Bureau of Mines Report of lnvestigations/l984 

A Short-Range Locator System 

for Detecting Trapped Miners 

By William E. Webb, R. H. Church, 

Walter E. Pittman, Jr., and James R. Boyle, Jr. 

UNITED STATES DEPARTMENT OF THE INTERIOR 
UNITED STATES DEPARTMENT OF THE INTERIOR 

William P. Clark, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 

Research at the Tuscaloosa Research Center is carriedout under a memorandum of agreement between the 

Bureau of Mines, U.S. Department of the Interior, and the University of Alabama. 
Library of Congress Cataloging in Publication Data: 

i 

A short-range locator system for detecting trapped miners. 

(Bureau of Mines report of investigations ; 8844) 

Bibliography: p. 18-22. 

Supt. of Docs. no.: I 28.23:8844. 

1. Mine rescue work-Equipment and supplies. 2. Radio-Transmitters and transmission. 3. Mine communication systems. I. Webb, 

William E. 11. Series: Report of investigations (United States. Bureau of Mines) ; 8844. 

. 
CONTENTS 

Page 

Abstract ....................................................................... Introduction .........e......................................................... 

Technical background ........................................................... Short-range locator system requirements ...........................e.........m.m 

Range .....................eee...............em............................... 

Frequency ...............m.............e..................e.................m. 

Power supply ..................................m..................a....m.m.... 

Cost ...............*..*..............m..........mmmm...........m............. 

Worker acceptance ............................................................ Design of transmitters ........mm.................................m...........m. 

Test results ...................m.....m.......m.m............................... 

Propagation through air... .............emmm..m..m.......e..........m......... 

Propagation through air-rock path ............................................ Propagation through rock ...............m..................m.................. 

Underground test. ..................em.m.........mm..........mmm.........m.... 

Prototype transmitter .....................m.....................m.............. 

Additional features ...........m................................................ 

Battery conservation circuit ................................................. Personal coding ............................m.......................m......... 

Rescue team units .................mm..........................e........m..... 

Safety considerations ................m...........mm.....................m...... 

Permissibility. .......em.......m..................m......................mm

Ignition of blasting materials.... ...........m..m.....m.....m.........e...... 

Conclusions ..........ee...ee.....m........mm.................................

Bibliography ........m...e...............................m.....m.....mm....m

ILLUSTRATIONS 

Schematic representing 660-kHz AM transmitter (unit 1). .................,, 

Schematic representing 27-MHz transmitter (unit 2) ........................ Schematic representing 27-MHz power amplifier (unit 3).. ................,, 

Schematic representing 27- to 60-MHz transmitter.. ........................ Unit 1 being placed in box ................................................ Overburden being placed over transmitter .................................. Test holes being drilled .................................................. Plan and elevation view of test holes at the Nickel Plate site ............ Plan and elevation view of test holes at the Drummond site ............... Transmitter being lowered into test hole .................................. Signal strength measurements being taken at toe of highwall ............... Typical plot of signal strength versus range through rock ................. Plot of signal strength versus range for airpath .......................... Schematic representing underground test location .......................... Transmitter placed inside cap lamp battery ................................ Schematic diagram of cap lamp transmitter ................................. Cap lamp battery voltage versus time of discharge ......................... Schematic diagram of low-voltage sensor ................................... Low-voltage sensor operational setup. ..................................... Cap lamp battery voltage versus time with automatic shutdown of lowvoltage sensor ........................................................... Light sensor output volts with low-voltage sensor ......................... 
TABLES 

Page 

1. Transmitter signal range in air, unit 2................................... 7 

2. Signal range of transmitter covered with varying thicknesses of rock, 

unit 2................................................................... 7 

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

A ampere pH microhenry 

dB/m decibel per meter MHz megahertz 

f t feet min minute 

g gram mV millivolt 

h hour pv microvolt 

Hz hertz mW milliwatt 

in inch S2 ohm 

kHz kilohertz pct percent 

ki2 kilohm pF picofarad 

mA milliampere V volt 

PF microfarad W watt 

mH millihenry 

4 
A SHORT-RANGE LOCATOR SYSTEM FOR DETECTING TRAPPED MINERS 

By William E. Webb, R. H. Church, Walter E. Pittman, ~r., 

and James R. Boyle, Jr, 2 

ABSTRACT 

As part of the Bureau of Mines health and safety research program, a 

short-range locator system, capable of locating miners who have become 

trapped following a mine accident, has been devised, The system consists of a lightweight, low-power radio transmitter incorporated into a 

miner's cap lamp battery and a directional receiver carried by rescuers. 

