

A sensor excitation circuit was added which supplies a regulated 

8 volts to the Abirko "Flowmaster'! Air Velocity Sensors. A fuse holder 

was installed in the side of the subsurface station cabinet near the sensor 

input connector. The fuse was connected in the sensor excitation circuit 

to protect the voltage regulator if the sensor chassis should get shorted 

to the station circuit ground. (The plus side of the sensor supply is 

connected to the sensor chassis and requires insulation from the station 

circuit ground . ) 

The sensor threshold circuits were referenced to the plus 8-volt 

supply and the RISE/FALL switch rewired to accommodate the sensor output 

voltage which decreases (changes from plus 8 volts toward ground) as the 

air velocity into the sensor increases. 

The 2qonductor phone plug provided on the Abirko anemometer 

for a remote meter was changed to a 3-conductor to provide sensor 

excitation input as well as the output to the subsurface station sensor 

input. A bypass capacitor was added in the anemometer circuit to 

prevent EM signals radiated by the subsurface station transmitter from 

affecting the sensor readings. 

The transmitter antenna coupling and tuning circuit were modified 

to facilitate tuning in the mine and to improve antenna matching at the voice 

uplink carrier frequency. The fouy-conductor cable between the station 

cabinet and the antenna tuning unit was replaced with a six-conductor cable. 

One of the added wires was used to connect a second tap on the output 

transformer to the VOICE mode antenna tuning capacitor. The other was 

connected to a remote tuning indicator light mounted in the tuning box. 

The subsurface voice downlink receiver squelch circuit was 

modified to provide a wider range of squelch control. A three-position 
gain control switch was added to provide reductions of 20 and 40 dB in 

receiver gain for use in strong signal locations such as the Bruceton 

mine. 

A number 12 AWG conductor was added in parallel to the printed 

circuit ground strip on the receiver PC card to improve receiver stability 

and reduce the tendency to oscillate. In addition, certain ceramic coupling 

and bypass capacitors which were determined to be microphonic, were 

replaced with Mylar film capacitors . 

In addition to the modifications listed above, several components 

were changed or replaced to provide more stable or reliable operation. 

As an example, some electrolytic coupling and filter capacitors were 

replaced with monolythic capacitors to reduce effects of leakage currents. 

The DATA/REPLY VCO1s were retuned to provide a + 15 Hz 

frequency shift about their center frequencies to allow a wider variation in 

environmental temperature before oscillator frequency drift becomes a 

problem. 

2.3 Surface Station Modifications 

The receiving antenna configuration of the surface station was 

extensively modified to provide better overall performance and greater 

operational flexibility. Preamplifier units were fabricated for three ferrite 

core receiving antennas. The antennas were mounted on the lids of the 

preamplifier boxes and covered by protective fibreglass domes similar to 

the ones used to house the downlink receiving antennas in the subsurface 

stations. 

The preamplifiers incorporated a FET input circuit, a 1 kHz highpass filter, a voltage amplifier, an output emitter follower, and a balanced 

line matching transformer. A balanced RG- 22/U cable connected each 
prc.nmplific r to a 1 ightning protection unit located nca 1- t.h t. inp~lto 

the surface. rc.ct.ivcr. The same cables provided powcr Cor tht. prcXa~nplificrs from a source in the surface receiver. Thc prt\,znTps arc 

cnpabl~. of driving cables of at least 4000 foot length. 

Thc lightning protection unit contains magnetic gap type 

surge arrestors. series bifilar inductors and 50 watt zener diodes to 

limit voltage transients appearing at the receiver input terminals . 

In addition, fifty watt zener diodes are used from each line to ground 

in the preamp units to provide protection for the preamp circuits. 

The antenna coupler module in the surface receiver was 

modified by the addition of three balanced input transofrmers to match 

the preamplifiers. The fourth channel (A-4) in the antenna coupler 

was left unmodified to accommodate high impedance loops or as a convenient 

input for test signals, etc. A fuse was installed in the antenna coupler 

module and connected in the circuit which supplies 1% volts to the pre - 

amplifier s . 

The alarm circuit was modified to permit control of the alarm 

signal volume by the front panel VOLUME control. A 1 kHz tone burst 

is derived from the clock circuitry in each decoder/display module and 

connected through a diode "ORt' circuit to the audio amplifier circuit in 

the surface station. 

