UNDERGROUND MINE COMMUNICATIONS 

J. N. Murphy and H. E. Parkinson 

Reprinted from Proceedings of the IEEE, Vol. 66, No. 1, January 1978 

U. S. Government work not protected by U.S. copyright 
PROCEEDINGS OF THE IEEE, VOL. 66, NO. 1, JANUARY 1978 

Underground Mine Communications 

JOHN N. MURPHY, SENIOR MEMBER, IEEE, AND) HOWARD E. PARKINSON 

Invited Paper 

~bstract-underground mines are typically extensive labyrinths that I. INTRODUCTION 

employ many people working over an area of many square miles; extensive analysis of mine-communications systems has identified specific A. The .Nature of Mining 

problem areas, in particular the excessive times required to lock key HILE THERE ARE many similarities among coal minpersonnel underground, the inadequacy of existing phone systems in 

terms of capacity and privacy, and the inability to communicate with ing techniques anywhere in the world, there are also 

men on the move with wireless communications, as is taken for granted striking differences, the most significant of which is 

on the surface. A review is presented of the existing systems, the prob- that in the U. S. approximately 95 percent of the underground 

lem definition, and the various approaches that have been or are being coal is nlined by the room and pillar technique, while in Euroinvestigated to solve these problems. pean countries approximately 9 5 percent of the underground 

coal is niined by the longwall technique. Other coal-producing 

Manuscript received June 8, 1977; revised August 28, 1977. areas of the world may use either of the above methods or vari- The authors are with the Industrial Hazards and Communications 

arou~. Pittsburgh Minin~ and Safetv Research Center. Bureau of Mines. ations these A exhaustive the 

6. s.-Department of the interior, piitsburgh, PA 152 13. ' 

mining techniques can be found in [ 1 1, [2], but for the purU.S. Government work not protected by U.S. copyright 
LEGENO 

=check curlam %permanen! stopplng 

----. Line brattics - A~rflow 

Fig. 2. Plan view of working section showing crosscuts. 

poses of ti-is paper, the fcaowing simplified descriptions will 

suffice. Mod, but nu. all, c3zl seams are horizontally bedded 

depcLi:s that re rairly flat and c. sinuous and that do not 

usuelly vary significantly in thickness for a given deposit; 

hence, the coal extraction process typically involves constructing tunnels or entries in coal, or mostly in coal, when the seam 

is not thick enough to provide adequate clearance for certain 

types of equipment. 

In room and pillar minings, a number of parallel entries are 

driven through the coal as shown in Fig. I ; the entries are used 

for ventilation, men and materials handling, and emergency escapeways. In the U.S., which is the principal user of this technique, Federal mining law 131 requires that a crosscut or connecting tunnel at right angles to the main entries be installed 

every 30 m; the result is the maze of entries and blocks of coal 

as shown in Fig. 2. In an actual mining operation after this set 

of main entries is developed some distance, mining is then extended to the left and right of this area to further extend the 

size and complexity of the labyrinth. Extraction of the coal is 

'ria electrically powered machinery, such as the continuous 

side to comply with Federal mining laws [3 ] . The emphasis 

on the single-entry versus the multiple-entry tipproache has 

been stressed primarily because of the differences in the way 

that wireless communications can be implemented, as discussed below. 

While this paper is directed primarily toward coal mine communications, basically the same problems and the same types 

of solutions are applicable to' other types cf undergrtiund 

mines; there are other types of mining methods, depeilding on 

the nature of the mineral depo~it and the surrouni;n~ geoldgi. 

cal conditions, but either single or mu1;:ple tur. !els c rist, pe. . 

haps in the vertical as well as the horizci .tal axis. For : ther 

mining methods 14 ] , : 5 ] several refccences are provided. Sone 

of the work described in this paper relates to noncoal mine experimental work. 

Most coal mines are large undxgrouncf complexes, many of 

which are 50 km2 or more in area and have up to 16 working 

sections operating simultaneously. Production from such a 

mine may be 3-million tons per year; in the U.S., the vast majority of underground coal is mined primarily with continuous 

mining machines and shuttle cars. 

B. An Overview of Communication Services 

Communications are necessary to achieve coordinated work. 

There is no problem while members of the work crew are within 

range of each other's voices; but immediately upon entering a 

portal to the underground, the work crew is separated from 

the surface workers and Federal law calls for communications. 
M08n bell headplrcr appror$malely l,i.?J 

Fig. 4. Plan view of typical longwall section with intake and return 

airway. 

Fig. 5. Typical longwall shearer. 
Shaft bottom 

Fig. 7. Typical pager-phone,system. 

C. Bell Signaling system (Fig. 7). The other is the multiple pair (mostly six-pair) 

At the portal, when there is a hoist, bell signals are used be- figure-8 type cable, with pairs used also for equipment control 

tween those requesting the cage and the hoistman. Recently, and monitoring. Of course, dial-type phones require multiplecages have become automatic elevators, and phone-type com- pair cables with enough pairs from the surface to satisfy the 

munications are provided at the working level and in the cage. 

total underground phones. Branch circuits are a single pair 

with three or four phones in a party-line mode. However, bell signals are simple and transcend language barriers. 

D. Phone Systems 

Telephones are the simplest and most reliable form of communications underground and from subsurface to surface. For 

many years magnetotype phones have been used, but local 

battery-powered loudspeaking telephones, with the advantage 

of transistor amplifiers that enhance the operating range, are 

by far the most prominent (Fig. 6). A few mines have central 

battery dial phones; however, the phones have not been robust, and phone companies will not go underground to provide 

service. 

There are two basic wiring systems: One is a two-wire cable 

having two separate twisted wires, generally #16 gauge, with 

phones tapped-in anywhere along the line, and with branches 

going throughout the mine to form a common talk party-line 

E. Carrier Current Systems 

Mines with electrified railroads often have trains going in one 

direction or another every 10 min, and FM carrier communications over the trolleywire-rail power circuit are used for dispatching (Fig. 8). In many cases this is the most dependable 

communication system. Compared with the telephone circuits, 

it has a better insulation and an appreciably higher mechanical 

strength. Damage caused by roof falls is very rare, and if damage does occur, it is quickly repaired so as to restore the 

haulage. 

Signals are capacitively coupled to the trolley wire, and modified trolley voltage provides operating power for communication circuits. Other coupling circuits transfer signals between 

the trolley and the phone line. The dispatcher's unit is coupled and balanced to ground across the phone line, and at a re- 
coupler interconnects the audio from the carrier, placing it on 

the phone line and the audio in the phone line, applying it to 

the carrier. 

Another system combines the features of pager phones and 

dial phones to provide a private line with a selective page and 

an all page. An interconnect joins the external surface dial 

phones with the underground permissible phone system. 

G. Other Services 

The communication lines often are utilized for control and 

monitoring. Polarity-sensitive relays are coupled to phone 

lines as a means of opening, closing, and indicating the status 

of circuit breakers. Power lines are utilized by superimposing 

a carrier with frequency shift to control breakers and to monitor the flow of power and air into the mine. Complex monitoring and control are accomplished by tone multiplex with 

frequency-shift keying to allow channel monitoring and bidirectional simultaneous transmission of a control and indication signal. A limited use is made of dc pulsing, pulse counting, and pulse length for telemetry purposes. A coaxial-cable 

frequency-multiplex system, controlled by a computer, forms 

an integrated whole-mine communications and monitoring network that is capable of satisfying voice, supervisory, TV, and 

other requirements for mine communications. 

11. COMMUNICATION APPARATUS 

A. Pager Phones 

Pager phones operate on a two-wire party-line system. They 

are simple to install, reliable in operation, and easy tomaintain. 

Fig. 9. Elemental pager phone schematic. 

All the phones have a self-contained battery, a microphone 

amplifier, a pager amplifier, a paging speaker, and associated 

signaling and switching circuits. 

A generalized schematic is shown in Fig. 9. A dc signal applied to the two-wire line by the station initiating the call actuates the solid-state switch in other phones, thus causing amplified voice to be broadcast from the speakers. The handset i~ 

used for party-line communication between the stations. When 

a "page" switch is actuated, a dc voltage is impressed upon all 

phones. The dc voltage actuates a solid-state switch to apply 

battery to the paging amplifier. With the "page" switch activated, the person initiating a call depresses the press-to-talk 

switch and calls out the desired person or location. The pressto-talk switch applies battery to the handset transmitter amplifier, amplified voice is connected to the activated paging 

amplifiers, and the "call" is broadcast throughout the phone 

system. After the page is completed, the "page" switch is deactivated, and with the press-to-talk switch depressed, the amplified handset-transmitted voice is applied to the handset receiver. Two-way party-line conversations are possible between 

all phones having the press-to-talk switch depressed. 