Prototype transmitters have been constructed, and the propagation of 

their signals has been investigated, The detectability of the signal in 

underground coal mines has also been investigated, This research indicates that a short-range location system would serve as an economical 

and practical means of locating miners from within the mine following an 

accident, 

- ' physical scientist, Tuscaloosa Research Center, Bureau of Nines, Tuscaloosa, AL; 

professor of electrical engineering, University of Alabama, Tuscaloosa, AL. 

2~ining engineer, Tuscaloosa Research Center. 

3~echnical information specialist, Tuscaloosa Research Center; professor of history, Mississippi State University for Women, Columbus, MS. 
INTRO 

Experience in postdisaster rescue work 

has indicated a need for a reliable means 

of quickly locating trapped miners. This 

was particularly evident after the 1968 

explosion at the Consol No. 9 Mine in 

Farmington, WV, where the whereabouts and 

condition of 76 men were unknown. This 

event prompted the enactment of the Coal 

Mine Health and Safety Act of 1969. The 

t ragedy at Farmingt on focused national 

attention upon the problems of mine 

safety, including those of postdisaster 

rescue. 

At the request of the Bureau of Mines, 

the National Academy of Engineering 

created the Committee on Mine Rescue and 

Survival Techniques to "conduct a study 

program to assess the technological capabilities that can be applied to survival 

and rescue techniques following mine 

disasters." The Bureau was particularly 

interested in the possible application of 

new technology to the problem of mine 

safety. The Committee on Mine Rescue and 

Survival made many recommendations for 

research in communications and mine rescue. The suggestions in regard to communication systems led to several Bureau 

programs to investigate through-the-earth 

commnications and trapped miner location 

systems. 

Most of the work on electromagnetic 

location was directed toward devising 

relatively long-range systems for locating an underground transmitter from the 

surface. Although these systems would 

undoubtedly prove to be of great value in 

future rescue operations, they would 

likely have several limitations: 

1. Owing to the expense and the 

weight added to the miner's personal 

equipment, it might not be economically or operationally feasible to 

equip every miner with a transmitter. 

Thus, the availability of the transmitter to the trapped miner would be 

uncertain. 

2. The trapped miner would have to 

take some action, such as deploying an 

antenna, that might be impossible if the 

miner were injured or confined. 

)DUCT ION 

3. Since highly specialized location 

equipment would be required, it is possible that 24 to 48 h would be needed to 

I move the system to the site of a disaster 

and deploy it. Because of these limitations, it is believed that a lightweight, 

inexpensive, short-range location system 

for use underground is needed to complement the long-range (underground to 

surface) systems being studied. 

The short-range system devised by the 

Bureau consists of a low-power radio 

transmitter carried by each miner as an 

integral part of the cap lamp battery 

pack, capable of being detected by a simple receiver utilized by the rescue team. 

The transmitter has a range of 100 to 200 

ft and would operate continuously, so 

that activating the system would require 

no action on the part of the trapped 

miner. Following a disaster, the transmitter would serve as a radio beacon to 

help rescue personnel and locate trapped 

miners under or behind roof falls or barricades, or in other inaccessible places. 

It is believed that such a system would 

enhance the chance of postdisaster survival by reducing the time required to 

locate trapped and possibly injured personnel. Rapid location of victims has 

the added advantage of reducing the time 

rescue personnel are exposed to a potentially hazardous environment. Thus, the 

locator could be a significant factor in 

enhancing the safety of the rescue team 

even in cases where the victims failed to 

survive the initial accident. 

This report describes the design and 

testing of a short-range radiofrequency 

location system that could be housed in 

the miner's cap lamp and powered from the 

cap lamp battery. The locator is intended as an inexpensive add-on to the 

cap lamp and has been designed so that it 

can be retrofitted into existing battery 

cases. Questions of technical and economic feasibility, worker acceptance, and 

permissibility are addressed. 