All type CD4009AE integrated circuits were replaced with 

type CD 4049AE to improve reliability. This change necessitated a 

reduction in value of the biasing resistor in the 1 kHz clock oscillator 

to maintain the proper frequency. 

To reduce the false reply probability, a type CD4017AE, 

decoded decade counter was added to the output of each reply pattern 

decoder and wired to require a total of 4 out of a possible 6 input 
pulses before it activated the reply indicator. The reply driver time 

constant was tailored to be approximately 5 seconds. These modifications seemed to completely eliminate the problem of ambiguous replies 

(2 or more showing simultaneously) that was previously observed. 

The FM voice uplink receiver squelch circuit was modified to 

provide a wider range of squelch threshold control. 

The phone coupling circuit was modified to incorporate a coupling 

transformer and balanced 2-wire phone line for paging and voice telephony. 

A solid state paging circuit, similar to the one used in the subsurface 

stations, replaced the electromechanical paging relay. As in the subsurface stations, the single pole paging switch was replaced with a 

double pole unit which simultaneously connects circuit common to one 

side of the phone line and t 12 volts to the other side. 

Laboratory Tests 

After incorporation of the modifications described previously, the 

system was tested in the laboratory. Gain, frequency response, and maximum 

signal capability of the surface ferrite antenna preaplifier were measured. 

Capacitors and resistors were installed to simulate 4000 feet of cable 

between the preamp and the surface station and the resultant loss measured. 

Since the loss was less than 1 dB, the preamplifiers could probably be 

located up to 2 or 3 miles from the surface station and still provide satisfactory operation. 

Operational tests of the modified anemometers and sensor input 

circuits were conducted. These tests revealed need for a bypass 

capacitor in the anemometer to eliminate the effects of EM signals 

radiated by the subsurface station transmitter. 

The new solid state paging circuit which replaces the electromechanical paging relay was tested to verify that it would operate properly with paging 
voltages of either polarity ranging from 5 to 50 volts. 

The subsurface station voice downlink receivers were functionally 

tested by inducing EM voice signals into their receiving antennas. The 

output of a tape recorder was connected to a small ferrite loop antenna 

which acted as the EM signal source. The receiving antenna orientation 

was varied, together with the amount of coupling to the EM signal source 

to provide a wide range of signal amplitudes and signal-to-noise ratios 

for testing operation of the receiver gain and squelch controls. 

The three subsurface stations and the surface station were set 

up at different locations within the laboratory building and interconnected 

with a 2-wire cable. Functional tests were conducted to verify proper 

operation of voice paging; voice telephony; FM voice transmission and 

detection; reply message transmission, detection and display; and 

data sensor status transmission, detection and display. 

No actual EM transmission tests were performed since the 

antennas remained in their original Bruceton installations ; however, the 

transmitters were tested using the dummy loads in the Test/Control 

fixture. 
3.0 BRUCETON TESTS AND DEMONSTRATION 

Before reinstalling subsurface station equipment in the mine at 

Bruceton, the subsurface stations were set up at different buildings on 

the surface and connected to the surface station in Building 07 by a common 

twisted pair cable. The distance between stations was approximately 

70 to 100 feet. 

Functional tests of the voice paging, voice telephony, Reply 

messages, and Sensor status monitoring were performed by WGL and 

USBM personnel. The Data/Reply VCO in Station 2 was found to be 

unstable during these tests. The integrated circuit (CD4007AE) was 

replaced and the oscillator frequency observed. Stability appeared to be 

greatly improved. 

3.1 Surface Receiving Antenna Deployment 

The three surface ferrite core receiving antennas and their 

associated preamplifiers were deployed as nearly above the three subsurface stations as possible. Roads, buildings, property line s, and a 

limited amount of RG-22/U Twinax available (2000 feet) determined the 

choice of locations. 

Figure 3-1 is a plan view showing the location of the subsurface 

stations, the surface station, and the receiving antenna locations. Antenna 

A-1 was located at the edge of the fire break along the boundary fence 

near the large water storage tank (Structure 0-66) on the hill. It 

was approximately 540 feet from the point directly above Station No. 2. 

To place the antenna closer to the desired location would require going 

outside the USBM property fence. 