With the advent of solid-state switches, the local voltage is 

provided by standard 12-V lantern batteries both for signaling 

and for powering the local amplifiers. Generally, the phone 

line is a twisted pair of #16 AWG, solid-conductor wire, doubleinsulated at 600-V dc. Splices are seldom made with special 
MURPHY AND PARKINSON: UNDERGROUND MINE COMMUNICATIONS 3 1 

Tw~sted polr, phone l~ne 

t 

Coupler 

Bells 

Transm~tter 4 I 

r--------. -- 

~ece'lver - 

Magneto 

7 300 or600 V dc Trolley w~re LEGEND 

D-D~scr~m~nator 

L- L~m~ter 

MO-Modulator 

Osc~llator 

PA- Paging amp 

S- Squelch 

Sp-Speaker 

- - Power M 0 M-Mtcrophone ampl~f ~er 

r7 Low-level dc 

Power 

supply 

Fig. 10. Elemental schematic of magneto phone. Fig. 11. Block diagram of 300/600V-dc carrier unit. 

connectors, and the wires are generally twisted together. The 

Code of Federal Regulations identifies the requirement for 

lightning arrestors to be used on ungrounded, exposed telephone wires within 100 ft of the point where the circuit enters 

the mine. Gas-filled arrestors are in general use as a replacement of the typical telephone-type carbon-block surge arrestor. 

In an average 1 -million-ton mine there are 30 to 40 phones, 

and the line impedance is about 100 a. A phone can impress LEGEND 

1 to 2 V of audio into this line. As the mine develops, the ba( Rectif~er 

miles of twisted pair increase; the limiting factor is the ability 

to signal the paging amplifier to turn on. The application of ' - / Approx~mate 

an electronic switch, in place of a low-voltage relay, has ex- 1500 meter 

tended the operating range. Fig. 12. Typical trolley system with locomotives and rectifiers. 

B. Magneto Telephones 

The magneto telephone has a battery, hand generator (magneto), bells, hook switch, line coupler, transmitter, and receiver. The battery supplies dc to the transmitter, and the 

hand generator impresses ac on the line where it is sensed by 

the bells as shown in Fig. 10. 

The magneto generates about 100 V at 20 Hz and is cranked 

to. produce the desired long and short rings. The hook switch 

is activated when the receiver is lifted from the hook. When 

the receiver is hung up, the battery voltage is removed from 

the transmitter. 

Magneto phones are placed in a ladder system similar to that 

shown in the pager-phone system. The phones are not con)- 

patible with pager phones, and because there is no amplification of the transmitted voice, the received signal is often weak 

and noisy . 

C. Carrier Current Phones 

The principles of the carrier apparatus are relatively straightforward. A narrow-band FM-type modulation utilizes about 

8 kHz of bandwidth. The typical transmitter (Fig. 11) 

is comprised of a carbon button microphone driving a low-level 

modulator-oscillator that is coupled by a buffer amplifier to a 

power switching amplifier. It is followed by a filter to reduce 

harmonics contained in the square wave and to reshape the 

output to a sine wave. The final stage of the filter is series 

tuned; the output is nominally 25 W at an impedance level of 

25 a. 

The most popular receiver is a tuned RF type. A11 input 

filter is impedance-matched and capacitor-coupled into two 

stages of RF amplification that are coupled by a transformerfilter into a limiter amplifier working into a discriminator. A 

squelch circuit is interposed before the speaker amplifier to silence the speaker at all times except when a clearly recognizable, about 5 to 10 mV, signal is being received. The audio 

output is 6 W into 16 a. A.talk-back switch allows the operator to check the transmitter and receiver. New models have 

phase-lock receiver circuitry. 

Carrier units operate either directly from the dc trolley power 

or from a lead acid storage battery and a charger for charging 

the battery from the dc trolley. The electrical environment is 

extremely severe. The trolley is either 300 or 600-V dc, and 

transient peaks of 12 000 V with pulse durations of 2 ms are 

not uncommon. The nominal line voltage fluctuations are in 

the f 20-percent range. 

The trolley wire system is not a good communication path. 

The trolley is located at a minimum of 20 cm from the roof. 

The roof conductivity is about mho/m. At a carrier frequency of 100 kHz the calculated attenuation rate for an unloaded trolley wire is about 1 dB/km. The carrier unit typically 

has a 80-dB operating range; thus the anticipated range would 

be 80 km. However, a realized range is about 10 km. The difficulty in realizing long range communication coverage is associated largely with the rectifiers and loads connected to the 

trolley wire. 

At 100-kHz carrier frequency, the solid-state rectifiers commonly used to power the trolley act as impedances of about 

1 shunting the trolley wire rail, and are a severe hindrance 

to the propagation of carrier signals (Fig. 12). The motors of a 
produce less noise than the mercury vapor rectifiers which 

they are replacing, and along with recent changes in squelch 

circuits, receivers are operating successfully at 0.3 mV. 

A few special carrier systems have been made. The voice 

equipment, separated into a transmitter and a receiver with a 

dc bias voltage replacing the microphone and a polar relay replacing the loudspeaker,is used in an FSK mode to control and 

indicate "off-on" operation of devices (Fig. 13). One unique 

device uses the polarity of the 60-Hz power envelope to actuate a carrier switch. The presence of the carrier signal during 

the positive half cycle switches a device on, and the presence 

of the signal during the negative half cycle switches it off. Another piece of equipment is an electricclock-type scanner with 

a permanent magnet attached to the end of a modified clock 

hand and the numbers replaced by reed switches. The transmit 

and receive clocks are started in synchrony, and the number 

positions gate the input and output for a channel with either 

an upshift or a downshift pulse for control and indication. 

When used for control, the carrier apparatus is often coupled 

into the ac power mains. The signal is coupled inductively by 

running a special signal wire beneath the main powerline. The 

signal wire is generally 300 m long. The transmission range is 

10 to 20 km. 

Through advanced solid-state control, scanning and counting 

circuits are available, but very few mines have developed the 

skills required to service the more complicated electronic instrumentation. There are belt control systems using advanced 

solid-state circuitry that are modularized and have diagnostic 

indicators built in to assist the electrician in the replacement 

method of troubleshooting. 

tion approaches 1500 m, and there are several dead spots experienced through the length of the shaft. A charger is provided, 

and batteries are generally changed on a weekly basis. 

Paging at LF is accomplished by inductive principles (Fig. 

14). A pocket-size page receiver is activated when a signal is 

sensed by a small ferrite loop mounted inside a molded plastic 

case which also encloses an FM receiver, volume and squelch 

control, and loudspeaker. The pagers are either 88 or 100 kHz, 

whichever is the frequency of the mine trolley-wire carrier 

phone system. The paging signal is connected to the dc trolley 

wire and rail. A tone encoder is used for selective paging and 

for alerting that a page message will follow. The paging range 

is a function of the LF current flowing in the trolley wire. A 

few units are in operation, and because they are on the haulageway carrier phone frequency, these are mostly used by roving miners to monitor the haulageway communications. 

Some fan monitoring systems use aboveground UHF highband radios. They operate on a single radio channel with signal flow in one direction at a time. The equipment is standard 

alarm telemetry products available at the time of installation. 

The systems have discrete audio tones to send indication and 

control information. The office has a very simple printer that 

provides hard copy of the status of the fan and the circuit 

breaker. 

E. Dial and Pager Phone Corn bined 

The extension of the surface bell-type telephone to underground coal mines has two disadvantages: The potential hazard, in a methane environment, from the 120-V 20-Hz bellringing voltage; and the inability to locate a person who is not 
F. Limitations of Existing Systems 

Underground communication has been very effective but 

provides very minimal service. Recent studies have specifically 

defined the areas where service could be improved. Three key 

operational parameters were observed [6], [7] : 

1) the time to reach key personnel underground; 

2) the traffic density or availability of phone lines as a function of time during the working shift; 

3) reliability and/or maintainability of the existing communications equipment. 

Surveys have consisted of full-shift monitoring of mine telephone lines and carrier current rail haulage communication circuits, followed by detailed analysis of these recording to ascertain if there are particular problem areas. As an example, 

consider the results from the survey of a 4500-ton-per-day 

mine. As seen in Fig. 15, the heavy line shows the percent of 

time each hour the phone is in use, with a maximum value of 

approximately 49 percent during the second hour of the shift. 

However, when the data are replotted (dashed line) for each 

quarter hour, it can be seen that there are times when the system is used 71 percent of the time. With this kind of system 

there is a 30-percent chance of finding a busy signal on a given 

call. This is considerably worse than the one chance in a thousand which is the standard for commercial telephone circuits. 

It is estimated that a six-channel phone system would be required to bring the system to commercial standards. 

Further explanation of the data shows that the duration of a 

given call, or attempted call, is very short. Fig. 16 shows for 

on the development of trapped-miner location systems employing a miniature electromagnetic transmitter or seismic signaling 

and detection on the surface. This work is the outgrowth of a 

study by the National Academy of Engineering on Mine Rescue and Survival [8]. 

A careful review of the requirements for an emergency communications system shows that 

1) the equipment must be readily available at the time of 

emergency; 

2) it must be routinely checked and maintained; 

3) miners must know how to operate equipment; 

4) they must know where it is in the event of an emergency. 

