breathing apparatus [l] for protection against toxic 

fire gases and save precious time needed for a 

successful escape from the mine fire [2]. The ideal 

fire warning system would also have low installation 

and maintenance costs, be reliable, able to withstand 

rough treatment, and tolerate the harsh mine environment. 

where H = the magnetic field strength, A/m 

I = the antenna current, A, 

M = the number of turns, 

A = the area of the antenna, m2, 

G = an attenuation constant, 

n = the constant 3.14 (pi), and 

Z = distance through the transmitting medium, m. 

Currently used underground mine fire warning systems The transmitter system consists of a low frequency 

such as telephones, ultra high frequency (UHF) and audio signal generator operating at 630 Hz coupled medium frequency (MF) radio, visual or audible alarms with a high power audio amplifier (fig. 1) which 

or the most commonly used "stench" system all have 

their 1 imitations and shortcomings. - 

Telephones are commonly used in coal mines where the 

miners are closely grouped at the working face and not 

dispersed as they often are in metal or nonmetal 

mines. The noise of a ringing telephone or other 

audible alarm can be masked by the noisy mine 

environment, or its ring becomes so familiar a sound 

that it is ignored by the miner. This form of 

communication can be rendered dysfunctional when the 

wire is severed by rock falls, fire, or explosion. 

UHF radio and visual alarms require line-of-sight 

for signal reception and miners in remote parts of the 

mine will not be made aware of the warning signal. MF 

radio requires an extensive electrical conductor 

system for signal transmission [3]. In mode~n mines 

where diesel equipment is used, conductors such as 

power, air, and water 1 ines might be absent, and the 

cost of installing wire conductors throughout the mine 

for MF radio would be very costly. Fig. 1. Fire warning system transmitter. 
A. Receiver 

Fig. 3. First generation ULF receiver. 

In initial trial tests of the fire warning system a 

transmission frequency of 1950 Hz was used, however a 

transmission frequency of 630 Hz was ultimately chosen 

because it is low enough to more easily penetrate 

overburden, while still operating within constraints 

of our power amplifier and loop antenna system. Since 

630 Hz lies between the tenth and eleventh harmonics 

of the standard 60 Hz power line frequency used in the 

United States, it also offers some immunity to 

electrical noise, although the ambient noise level is 

In order to detect the transmitted signal and warn 

the miner of a possible emergency the Bureau has 

developed a prototype receiver circuit which will 

eventually be integrated into the miners cap lamp 

battery unit. The system will detect the pulse 

modulated signal from the transmitting system and 

flash the cap lamp until the miner acknowledges the 

warning and returns the lamp to normal operation. 

The receiver circuit consists of four major 

sections: 1) the receiving antenna, 2) the amplifier, 

3) the phase-locked-loop detection system, and 4) the 

cap lamp flashing system. Each section is described 

in detail below. 

The receivins antenna consists of a 17 cm (7 inch) 

long and 0.635"cm (1/4 inch) diameter ferrite core 

wrapped with several hundred turns of #30 AWG enameled 

copper wire. The receiver antenna, is capacitively 

cou~led to the in~utransformer of the receiver and 

tuned to operate at the 630 Hz resonant frequency. 

The transformer steps up the voltage by a factor of 

ten, after which a selective bandpass amplifier 

filters the received signal and drives two gain stages 

designed to amplify the filtered signal to the desired 

level for detection by the phase-locked-loop (PLL) 

detector circuitry. The prototype receiver has the 

amp1 ified signal fed to a pair of PLL signal 

detectors. The first PLL detects the 630 Hz carrier 

signal. The output of the first PLL drives the input 

of a second PLL for detection of the 1 Hz modulation 

frequency. Four cycles of the 1 Hz modulated signal 
must be received by the counter before the system 

recognizes it as a valid warning and activates the cap 

lamp flasher to warn the miner of an emergency. A 

reset button allows the miner to acknowledge the 

warning and return the cap lamp to its normal 

operation. Additional PLL stages could be added to 

provide mu1 tip1 e warning signals or digital data 

transmission capabilities if desired. 

B. Transmitter 

The Bureau's prototype ultra-low frequency 

transmitter for encoding signals consists of a pair of 

signal generators, a power amplifier, a transformer, 

and loop antenna. The primary signal source generates 

a 630 Hz sine wave output that is triggered by the 

second signal source operating at the 1 Hz modulation 

frequency. The output of the primary signal source 

feeds the input section of the 1000 watt audio 

frequency range power ampl i f i er . 