Antenna A-2 was located near the edge of the road just below 

Building No. 41. It was approximately 400 feet from the point on the 

surface above Station No. 2. If additional Twina-x cable were available, 

A-2 could be placed directly above Station No. 2. 

Antenna A-3 was located approximately 100 feet from the point 

above Station No. 20. Buildings and roads prevented any closer placement 

to the desired location. 

The lightning protection network was installed in the large metal 

JIG box on the concrete bulkhead outside Building 07. It was earth 

grounded through a heavy copper wire to a copperweld rod driven into 

the ground directly below it. 

Observation with an oscilloscope of the amplified signals from the 

three preamplifiers revealed that A-1 was quite free from 60 Hz synchronous noise, A-2 had a high level of 60 Hz synchronous noise, and 

A-3 had a moderate level. Antenna A- 2 was oriented to minimize the line 

synchronous noise; however, the level was still objectionable. 
3. 3 Installation and Checkout of Subsurface Stations 

All three subsurface stations were reinstalled at their original 

locations ( s e e F i g u r e 3-1) and connected to their antennas, chargers. 

and phone line. Air velocity sensors were installed at Stations #1 and #20. 

No sensor was available for Station #? so it was set up to give a NORMAL 

indication with no sensor connected. 

During the system checkout several problems were encountered 

and repaired. The most severe problem encountered was chatter of the 

antenna tuning relay when the transmitter was properly tuned in the DATA/ 

REPLY mode. This was determined to be caused by EM coupling from the 

transmitter antenna into the phone line where it was rectified by the 

steering diode bridge and activated the paging circuit. Activation of the 

paging circuit caused the transmitter to switch to the VOICE mode and the 

antenna relay to close. As soon as the antenna r e l a y t r a n s f e r r e d to the 

voice position, the EM feedback loop was broken, the unit switched back 

to the DATAIREPLY mode and the cycle began again. 

The problem was solved by installation of a lowpass filter at 

the input to the steering diodes from each side of the phone line to ground. 

No antenna current indication could be obtained for Station #20 in 

the VOICE uplink mode. The problem was finally traced to a piece of dirt 

lodged between the armature and the coil of the antenna tuning relay. Removal 

of the dirt cured the problem and the station operated normally. 

During checkout of Station # 2, the DATAIREPLY VCO had to be 

retuned because it was low in frequency by about 10 Hz. Further monitoring 

indicated it was still drifting and behaving in an erratic manner. After 

demonstration of system operation, it was removed from the mine and 

returned to WGL in Boulder for further testing and repair. 

3.3 Demonstration of Svstem to USBM 

After completion of system checkout, WGL personnel demonstrated 
system operation to Mr. Bob Bradburn of USBM, who had been designated 

by the T. P. 0. as his representative. 

Voice paging, voice telephony, data status monitoring, and coded 

reply messages were received at the surface station via phone line from 

each subsurface station. The phone line was then disconnected and 2-way 

EM communications were demonstrated. Mr. Bradburn asked questions 

via the direct audio voice downlink, and coded reply message responses 

were received from the subsurface stations. 

The FM voice uplink was also demonstrated. Voice quality and signalto-noise ratio were acceptable from Station No. 1 using Antenna A-1 (the closest 

one), which was about 540 feet away. Signal-to-noise ratio was not 

adequate from Station No. 1 using the other two surface receiving antennas. 

FM voice uplink signals from Station No. 2 were very marginal because of the high level of 60 Hz synchronous noise at surface antenna 

location A - 2. 

FM voice uplink from Station No. 20 was acceptable after repair 

of the antenna tuning relay (previously discussed); however the signal-tonoise ratio was not adequate for high quality voice communication. 

After completing the demonstration at the surface station, Mr. 

Bradburn went into the mine for demonstration of the subsurface stations. 

Three -way paging and voice telephony were conducted between the surface 

station and two subsurface stations. Again, two-way through-the-earth 

EM communications were demonstrated with Mr. Bradburn operating the 

subsurface station. 
4.0 COTJC LUSIONS, RECOMMENDATIONS, AND FUTURE APPLICATIONS 

The time allotted for the reinstallation of the system and demonstration to USBM did not permit extensive in-situ testing of the modified 

system. The conclusions presented herein are based, therefore, on a 

limited number of experiences with the system, rather than statistically 

meaningful figure s and data. 