It has been reasoned that the best emergency communications 

system is an operational one which is functional under emergency conditions. 

The most desirable form of communications to reach key 

personnel on the move in a coal mine is wireless, either twoway or personal paging. However, the underground mining industry cannot take for granted the utilization of wireless communications as can their counterparts on the surface. As an 

example, at 27 MHz reliable communication in a mine entry is 

limited to about 30 m. Three options are available to the ur 

derground mine operator: 1) to use frequencies that are hig 

enough to utilize the entries as waveguides, 2) to use frequencies that are low enough that propagation through the strata 

can be ensured, or 3) install a special conductor or leaky feeder 

(which in some cases may not be operationally acceptable). 
PP.OCEEDINGS OF THE IEEE, VOL. 66, NO. 1, JANUARY 1978 

M~ne generated nolse 

30 - 

40- 

FREQUENCY, KHz 

Fig. 17. Electromagnetic noise in spectrum in the face area of a coal 

mine. 

Each technique has advantages and disadvantages, which will 

be discussed; however, for determination of the optimum frequency, it is necessary to quantify the EM noise environment 

in coal mines. 

A. Electromagnetic Noise 

The need for improved con~munication systems in mines is a 

long standing problem; during normal operation of a mine, the 

machinery used creates a wide range of many types of intense 

EM interference (EMI), and ambient EM1 is, therefore, a major 

limiting factor in the design of a communication system. However, under emergency conditions when all the power in a mine 

is cut off, the residual EM noise is not a problem. 

EM noise generated in mines is generally a nonstationary, 

random process. Therefore, the most meaningful parameters 

for EM noise generated in mines are statistical ones. In the 

work by the National Bureau of Standards [9], five time and 

amplitude statistics have been used in order to unravel the 

complexities included in the EM manmade noise in mines. 

Ambient magnetic-field noise spectra covering frequencies 

from 100 Hz to 100 kHz are given for several underground 

coal mine locations. Data have been developed for magnetic 

field noise on the surface above the mine, noise in the mine 

face area, noise radiated by specific equipment, the voltage 

spectrum found on a 600-V dc trolley wire, and noise picked 

up simultaneously on loops and on roof support bolts [9g]. 

Extensive work has been conducted in the development of 

data collection techniques suitable for underground mines and 

in the qualification of noise conditions from representative 

mines [9] . Data reference [9g] was taken in U.S. Steel's Robena No. 4 

Mine, Waynesburg, PA. Fig. 17 shows a spectrum measured in 

the face area (Spectral Resolution 125 Hz) about 10 m behind 

a continuous-mining machine in full operation. The machine 

was powered by 600-V dc. For the curve shown, the antenna 

sensitive axis was- oriented for a vertical moment. The field 

strengths measured were about 39 dB above 1 pA/m (39 dB 

pA/m) at 10 kHz and show system noise with the antenna terminals shorted. In a-ddition, the two horizontal antennas recorded mine noise spectra (not shown) that were lower in amplitude by about 10 dB at I00 kHz and about 35 dB at 10 kHz. 

Spectra taken in haulageways in the mine tended to show 

magnetic-field strengths typically 60 to 70 dB pA/m up to a 

few kilohertz, which then decreased sharply above 8 to 12 kHz. 

One exception was a spectrum taken near a dc motor-driven 

hydraulic pump (car pull). This spectrum peaked at 78 dB 

pA/m at 1000 Hz, dropped t o 4 7 dB pA/m at 10 kHz, and was 

down to 25 dB pA/m at 30 kHz. 

As seen in Fig. 17 the EM noise amplitude decreases with increasing frequency; however, three propagation mechanisms 

must be considered: 1) through the strata, 2) through the entries supported by metallic structures and conductors, and 3) 

through the entries where they serve as a "waveguide." Each 

of these mechanisms is discussed below. For the latter two 

cases, it would appear from the data presented above that selection of frequencies >>lo0 kHz would be desirable; however, for situations in which the propagation is through the 

strata, attenuation varies inversely with frequency. A complete 

description of through-theearth propagation can be found in 

[lo]. Because of the lower attenuation, the lower the frequency, the better the signal-to-noise ratio will generally be, 

despite the higher amplitude noise levels. In-mine noise levels 

at higher frequencies are typically the same as in other industrial operations. 

B. Wireless Comnzunications 

This section covers voice communications within the mine, 

and voice or code communications through-the-earth to the 

surface or to another level of the mine. For real-time voice 

transmission, minimum frequencies are about 30 kHz-in the 

LF range the higher the frequency the better the coupling efficiency; in the UHF band, the radiated wave propagates in the 

"waveguide" formed by the mine opening. Hence, selection of 

op tirnum frequencies is dependent on the relative efficiency of 

propagation and the noise level, which together give the optimum signal-to-noise ratio. There are some practical considerations. At the lower frequencies, signal propagation is supplemented via coupling to conductors that may be in the mine 

entries, and antenna efficiency is not necessarily compatible 

with sizes that are portable. In the UHF band, attenuation is 

relatively low in straight mine entries and is significantly higher 

when the signal propagates around a corner or when a massive 

piece of machinery is in the path of propagation. The following is a discussion of experimental and theoretical analysis of 

1) UHF and 2) VF, both through-theearth and in the mine, 

and 3) the use of leaky feeder transmission lines to support 

prop<agation in a controlled manner. Random coupling to miscellaileous conductors in a mine entry is not covered in detail, 

but increases in range approaching two orders of magnitude 

have been obtained by these parasitic couplings. 

Sel'ection of Optim um Frequency: 

C/HF using the mine entry as a waveguide: A comprehensive theoretical study has been conducted [ 11 ] of UHF radio 

communication in coal mines, with particular reference to the 

rate of loss of signal strength along a tunnel and from one tunnel to another around a comer. Of prime interest are the nature of the propagation mechanism and the prediction of the 

radio frequency that propagates with the smallest loss. The 

theoretical results have been compared with field measurements [ 121. 

At frequencies in the range of 200-4000 MHz, the rock and 

coal bounding a coal mine tunnel act as relatively low-loss dielectrics with dielectric constants in the range 5-10. Under 

these conditions a reasonable hypothesis is that transmission 

takes the form of waveguide propagation in a tunnel, since the 

wavelengths of the UHF waves are smaller than the tunnel dimensions. An electromagnetic wave traveling along a rectangular tunnel in a dielectric medium can propagate in any one of a 

number of allowed waveguide modes. All of these modes are 

"1oss:vr modes" because any part of the wave that impinges on 

a wall of the tunnel is partially refracted into the surrounding 

dielectric and partially reflected back into the waveguide. The 

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

the light-pipe modes in glass fibers in which total internal re- 
MURPHY AND PARKINSON: UNDERGROUND MINE COMMUNICATIONS 

7-7-- 7-7 

Halfwave antennas 

Or~entat~on (HH) 

-1 

1 

1,000 2,000 3,000 4,000 

FREQUENCY, MHz 

Fig. 18. Total loss for various distances along a straight tunnel. 

flection occurs at the wall of the fiber, with zero power loss if 

the fiber and the matrix in which it is embedded are both lossless. It is to be noted that the attenuation rates of the waveguide modes studied depend almost entirely on refraction loss, 

both for the dominant mode and for higher modes excited by 

scattering, rather than on ohmic loss. The effect of ohmic loss 

due to the small conductivity of the surrounding material is 

found to be neghgible at the frequencies of interest. 

The overall loss in signal strength in a straight tunnel is the 

sum of the propagation loss and the insertion losses of the 

transmitting and receiving antennas. The overall loss for the 

horizontal transmit horizontal source antenna (HH) orientation is shown in Fig. 18, where it is seen that the optimum frequency for minimum overall loss is in the range 500-1000 

MHz, depending on the desired communication distance. 

The theoretical results for the three different antenna orientations for frequencies of 4 15 MHz and 1000 MHz are compared with the experimental data in Figs. 19 and 20. It is seen 

that the theory agrees quite well with the general trend of the 

data. 

Experimentally, Goddard [ 121 found that the significant 

propagation characteristics are as follows: 

1) attenuation (in decibels) increases nearly linearly with increasing distance ; 

2) horizontal polarization produces significantly lower transmission loss at a given distance than does vertical polarization ; cross polarization produces a loss intermediate between horizontal and vertical ; 

3) transmission loss decreases significantly at a given distance as the frequency is increased from 200 to 1000 MHz. 

With the main tunnel measurements as a reference, data were 

also obtained around comers. Observed corner attenuation is 

shown in Figs. 2 1 and 22 for 4 1 5 and 1000 MHz, respectively. 

Corner attenuation is plotted in db relative to the horizontally 

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

1 

KEY Frequency-415MHz - 

Theory - 

Antenna - 

Experiment orlentatlon - 

HH - VH 

vv - 

- 

- 

I 

0 150 300 450 600 750 900 

DISTANCE, meters 

Fig. 19. Propagation loss in a straight tunnel in high coal at 41 5 MHz. 