In order to provide the maximum power to the multiturn loop antenna a set of matching capacitors is used 

to cancel the effect of the antenna's inductive 

reactance on the system. By measuring the inductance 

of the loop antenna using a inductance meter, the 

capacitance value needed to maximize power transfer 

can be obtained from the following formula: 

where f = frequency, Hz, 

L = antenna inductance, Henries (H), and 

C = matching capacitance in Farads (F). 

After selecting the approximate series capacitance 

required to tune the antenna, the system is fine tuned 

to the resonant frequency by adding or removing 

capacitors as necessary. The use of a transformer to 

match impedance of the transmitting antenna to that of 

the amplifier can improve efficiency of the system 

when the impedance of the large transmitting antenna 

is higher than the optimum output impedance of the 

ampl ifier. 

111. INITIAL FIELD TEST RESULTS 

Initial field tests of the electromagnetic fire 

alarm system were made at a local Minneapolis 

sandstone mine which had 15 m of overburden made up of 

sandstone, limestone, and glacial till. The 

transmitter and a 30 m diameter six-turn loop 

transmitting antenna were located on the surface and 

the receiver unit was carried underground. For the 

initial tests, an 8 ohm speaker was used in 

conjunction with an audio frequency receiver to allow 

the operator to hear a continuous tone signal. The 

first tests were made at low power levels (2 w). The 

signal was received through about 15 m of overburden 

with an area of reception about 10 times the area of 

the transmitting loop antenna. The results of this 

first test were encouraging enough to justify 

additional field tests of the system. 

A second series of tests was made in the 

Tower-Soudan underground iron mine located in northern 

Minnesota. Unlike the first test, the transmitting 

antenna was placed underground in a stope above the 

27th level drift, located 762 m from the main shaft. 

The transmitting antenna was made up of 10 turns of 

#10 copper wire formed into a 30 m diameter loop. The 

transmitter (fig. 1 ) and receiver (fig. 3) were used 

in the tests. 

It is possible to establish the current generated in 

a ferrite core receiving antenna by measuring the 

voltage across its leads. This was done while the 

receiving unit was placed at the center of the 

transmitting loop antenna. It was established that 

the ratio of the power in the receiving antenna to the 

power in the transmitting antenna, in decibels, was a 

constant value regardless of the transmitting power 

levels, provided distance between antennas was 

constant. 

Receiving antenna power levels were then established 

at various points throughout the mine at various 

transmission power levels. The ratio of the power 

level in the receiving antenna to the power level in 

the transmitting antenna, in decibel loss, is the 

basis for establishing power loss at various 

distances. The results of these measurements are 

shown in figure 4 and indicates that the through-therock electromagnetic signal decays with distance in a 

manner similar to an inverse cubic function. The 

graph also indicates maximum signal reception through 

rock at this mine site was in excess of 762 m. 

Maximum transmitter power level was 53.3 w. 

Distance from Transmitter (M) 

Fig. 4. Graph of signal attenuation at various 

distances. 

Another in-mine test of the electromagnetic fire 

alarm system was conducted at a mine near Ogdensburg, 

NJ. A 10-turn, 30-m-diam transmitting antenna was 

positioned at two different locations on the surface. 

At one site the antenna was positioned on overburden 

of water saturated saprolite (a claylike residuum of 

limestone dissolution). At the other site, the 

antenna was positioned on marble (fig. 2). For this 

test a hand held 30-cm-diam 500-turn loop antenna was 

used to measure the magnetic field strength at depth. 

The measured magnetic field strength at depth was 

compared with the theoretical (free space) value of 

magnetic field strength at depth. The ratio of the 

two values of field strength permitted determination 

of the attenuation constant. When compared, it was 

determined that the marble had lower attenuation 

characteristics than saprolite and was correspondingly 

more transparent to electromagnetic waves. 

Tests at the mine and the two sites in Minnesota 

were conducted at a carrier wave frequency of 1950 Hz. 

Theory predicts that if lower values of frequency are 

used, the signal attenuation will be less, because 

skin depth of the signal (the depth at which the 

signal equals l/e or 36 pct of its original value) is 

inversely proportional to both frequency and 

conductivity as shown by the following equation: 
The following field tests of the electromagnetic 

fire warning system were made using the high gain 

receivers (6000 and 12000 gain) that can be equipped 

with phase lock loops for receiving pulse modulated 

630 Hz signals. For gathering test data on EM signalto-noise ratios, a voltmeter or portable oscilloscope 

were used to monitor voltage output of the receiver 

circuit. 