An extensive review of current stateof-the-art location systems has been compiled and is presented as a comprehensive 

bibliography. 
TECHNICAL BACKGROUND 

The 1970 report of the National Academy 

of Engineering recognized that there were 

two aspects to the problem of trapped 

miner location. The first problem was to 

locate the trapped miner from the surface 

so that rescue efforts could be initiated 

and directed toward the proper area in 

the mine. The second problemwas to 

guide rescue teams to trapped or injured 

miners who might be within a few feet but 

buried under rock or obscured by smoke or 

debris. It was first assumed that one 

system could serve both functions, but 

early work soon indicated that the requirements for the two applications were 

largely incompatible. The long-range 

locator requires a low-frequency (kilohertz) transmitter with sufficient power 

to be capable of penetrating hundreds of 

feet of rock and soil. The short-range 

locator should be small and lightweight 

so that it can be carried on the miner's 

person; therefore, it must be low power 

and operate at a high frequency (megahertz). It is clear that two separate 

systems are required. 

One approach to the short-range locator 

would be to use a passive system, similar 

to that used for security in libraries 

and retail stores. The passive locator 

consists of an antenna and a nonlinear 

element, usually a diode or ferromagnetic 

element. The device is irradiated at one 

frequency but because of the nonlinearity, is capable of generating and reradiating a different harmonic frequency. 

SHORT-RANGE LOCATOR 

The short-range locator (SRL) is intended to serve as a personal beacon to 

aid rescue personnel in locating trapped 

miners in a postdisaster environment. 

Discussions were held with Bureau, Mine 

Safety and Health Administration (MSHA), 

company, and union officials as to what 

constitutes an effective SRL. Since 

every miner would be provided with an 

SRL, the transmitter must be extremely 

small, lightweight, and inexpensive. 

Some of the factors considered in the 

design of the SRL were range, frequency, 

power supply, cost , and worker acceptance 

(size and weight). 

The passive transponder would be worn by 

the miner. The rescue team would carry a 

transmitter and receiver tuned to the 

appropriate harmonic. When near a transponder, a signal would be detected by the 

rescue team receiver. 

During the early stages of this project, a survey was made of the commercially available passive transponders, so 

that an existing commercial system could 

be modified for use underground. The 

existing passive systems were found to be 

limited to ranges of a few feet at most, 

and within limitations of practical size 

and transmitter power, the range could 

not easily be extended. It also soon 

became apparent that a passive system was 

not likely to be useful in the electromagnetic noise environment of a typical 

mine. 

An alternative to the passive transponder would be either an active beacon 

or an active transponder. The transponder would have the advantage of only 

transmitting when it was interrogated. 

It would therefore be less of a source of 

electromagnetic interference than would a 

continuously operating beacon. On the 

other hand, the transponder would require 

considerably more complex circuitry. It 

was determined that the advantages of a 

transponder would not justify the additional size and expense, and that an 

active beacon was the most realistic 

technical approach. 

SYSTEM REQUIREMENTS 

RANGE 

The SRL system should be able to operate over a range of at least 100 ft in 

air and through 10 or more feet of rock. 

This range would be adequate in most 

post-mine-disaster scenarios that can be 

envisioned. Slightly greater ranges 

might be desirable; however, the range 

should not be too great, in order to 

avoid interference between different 

transmitters. 
FREQUENCY 

The operating frequency of the SRL is a 

compromise between several constraints. 

In general, lower frequencies propagate 

better through the ground but require 

larger, heavier units. High-f requency 

transmitters have the advantage of using 

smaller components and requiring shorter 

antennas, thereby allowing use of the cap 

lamp cord as an antenna. Another factor 

to be considered is that at certain frequencies the mine tunnels may act as 

dielectric wave guides. The determination of the optimum frequency of operation was one of the objectives of this 

project. 

POWER SUPPLY 

As designed, the SRL is powered by 

and incorporated into the miner's cap 

lamp battery. This is desirable since 

a separate power source would more 

than double the weight of the transmitter. Also, using the cap lamp battery 

eliminates the maintenance required to 

keep a separate battery charged or periodically replaced. The lamp battery is 

always fully charged at the beginning 

of each shift. It should be noted that 

the power consumed by the SRL will be 

negligible compared with that used 

by the cap lamp and, therefore, will 

not significantly reduce the length of 

time that the lamp will operate between 

rechargings . DESIGN OF T 

SRL transmitters were constructed to 

operate at 660 kHz, 27 MHz, 45 MHz, 50 

MHz, and 60 MHz. 

