RI 9641 REPORT OF INVESTIGATIONS/1997

Evaluation of a Signaling and Warning 

System for Underground Mines

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health
Report of Investigations 9641

Evaluation of a Signaling and Warning

System for Underground Mines

Ronald S. Conti and Robert G. Yewen

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Pittsburgh Research Center

Pittsburgh, PA

May 1997
International Standard Serial Number

ISSN 1066-5552
CONTENTS

Page

Abstract ...................................................................................... 1

Introduction .................................................................................... 2

TeleMagnetic Signalling Systems and technology overview .................................................. 3

Lake Lynn Laboratory ............................................................................. 5

System design, installation, and evaluation .............................................................. 5

Loop antenna introduction ....................................................................... 5

Loop antenna design considerations ................................................................. 5

Receiver reference inductor ....................................................................... 8

7.5-meter-diameter loop ......................................................................... 8

15-meter-diameter loop .......................................................................... 9

25-meter-diameter loops ......................................................................... 9

First 25-meter loop .......................................................................... 9

Second 25-meter loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Site-specific engineering overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Response to underground mine fires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Appendix.—Resonance nomograph chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

ILLUSTRATIONS

1. Conceptual representation of a mine, depicting capabilities of the communication and signaling system ............... 4

2. Stratigraphic column in mine area ................................................................ 6

3. Plan view of the Lake Lynn Experimental Mine depicting the locations of loop antennas ......................... 7

4. Receiver reference inductor mounted on a lead-acid-type cap lamp ......................................... 8

5. A previous 7.5-m loop antenna in old workings of Lake Lynn mine site ..................................... 9

6. Electrical schematic of 7.5-m loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

7. Signal strength as a function of distance, 7.5-m loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

8. 15-m loop outside location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

9. Electrical schematic of 15-m loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

10. Signal strength as a function of distance, 15-m loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

11. Location of first 25-m loop antenna mounted at the roof between A- and B-drifts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

12. Location of existing 25-m loop antenna positioned around a pillar near the portal of D-drift . . . . . . . . . . . . . . . . . . . . . . . 11

13. Electrical schematic of 25-m loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

14. Signal strength as a function of distance, 25-m loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

15. Block diagram of TeleMagnetic signaling system for Lake Lynn Laboratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

16. Flowchart of Lake Lynn antenna system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

A-1. Resonance nomograph chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cm centimeter min minute

H henry mm millimeter

Hz hertz mV millivolt

kg kilogram s second

kHz kilohertz V volt

km kilometer W watt

km square kilometer FF microfarad 2

kW kilowatt FV microvolt

m meter S ohm

m square meter % percent 2

mH millihenry

Mention of any company name or product does not constitute endorsement by the National Institute for Occupational Safety and

Health.

To receive additional information about mining issues or other occupational safety and health problems, call 1-800-35-NIOSH (1-

800-356-4674), or visit the NIOSH Home Page on the World Wide Web at

 http://www.cdc.gov/niosh/homepage. html
EVALUATION OF A SIGNALING AND WARNING SYSTEM

FOR UNDERGROUND MINES

By Ronald S. Conti and Robert G. Yewen 1 2

ABSTRACT

Underground mines rely on alarm systems, such as stench gas, audible or visual alarms, pager phones, telephones,

and messengers to warn miners of a fire or other emergency. These systems are often slow, unreliable, and limited

in mine coverage. This report describes the evaluation of a wireless signaling and warning system for underground

mines. This system is applicable to both coal and noncoal mines. The work was conducted by the National Institute

for Occupational Safety and Health in cooperation with TeleMagnetic Signalling Systems (TSS) under Cooperative 3

Research and Development Agreement No. BOM-CRDA-6200-0119.

A TSS wireless ultra-low-frequency electromagnetic signaling system was installed at the Experimental Mine at

Lake Lynn Laboratory near Fairchance, Fayette County, PA. A commercial smoke sensor was interfaced to a remote

portable transmitter, and the alarm of the sensor was used to trigger the central evacuation and paging transmitter

system during experimental mine fires. The underground/surface receivers flashed cap lamps and activated remote

devices, such as strobe lights, within 30 to 40 s after the encoded signal was received. Evaluation results showed

full-mine coverage of the electromagnetic field and that the encoded signal was received at the farthest point

underground and on the surface perimeter.

Fire prevention engineer, Pittsburgh Research Center, National Institute for Occupational Safety and Health, Pittsburgh, PA. 1

Director of research, TeleMagnetic Signalling Systems, Unionville, Ontario, Canada. 2

This research originated under the former U.S. Bureau of Mines prior to transferring to the National Institute for Occupational Safety and Health in 1996. 3
2

INTRODUCTION

Mine Safety and Health Administration (MSHA) statistics miners received an evacuation message [Stowinsky 1996]. This 4

indicated that 107 underground fires occurred in coal and metal/ delay in evacuating the mine could have been catastrophic had the

nonmetal mines from 1990 to 1995 in the United States, causing fire been further developed and escape routes blocked. The mine

32 injuries. Historically, mine fires have resulted in fatalities, eventually had to be sealed and abandoned.