KEY 

Frequenc y-I,OOOMHz 

Theory - k\ orlentatlon Ani; 1 

180 l I 1 I I I 1 0 150 300 450 600 750 900 

DISTANCE, meters 

Fig. 20. Propagation loss in a straight tunnel in high coal at 1000 MHz. 

0 5 10 15 20 25 30 35 40 

DISTANCE, meters 

Fig. 2 1. Attenuation of a single corner at 41 5 MHz. 

0 I111I1I 

0 IIIIIII' 

10"- KEY - I,OOOMHz 

," 20 - Pola rlzallon - 

AV v - .V OH n n - 

- 

80 - 

90 IIIIIII 

0 5 10 15 20 25 30 35 40 

DISTANCE, meters 

Fig. 22. Attenuation of a single corner at 1000 MHz. 

," 20 

1 ) signal attenuation immediately around a corner is consid- attenuated. Consequently, the signal existing at any point can erable at all three frequencies; be reasonably assumed to have followed the path with the least 2) complete signal depolarization is observed around the number of corners. The transmission loss at any point along a corner. cross tunnel can then be estimated by adding the attenuation 

Because of the high attenuation of a single corner, propagation in the straight tunnel to the transmission loss corresponding to 

around multiple corners is expected to be even more severely the distance along the main tunnel back to the transmitter. 

KEY - I,OOOMHz - Pola rlzatlon - 
PROCEEDINGS OF THE IEEE, VOL. 66, NO. 1, JANUARY 1978 

Subsequent work by Wait et al. [ 13 ] analytically considered 

EM wave propagation inside an empty rectangular mine tunnel 

where imperfect walls are considered. The modal expansion of 

the fields is complicated by the coupling of the basic modes by 

the imperfect walls. They have developed a geometrical ray 

analysis and applied same to the rectangular waveguide when 

all four walls are imperfectly conducting. They have shown 

that the percentage increase in modal attenuation due to a typical wall roughness for mine tunnels increases with frequency, 

although the overall attenuation is always a decreasing function of frequency due to a more grazing incidence of rays on 

the guide walls. 

Two functional requirements exist for VF communications 

in underground mines: 1) to provide a link through the earth 

to the surface, and 2) to provide in-mine communications. 

The extensive analytical investigation of EM location schemes 

relevant to mine rescue is found in selected publications which 

are summarized below; this work also includes, because of similarity, analysis of electromagnetic detection and through-theearth c~mmunications. The review by Wait [14] of analytical 

techniques related to propagation in the earth provides a good 

summary. 

I) CW Transmission with Loop Antennas: Transmitting antennas that have proved successful in location tests have consisted of either single-turn or multiple-turn loops, usually deployed in a horizontal plane on mine floors. Normally, the 

loop is sufficiently small that it can be treated as a magnetic 

dipole. The magnetic moment is NIA , where N is the number 

of turns, I is the loop current, and A is the loop area. 

a) Horizontal loop (vertical magnetic dipole): The null location method utilizes a loop antenna, which is deployed by 

the miner in the horizontal plane and excited by a CW transmitter at a relatively low frequency. A null in the horizontal 

magnetic field exists directly above the transmitting loop. A 

small loop receiving antenna can be used to search for the null 

at the earth's surface, and the performance of an actual system 

has been evaluated experimentally in both coal and hardrock 

mines 151 - [ 171 . Essentially the same method has also been 

used to survey underground quarries [ 181. 

b) Homogeneous earth: The surface magnetic fields of a 

small buried horizontal loop (vertical magnetic dipole) have 

been examined analytically by Wait [ 191 for a homogeneous 

half-space model of the earth. He has shown that the location 

of the loop can be determined from the complex ratio of the 

horizontal to the vertical magnetic field at a point on the surface, provided that the frequency is sufficiently low that the 

fields have a static-like behavior. Even if the usual null search 

were used, the information contained in the complex ratio 

might be useful in reducing the time required in the search for 

the null in the horizontal magnetic field. The above formulation and numerical results can also be applied to downlink 

transmission by application of reciprocity. 

Although the transmitted field strength is normally computed 

for a specified loop current I, the power required to maintain 

the specified current is also of importance. To calculate the 

required power, the input impedance of the loop is required. 

Wait and Spies [20] have calculated the input impedance of a 

loop above a homogeneous earth and related this impedance to 

the power requirements for a downlink communication system. 

c) Two-layer earth: Wait and Spies [2 1 ] have also considered the effect of earth layering on the location configuration 

by computing the complex magnetic field ratio at the surface 

Null line for vertical 

magnetic field Air 

Magnetic field l~nes 

Fig. 23. Magnetic field lines for a vertical loop source (horizontal magnetic dipole) 

when the vertical magnetic dipole is located in a two-layer 

earth. The null in the horizontal magnetic field is unaffected, 

but the structure of the fields away from the null is considerably modified unless the frequency is sufficiently low. 

d) Vertical loop (horizontal magnetic dipole): Wait [22] 

has als~o considered the surface magnetic fields of a buried vertical loop (horizontal magnetic dipole). The primary advantage of the horizontal magnetic dipole in location is that the 

overhead null occurs in the vertical rather than the horizontal 

magnetic field, as shown in Fig. 23. Consequently, the atmospheric noise, which has a smaller vertical component, is less of 

a problem. The disadvantages of a horizontal magnetic dipole 

are that the surface null is a line rather than a point and that a 

vertical. loop configuration may be more difficult for a trapped 

miner t.o implement [23 ] . Another reason that the analytical solution for the horizontal magnetic dipole is useful is that it can be combined with 

the vertical magnetic dipole solution to yield the solution for a 

magnetic dipole as an arbitrary orientation with respect to the 

earth surface. Consequently, the effect of a tilted tunnel or 

earth surface on location can be estimated. Such effects have 

been examined both analytically and experimentally by Olsen 

and Fairstad [ 171 . 

2) C'W Transmission with Linear Antennas: The horizontal 

wire antenna has been shown experimentally to be effective 

for both downlink [24], [17] and uplink [25] transmission. 

One disadvantage of the horizontal wire antenna is that some 

type of grounding is required at the ends to allow sufficient 

current flow. However, Farstad [25 ] has successfully demonstrated the use of roof bolts for grounding in uplink 

transmission. 

a) irnfinite line source: The two-dimensional infinite line 

source model has analytical advantages over the more realistic 

finite line source considered later. The two-dimensional model 

is valid when the wire is sufficiently long and the observer is 

not 10ci:ted near either end. 

b) lHomogeneous earth: The subsurface fields of a line 

source on a homogeneous half-space have been analyzed by 

Wait arrd Spies [26], and numerical values have been computed for a wide range of parameters. The complex ratio of 

the vertical to the horizontal magnetic field has been shown to 

be diagnostic of the position of the receiver relative to the 

source. A location scheme involving two line sources, one with 

variable excitation, has been described by Wait [27] . By 

changing one line current, a null in the vertical magnetic field 

can be swept through the earth. Where the miner detects a 

null in ithe vertical field, he signals to the surface. Such signaling could perhaps be done seismically by a hammer blow. 

c) Two-layer earth: The subsurface fields of a line over a 

two-layer earth have also been analyzed by Wait and Spies 
MURPHY AND PARKINSON: UNDERGROUND MINE COMMUNICATIONS 3 7 

[28]. The numerical calculations reveal that the subsurface 

field structure can be considerably modified by the layering. 

d) Curved earth: The feature of a curved earth has been 

treated analytically [29] , [30] by treating the problem of radiation of a line source at the surface of a circular cylinder. 

The radius of the cylinder is chosen to match the radius of curvature of the local topography. The calculations indicate that 

small curvatures have little effect on the subsurface fields, 

but that large curvature affects both field structures and 

magnitudes. 

e) Finite-length line source: In order to handle the finitelength-line source analytically, the antenna is subdivided into 

short pieces and the total fields are summed numerically. The 

antenna is assumed to carry constant current, which is normally 

a valid assumption for insulated antennas grounded at the ends. 

f) Downlink transmission: The formulation of the subsurface magnetic field has been simplified for efficient computation for the surface of a homogeneous half space [3 1 ] . Calculations reveal that for a cable length roughly twice the observer 

depth, the fields below the cable center are essentially those of 

an infinite line source. This has an important practical implication in that nothing is to be gained in field strength by making the cable longer. However, a longer cable will result in 

greater volume of coverage. The subsurface electric fields of 

the same configuration have also been computed [32]. The 

electric fields are important when reception is with a grounded 

cable rather than a loop [24]. 

g) Uplink transmission: The horizontal wire antenna has 

also been shown to be useful in uplink transmission where roof 

bolts can sometimes provide convenient grounding points. To 

account for possible tilts in either the mine entry or the earth 

surface, the case of a tilted finite line source has been analyzed 

[33] . Calculations of a magnetic-field component at the surface were made for a wide variety of parameters. These are 

the components of interest in miner detection and location 

when small loops are used for reception. Measurements by 

Farstad [25] of a horizontal wire in a hardrock mine have demonstrated good signal detection for antenna depths greater 

than 900 m. In fact, the location of the overhead null in the 

vertical field, which may be useful in location, was also shown 

to be feasible in rough terrain. Calculations indicate that the 

infinite wire result is not reached until the cable length is several times greater than the cable depth. Thus, the cable should 

generally be made as long as possible to achieve maximum signal strength. 