The 660-kHz transmitter (unit 1) is 

shown schematically in figure 1, It employed a single 2N222 transistor as an 

inductively coupled oscillator and a 

single-stage power amplifier. The oscillator was amplitude modulated by switching its collector supply with the 555 

timer (integrated circuit (1C)-2). The 

original transmitter produced a steady 

audio tone at 940 Hz, Early tests showed 

that the steady tone was not easily 

detected and could be confused with the 

The main disadvantage of using the cap 

lamp battery is that it requires the 

transmitter to operate on 4-112 V. While 

this is not an insurmountable obstacle, a 

slightly higher voltage would simplify 

circuit design. 

COST 

Since each miner would be equipped with 

an SRL transmitter, it is necessary that 

they be inexpensive. Here again the use 

of the cap lamp battery eliminates the 

expense of both a second battery and a 

separate case. It was estimated that an 

SRL transmitter could be incorporated 

into a cap lamp at approximately $10 

additional cost for each unit, based upon 

component cost and circuitry. 

WORKER ACCEPTANCE 

The SRL would be very small and lightweight so as not to encumber the miner 

with additional equipment. The prototype 

SRL weighed 67 g; it was implemented with 

discrete components and could be repackaged into a much smaller integrated circuit. Incorporating the SRL into the cap 

lamp battery pack would not make the 

miner aware of its presence. The unit 

would operate continuously and would 

require no action on the part of the 

miner. This would avoid the possibility 

of a worker's forgetting to turn on the 

unit when going underground or of its 

being turned off accidentally. 

RANSMITTERS 

beat note that resulted when the receiver 

was tuned slightly off a distant station. 

A second 555 timer (IC-1) was therefore 

used to modulate IC-2, This circuit produced a wavering tone that switched 

between 235 and 940 Hz at a rate of about 

0,6 Hz, This tone proved to be quite 

easy to detect audibly and could not be 

confused with any type of interference, 

The 27-MHz transmitter (unit 2) is 

shown in figure 2, It employed two 

ECG222 field effect transistors (FET1s), 

one as a crystal-controlled oscillator and the other as an emitter follower to drive the antenna, The antenna 
7 nn 7pF[ 2 "" 

ECG 186A 

TI RFC (slug tuned) prlmary 

8 turns No. 20 AWG enameled secondary 

T2 I- pH RFC (slug tuned) prlmary 

40 turns No. 32 AWG enameled secondary 

ANT Ferr~te loop- st~ck 

ANT 

FIGURE 1. - Schematic representing 660-kHz 

AM transmitter (unit 1). 

Fl GURE 2. - Schematic representing 27-MHz 

was a wire approximately 6 ft long. In 

this circuit, the two 555 timers were 

replaced with a single 556 dual timer. 

It was found that performance was improved by operating the oscillator from a 

constant supply and modulating the 

emitter follower. A switching arrangement was provided so that either the 

wavering tone or an unmodulated carrier 

could be transmitted. 

A second 27-MHz transmitter (unit 3) 

was identical to unit 2 except for the 

addition of one stage of power amplification. The output amplifier, shown in 

figure 3, used a single ECG186A power 

transistor operated as a class B power 

amplifier. 

A transmitter capable of operating 

throughout the range of 27 to 60 MHz was 

also constructed. This transmitter (unit 

4) is shown schematically in figure 4. A 

crystal oscillator drove a singletransistor output amplifier. The amplifier was modulated by the 556 timer. 

Switch 1-A allowed either a modulated 

signal (beep) or unmodulated carrier to 

be selected. The operating frequency was 

selected by changing the plug-in oscillator coils and/or crystals. Available 

frequencies were 26.995, 40.000, 45.000, 

50.000, and 60.000 MHz. 

I "cc 

95p H 

02 10 K4 ANT 

T3 

EGG I86A 

FIGURE 3. - Schematic representing 27-MHz 

transmitter (unit 2). power amplifier (unit 3). 
- 

FIGURE 4. - Schematic representing 27- to 60-MHz transmitter. 

TEST RESULTS 

Four tests were conducted to evaluate the performance of the prototype 

SRL transmitter. These tests consisted 

of -- 

1. Measurement of the signal strength 

propagation through air; 

2. Measurement of the signal strength 

propagation through air and broken rock; 

3. Measurement of the propagation 

through solid rock; 

4. Measurement of propagation underground. 

PROPAGATION THROUGH AIR 

The approximate range of the transmitter i n a i r was determined by detecting 

the signal with a multiband communicat ions receiver. The 660-kHz 

transmitter (unit 1) was found to have 

a range of approximately 60 ft when 

placed directly on the ground. This 

could be extended to several hundred 

feet by connecting a 20-ft wire antenna to the end of the ferrite loopstick antenna. The poor performance 

of this transmitter was attributed to 

the combination of low power and inefficient antenna. The 27-MHz transmitters (units 2 and 3) could be 

detected at distances of approximately 

3,300 to 5,000 ft, with the transmitter located at about a 30-ft elevation. Table 1 shows typical results 

through the air for a handheld 27- 

MHz transmitter (unit 2). Although the 

field strength voltage fell below 40 pV 

at 600 ft, the tone was still audible 
for 1,500 ft. The unit 4transmitter 

could be detected at ranges of up to 

1 mile. 