4.1 Conclusions 

The modifications incorporated in the system have improved the 

performance and quality of the paging and voice telephony operational 

mode of the system. The modifications to the subsurface voice receivers 

have reduced the objectionable noise output and have improved reception 

of downlink voice signals. 

The addition of guide rails and test switches have improved the 

maintainability of the subsurface stations. The Test/ Control Fixture 

provides a useful tool for setup and testing of the subsurface station 

electronics modules outside the mine. Addition of regulators, filters, 

and trans istor switches in the subsurface transmitters, together with 

replacement of the CD4009AE and CD4010AE integrated circuits should 

improve the reliability and performance of the system. 

Addition of the CD4017AE counter in the Reply message decoders 

should greatly reduce the probability of false or ambiguous replies. The 

clarity of the reply certainly justifies the additional delay (approximately 

5-10 seconds) before the message is displayed. 

The addition of preamplifiers to the surface recr iving antennas 

permits better selection of a suitable location for them. The limitations 

imposed by surface developments such as buildings, roads, power lines, 

parking lots and fences, together with a limited amount of Twinax cable 

on hand, did not permit sufficient optimization of the signa-1-to-noise ratio 
for good quality voice uplink signals. Previous theoretical studies [ 1 ] 

have shown that the limitations imposed by permis sibility requirements 

and high surface noise environments would restrict the usefulness of such 

systems to mines with very shallow overburdens and/or very low conductivic,. 

One system problem presently remains unresolved, the instability 

of the subsurface station DATA/REPLY VCO1s. The present VCO uses 

a type CD4007AE COS MOS integrated circuit in a voltage controlled 

multivibrator developed by RCA. With a regulated voltage source and 

components selected to minimize temperature effects , the circuit exhibits 

excellent short term stability and acceptable temperature stability . The 

drift during warmup, however, and unexplained frequency changes when the 

unit has been turned off for a period of time, make it unsuitable for long 

term unattended operation. WGL is presently investigating other circuits 

suitable for this application under an in-house funded research program. 

4. 2 Recommendations 

It is recommended that USBM personnel exercise the present system 

on a regular basis (say, weekly) to accumulate performance statistics 

and a record of operational characteristics. Performance of each of the 

various operational modes should be tested and the results recorded. 

System maintenance should also be recorded in detail, since component 

failures or the need for frequent adjustments would indicate the need 

for adesign change in future systems. 

The use of a system concept which employs independent, modular 

subsystems is recommended for future systems. This concept permits 

tailoring the system to the requirements of a particular mine or installation, 

facilitates maintenance, and provides flexibility for future expansion or 

modification. 
4. 3 Future Applications 

The overall objective of this task was to develop a quasi-hardened, 

fixed location, in-mine communications subcenter for use in normal mine 

operations and for post disaster rescue operations. Information from a large 

number of subcenters was to be gathered together on a single display at 

the central station. The earth overburden is used as a 2-way communication 

medium, but the signal path can be changed to use existing mine phone lines 

or to use both links simultaneously. 

This task has now been completed including the design, fabrication, 

installation, and demonstration of an engineering model of the system using 

three subcenters. It is useful, in retrospect, to re-evaluate the basic system 

concepts based on the results achieved, to discuss problem areas, and to 

consider potential applications in operating mines. 

1. The basic concept of transmitting digital data and beacon 

codes through the earth to a central collection point from 

multiple stations is entirely feasible, using different 

frequency allocations in the 3-5 kHz band. The signaling 

depth is limited by conductivity of the earth, the noise 

environment, and the antenna moment. Theref ore, the optimum 

system should utilize a transmitter power amplifier and 

antenna system designed to maximize antenna moment and 

still remain intrinsically safe. It may be desirable to 

increase the number of condition monitors available per 

station. This could be accomplished either by reducing the 

number of beacon replies or by increasing the code word 

length. 

2. The use of the two-wayvoice link via phone line is useful 

since it permits an interface to the system from remote 

conventional mine pagers, provides a redundant link to the 

surface, and is a convenient means to check out both the 

phone line and through-the-earth links from a single point 

on the surface. 