injuries, and economic losses totaling hundreds of millions of Survival for an underground miner during a fire or other

dollars. A primary research thrust of the National Institute for emergency can be measured in terms of minutes. An emergency

Occupational Safety and Health’s (NIOSH) Pittsburgh Research warning that arrives late can result in tragedy. Existing warning

Center is to enhance the safety of the mine personnel by systems for underground mines, such as horns, sirens, stench gases

preventing disasters, such as fires and explosions. through the ventilation system, or messengers (other miners), can

Federal regulations [30 CFR 57 and 75 (1994)] require that be slow and ineffective. This prompted the development of 5

each mine have (1) a program of instruction to include location wireless, ultra-low-frequency electromagnetic signaling

and use of fire-fighting equipment, escapeways, exits, and routes technology for warning and paging systems for the mining

of travel; evacuation procedures; and fire drills; and (2) automatic industry.

fire-warning devices that upon activation would provide an As early as 1899, Nikola Tesla suggested the use of what is

effective warning signal to all work locations, or at a manned today described as extremely low frequencies for worldwide

location where personnel have an assigned post of duty and have communication using an earth medium [Wheeler 1961].

telephone or equivalent communications with all personnel who Pioneering research was conducted by the former U.S. Bureau of

may be endangered. Mines (USBM) on the propagation of radio waves through the

A fire occurred on a belt drive roller on March 7, 1988, earth and detection of trapped miners [Dobroski and Stolarczyk

[Strahin et al. 1990] in the Marianna No. 58 Mine, Washington 1982; Shope et al. 1982; Geyer et al. 1974; Anema 1976; Lagace

County, PA. During the developing fire, the following sequence et al. 1980; Westinghouse Georesearch Laboratory 1973; Lagace

of events occurred: et al. 1982]. Subsequent USBM research [Hjelmstad 1990;

• At approximately 7:00 p.m., the belt stopped and was electromagnetic signals from 630 Hz to 2 kHz could be

restarted. transmitted through mine rock for distances as great as 1,645 m to

• At 9:08 p.m., a high carbon monoxide warning in the an intrinsically safe receiver. The prototype wireless system of the

tailpiece cleared and was never investigated. previous research used off-the-shelf components and state-of-the-

• At 10:35 p.m., a miner smelled smoke, which was art technology to ensure high reliability and low cost. The

immediately followed by light smoke, which quickly changed to technology enabled simultaneous and instantaneous warning of all

a heavy black smoke. Upon investigation, this miner observed personnel, regardless of their underground location or work

fire coiled around the belt drive roller. Fire-fighting attempts with activities, by flashing their cap lamp.

fire extinguishers were unsuccessful, water was not readily An Australian mining industry research initiative resulted in the

available, and the belt drive deluge system had been turned off. commercial availability of a "paging" system for under- ground

• At 10:50-10:55 p.m., miners were notified to evacuate the mines [Zamel 1990]. The Personal Emergency Device

mine. During this period, 27 miners donned their self-contained communication system is a "through-the-earth" transmission

self-rescuers and began their escape. It required almost 80 min for system that enables communication of specific messages with

the miners to reach the surface. Four miners were treated for individuals underground, no matter what their location and

possible smoke inhalation. without dependence on cables or wiring underground. This

All three working sections were equipped with a telephone at surface loop antenna, and portable receiving units integrated with

the time of the fire. No one was present to answer the telephone

on two of the sections, and a telephone call was ignored on the

other section. A carbon monoxide warning light was energized

near the belt feeder; however, no one was present to see it.

Approximately 10 min passed before all

 Mine fire statistics were obtained from files maintained at MSHA’s Denver 4

Safety and Health Technology Center, Injury and Employment Information

Branch, Denver, CO.

 Code of Federal Regulations. See CFR in references. 5

Hjelmstad and Pomroy 1991] showed that ultra-low-frequency

system consists of a personal computer, low-frequency transmitter,

the cap lamp battery. It functions with a carrier wave frequency of

1,000 Hz and employs a frequency-modulated signal for

transmitting messages entered to the transmitter from a personal

computer. Specially designed software makes it possible to send

a message up to 32 characters long for readout on the personwearable receiver with its liquid crystal display (LCD). Messages

can be directed to an individual, to a group, or to all underground

personnel. When a message is received, the cap lamp flashes and

the miner can then read the message from the LCD on top of the
3

lamp battery. A message is displayed for 1 min. For example, Fayette County, PA, in November 1990. An approximately 1.6-

with a surface loop antenna approximately 2 km long, a signal km-long copper wire formed a surface loop antenna above the

was received 1 km in depth and laterally for a 1- to 2-km radius. mine. Several messages were typed into the computer and clearly

The first demonstration of the system in the United States was received throughout the underground mine workings 50 to 100 m

conducted at our Lake Lynn Laboratory near Fairchance, below the surface.

TELEMAGNETIC SIGNALLING SYSTEMS AND TECHNOLOGY OVERVIEW

Under a Cooperative Research and Development Agreement paging stations are available as a standard "Fire Pull Station."

(CRADA) with TeleMagnetic Signaling Systems (TSS), They can be installed at any strategic location in the mine or on the

Unionville, Ontario, Canada, a research effort at Lake Lynn surface. During an emergency event, the station can activate the

Laboratory was established to further develop the signaling system. warning signal.