3) Pulse Transmission with Loop Antennas: It is also possible to pulse or shock excite a loop antenna for use in electromagnetic location. In this case the loop current is a pulse 

waveform rather than a CW or time-harmonic waveform. The 

transmitter could be a single battery-switch combination or a 

more sophisticated waveform generator. 

a) Horizontal loop: The geometry of interest in location is 

a vertical magnetic dipole for the buried transmitting loop. An 

overhead null exists in the horizontal magnetic field as it did in 

the CW case, and a pulse system has been tested using the null 

technique [34] . However, the waveshape distortion which occurs in propagation to the surface contains information on the 

loop location including the depth. No experimental attempt 

has yet been made to utilize the waveshape information. All 

of the following results assume a homogeneous half-space 

model for the earth and an earth conductivity which is independent of frequency. 

b) Vertical magnetic dipole: As in the CW case, the solution simplifies when the loop is sufficiently small to be treated 

as a magnetic dipole. The case of an impulsive or delta function loop current was treated first [35 ] because it is the most 

basic transient excitation. Calculations of the vertical and horizontal magnetic-field waveforms at the surface were performed 

for a wide variety of parameters. It was shown that the waveshapes contain information on the loop location and that a 

knowledge of earth conductivity is an aid in interpreting the 

waveshape information. Similar results for step-function excitation have been obtained by integration of the impulse response [36]. These waveforms contain an equal amount of 

location information. An exponential excitation is also of interest since it is the current waveform which results from discharge of a capacitor into a resistive loop. 

Responses for exponential excitation have been obtained 

from impulse responses by convolution [37] . The waveshapes 

are influenced by the time constant of the exponential, but 

the location information is preserved. 

The possibility of passive detection and location has also 

been analyzed. The transmitting loop at the surface sends out 

a pulse which excites a current in the scattering loop which is 

set up by the trapped miner. This current radiates, and the receiving loop (or loops) at the surface hopefully detects this reradiated field. calculations reveal that the reradiated signal 

contains location information but is of very low strength. A 

more practical system might allow the miner to modulate the 

loop impedance in some manner while some sophisticated signal processing is employed at the surface to increase the signalto-noise ratio. This idea has never been explored theoretically 

or experimentally, but there has been some interest in passive 

detection [23]. The obvious advantage is that no power is required by the miner. 

c) Finite-size loop: It is often desirable to make the transmitting loop quite large in order to increase signal strength, 

particularly in the downlink case where a large area is normally 

available. In such cases, the usual magnetic dipole approximation may not be valid, and the finite loop must be taken into 

consideration. Calculations of the transient magnetic fields 

(both uplink and downlink configurations) have been made for 

various sizes of loops [39]. In general, the response waveforms became more spread out and less peaked as the loop size 

is increased. 

d) Vertical loop (horizontal magnetic dipole): As in the 

CW case, there are two main reasons for analyzing the pulsed 

horizontal magnetic dipole. First, it may be a useful source 

for location because it has an overhead null in the vertical magnetic field for which atmospheric noise is less of a problem. 

Second, the solution can be combined with that of the vertical 

magnetic dipole to yield the solution for a magnetic dipole at 

an arbitrary angle to the earth-air interface. The configuration 

that has been analyzed [40] for a horizontal magnetic dipole 

and the loop current was an impulse. The surface magneticfield waveforms were computed and were found to contain information on loop location. 

4) Pulse Transmission with Linear Antennas: As with loops, 

only a small amount of experimental data is available for transmission of pulses with horizontal wire antennas [24]. The following is a summary of the limited analytical results available 

for downlink transmission of pulses with line sources. 

a) Infinite line source: The simplified two-dimensional 

model of an infinite line source on a homogeneous half-space 

has been analyzed [4 1 1. The subsurface electric and magneticfield waveforms were computed for an impulsive source current, and the waveforms were generally found to become 

stretched out and attenuated as the observer moves away from 
PROCIEEDINGS OF THE IEEE, VOL. 66, NO. 1, JANUARY 1978 

Air L~ne source 

Earth 

@ C~rcular cylrnder 

inhomogeneity 

0 

Observer 

Fig. 24. Two-dimensional geometry for a cylindrical discontinuity 

the source. If desired, results for other current waveforms 

could be obtained by convolution. 

b) Finite-length line source: A finite-length line source on 

a homogeneous half-space has also been considered [32] . The 

subsurface electric-field components were calculated for a stepfunction current. The horizontal electric field corresponds to 

the component measured by Geyer [24] with a grounded cable receiver, and at least quantitative agreement was obtained 

for the waveshape. 

5) Scattering by Obstacles: Generally the overburden is not 

homogeneous, and inhomogeneities can distort fields and cause 

errors in source location. Layered earth models, which were 

mentioned earlier, mainly provide insight in to field strength effects. Such models provide no prediction in source location 

errors because their horizontal uniformity does not produce 

any shift in the overhead null. To examine source location errors, models that are nonuniform in at least one horizontal direction are required. 

a) Two-dimensional geometry: Even two-dimensional geometries can be quite complicated analytically because of the 

effect of the air-earth interface on the scattering by a cylindrical inhomogeneity . A problem that has been successfully 

treated by Wait [42] and Howard [431 using different methods is illustrated in Fig. 24. A line source is located at the surface, and a cylindrical inhomogeneity (which could represent a 

pipe, rail, or elongated ore body) causes a distortion of the 

subsurface field. Such distortion could affect the feasibility of 

the location scheme outlined by Wait and Spies [26], which 

relies on the complex ratio of the vertical to the horizontal 

magnetic field to determine position. The solutions of Wait 

[42 I and Howard [43] agree quite closely in a common range 

of validity, and their numerical results indicate significant location errors for certain ranges of parameters. 

b) Three-dimensional geometry: Three-dimensional geometries are of such complexity that either an approximate or a 

highly numerical treatment is required. A given configuration 

can be made three dimensional by the introduction of a finite 

source (such as a magnetic dipole) even though the inhomogeneity may be two dimensional (such as a long pipe). For example, Stoyer [44] has treated the effect of a half layer on the 

fields of a buried vertical magnetic dipole. The fields are three 

dimensional even though the overburden is two dimensional. 

Stoyer's calculations reveal that significant location errors can 

occur unless the dipole is located far away from the layer 

boundary. 

c) Cylindrical obstacle: The effect of an infinite circular 

cylinder on the surface magnetic fields of a buried vertical 

magnetic dipole has been examined [45]. The conductivity of 

the cylinder is arbitrary, but the frequency is assumed to be 

sufficiently low that currents in the overburden can be neglected. Calculations reveal that significant location errors can 

result if the cylinder is sufficiently close to the interface. A 

rather complicated treatment of the problem that considers 

AI r 

Observer n w\\@\\q\b'\\vA\a\Bi\ 

@I Enrth 

Clrcular cylrnder 

Fig. 25. (Circular cylindrical discontinuity with a vertical magnetic dipole source. 

overburden currents and the air-earth interface effect has been 

presented by Howard [46] . 

The effect of a finite-length cylinder has also been considered in an approximate treatment. The infinite cylinder is 

now a finite length, and the small loop source is replaced by 

either a finite-length line source [47] or a long narrow loop 

[48 I . Calculations again reveal significant location errors 

when the cylinder is near the surface. Also, the calculations 

reveal that the cylinder must be extremely long before the results approach those of an infinite cylinder. 

d) Spherical obstacle: The effect of a spherical obstacle 

(such as an ore zone) on the surface magnetic fields of a buried 

vertical magnetic dipole has also been examined [49], and 

overburden currents are included in the treatment. The calculations reveal that location errors caused by small spheres are 

small. 

The above treatment has been specifically applied to the calculatiorl of the shift in the null of the horizontal magnetic 

field when the sphere is near the surface [50]. Such a treatment could apply to manmade obstacles such as vehicles or 

machinery. The calculations reveal some secondary nulls in 

some cases, but the null shifts are still small for vehicle-size obstacles. Farstad [25] has observed experimentally that the 

presence of a van has only a very localized effect on the surface ma~gnetic fields. 

e) Prolate spheroidal obstacle: The effect of a prolate 

spheroidal obstacle on the surface magnetic fields of a buried 

vertical magnetic dipole has also been examined [5 1 I . The geometry is the same as that in Fig. 25 except that the cylinder 

is replaced by a prolate spheroid. The prolate spheroid is a 

useful shape to analyze became the axial ratio can be varied to 

obtain shapes ranging from a sphere to an infinite cylinder. 