TABLE 1. - Transmitter signal range in air, unit 2 

(27 MHz) 

Distance Field 

in air, ft strength, pvl 

7 7. ....a*. 

124....... 

166ee**aae 

215amamee* 

260.. ..... 300. ...... 345....... 

400....... 

480.. ..... 515..eeeee 

601.. ..... Background 

775 

455 

145 

135 

109 

86 

66 

84 

69 

42 

28 

noise = 32 pV. 

PROPAGATION THROUGH AIR-ROCK PATH 

To evaluate the performance of the SRL 

through a propagation path similar to one 

that might be encountered after a roof 

fall, the transmitters were buried under 

a pile of broken rock, and the range at 

which the signal was just detectable was 

measured at various depths of the debris 

thickness . The tests were conducted at a coal surface mining operation in Tuscaloosa 

County, AL. The prototype transmitters were enclosed in a wooden box to 

prevent damage (fig. 5). Each transmitter in turn was enclosed in the box, 

placed on the floor of the strip pit, 

and covered with overburden consisting 

of broken shale and fire clays 

(fig. 6). 

For unit 1 (660 kHz), no decrease in 

the signal strength or range was detected 

for broken rock coverings of 10 ft. Typical results for a 27-MHz transmitter 

(unit 2) through the air-rock path are 

shown in table 2. 

TABLE 2. - Signal range of 

transmitter covered with 

varying thicknesses of rock, 

unit 2 (27 MHz) 

Thickness Maximum 

of rock, ft range, ftl 

8.0............... 350 

9.0............... 350 

10.0. ............. 350 

lAir path plus rock thickness. 

As a test of the validity of these results, they were fitted to an equation of 

the form 

Sm = -&2 E - ad, (1) 

where Po = effective radiated power at 

the transmitter, 

Sm = minimum detectable signal 

power, 

R = range, combined air-rock 

path, 

a = attenuation coefficient of 

rock, 

and d = rock thickness. 

Equation 1 can be written 

where U = -21n R, (3) 

and 

A linear regression of d on U gave an 

excellent fit, with a value of a of 2.6 

dB/m from the broken rock. 
FIGURE 6. - Overburden being placed over transmitter. 
PROPAGATION THROUGH ROCK 

To measure the propagation through 

solid rock, tests were conducted at the 

Nickel Plate Mining Co, , Inc, Pit No, 3 

in Jefferson County, AL, and at Drummond 

Coal Co, Kellerman Pit in Tuscaloosa 

County, AL, At each site a series of 

vertical holes 6-3/4 in, in diameter and 

50 ft deep were drilled along a line 

extending at 45O back from the highwall 

(fig, 7). Location of the holes at the 

Nickel Plate and Drummond sites are shown 

in figures 8 and 9, respectively, The 

transmitter was lowered into each hole to 

a depth level with the floor of the pit, 

and the signal was measured at the face 

of the highwall, Figure 10 shows the 

transmitter, enclosed in a plastic case 

to protect it from water and dirt, being 

lowered into a test hole, Figure 11 

shows signal strength measurements being 

made, 

The signal strength meter consisted of 

a commercial commnications receiver that 

was modified by disconnecting the 

squelch control and adding a meter to 

read the automatic gain control (AGC) 