Initial meetings were held during the summer of 1992 to develop The system is designed to produce a low-frequency

a research plan and formalize the working agree- ment. The electromagnetic field with a programmable carrier output frequency

research plan included a simple and inexpensive system using a about 2 to 3 kHz. Operating the system at this low frequency

small surface transmitting antenna (less than 2 m in diameter), an enhances the ability to penetrate through most geology of sand,

underground receiver/transmitting antenna (15 to 50 m in soil, water, and rock with little difficulty. The carrier is modulated

diameter), and underground/surface person-wearable receivers for with low-frequency tones from 4 to 40 Hz using a unique signaling

full mine coverage. Once the system was designed, it must be protocol to address individual receiver units.

easily integrated with existing fire detection and warning systems, The transmitter and antenna system is usually installed on the

turn devices such as strobe lights on or off, and its frequency surface of the mine using a loop antenna. A custom-designed loop

cannot initiate electric blasting caps. Lastly, it can alert all miners antenna at the point of installation is engineered around

of an emergency. environmental and geological influences on the electrical

The first-generation product produced by TSS [Yewen 1987] characteristics of the loop. For example, if the loop antenna is

is aimed at mine evacuation and paging. This underground lying on an overburden that is high in various salt contents, then

signaling system transmits an emergency warning that can quickly at times of heavy rain or melting snow, the earth beneath the loop

reach the underground worker. The low-frequency electromagnetic antenna can become very conductive. This drives the impedance

field has a distinct advantage because it can penetrate kilometers of up, thus reducing the effective ampere-turns. Low-lying areas of

soil and rock to reach the most remote shaft or tunnel, which makes overburden with this characteristic should be avoided.

it ideal for underground signaling and paging systems. This Influencing geologies such as sulfide ore bodies can also affect

system consists of a low-frequency transmitter that can be the impedance of the loop antenna. When possible, the loop

strategically placed to create an electromagnetic signal that can antenna should be installed strategically at a comfortable distance

completely envelop most mines without the use of repeater from the influence. A recommended distance, depending on the

systems. However, some mines may require a repeater system for size of the ore body, is 200 to 600 m.

complete mine coverage. Figure 1 shows a typical configuration Other influences, such as diabase dikes, have a different effect.

of the wireless system for a mine. The transmitter loop antenna is The diabase dike is magnetic and deviates the field. These dikes

on the surface, and a receiver/transmitter loop antenna is effectively form a magnetic wall. If possible, the loop antenna

underground. The person-wearable receivers are small, lightweight should be placed where the electromagnetic field does not have to

modules incorporated into the miner's cap lamp assembly. Upon penetrate the dike directly, but will still reach its target radius to the

receiving an emergency or paging signal, the cap lamp begins to mine.

flash, which in turn alerts the miner to evacuate the mine or call the A microprocessor-based center controls the operation of the

surface for a message, depending on which signal is received. To system. It encodes the through-the-earth transmitted signals in such

discriminate between the two signals, the miner would observe the a way that the desired in-mine receivers turn on. The system

"on" status of the red light-emitting diode (LED) mounted in the paging capability can be programmed to contact up to eight

lamp assembly receiver module. For example, for a paging signal, individual person-carried receivers in the mine, such as supervisors

the miners' cap lamp would flash and the red LED would and maintenance personnel. A control station is 

illuminate; if the red LED is not illuminated, the miner is alerted

that an emergency may exist.

Further coverage can be provided by installing receivers on

vehicles or in refuge stations and work areas. Due to the high level

of electrical noise (electromagnetic interference/radio frequencies)

generated by some vehicles, specialized low-noise antenna receiver

systems are used. The system has the potential for special receivers

with the ability to relay gas sensor alarms, seismic alerts, and

addresses to control fans and pumps. Remote emergency wireless
4

Figure 1.—Conceptual representation of a mine, depicting capabilities of the communication and signaling system.

also available as a system remote module that can be mounted at mine fire preparedness, the first installation at an operating U.S.

the surface transmitter location and can be addressed and controlled mine was completed. Dravo Lime's Black River Mine near

from existing personal computers or programmable logic control Maysville, KY, tested the system on February 7, 1996, and

systems. acquired 12 person-wearable lamp receivers for key personnel.

The TSS receiver technology has been designed to a module Each cap lamp module was retrofitted with a receiver. The Black

format to retrofit existing cap lamps. The receiver module River Mine is a room-and-pillar underground limestone mine with

technology is encapsulated to protect it from harsh environ- ments an area of approximately 3.7 km and 198 m of overburden. The

in an injection-molded, impact-resistant plastic case. Cap lamp mine employs approximately 135 employees (40 underground

manufacturers are not opposed to the retrofits. TSS is currently miners per shift) and utilizes diesel equipment and conveyor belt

seeking MSHA approval for use in U.S. underground coal mines. haulage. A 50-m loop with an applied power of 50 W is used to