Unfortunately, the mathematical difficulties only allow the 

case where overburden currents are neglected and the spheroid 

is perfectly conducting to be handled conveniently. Calculations indicate that the strength of the anomalous fields increases as the length of the spheroid is increased. However, 

the location errors are still small unless the obstacle is close to 

the interface and in the vicinity of the source loop. 

D. Leaky Feeder Transmission Lines 

1) Theory: There has been international interest in the application of leaky feeder transmission lines as a means of extending the propagation of radio underground. From an analytical viewpoint, the majority of the initial work has heen in 

Europe with two principal pioneers--Professor P. Delogne of 

Belgiunn [52] , working in collaboration with the personnel of 

the Institute of National Extractive Industries (INIEX); and 

Dr. David Martin [53] of the National Coal Board of U.K. 

One excellent summary of work in the area of leaky feeder 

transm.ission lines is found in the proceedings of the April 

1974 Colloquium on Leaky Feeder Radio Communications 
MURPHY AND PARKINSON: UNDERGROUND MINE COMMUNICATIONS 39 

[ 54 1 ; also, numerous papers in the Proceedings of the In ternational Conference, "Radio: Roads, Tunnels, and Mines" [55] 

relate .to the use and application of leaky feeders. Additional 

work has been done by Martin [56], Delogne [57], and Wait 

[58] in the analysis of waveguide systems. 

There are several imp ortan t considerations in the selection 

and application of waveguides relative to the type and the operating frequency. Let us first review the types of waveguides; 

the simplest is a single wire suspended in the mine entry [59] , 

although it has been found under certain selected conditions in 

European mines, to be advantageous to use a twoconductor 

transmission line which will support two types of dominant 

modes, monofilar and bifilar [60] . In the monofilar mode, 

the forward current is carried by the transmission line, and the 

return current is carried by the tunnel walls or structure. In 

the bifilar mode, the return current is carried by the outer conductor. The advantage of the monofilar mode is that it is readily excited, or received, by an antenna located in the tunnel; it 

has the disadvantage of higher attenuation due to loss in surrounding rock or mine structure. An important consideration 

in these systems is the conversion from the monofilar or asymmetrical mode to the bifilar or symmetrical mode. Not only is 

the mechanism of mode conversion important, but in some installations how frequently the mode converters are inserted 

into the line is also important; if the transmission line is installed near the mine structure or walls, the attenuation of the 

monofilar mode will be high and reinforcement will be required frequently. If the means of mode conversion is incremental rather than continuous, as it is in the INIEXIDeryck 

[6 1 ] systems, the number of mode converters needed to maintain the level of the monofilar mode may become excessive in 

difficult installations. 

The actual transmission line may be either a two-wire line, as 

in the INIEXIDeryck system, or a coaxial cable. The attenuation and excitation of the modes on a two-wire line have been 

studied both analytically [62], [63] and experimentally [6 1 ] . When the transmission line is a coaxial cable, the bifilar mode 

propagates between the inner and outer conductors, and the 

monofilar mode propagates via the outer conductor and the 

tunnel walls. Mode conversion is accomplished either discretely , 

by spaced discontinuities in the cable as in the INIEX/Delogne 

system [60] , or continuously through a loosely braided outer 

conductor [64], [65] or through spaced slots in a solid external shield of a coaxial cable [66] ; a good comparison of bifilar 

and monofilar transmission lines has been made by Martin 

[56], and a summary of various coaxial cable leaky feeder systems has been prepared by Lagace et al. [67] . The detailed 

analysis of conductors in tunnels of circular and rectangular 

cross section has been implemented by Wait et al. [62] , [68 I , 

[69], [58] ; it is interesting to note that Martin's design [641 

has equalized the phase velocities between the monafilar and 

bifilar modes and that Wait [58] has shown that the phase velocity of the single-wire mode varies with the proximity of the 

transmission line to the tunnel wall. 

2) Experimental Results: As discussed above, the location 

of the transmission line relative to the mine wall does affect 

performance; in particular the monofilar mode is attenuated. 

If the mine or tunnel geometry permits installation of the 

transmission line at least 0.5 m from the wall, these interference effects are minimized or are negligible. However, in the 

rectangular entries of many U.S. mines, the height of the coal 

seam significantly restricts free space and dictates installation 

at or near the wall of the mine. In these instances, bifilar transmission lines such as TV twin lead are unacceptable, as perhaps 

are some of the coaxial cables with discrete mode converters, 

such as the INIEX system. A good analysis of the effect of external structures to leaky feeder cables is given by Cree [70]. 

Another consideration in the selection of a leaky feeder 

transmission line is the selection of the optimum frequency, 

but before one can select the frequency the operating conditions must be defined. In the majority of European mining 

conditions, especially coal mining, the single-entry longwall 

method is used, as shown in Fig. 4. The communications requirements are between men in the intake and return entries 

and across the face or between these men and the surface. The 

signal radiated from the leaky feeder transmission line must be 

able to couple to a portable antenna only across the width of 

the entry, about 4 m and vice versa. 

Typically, there is a rather high total system loss in the operation of a leaky feeder system. The coupling laterally from 

the main axis of the cable varies inversely with frequency; 

hence, the farther laterally from the transmission line one 

wishes to communicate, the higher the frequency required. 

Contrast this requirement with the fact that the higher the frequency, the higher the loss in the coaxial cable. Hence, frequency has to be optimized-it should only be high enough to 

obtain the desired range of lateral coupling to the waveguide. 

In the case of a European longwall, the optimum frequency 

would be chosen to meet the operational constraint of coupling to a transmission in the same entry. Higher frequencies 

would be necessary in U.S. longwall with the requirement for 

multiple entries, and a desire to establish communications in 

more than one entry. The selection of optimum frequency is a 

subject of debate. For example, Martin recommends [7 11 a 

frequency of 30 MHz or above to ensure efficient coupling to 

the loose braid cable used in Britain, while INIEX advocates 

[52] the use of 5 to 10 MHz although they have had difficulty 

in obtaining suitable portable transceivers to operate in this 

band. In the U. S. room and pillar mining method (Fig. 1 ) in a 

working section 200 X 600 m, to obtain coupling to all parts 

of the section would require the use of UHF frequencies (450 

MHz); as shown above, with a centrally located transmitter, 

this coverage can be completed via wireless propagation and 

eliminates the need to continuously advance a leaky feeder 

system in a rapidly advancing mining operation. Details of implementation are discussed below. 

To this point there has been no discussion of active repeaters 

or in-line amplifiers to extend the range of coverage. Without 

repeaters, the size and cost of the transmission line must be selected for acceptable performance; alternatively, line losses can 

be overcome with active equipment. Several techniques have 

been used. 1) Borrowing from conventional mobile radiocommunications practice, further individual fixed-base stations 

are installed at intervals as necessary to provide the total range, 

all stations being under a common remote control with the first. 

Such a system has been in use by the British Coal Board at the 

Longannet mine since 1970 [56]. 2) A series of one-way inline repeaters, such as the daisy-chain system developed by 

Martin shown in Fig. 26, is effective; it does have a slight disadvantage that an audio return line is required and, when 

branches are required, the system can become complex. Martin 

has recently developed a bicoaxial system [ 7 1 ] that appears to 

eliminate the problems of the daisy chain while maintaining 

the advantages, at the expense of a slightly more expensive system installation. 3) Multiple-frequency repeater schemes have 

been used successfully; the simplest uses one transmitter and 
I Push to page I Y 

control cansale R1,7-456 825 MHZ T,, T~ H,nd,e ' RT- 5 MHz M 

Fig. 28. UHF/VHF repeater system. 

one receiver 172 ] as shown in Fig. 27. Where extended range 

is required, the multiple-frequency system as shown in Fig. 28 

has been used successfully [73]. Phone line I 

Haulage roy 

Fig. 31. Call alert paging system. IV. HARDWARE DEVELOPMENT AND EVALUATION 

Initial work has shown that no single communication system 

underground meets all requirements. However, there seems to 

be some particular type of communication technique applicable to all problems that have been identified; hence, we find 

the use of hybrid systems provides the most realistic means of 

implementing all the communications requirements. From the 

surveys described above the need has been identified for the 

following types of communications [74] : 

5) improved hardware that is more maintainable and reliable 

under emergency conditions. 

A. Roving Miner Paging 

I) Along-the-Roof Paging: The primary carrier current frequency for communications systems in U.S. mines is 88 to 

100 kHz. Experiments have shown that, with the proper sensitivity receiver, these carrier current signals could be received 

well beyond the entry in which the trolley wire is placed; in 

fact, in many mines carrier current signals are clearly receivable 

eight entries away from the trolley wire. In working sections 

where dc face equipment is employed (which is powered from 

the trolley wire), the carrier current signals are also readily detectable in the face area. However, where dc face equipment is 

not available, it has been found that through the use of a 

1) improved methods to reach roving miners, either one-way 

(paging) or two-way (walkie-talkies); 

2) improved telephone systems that provide additional channels and perhaps a secure supervisory channel; 

3) improved haulageway communications in terms of both 

reliability and maintainability ; 

4) mine monitoring systems to identify potential problem 

areas in underground workings; 
y y 

is being paged will hear the message. However, in case of an 

emergency, an all-call feature is available that permits notification of all personnel simultaneously. Various encoding schemes 

can be used, from a simple pushbutton for each particular 

pager to a more sophisticated scheme shown in Fig. 30 in 

which the dial telephone on the mine site is used as the encoding device. 