voltage, The meter was calibrated by 

injecting into the receiver a signal of 

known amplitude from a VHF signal generator, Signal levels from 0.1 pV to 20 

mV were used, Since the AGC voltage is a 

nonlinear function of the receiver 

signal, a computer program was written to 

convert the meter reading to signal 

strength, This program used a four-point 

interpolation on the stored calibration curve, The sensitivity of the 

signal strength meter was the same 

through all frequency tests, approximately 0-1 pV, 

To ensure that the measured signal was 

actually propagating through the rock and 

not leaking out of the tops of the holes, 

the signal strength was also measured 

above the holes, No signal was found 

once the transmitter had been lowered 

more than 20 ft into a hole, even 

with the hole open. As an added precaution the holes were covered with a 

metal plate while measurements were being 

made, 

Several tests were conducted on the 

three 27-MHz transmitters (units 2, 3, 

and 4). It was found that at the Nickel 

Plate site the audio tone was strongly 

audible from station 4, a distance of 

43.9 f t , and weak but clearly audible 

from station 5, a distance of 62.8 ft, A 

faint signal could sometimes be detected 

from stations 6 and 7, distances of 

approximately 95 and 110 ft, The useful 

range through solid rock was judged to be 

approximately 60 f t, 

Tests at the Drummond site gave similar 

results, An audible signal could be 

detected from stations 1 through 5 (51-6 

f t maximum rock thickness ) , Occasionally 

a faint signal could be heard at station 

6 (62.6 ft), The apparent decrease in 

range was attributed to the higher moisture content of the rock at the Drummond 

site, as evidenced by water seeping from 

the face of the highwall, In order to 

verify this, core samples were collected 

at both sites, The Nickel Plate sample 

contained 1-5 pct moisture, the Drummond 

site 9 pct moisture, When signal 

strength measurements were made, the 

transmitter modulation was turned off and 

the strength of the unmodulated carrier 

measured at the face of the highwall. 

Usable data were obtained at stations 1 

through 5 at each site, 

To determine the attenuation coefficient of the rock, it was assumed that 

the power density (P) at the receiver was 

given by 

where R = range through rock, 

and 

Po = effective radiated power 

at the transmitter, 

a = attenuation coefficient 

of rock 

G~ = antenna gain of the 

transmitting antenna in 

the direction of the 

receiver, 
PLAN VIEW 

PLAN VlEW 

Transmil 

SECTION VIEW 

I 

SECTION A-A VIEW 

FIGURE 8, - Plan and elevation view of test FIGURE 9. - Plan and elevation view of test 

holes at the Nickel Plate site, holes at the Drummond site. 
FIGURE 11. = Signal strength measurements being taken at toe of highwall. 
The measured signal strength was proportional to the square root of the power 

density at the receiver, i,e., 

where GR = receiver antenna gain, 

and k = sensivity factor for the 

receiver. 

From equations 5 and 6, 

Here, So is a constant involving GR , GT , and k. Care was taken to keep the orientation of the transmitter and receiver 

antenna the same for all measurements so 

that So would not vary. 

The signal voltages were fitted to 

equation 8, and from this the best value 

of a was determined. In general, the 

experimental data fit this equation extremely well despite the fact that the 

"far field" assumption implied in its 

derivation was not strictly met by the 

actual experimental geometry, A typical 

plot of signal strength versus range is 

shown in figure 12. The data points are 

the measured field strengths; the solid 

line is the best fitting theoretical 

curve, For comparison, a plot for propagation in air is shown in figure 13, 

The measured attenuation coefficient for 

RANGE, f t 

FIGURE 12. - Typical plot of signal strength 

versus range through rock. 

d i00 I 200 I 300 400 

1 

500 600 

RANGE. ft 

the airpath was 0.0001 (essentially 

zero), which added considerable con- FIGURE 13. - Plot of signal strength versus 

fidence in the data reduction technique. range for airpath. 
Data from the Nickel Plate site fit the KEY 

theoretical curve exceptionally well, the M RD Maximum received distance 

average error at a point being less than 

6 pct. The attenuation coefficient was 

found to be 1.93k0.02 dB/m. Data from 

the Drummond site were less reliable. 

The value of the attenuation coefficient 

could not be determined with any precision, but it appeared to be at least 

twice as large as at the Nickel Plate 

site. 

Unit 4 transmitters were also tested 

at higher frequencies at both sites. At nsmitter 

the Nickel Plate site, an audible signal 

from station 1 (20.6 ft) and occasionally 

a weak signal from station 2 (27.0 

ft) could be detected, indicating that 

the attenuation is much greater in the 

45- to 60-MHz range than at 27 MHz. 

No detectable signal was obtained at 

the Drummond site, where the minimum 1 I 4) 

rock thickness was 31.5 ft. Reliable 

measurement of attenuation coefficient in 

the 45- to 60-MHz range could not be 

A-MRD 

165 ft 

obtained. 