The receiver circuit consists of high-quality factor (Q) resonated generate an effective electromagnetic field (ampere-turns) for full

inductors that function as receiving antennas connected to a high- mine coverage at this site. Because of the success of its present

gain amplification circuit utilizing noise reduction processing and system, Black River Mine plans to augment its cache of persondemodulating circuitry to process the signal. The receivers may be wearable lamp receivers for more miners.

interfaced with fans, pumps, etc., for control purposes, or they can

monitor the operation of these devices. The NIOSH Pittsburgh Research Center conducts open industry briefings

As a result of demonstrating the capability of the TSS system

and the work conducted under the CRADA during briefings on 

6

2

6

on mine fire preparedness to enhance the mining industry’s awareness of the

dangers of underground mine fires and convey research findings on the latest

methods for detection, control, and extinguishment of mine fires.
5

Installations at three gold mines in Ontario, Canada, have also electromagnetic signal could be transmitted through the mine rock

been successfully completed. Together these mines acquired more for a distance of 2,073 m. A 100-m-diam loop antenna (two turns)

than 300 lamp receivers for their underground personnel. The with an applied power of 80 W was used to generate the

installations also use remote strobe light receivers on jumbo drills electromagnetic field. Additional tests are planned to further

and on rampways to signal vehicles traveling in and out of the develop the loop and provide full mine coverage.

mine.

A recent test at a Canadian metal mine, products of which

include zinc, gold, and platinum, showed that the low-frequency

LAKE LYNN LABORATORY

The NIOSH Pittsburgh Research Center's Lake Lynn The average dimension of the old workings is 10 m high with

Laboratory [Mattes et al. 1983; Triebsch and Sapko 1990], 15-m-wide entries and crosscuts on 30-m centers, leaving 15-m

formerly a limestone mine, is now a multipurpose research facility square pillars for support. The old workings have an area of

used to conduct mining health and safety research. The mine is approximately 130 km .

geologically in the Greenbriar Limestone Formation (locally The new entry dimensions of the underground mine range from

known as the Wymps Gap Limestone). A repre- sentative 1.8 to 2.4 m high and 5.3 to 6.3 m wide. The average dimensions

stratigraphic column of the strata above and below the mine is are 2.1 and 5.8 m, for an average cross-sectional area of 12 m .

shown in figure 2. The underground configuration of the new entries covers

Lake Lynn Laboratory's underground layout, including the approximately 95 km , with an overburden ranging from 50 to

locations of the electromagnetic system antenna loops, is shown in 100 m.

figure 3. Initial mining was done by surface methods and created

900 m of high wall and almost 60 km of gently sloping quarry 2

floor.

2

2

2

SYSTEM DESIGN, INSTALLATION, AND EVALUATION

LOOP ANTENNA INTRODUCTION The power requirements will be determined by the electromagnetic

The antenna used at Lake Lynn is a large air core loop the desired coverage area. This is influenced primarily by the

constructed with multiple conductor control cable. Burying the geology between the transmitting antenna and receivers. The

loop would not significantly impact the loops' characteristics. This applied power to the loop will result in ampere-turns in the loop

cable is laid down in a circle with the two ends brought together antenna. The electromagnetic field or force produced by the loop

for connections. Each conductor (both ends) is then numbered, is a direct result of the ampere-turns. Ampere-turns are the current

and each wire is connected to form a continuous multiple-turn in the loop times the number of loops or turns.

loop. For example, a continuous loop with 14 turns would require For this application, the following factors were considered:

15 conductors connected together. The circular loop must be as

round as possible to project the electromagnetic field around its • Design of a transmitting loop antenna using standard

entire circumference. For example, if the loop antenna were polyvinyl chloride (PVC) jacketed control cable using the smallest

teardrop-shaped, the electromagnetic field would be projected as a wire size, typically 2.1 mm in diameter.

teardrop, but opposite to the shape of the loop antenna. The • Installation of the transmitter loop antenna must be

shortest coverage of the electromagnetic field would be projected economical, taking up the least amount of surface area (15 to 50 m

off of the pointed end of the teardrop-shaped antenna. in diameter or less).

LOOP ANTENNA DESIGN CONSIDERATIONS coverage can be obtained with minimal applied power

The direct-current (dc) resistance (R) of this loop must be it will have the most ampere-turns for applied power.

minimized; therefore, a large wire diameter or gauge, such as

2.6 mm or even 3.3 mm, is desirable. Nevertheless, 2.1-mm-diam

copper wire is generally sufficient for most applications. For

example, 2.1-mm- and even 1.6-mm-diam wire is adequate when

the electromagnetic field of coverage is less than a few kilometers.

The number of turns, diameter, and conductor size of the loop are

determined by the signal's strength requirements at the receiver site.

field needed to produce minimum signal strength in all receivers in

• Efficiency of the loop antenna is critical so that full mine

requirements to the loop (75 to 100 W). If the loop has a high Q,
6

Figure 2.—Stratigraphic column in mine area (Lake Lynn Laboratory).
7

Figure 3.—Plan view of the Lake Lynn Experimental Mine depicting the locations of loop antennas.