Tests of the electromagnetic fire alarm system were 

made at two Louisiana salt mines where the overburden 

was known to be made up of salt water saturated earth 

material. The highly conductive overburden is 

believed to present a most difficult condition for 

electromagnetic signal transmission, and this proved 

to be true even at a the lower transmission frequency 

of 630 Hz. 

The transmitter and 93 m diameter loop antenna of 12 

turns was deployed on the surface above the workings 

of the first mine (fig. 5). The transmitted 

electromagnetic signal pulsed on/off for 30 second 

intervals penetrated to the 365 m level and was 

received at sites in the mine which indicate that full 

mine signal coverage of the fire warning signal was 

possible. In reaching to the 365 m level the signal 

passed through two levels of the mine in which the 

Department of Energy had stored 95 million gallons of 

petroleum, and also through 45 m of salt water 

saturated overburden and 320 m of cap rock and rock 

salt. 

mitter antenna placed underground. 

A gypsum mine in Iowa was a site at which tests were 

made to determine the effects of antenna design on the 

performance of the fire alarm system. This mine has 

188 m of overburden made up of 60 m of limestone above 

the gypsum ore body, 60 m of shale and about 60 m of 

glacial till. Like the salt mines this was a room and 

pillar mine and perhaps more typical of the many room 

and pillar mines in the United States. The mine 

under1 ies farm1 and and to simp1 ify test conditions the 

transmitter antenna was placed underground around mine 

pillars. Two antennas were used to gain information 

on electromagnetic signal transmission characteristics 

as a function of antenna design. One antenna was a 93 

m diameter loop of 12 turns, the other 48 m diameter 

loop of 8 turns. Field strength meters were used to 

evaluate reception on the surface. Electromagnetic 

signal transmission from a surface transmitter and 

antenna would encounter the same rock strata and 

therefore approximate the conditions of the test. 

Field strength measurements were made at sites on 

intersecting surface roads overlying the mine. 

Reception at the surface was good and the electromagnetic signal was detectable at over a mile away 

from the center of the underground transmitting loop 

antenna. The limits of the surface reception area had 

an elliptical shape for transmission from both types 

of antennas. The major axis of the ellipses is 

thought to correspond to a preferred orientation of 

joints in the limestone overlying the ore body, 

because flat surfaces which exist on mine pillars have 
magnetic moment of .9*10"~~' which is about 4 times 

greater than the magnetic moment used in the above 

mentioned test. 

,I."..l*ULF*n".I.C*Itul 

I,.,,.,.".",., ... 9- .-*I-- 

Fig. 6. Map of reception limits at ypsum mine 

(1) Magnetic moment of 129*103 ATM yields 

2.3*106 m2 area of reception; 

(2) Magnetic moment of 60*lo3 ATM' yields 

1 .2*106 m2 area of reception. 

finalized and the system made commercially available. 

REFERENCES 

[I] FMC Corp. Mine Shaft Fire and Smoke Protection 

System, Volume I. Design and Demonstration. BuMines 

Open File Report 24077, July 1975, 407 pp. 

[2] Ontario Provincial Government. Improving Ground 

Control and Mine Rescue; The Report of the Provincial 

Inquiry Into Ground Control and Emergency Preparedness 

in Ontario Mines, ISBNO-7729-1064-2, 1986, 108 pp. 

[3] L. G. Stolarczyk, A Medium Frequency Wireless 

Communication System for Underground Mines (contract 

H0308004, A.R.F. Products, Inc.). BuMines OFR 115-85, 

1984, 221 pp.; NTIS PB 86-134103. 

[4] T. L. Muldoon, T. Lewtas, and T. E. Gore. 

Upgrade Stench Fire Warning System, System Development 

Prototype Tests (contract H0292002, Foster-Miller 

Assoc., Inc.). BuMines OFR 136-81, 1981, 142 pp.; 

NTIS PB 82-122128. 

[5] W. H. Pomroy and T. L. Muldoon. Improved Stench 

Fire Warning for Underground Mines. BuMines IC 9016, 

1985, 33 pp. 

[6] H. K. Sacks, Trapped Miner Location and 

Communication System. Ch. in Underground Mine 

Communication (In Four Parts). 4. Section-to-Place 

Communication, BuMines IC 8745, 1977, pp. 31-43. 
[7] E. C. Jordan and K. G. Balmain, 1968, 

Electromagnetic Waves and Radiating Systems, PrenticeHall, EE series, Second Edition, 753 pp. 

[8] James R. and Jill Wait and A. David, 

Electromagnetic Fields In Mine Environments. BuMines 

OFR 32-81, 1980, 448 pp.; J0199115.