UNDERGKOUND TEST 

A 27-MHz transmitter was taken under- Not to scale 

ground to determine the range. The test FIGURE 14. - Schematic representing underwas conducted at the North River Energy 

Corp. No. 1 Mine in Fayette County, AL. ground test locat ion. 

The transmitter was placed in an entry, 

and the maximum distance at which it and in crosscuts. The maximum distance 

could be detected was determined. Mea- ranged from 158 to 230 ft as shown in 

surements were made both down the entry figure 14. 

PROTOTYPE TRANSMITTER 

A demonstration transmitter (fig. 15) 

was constructed and incorporated into a 

miner's cap lamp battery. The circuit 

(fig. 16) is essentially the same as that 

of the unit 2 transmitter used in the 

propagation tests. RFCl and RFC2 are 

inductors to isolate the RF circuits and 

lamp cord (which serves as an antenna) 

from the battery. No modification to the 

lamp was required. 

This unit exhibited open-air and underground ranges comparable to those of the 

other 27-MHz transmitters. This frequency allowed for use of the cap lamp 

cord as an antenna, thereby significantly 

affecting circuit simplicity, and allows 

for use of conventional CB broadband 

receivers that are approved as intrinsically saf em 
FIGURE 15. - Transmitter placed inside cap lamp battery. 
FIGURE 16. - Schematic diagram of cap lamp 

transmitter. 

I 

0 200 400 600 800 1,000 

TIME, min 

FIGURE 17. - Cap lamp battery voltage versus time of discharge. 

to turn off the lamp. For this reason, a 

circuit could be installed to sense a 

low-voltage condition, which would automatically turn off the cap lamp. 

The low-voltage sensor is shown in figure 18. When the battery voltage exceeds 

the zener voltage of zener diode Dl, current flows through R2 and Dl, biasing 

transistor Q, into saturation. The gate 

of the silicon controlled rectifier 

(SCR) is low, and the SCR is in its 

nonconducting (off) state. When the 

battery terminal voltage drops below the 

zener voltage, the base current to Ql is 

blocked and Ql is cut off. The SCR gate 

voltage goes high and the SCR fires, 

drawing a large current that blows the 

fuse, disconnecting the cap lamp. Diode 

D2 bypasses the fuse when the battery is 

being charged. 

The low-voltage sensor was tested and 

found to operate reliably. The test 

setup is shown in figure 19. A computer 

was programmed to record the terminal 

voltage of the cap lamp battery and the 

luminous output at 5-min intervals and to 

record the data on magnetic tape. Thus, 

the test could be run unattended. Relay 

1 allowed the computer to terminate the 

test after a preset time as measured by 

the internal clock. This was done to 

prevent damage to the battery in case the 

voltage sensor failed. 

The battery voltage and light output 

for a typical test are shown in figures 

20 and 21. In this test, the trigger 

2-A I I 

fuse 

-- - 

R FC I 

FIGURE 18. - Schematic diagram of low-voltage 

sensor. 

Light-tight enclosure 

Transmitter 

and volt sensor 

I I IOV 

I Computer I 

~na~d~ue/ 

dig~tal 

channel I 

FIGURE 19. - Low-voltage sensor operationa1 setup. 

1 

Relay 

closure 

Analogue / 

digital 

channel 2 
3 1 I 1 i I 1 I I I I 

0 200 400 600 800 1,000 

TIME, min 

FIGURE 20. - Cap lamp battery voltage versus 

time with automatic shutdown of low-voltage 

sensor. 

0 200 400 600 

TIME, min 

FIGURE 21. - Light sensor output volts with 

low-voltage sensor. 

level was set at 3.5V. Any other 

trigger level could be selected by using 

a zener diode with that voltage. A 3.25- 

V trigger level allowed the lamp to run 

for about 15 h. 

The desirability of an automatic turnoff feature is open to some question. 

One can envision situations where an 

unconscious miner, whose lamp was unnecessary anyway, might be saved by extra 

hours of SRL operation. Scenarios can 

also be postulated in which a few minutes 

of light might be critical. It is felt 

that this is a question that deserves 

additional study. 

PERSONAL CODING 

A simple modification of the pulser 

circuit would allow each unit to transmit 

a unique pulse code so that the trapped 

individual could be identified. This 

feature might or might not be desirable. 