A loop antenna adequate for a small mine such as that at Lake (ac) resistance (Z) of the loop. Large loops and loops with a large

Lynn can be typically 15 to 25 m in diameter and use 10 to number of turns will have a larger amount of dc resistance. The ac

15 turns. Such a loop antenna with adequate applied power to resistance is the sum of the losses such as dc resistance of the wire,

provide 250 mV in a receiver reference inductor at 10 m will soil conductivity near the loop (shorted turn effect of the

produce approximately 0.5 mV at 1 km. This is a generic conductive soil on which the loop is lying), and ac resistance of the

approximation because of the different types of geological capacitors used to series resonate the loop.

influences on the loop antenna. A site-specific application engineering of a loop involves

Larger mines pose a challenge in that the larger the loop, the resonating the inductor with capacitors, then determining the

larger the magnetic moment and the larger the field. However, the overall impedance to adjust the transmitter's output stage

larger the loop, the more it couples into conductive earth, resulting accordingly. Establishing the Q in the transmitted inductor in its

in a lower Q. Therefore, a point is reached where the loop size is target location is always desirable. This is easily ac- complished by

optimal. This implies that a loop antenna as large as possible resonating the loop using an oscilloscope, a frequency generator,

should be used. However, at the point where applied power rises and a frequency counter connected to a capacitance decade box in

considerably, resulting in less of a rise in ampere-turns, one would series with the loop. Adjusting the decade box and frequency

conclude that the loop antenna is too large. The most effective while observing the waveform until it shows the most current

solution to this is multiple smaller loops. drawn at the desired frequency will establish resonance. Then, by

The loop antenna is series-resonated with high-voltage establishing the voltage in the loop (V ) and dividing it by V , Q is

capacitors. High-voltage rated capacitors are needed due to the established. By using the resonance nomograph chart shown in the

high voltage generated in the loop. A loop with a Q of 40 and an appendix, capacitance (C), frequency (ƒ), or inductance (L) can

applied voltage (V ) of 70 V (peak to peak) will generate a loop establish the best Q. A

voltage of 2.8 kV (peak to peak). The Z of the loop that is 15 m in diameter with a Q of 40 would

The Q is directly related to the impedance or alternating-current be calculated by the reactance (X ) divided by the Q. X is derived

L A

L L
XL ' 2BƒL,

P ' E 2

Z ,

P '

VA

2

0.707

2

XL

Q

.

8

(1)

(2)

(3)

 Figure 4.—Receiver reference inductor mounted on a leadacid-type cap lamp.

by the L and frequency (ƒ) used and is given by the following conductivity will attenuate the signal more rapidly as distance from

expression:

where X ' reactance of the loop, S, L

L ' inductance of the loop, H,

and ƒ ' frequency of the loop, Hz.

For a typical 15-m loop that is series-configured at 15 mH with a

3-kHz carrier signal, X ' 283 S. Therefore, this con- figuration L

at a mine site that requires a 2- to 3-km radius of field around the

loop will use the following power expression:

 where P ' applied power of the loop, W,

E ' applied voltage in the loop, V,

and Z ' impedance of the loop, S.

Equation 2 can be rearranged to yield the following expression:

Substituting for E in equation 3 is the applied voltage to the

loop antenna at half power. Therefore, the effective root-meanssquare value equals 24.7 V. The impedance of the loop, Z, can be

calculated by dividing the reactance (X ' 283 S) by Q (40), L

yielding a Z of 7.08 S. The applied power to the 15-m loop is

87.2 W.

This antenna design is unique to its location because the

environment and geology at the point of installation affect its

electrical characteristics. The above example of a 15-m loop that

provides a Q of 40 and a Z of 7.08 S, if laid on wet conductive

ground, will require more power to achieve the same results. For

example, if the Q is reduced to 30, the Z will increase to 9.4 S;

therefore, the applied power will decrease to approximately 65 W.

The aim of the site engineering exercise for the transmitter is to

construct a loop that creates the largest low-frequency

electromagnetic field requiring the least amount of power under

several practical considerations. Geology containing high

the loop increases. Geologies with magnetic materials or

ferromagnetic qualities influence the direction of the field's line of

force and can distort the field or area of coverage.

RECEIVER REFERENCE INDUCTOR

To establish reference to the transmitted electromagnetic field

strength, a receiver reference inductor with a high Q was used. The

receiver reference inductor (a loop stick antenna) used for the lowfrequency application was constructed on a 14-cm-long by 0.79-

cm-diam, high- permeability ferrite rod. This ferrite rod was

wound with 0.32-mm-diam magnet wire to an inductance (L) of

about 100 mH and resonated to 3 kHz with a polystyrene capacitor

to a Q of 100. These specifications for the inductor were used for

the reference because they were the same specifications used for the

person-wearable receivers attached to the cap lamp modules shown

in figure 4. The in- ductor antenna requires approximately 500 FV

for dependable operation in some mounted receiver modules (when

mounted into the person-wearable cap lamp), although it can

receive a signal as low as 100 FV without electrical noise.

7.5-METER-DIAMETER LOOP

The first transmitter loop antenna to be set up at the Lake Lynn

mine site measured approximately 7.5 m in diameter.

Figure 5 shows the loop setup on the floor in the old workings of

the mine between two pillars. This loop was resonated at 3 kHz,

an

d

its

m

ea

su

r e

d

i n

d

uc

t a

nc

e

w

a s

2

5

m

H.