3. The performance of the NBFM voice uplink through the 

earth using 7 kHz carrier was marginal. This is attributed 

in part to the bandwidth limitation of the antenna system, 

but mostly to the high noise environment existent over the 

Bruceton mine. Acceptable operation of this link through 

1000 feet of overburden with conductivities of 10-2 mhoslrneter, 
using an intrinsically safe transmitting system is 

questionable unless the surface noise environment 

is extremely low. The NBFM link can be used on the 

phone line and by using different carrier frequencies 

provides additional 2-way voice communications on 

separate channels using a single phone line. 

4. The direct audio voice downlink function is useful in 

emergencies, since subcenters equipped with direct 

audio receivers and beacon code transmitters 

provide two-way links to the surface. For daily 

operations this link may be impractical because the 

surface transmitter requires deployment of an antenna 

over the desired coverage area. This may be impractical 

for large mines; however limited coverage can be 

provided over certain areas using mobile equipment 

deployed only for an emergency. Such a system has 

added merit when individual miners are also equipped 

with manpack voice receivers. 

5. It is also possible to incorporate a call-alert feature 

in the system whereby the phone line is used to activate 

the subcenter transmitter and by selective addressing 

codes alert miners equipped with portable receivers. 

This function was not included in the present system 

but could be incorporated in future systems to enhance 

their usefulness. 

Based on our experience with the present system and the above 

analysis, it is possible to define second generation prototype equipment 

which could be fabricated and demonstrated in an operating mine. 

The surface center would provide basic functions and incorporate 

features as follows. 

2-way simplex voice commu.iiications via phone line to 

any one or all subcenters using direct audio or on any 

one of six separate channels using narrowband frequency 

modulation (NBFM) . 

e Selective calling via phone line and subcenter EM 

transmissions to any one of 30 addressable call-alert 

receivers plus one common all-call address. 
(B Receiver system for sensing and displaying the 

through-the-earth transmissions from each subcenter for automatic monitoring of in-mine 

condition sensor and for manually generated code 

responses. 

cs Handset for normal voice communications and 

loudspeaker for paging and audible alarm. 

s Indicator lights to display three (3) levels of 

condition sensor data from 3 sensors at each subcenter. 

e Display lights to indicate reception of any one of 

three beacon codes from each subcenter. 

e A means of indicating which of the six (6) NBFM 

channels are in use on the phone line. 

e A method of calling any one of 30 addressable 

call-alert receivers located within detectable 

range of a subcenter which automatically retransmit such signals using a through-the-earth 

mode. 

e A means for generating a distinguishable all-call 

signal which, when retransmitted by all subcenters, 

can be received by every call-alert receiver within 

detection range of the subcenters . 

Each in-mine subcenter should be designed on the basis of 

intrinsic safety and would provide: 

e 2-way voice communications via phone line to any other 

station using direct audio or on any one of six channels 

using NBFM. 

9 Full-wave type transmitter and loop antenna capable 

of transmitting 1) automatic condition data from a 

single sensor, 2) coded beacon responses, 3) callalert signals to portable receivers. 

9 A direct audio voice receiver and antenna system 

capable of receiving voice signals and call alert 

signals which are transmitted through the earth from 

the surface. 
a Operation from a .f 2-volt battery which is continuously 

trickle-charged from the 440 volt AC power sled. 

s Automatic condition monitoring of 3 sensors at approximately 10 minute intervals using three adjustable sensor 

ranges to indicate normal, caution, and alarm conditions. 

a A means to interface voice signals received on any 

voice channel to the 470 MHz transmission system in the 

mine. 

o A full-wave class S transmitter and resonated loop 

antenna system capable of retransmitting throughthe -earth the call-alert signals received from the 

surface center, the three coded beacon responses and 

the automatic condition monitor data. 

A means of indicating which of the six (6) NBFM 

channels are in use. 

A handset for originating and receiving voice 

transmissions via the phone line. 

a A direct audio voice receiver and loudspeaker for 

receiving through-the-earth voice transmissions . 

The portable call-alert receivers would provide both visual 

and aural means to indicate reception of a preselected address code and 

be capable of distinguishing between normal call-alert signals and an allcall signal. 