2) Call Alert Paging: This function is somewhat similar to 

the "bleep-bleep" paging developed for U.K.3 mines some 

years ago. 

A simple call alert system is illustrated in the block diagram, 

Fig. 31. From an office, a signal outside the audible voice 

range is applied onto the telephone lines to alert an individual 

that he should call the office. The keying receiver at the section telephone receives the signal and activates a ULF transmitter that causes a current to flow in a one turn loop that is 

formed by wrapping a wire around a piUar of coal; thus a magnetic field that permeates the region within about 300 m of 

the loop is set up. A small pocket-type receiver containing a 

pickup loop responds to the presence of the ULF signal by a 

flashing light and an audible tone, notifying the person alerted 

to call the office. 

A more complex version has been developed that allows a 

call to originate from any dial phone in the mine phone system 

(Fig. 32). All underground phones have call-alert features and 

a coded PSK signal to permit mine-wide selective calling. 

When an individual receives an alert, he dials a precode and 

then his own alert number and the PBX automatically dials the 

party who initiated the "alert" call. 

yy pp

moves the data to the modulator, keys the transmitter, and 

connects the transmitter to the loop. The system is intrinsically safe and develops a minimum of 10 W into a 120-m 

loop. When a message is received, the "receive message 

alarm," both audio and visual, is activated to alert the operator of an incoming message. The received message is displayed on the "receive message" display. The delayed message 

transmission is at one character per second. 

4) Remote Access Monitoring: In recent developments the 

monitoring of air velocity, methane, carbon monoxide, and 

temperature has become associated with underground mine 

telephones. A system has been developed to remotely access 

the sensor information from the underground and to transmit 

it via VF to the surface. 

B. Remote Access Monitor 

This is very simple monitoring. Each of the environmental 

monitor sensors has a preset threshold. The monitor can, 

therefore, indicate two conditions for each sensor. If the environment at the monitor is below the sensor threshold, the 

monitor indicates a "normd" condition. If the environment 

has exceeded the threshold of any of the sensors, then the 

monitor indicates an "abnormal" condition for each affected 

sensor. The selection of sensors for use in the mining environment is critical to the success of such a system [75], [761. 

To activafe the underground receiver, a surface transmitter, 

located approximately over the underground loop, is keyed to 

transmit a triggering signal of at least 12 s duration. Upon receiving a signal, the receiver will apply power to the sensors; 
Fig. 34. Surface receiver for detection of trapped miners. 

after 2 min the encod.er is activated, and a coded data word is 

formed which indicates the status of the sensors. After the 

data word is formed, the VF transmitter is keyed to transmit 

the data word three times in succession. After the completion 

of the transmission, the activating receiver shuts down power 

and awaits the next triggering signal. The surface monitor is 

the VF receiver that is used to locate a postdisaster alert signal. The receiver has both visual and audible indication of the 

undergroun:! zsssage. 

Seismic Trupped Miner Signaling: In the seismic system, the 

trapped miner signals on the mine floor with a timber or sledge 

hammer, and multiple geophone arrays on the sbrface can detect signals in the majority of areas in overburdens less than 

150 m deep. Various signal-processing techniques enhance 

the signal-to-noise ratio; the predominant noise sources during 

a rescue operation are surface-generated noises from moving 

equipment, people walking, and power lines. A variety of 

processing schemes have been tried [77] ; the computation of 

desired. However, until a suitable alternative is available, the 

seismic scheme does provide the miner with an additional degree of protection. The Mining Enforcement and Safety Administration (MESA) does maintain a seismic rescue system as 

part of its Mine Emergency Operations group [78]. Additional work is nearing completion ih terms of optimizing the 

hardware. 

The advantages and disadvantages of the seismic approach 

have been identified above. Also in the work described above, 

the propagation of VF signals through the overburden has 

been thoroughly analyzed. The advantages of an electromagnetic scheme for the detection and location of trapped miners 

are that it would not require the timeconsuming deployment 

of geophone arrays-in fact, the site could be scanned by helicopter-and that the propagation of a VF signal through the 

overburden would not be so susceptible to typical overburden 

anomalies as is the seismic approach. The disadvantage is that 

a piece of special equipment (a transmitter) is required 

underground. Considerable efforts have been directed toward 

development of suitable hardware and evaluation of same in 

operating mines [79]. The present configurations consist of a 

transmitter [80] with dimensions of 6.25 X 3.2 X 1.5 cm, 

which, when connected to an external loop of wire 25 m long, 

generates a peak magnetic moment of 1000 A . mZ. The 

transmitter and the antenna have been packaged two ways, on 

the left as an attachment to the top of the cap-lamp battery, 

and on the right as a self-contained unit to be worn on the 

miner's belt. 

A recent modification to the system has been to incorporate 

an inductive voice receiver into the transmitting package so 
Detector Modulator I 

Fig. 36. Block diagram of refuge shelter communications. 

that, via a longwire or loop antenna on the surface, voice messages can be sent to the miner; he responds via code with his 

beacon transmitter. To support these transmitters, surface 

equipment has been developed-a receiver for handheld or helicopter-borne use. 

C. Two-way Communications with Roving Miners 

I) Two-way Wireless: Where possible, the establishment of 

two-way communications without the need to install, maintain, and extend or relocate leaky feeders is certainly desirable. 

As discussed above, there are two options-the use of UHF or 

LF. Presently available UHF portables (460 mHz) are small 

and also have small antennas. The recently introduced Mx 

series from Motorola is only 7.21 cm w X 3.58 cm d X 15.32 

cm h in the 1-W model and has been certified by MESA to be 

intrinsically safe for use in gassy areas of underground mines 

[81]. The limitation on the operation of this type of radio in 

LEGEND 

HCOOI = hhrfactory cwempe ~h+=rrll~l cmmpe m-ge 

Fig. 37. Predicted UHF wireless radio coverage. 

a room and pillar section (Fig. 37) is the intrinsic electrical 

noise of the receivers. 

In coal seams that are 1.5 m or higher using walkie-talkies 

with a transmitting power of 2 W and receiver sensitivity of 

0.5 pV for 20 dB of quieting, the operating range is about 90 

m. With such a system the signal losses of UHF (450 MHz) in 

a straight entry restrict operations to about 550 m. However, 

in room and pillar workings, if the communicators are not in 

the same entry, there is a comer loss of about 60 dB, and additional losses due to depolarization of signal increase the overall 

antenna coupling loss to about 45 dB. A distance of 90 m 

around a comer is equivalent in signal loss to 550 m of 

straight-line communications. Though a range of 90 m is useful, the required range is more like 180 m. The range can be 

doubled by placing a repeater in a central location. 

To operate in the repeater mode, the portable units transmit 

on fl and receive on f2. This allows a centrally located repeater station to pick up fi transmissions and retransmit them 

for fi reception by other personal radios, thereby doubling the 

roving-miner-to-roving-miner range. 
One exception is the rescue team radio developed by the 

South African Chamber of Mines [82] which operates on 335 

kHz single sideband. These units, Fig. 38, are 94 mm h X 

222 mm wX 2571 mm d, weigh 3.7 kg, and require a 

bandolier-type antenna which is elliptical (660 X 420) mm and 

weighs 0.9 kg. The system is not small enough to have a man 

carry every day. Work is currently underway in the development of a smaller LF transceiver operating at 520 kHz, but 

this device will still require a bandolier antenna; unfortunately 

it appears as though at these lower frequencies there is no way 

currently available to reduce antenna size and obtain acceptable radiation efficiency. Anticipated range from the 520-kHz 

transceiver is 450-m radius in a room and pillar section. 

Areas where either UHF or LF radio can be used underground beneficially, even with the limited range, are: 1) rescue 

team radio communications from fresh air base to rescue team, 

2) section communication between miners, and 3) on haulageways between motorman and helper (snapper) who must 

couple and uncouple cars of coal [83]. 