T

he control cable used for this loop consisted of 30 con- ductors of

2.1-mm-diam stranded wire and measured ap- proximately 46 m

long. This cable was laid in a circle and looped around twice to

form the antenna. The loop was connected to form two 30 turns;

the two loops were then connected in parallel to give the better than

effect of 2.6-mm-diam wire conductors. This 30-turn loop, shown

in figure 6, used the following parameters to establish resonance in

the circuit: L ' 25 mH, X L ' 471 S, Z ' 9 S, ƒ ' 3 kHz, and C '

0.11 FF. With the V A ' 65 V (peak to peak), a Q of 52 in the loop
9

Figure 5.—A previous 7.5-m loop antenna in old workings of Lake Lynn mine site.

will produce V ' 3,380 V (peak to peak). With the applied power conductors of 2.1-mm-diam stranded wire and measured L

at approximately 60 W, the signal was tested at the farthest point of approximately 47 m long. The cable was laid in a circle and

A-drift at the northeast corner of the mine. As can be seen from the looped around once to form the antenna. This 15-turn loop,

data in figure 7, a marked decrease in signal strength occurs as the shown in figure 9, used the following parameters to establish

receiver reference inductor moves farther away from the loop. A resonance in the circuit: L ' 15 mH, X ' 283 S, Z ' 6 S, ƒ '

signal loss of 86% resulted in moving from 100 to 200 m. 3 kHz, and C ' 0.18 FF. With the V ' 27 V (peak to peak), a Q

However, at 1,000 m, the signal is still within the 200-FV range. of 47 in the loop will produce V ' 1,269 V (peak to peak). The

Because full mine coverage was not achieved, a larger loop was signal was applied to the loop at approximately 15 W, and readings

built. were taken at the junction of D- and E-drifts and at the beginning

15-METER-DIAMETER LOOP The data in figure 10 show a signal strength of 32 mV at 100 m.

A 15-m-diam loop antenna, in addition to the 7.5-m loop, was moving to 200 m. This loop provided better than 500 FV to the

set up outside of the mine between the control building and the receiver reference inductor at both locations. Although the signal

quarry wall northeast of the mine entrance, as shown in figure 8. was adequate for coverage, a stronger signal was preferred.

The control cable used for this loop consisted of 30 Because the research plan called for a small surface transmitting

L

A

L

of D-drift.

However, a sharp decline in the signal strength is observed in

antenna (less than 2 m in diameter), the effort to develop it

continued.

25-METER-DIAMETER LOOPS

First 25-Meter Loop

The 25-m-diam loop (6 and 12 turns) was installed inside of the

mine, outside of the portal of A-drift, and suspended from the steel

support mesh attached to the roof, as shown in figure 11. The steel

mesh effects soon became apparent as the loop was resonated.
10

Figure 6.—Electrical schematic of 7.5-m loop.

 Figure 7.—Signal strength as a function of distance, 7.5-m

loop.

Figure 8.—15-m loop outside location.

Figure 9.—Electrical schematic of 15-m loop.

Figure 10.—Signal strength as a function of distance, 15-m

loop.
11

 Figure 11.—Location of first 25-m loop antenna mounted at

the roof between A- and B-drifts.
12

 Figure 12.—Location of existing 25-m loop antenna positioned around a pillar near the portal of D-drift.

Figure 13.—Electrical schematic of 25-m loop.

Figure 14.—Signal strength as a function of distance, 25-m

loop.

The signal generated in the antenna system (both 6 and 12 SITE-SPECIFIC ENGINEERING OVERVIEW

turns) for the lamp receivers was less than 100 FV about 100 m

from the end of A-, B-, and C-drifts. The TSS field strength meter Working with large inductors at ultralow frequencies can

could receive the signal throughout the mine; its sensitivity was present a unique set of problems. Textbook theory would point to

able to decode at 20 FV in a low-noise environment. The signal large loops at such a low frequency as a method to create a large

was suppressed for two major reasons. The first reason was the electromagnetic field. However, in most field tests and

location of the central transmit antenna. The steel mesh caused a experiments, we found that this was not practical or economical.

shorted turn effect on the loop antenna that drove the impedance up The adverse effect of the environment (e.g., moisture) is also

and lowered the Q of the loop. The second reason is seasonal due accelerated by increasing the diameter of the loop.

to the overburden holding more moisture in the spring months of Mine entries spread out over several kilometers and can be at

the year; however, this problem does not alone prevent a full- various depths. Some mine entries are in the sides of mountains

coverage signal. and even extend under lakes and streams. This creates a difficult

Second 25-Meter Loop larger mines. Further complications can be introduced by the

The 25-m loop was then relocated around the base of a pillar, It was also determined that, although advances could be made

as shown in figure 12, in the area where D-drift begins. This 14- toward getting the signal into the mine, processing intelligent

turn loop, shown in figure 13, used the following parameters to modulation protocol at 3 kHz can present a problem. A simple

establish resonance in the circuit: L ' 30 mH, X ' 561 S, Z ' 100% amplitude modulation (AM) format for the first system or L