The situation might arise where a rescuer 

had to decide which of several trapped 

miners to rescue. Mine personnel who 

might be responsible for rescue operations have indicated that they would prefer to make such a decision without knowing the identities of the individuals 

involved. Concern has also been expressed that workers would fear that the 

locator was being used to monitor them 

during normal operations. 

Other types of coding are also possible. For example, the short-range 

locator could be equipped with a selective code allowing miners to indicate 

that they (1) were in no immediate danger 

or (2) required prompt assistance. A 

third code would indicate that the miner 

had taken no action and presumably was 

unable to do so. 

RESCUE TEAM UNITS 

As presently perceived, all SRL's will 

operate on the same frequency so that a 

f ixed-frequency receiver can be used. A 

second frequency should be available for 

use by rescue team personnel so that 

their locators would not interfere with 

search operations. 
SAFETY CONSIDERATIONS 

PERMISSIBILITY 

The question of permissibility has been 

discussed with personnel at MSHA Laboratories, Wheeling, WV, and the MSHA field 

office, Birmingham, AL. Because of the 

low voltages and power involved, no reason is foreseen why the SRL should not be 

intrinsically safe. 

IGNITION OF BLASTING MATERIALS 

One hazard associated with the use of 

any radio transmitter underground is the 

possibility of accidently igniting blasting materials. Since the SRL will be 

carried by workers handling blasting 

material, the transmitter can be expected 

to come into close proximity to electrical blasting caps. It is therefore 

necessary to consider the possibility of 

accidental ignitions. 

The no-fire level of blasting caps obtainable in the United States is 40 mW or 

greater.4 A typical commercial blasting 

cap has a no-fire current of 250 mA and 

an internal resistance of approximately 

1 n.5 The no-fire level is therefore 

approximately 62 mW. The prototype SRL 

transmitter draws a supply current of 10 

mA at 4.5 V or a total power of 45 mW. 

Since the output stage is a class A 

amplifier, the maximum theoretical 

efficiency is 25 pct. The maximum radiated power is therefore less than 11 mW. 

Even if the blasting cap and transmitter 

are very close together, only a fraction 

of the radiated power could be coupled 

into the blasting circuits. It is clear, 

therefore, that the SRL will be incapable 

of igniting blasting materials. 

To verify that the SRL transmitter 

would not ignite blasting caps, the following tests were conducted. An electrical blasting cap was connected to a pair 

of long wires to simulate a blasting circuit. The SRL transmitter antenna was 

loosely entwined with the cap's leg wires 

to obtain maximum coupling between the 

transmitter and blasting circuit. The 

test was repeated with three caps. None 

of the caps detonated. During the test, 

the SRL signal was monitored to verify 

that the transmitter was operative. All 

caps used in the test were later detonated electrically to ensure that they 

were not defective. 

On the basis of these calculations and 

tests, it is believed that the SRL is 

incapable of causing accidental detonations even under the worst conditions. 

Before the SRL is used, additional tests 

should be conducted by persons familiar 

with the accidental ignition hazard. 

CONCLUSIONS 

Test data indicated that a simple, lowcost transmitter consisting of one integrated circuit and two transistors is 

capable of radiating a signal through 

65 ft or more of rock or through 12 ft 

of rock and several hundred feet of 

air. The transmitter can operate from a 

4.5-V supply and is therefore capable 

 h he Institute of Makers of Explosives 

(Atlanta, GA). Radio Frequency Radiation 

Hazards. Pamphlet 20, Oct. 11, 1956, 

24 PP* 

5~tlas Powder Co. (Dallas, TX) . Handbook of Electrical Blasting. 1976, 

93 PP* 

of being powered from a miner's cap lamp 

battery. The cost of such a unit would 

be low, and the unit would require no 

action on the part of the distressed 

miner. The proposed short-range locator 

is technically, economically, and operationally feasible, and it would substantially enhance the chances of survival of personnel trapped after a mine 

disaster. 

Tests indicated that 27 MHz is a usable 

frequency for the short range locator. 

At frequencies above 40 MHz, the attenuation of rock is too high to allow adequate penetration, while frequencies 
below 10 MHz require excessively large A number of optional features are posantennas. Additional investigation could sible, including various types of coding 

be conducted to select an optimum fre- and automatic battery conservation, 

quency in the 10- to 30-MHz region, Additional study is required to determine 

if these features are desirable and to 

The short-range locator can be made justify the additional cost, 

intrinsically safe and will pose no hazard to blasting material, 

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IN T.-BU.OF MIN ES,PGH.,PA. 27294