This prototype system should be fabricated using solid state 

circuitry whenever feasible and should utilize modular construction 

techniques so that certain basic functions can be easily expanded or 

omitted without affecting performance of remaining functions. Most 

of the basic subsystems required for this prototype system have already 

been developed and dem.onstrated. Areas where further work is necessary 

include: 
ar The design of a stable frequency generator for FSK 

data transmissions . 

e Development of an address code for the call-alert 

system and the portable receivers. 

s Breadboarding and testing the multichannel NBFM 

voice system for use on phone lirte and develop 

method indicating channel usage. 

a Developing methods which may be either hardwire 

or radio for relaying data from surface antenna 

locations to central station. 

a Interfacing voice signals between subcenter voice 

channel and 470 MHz transmission system in mine. 
5.0 REFERENCES 

[$I "Monitoring, Locating and Communication Sys tem for Normal . Mine Operating and for Post-Disaster Rescue Operations, l1 

J. W. Allen and R. F. Linfield. Submitted to USBM on Contract 

H0220073, 1 April 1973. 
PART 111 

ARMCO #9 LOCATION SYSTEM DEMONSTRATION 

BY 

R. F. Linfield 

CONTENTS 

INTRODUCTION 

TEST CONDITIONS AND RESULTS OBTAINED 

CONCLUSIONS AND RECOMMENDATIONS 

Page 

1 
1.0 INTRODUCTION 

A test and demonstration of the EM location system was conducted 

on June 2 and 3, 1973 for the Bureau of Mines at the Armco #9 coal mine 

near Bandytown, West Virginia. This effort was conducted as a supplemental agreement to the contract to cover the cost of equipment checkout 

and for engineering services required at the mine. 

Two different types of transmitters were furnished; 1) a portable 

half-wave transmitter mounted in the top of a miner's cap-lamp battery 

and using a 90 ' loop antenna enclosed in a reel, and 2) a full-wave belt 

type transmitter using a vertical magnetic dipole (VMD) antenna consisting 

of 500 feet of #12 AWG wire. Operating frequencies were 2900 Hz and 

2500 Hz. Spare transmitter units of each type were furnished to insure 

a successful demonstration. Three manpack location receivers were also 

supplied, along with one helmet type receiver which could be used to 

demonstrate the call-alert concept. 

A three-man team consisting of 1 engineer and two technicians 

departed from Boulder on June 1, 1973 to check out the system in the mine 

on Saturday, June and then to demonstrate its performance to approximately 

40 visitors on Sunday, June 3. 

The test results obtained were only qualitative but are presented 

here because the operational experience gain at this exercise could have 

useful implications at future real emergencies. 
2.0 TEST CONDITIONS AND RESULTS OBTAINED 

The Armco #9 Mine is located near Bandytown, West Virginia 

about one and a half hours ' drive from Charleston. The mine portal is 

on a ledge at about 1600 feet elevation. Seismic and EM equipment vans 

were located on this ledge near the portal. A 15 coal seam (of which 7' 

is mined) is exposed at the bottom of a 200' cliff cut into the mountainside. 

Access to the area over the mine could be gained via four-wheel drive 

vehicle or on foot. MTorking depth for the demonstration exercise was 

350' to 400'. Distance from the portal to the in-mine preselected 

location was approximately 1300 feet. The surface over the mine was covered 

with heavy vegetation (trees and bushes) which, along with the steep slopes, 

made it extremely difficult to traverse. Since the primary objective of 

this task was to demonstrate the EM location system capabilities, no 

quantitative performance data was taken, nor is the conductivity of the 

overburden known. Test and demonstration results are, however, useful 

from an emergency operations standpoint. 

The WGL team arrived on site at 10:CiO am on Saturday. One 

2900 Hz full-wave transmitter was operational within 10 minutes after 

two men arrived at the preselected in-mine location. This installation 

time includes the deployment of the 500' of #1 2 wire by two men. This 

lo op antenna was wrapped around three pillars of coal in a single turn 

rectangular configuration approximately 50 by 200 feet. The 2500 HZ halfwave transmitter was set up by one man in less than five minutes, including 

the deployment of the 90 ' reel antenna, into a circular configuration within 

an entry cross-cut. This separation between the centers of the two loop 

antennas was 50 feet, as scaled from the mine map. 

The antenna moments of the full-wave transmitter at 2900 Hz 

was about 20 times that of the half -wave system, at 2500 Hz. , i. e. , 

2 1800 as compared to 90 ampere-turn-meters . 
After installing the antennas and leaving the mine, the location 

receivers were operated outside and near the portal, 1400 feet away 

from the transmitters. The full-wave transmitter 2900 Hz was 

immediately detectable and a null line determined immediately by 

rotating the receiver loop in the vertical plane. The half-wave transmitter at 2500 Hz was not detectable outside the portal at the 1400 foot 

range. 