14.4 S, ƒ ' 3 kHz, and C ' 0.1 FF. With the V ' 45 V (peak to 7.5-m loop (starting at 4 Hz up to 30 Hz) was developed. This A

peak), a Q of 39 will produce V ' 1,802 V (peak to peak). With method was adequate for testing; however, complications can arise L

applied power at approximately 30 W, the following measurements with other electrical noise interfering with the transmitted

with the receiver reference inductor were taken: (1) the first modulation.

crosscut down C-drift measured 5 mV; (2) the junction of A- and

E-drifts measured 1 mV. The data in figure 14 show a signal

strength of 155 mV at 100 m and decreases 25 mV at 200 m. With

the achievement of a sufficient signal throughout the Lake Lynn

underground mine and surface perimeter with this antenna (a 25-mdiam loop around the base of a pillar near D-drift), the system was

further developed.

situation for a single-transmitter loop antenna system for some

geology at the mine site, as described previously.
13

Figure 15.—Block diagram of TeleMagnetic signaling system for Lake Lynn Laboratory.

The Lake Lynn system, depicted in figure 15, is also set up with compensate for the low signal, a 15-m loop was installed inside of

a remote control transmitter and central control console in the the mine, outside of the portal of A-drift, where the first 25-m loop

building. The signals are transmitted to the underground mine was initially mounted, and hardwired to the 25-m loop. The 15-m

through a 1.5-m loop antenna located outside of the building. Due loop, parallel-resonated for 8.4 kHz, was used as a receiver

to the overall radial distance of the present Lake Lynn 25-m loop antenna. Figure 16 is a block diagram of the various transmitter

with respect to the outside transmitter (a 1.5-m loop) and power and receiver devices of the completed system.

form, the transmitted signal from the central control console was

inadequate to reach the 25-m loop. To
14

Figure 16.—Flowchart of Lake Lynn antenna system.

 The Lake Lynn central control console is mounted on the wall At this point, the main transmitter transmits the paging signal

in the control building. This console controls all of the functions throughout the mine. The individual person-wearable cap lamp

of the remote control transmitter for the site and accommodates six receiver will flash its lamp once the paging signal is received. The

paging numbers (signals) plus the evacuation signal. These paging lamps will continue to flash for approximately 90 s. An encoded

numbers can be used for contacting personnel and/or other signal transmitted from the control building can also turn off the

auxiliary functions, such as turning on/off devices. The system flashing lamps. Several miners can wear the paging lamps on their

was initially set up to page five individual person-wearable cap helmet with the battery pack and receiver on their belts and can be

lamp receivers. The individual paging numbers can also be used paged at various times during the day or warned during an

to turn on and off remote receiver controllers connected to strobe emergency. Each cap lamp receiver will also respond to a global

lights. A microprocessor-based central console processes the warning or evacuation signal. It is recommended that a battery

encoded page number that is manually entered on its selector backup be installed for the complete system in case of a power

switch. The console then sends the page number to the remote failure.

control transmitter, which transmits the encoded signal into the The smoke sensor is interfaced with an inductor series-resonated

mine. Centrally in the mine is a microprocessor-controlled multi- transmitter (0.3-m-diam antenna) for 8.4 kHz (see figure 16). With

function receiver unit. Here the signal is decoded, and the correct the applied power of 10 W, the signal can be transmitted 800 m to

paging signal is sent to the main central loop transmitter. the 15-m loop when the sensor alarms.
15

RESPONSE TO UNDERGROUND MINE FIRES and response times of other fire sensors can be found in Conti and

A series of underground fire detection experiments was A pump-operated ionization smoke sensor used during the

conducted to examine the feasibility of interfacing commercial experiments was mounted on the rib, with the intake sampling

mine fire sensors with the TSS system to initiate an emergency point (a 1.3-cm-diam tube) at the roof in the center of the entry,

warning. The alarm modes of two different smoke sensors were 15.2 m inby the fire. In other experiments, a diffusion-type

used to trigger the signaling system during experimental mine fires. ionization smoke sensor was mounted on the roof in the center of

The fire detection experiments reported here were conducted in the entry, 15.2 m inby the fire.

A-drift. The normal airflow in the mine was reversed so that the In the control building, the data signal of the smoke sensor was

combustion products were exhausted through the main fan. The displayed on a video monitor, along with a continuous video

scenario studied was a slowly developing coal conveyor belt fire. coverage of the fire. Another video camera 274 m downstream of

Seven 220-V electric strip heaters, with a combined power rating the fire monitored a mannequin wearing a person-wearable cap

of 9.5 kW, were embedded into a 1.2- by 1.2-m coal pile lamp receiver.

containing 75 kg of Pittsburgh coal. Six 10.2- by 22.8-cm strips Under these test conditions, the smoke sensor alarmed at 15.2 m

of rubber conveyor belting, 1.1 cm thick, were evenly distributed because of the smoldering coal pile, usually within 1 to 2 min after

throughout the coal pile. Additionally, two 22.8- by 61-cm strips the first sign of visible smoke was observed from the coal pile in

of the same belt were placed on top of the coal pile, and the pile the entry. The encoded signal was received, and underground/

was seeded with approximately 0.75 kg of pulverized Pittsburgh surface receivers flashed cap lamps.

coal dust. Full electrical power was applied to the heating elements In other experiments, these encoded signals were also used

at the start of each test. Visible smoke from the coal pile was remotely to initiate inflation of a positive-pressure inflatable escape

usually observed in 2 to 3 min, with flames emanating from the device [Weiss et al. 1996] and turn on and off strobe lights and

coal about 9 min later. The strips of conveyor belting placed on other lighting fixtures. The strobe lights could be located at key

top of the coal pile ignited later during the tests. A detailed decision points along an underground escape route for miners.

description of the fire scenario 

Litton [1992].