The location receivers were subsequently transported to the 

hilltop over the mine workings via a 4-wheel drive vehicle. TWO surface 

locations over each transmitter were then determined within about 

20 minutes, beginning about 400 feet away at the point where the vehicle 

was parked. It has since been estimated that the surface location over 

the 2900 Hz transmitter could have been determined beginning at the 

portal and walking the null line (with deviations around the cliff) in 

approximately one hour. Most of this time would be spent climbing the 

steep slope and making a path through the thick underbrush. 

It turned out that the null location of the half-wave transmitter with 

the smaller antenna was easier to determine even though the signal amplitude was about 20 dB less than that obtained with the full-wave system. 

This is attributed to the smaller antenna generating a dipole moment 

which appeared as a point source, of the dimensions of the full-wave transmitting antenna approached the mine depth and appeared as a much larger 

source. 

The three team members independently located the half --wave 

transmitter, and these location points were all within five feet of each 

other. This location was paced off and was found to be about 15 feet 

downslope from the surveyed location. This error could be the result 

of a 30" slope of surface terrain. 
The null locations obtained on the full-wave transmitter by team 

members varied an estimated 20 feet with points on a line along the long 

axis of the subsurface transmitting antenna. The average location 

obtained was about 50' away from the other location and in the right 

direction. 

Upon leaving the surface area, periodic stops were made along 

the road to obtain a qualitative measure of detection range. 

The half-wave transmitter (2500 Hz) was detectable an estimated 

1000' from the null location. The full-wave transmitter was detectable 

up to about 3500 feet. 

A helmet receiver operating at 2900 Hz was used to demonstrate 

the call-alert concept whereby a location type transmitter can be used 

to alert miners. Signal reception was obtained in the mine from the 

full-wave transmitter. Outside the portal the unit was inoperative. The 

existence of other cables and conductors in the entry traversed probably 

contributed to the range achieved with this unit. 

During the actual demonstration on Sunday, June 3, one member of 

the WGL team was stationed inside the mine to describe and demonstrate 

the transmitter operation to the groups of visitors. Another team member 

was stationed on the hill and remotely demonstrated the null location 

process via a special TV link to the comm~nications van and TV monitors 

near the portal. The third team member set up spare transmitters and 

receivers near the portal so visitors could see the location equipment and 

hear the transmitted signals. 

The EM location system's transmitters caused considerable 

interference with other EM systems being demonstrated and therefore 

could only be operated intermittently during the demonstration. 
CONCLUSIONS AND RECOMMENDATIONS 

Although the objective of this task was primarily to demonstrate 

the EM location system capabilities to the Bureau of Mines, the experience 

gained on this exercise could be applied to future emergency operations. 

Since no quantitative data was obtained, only generalized conclusions can 

be given concerning the system's performance. 

The Armco #9 mine is typical of many such mines in West Virginia. 

At such mines the heavy vegetation and steep terrain could seriously increase 

the time required for surface searches of trapped miners, particularly in 

the larger mines where a considerable area must be covered. The use of 

airborne receivers in helicopters could reduce this search time if adequate 

signals and detection capabilities can be achieved and if weather is suitable 

for flying. Accurate surveying over this type of mine could be extremely 

difficult because of the terrain and vegetation. Locations obtained in a real 

emergency usually must be related to the mine map, using tie points to 

determine drilling points for life support and rescue. This could be time 

consuming because of the scarcity of the points from the mine workings to 

the surface. As more experience is gained with the EM location system, its 

accuracy may be refined using slope corrections. The system itself could 

then be used as a simple means for locating the points on the surface which 

could later be used to establish locations relative to the mine map during 

an emergency. The system also could be used to determine surface boundaries 

of mine workings. 

When the EM location system is operated in conjunction with other 

EM communication systems, the interference caused by one system on the other 

must be considered. The EM location transmitter does interfere with direct 

audio voice reception in manpack receivers, although the 10% duty cycle allows 

reception between signal pulses. A miner equipped with both an EM location 

transmitter and a voice receiver could of course communicate with the surface 

by shutting off his transmitter to receive voice messages.