SUMMARY AND CONCLUSIONS

The experiments at Lake Lynn Laboratory incorporated a larger loop antennas were successfully used in various highvariety of loop antennas ranging from a few meters up to 25 m in conductivity geological environments. At times, depending on the

diameter. Many tests were conducted and data collected on several configuration of a given mine, complete coverage seems impossible

loop specifications to ascertain the effects of the environment and due to the adverse conditions. To improve cov- erage, several

geology. The experiments were conducted through the course of transmitter [Zamel 1990] locations that can be controlled and

seasonal changes so that the data could be used to design a synchronized with unique low-frequency transmit modulation

transmitter loop that would work well in all environmental protocols could be used.

conditions. The moisture in the overburden above all of the loops The research has led to a unique application engineering format

at Lake Lynn Laboratory increased the conductivity in the strata, to design a loop antenna to a specific site. Special portable test

which resulted in increased signal attenuation. The steel support instruments and test loops have been designed and successfully

mesh on the roof for the first 25-m loop antenna increased the evaluated at Lake Lynn. The development has also led to several

conductivity directly adjacent to the antenna, whereas the shorted workable solutions, especially to the problem of limited surface

turn effect lowered the Q of the loop. Nonferrous metal would also access. This was solved with the addition of small-diameter

lower the Q because of the conductive nature causing a shorted underground repeater loops. A small dis- advantage is electrical

turn effect. power requirements and maintenance.

The transmitter loop antenna tests have led to a greater The development and testing of the modulation protocols have

understanding of the effects of geology and environment on the led to reliable operation without fear of false alarms that would

systems. These data will be valuable toward reducing the cost of often result from electrical noise in the earlier design. New

engineering systems for future sites. For example, it was protocols include a combination of both AM and frequency shift

determined that the soil conductivity increases the inductor's keying (FSK) modulation platforms, along with the addition of

impedance when close to the loop. It was also found that, multiple-band transmission and expansion of the frequency range

although increasing the diameter of a transmitter loop antenna from 630 to 8,970 Hz.

would theoretically increase the magnetic moment, it would also During underground fire tests, when the smoke sensor alarmed

increase the effects of the geology on the loop. Previous USBM to the products of combustion produced from the smoldering coal

research [Hjelmstad and Pomroy 1991] showed that pile, the alarm mode successfully triggered an 
16

emergency warning signal throughout the Lake Lynn complex. The technology displays the ability to transmit from com- pact,

Underground and surface receivers flashed cap lamps when the portable, and semiportable receiving and decoding equipment.

encoded signal was received. This paves the road to transceiver applications such as emergency

In other tests, these encoded signals initiated the inflation of an two-way messaging for refuge stations or mine rescue operations;

underground positive-pressure inflatable escape device. More remote monitoring of fire sensors, gas detectors, seismic sensors,

recent tests showed that the encoded signals may be used to turn on sump pumps; and remote control and monitoring of other mine

or off other devices, such as strobe lights, thus mapping an escape equipment, such as ventilation fans, conveyor belt systems, and

route for evacuating miners. The concept of strobe lights was haulage traffic control. Additional areas for research include

successful in experiments at the Lake Lynn mine, which is small transmitter designs for very large mines with and without adverse

and has a simple ventilation system. In a larger mine, the geology, vehicle paging and tracking, a motionless detector (to

uncertainties inherent in a complex ventilation system would alarm if personnel become motionless) or a receiver with manually

complicate this process considerably. Additional research would operated distress buttons, automatic "tag-in and out" with

be required to evaluate the feasibility of using these devices in addressable paging numbers, and tracking of underground miners

larger mines and incorporating an audio output with each strobe that could send out a beacon on command from a locator

unit. The cost of a signaling and warning system such as the one interrogator for mine rescue purposes.

installed at Lake Lynn Laboratory is approximately $20,000.

ACKNOWLEDGMENTS

The authors thank the following NIOSH Pittsburgh Research Slivensky, physical science technicians; Kenneth W. Jackson,

Center employees for their assistance during experimental setups: electronics technician; and Deepak Kohli, electrical engineer.

Richard W. Pro, Linda L. Chasko, and William

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18

Figure A-1.—Resonance nomograph chart [AC electronics: a step-by-step introduction 1983].

APPENDIX.—RESONANCE NOMOGRAPH CHART
DHHS (NIOSH) Publication No. 97-127

May 1997
20

Cover photo: Mine rescue team members responding to a mine

emergency. (Photo by Ronald S. Conti, National Institute for

Occupational Safety and Health, Pittsburgh Research Center)