2) Leaky Feeders: Where communication requirements are 

in excess of the ranges presently available, an alternative is to 

use a leaky feeder to extend range. There are a variety of approaches presently in use or under evaluation as described 

below: 

a) Longwire antennas: We have taken the liberty of including the following installation here because of the functional comparison-rather than leaky feeders they are in fact 

loop-to-loop inductive systems. Such loops have been used extensively in Europe with portable transceivers [84] to cover a 

tion with proximity to structures and debris on the exterior of 

the cable. 

c) The INZEX/Delogne system: While various techniques 

have been used to implement the exchange of electromagnetic 

field(s) between the coaxial transmission line and the cavity of 

the mine entry, in the INIEX/Delogne system a complete annular gap in the external conductor of the coaxial cable is used 

[52]. To reduce the uncertain loss of the gap, a circuit as 

shown in Fig. 39 is used. Two types of circuits are in routine 

use. In the first, which is selective, the gap is shunted by a capacitor, which reduces its impedance, while the capacitance 

effect is compensated at the operating frequency by a induction coil inserted in the external conductor. The bandwidth is 

about 20 percent. 

In a second type, wide-band operation is obtained. It consists of a transformer with the windings running in a direction 

so that no magnetic flux would be created by the coaxial 

mode if the number of turns was equal; a slight difference in 

the number of turns is sufficient to achieve radiation. 

The INIEX/Delogne radiators are available commercially as 

shown in Fig. 40; they are made so that the tuned elements 

can be easily replaced if frequencies are changed. The system 

has been installed principally in Belgium and France; other 

known experimental installations have been made in Gerinany 

and the U.S. 

d) Loose-braid cables: These cables, which have principally been designed around the work of Martin [7 1 1, are available from the British Insulated Cable Corporation and the 

Times Wire and Cable Corporation. The braid covers about 67 
External 

speaker 

Fig. 41. Block diagram of hoist phone. 

percent of the exterior, as opposed to 95 percent for the 

normal cable. 

e) Slotted shield cables: Slotted shield cable is designed so 

that the outer shield of the helical coaxial cable is milled away 

to provide slots or apertures about 0.5 X 0.1 cm about every 2 

cm. The cable is more expensive than some of the others described but has performed quite well in numerous applications; 

applications include the U.S. and Canada, as well as abroad. 

The selection of the type of leaky feeder depends on the 

type of requirement and whether an active or passive system 

is used. It appears as though in some cases the use of in-line 

repeaters with low-cost cable can be more cost effective than 

the larger, lower loss cable. Traditional repeaters have not 

been discussed in detail; reference has been made above to the 

British daisy-chain amplifier scheme [7 1 ] , which is commercially available with a line-powered repeater. 

D. Hoist Communications 

While most coal mines in the U.S. are less than 300 m deep, 

there is a trend toward deeper mines, and some metallic and 

nonmetallic mines are presently working at depths in excess 

of 1500 m. A need exists for improved hoist communications 

between the skip and the hoist operator. Two systems are 

presently used today: One uses a trailing cable to provide communications, and the second inductively couples the carrier 

current signal over the hoist rope. The former has limitations 

in terms of depth because of the amount of cable that can be 

trailed from the cage, and the latter has limitations at great 

depths because signal dead spots develop on the hoist rope. A 

technique evaluated to overcome this problem involves the use 

of the two-frequency concept where a dual-frequency transceiver simultaneously monitors two frequencies (approximately 30 and 52 kHz simultaneously) and selects the highest 

signal for use; hence, the null from a standing wave would not 

be at the same location for these two dissimilar frequencies. 

However, results of evaluations [87] have shown that with improved sensitivity the dual frequency is not required. Tests to 

date on a 1600-m-deep shaft have been quite successful. 

Transceivers have since been developed which combine a 

low-power transmitter and a sensitive receiver with battery, 

handset, and speaker into a'very compact unit. Fig. 41 is the 

block diagram of the unit. The current in the rope induces a 

voltage in the coupler which is applied to the RF amplifier. 

The amplified signal is fed into a balanced mixer and is mixed 

with a 430-kHz signal from the crystal-controlled injection 

oscillator. The resulting 455 kHz is amplified and filtered by 

a mechanical filter with 13-kHz bandwidth. After additional 

amplification, the audio is taken from a limiter-detector and 

modified to 0-dBm level for the handset earpiece and to a 
crophone is fed into an audio-clipper amplifier. The audio 

modulates two crystal oscillators using variable capacitance 

diodes to pull the crystals the desired 3 kHz. The two frequencies are injected into a mixer and produce a 52-kHz difference frequency. The 52 kHz is amplified and fed into a 

series tuned circuit which couples the signal to the hoist rope. 

The RF power into the coupler is less than 1 W. 

Coupler: The coupler, Fig. 42, transfers RF energy to and 

from the hoist rope. The toroid secondary, the hoist rope, can 

be considered to have one turn. The induced voltage creates 

a current flow through the rope, the cage capacitance to the 

shaft walls, and back through the conductive earth shaft walls 

to the hoist rope at the head frame. This current flow then induces a voltage ih the other coupler, which also encircles the 

hoist rope, as shown in Fig. 43. The inductance of the magnetic core toroid is series-tuned with a capacitor. This circuit 

is shunted with another capacitor, which improves the impedance match to the transmitter. The series-tuned circuit increases the V-A to produce increased induced voltage in the 

hoist rope. 

The increased voltage in the hoist rope, along with an improved sensitivity of the receiver, has eliminated the dead spots 

as the hoist rope is lengthened. 

Other approaches to hoist communications have been and 

are being implemented via leaky feeder cables in the shaft and 

in some instances with VHF and UHF transceivers. 

E. Improved Phone System 

Several mines have begun installing dial telephone systems in 

order to achieve additional channel capacity [88]. However, 

:   Vo~e i.,ne ornpl8fler 

- Monnlor TV Corner0 

Control 

Fig.45. Block diagram of mine communication and monitoring 

systems. 

the problems of installing and maintaining multiple-pair cables 

(perhaps 50 pairs) in underground coal mines are difficult at 

best. Additionally, dial-type telephones only provide a ringing 

signal or a page at the phone locations and suffer the disadvantage that, normally, people are paging an individual who may be 

beyond the audible range of any specific phone location. A 

new phone system has been developed that overcomes the 

problems of extending the multiple-pair cable and reaching a 

person beyond the range of hearing the phone. 

The phone shown in Fig. 44 has pushbutton dial capability 

and contains frequency synthesizers so that each channel is 

derived within the phone when required [89]. Also included 

in the phone, and operating as a part of it, is the call alert 

paging system. The use of coaxial cable for a phone line is a 

radical departure from present techniques; commonly a twoconductor twisted #12 wire is used. However, in view of the 

increased communications channels required both for voice 

communications and for mine monitoring, additional bandwidth must be achieved. One alternative is the use of additional conductors. However, the use of six pairs of wire with a 

messenger cable just to meet the phone requirements of a large 

coal mine is equal in cost to the cost of a 2.22-cm-diameter 

coaxial cable. The phone system with its ancillary functions 

has been designed around coaxial cable. Fig. 45 is an overall 
gy pg 

ore mine and are being demonstrated. Techniques and tech communication coaxial-cable system. The system utilizes fre- nology are available to overcome many of the operational 

quency division multiplexing in the 7.08-10.6-MHz band and problems of mine communications. mese techthe 49.6-53.1-MHz band to provide simultaneously 300 .iques are being reduced to hardware that be usable by 3000-Hz voice channels, several hundred serial sequence moni- the mining industry. 

tor and control signds (7.083 and 49.79 MHz, respectively), ,)Systems reliability: The reliability of a mine communiand 13 TV channels through the coaxial cable distribution sys- cations system can be divided into three areas: 

tem with loopback and automatic switchover to permit the 

signal to be inserted at both ends of the cable if the under- 1) the performance of the equipment itself; 

ground cable is broken. 2) the availability of power for the particular devices; 

me EM paging system is dialable from any phone, and all 3) the reliability of the waveguides of communications 

underground phones have a paging loop emanating a signal circuits. 

with a paging range in ~xcess of 300 m. When responding to a Th, new prototype equipment described has been designed for 

page, the responder dials a precede and its own page number- reliability and ruggedness; all solid state circuitry is and 

The system center automatically calls the phone that initiated attention is paid to environmental (dust, moisture, and 

the page. If the line called is an is actuated to vibration) problems, Additionally, all equipment is operated allow the page call to go through. either on primary battery supplies or, if it is powered from the 

F. Mine Monitoring ac main, has a battery that is float-charged so that in the event 

of power failure the systems will remain functional. The last 

I) System Configuration: consideration of the integrity or reliability of the communicaa) Mine monitoring: As the room and pillar mining method tions channels is most important and is perhaps the most vulpresents problems in terms of communications in all parts of nerable of all parts of the hybrid system. In most U.S. coal 

the mine, it also presents complications from an environmental mines, where the overburden is typically less than 300 m and 

mine-monitoring viewpoint. Idealistically, one would like to where many access points to the mine are provided for purhave monitors at every point in the mine. Realistically this poses of introducing power and/or ventilation, there is a 

cannot be achieved; the concept of sensor installation and unique opportunity to provide bbloo~back" so that there is an 

maintenance at a multitude of points is unrealistic. However, alternative path into the mine in the event of an emergency 

work has been underway to locate monitoring stations at key or failure of the communications channel. This approach is 

locations to identify potential problems at the station as well shown in the simplified schematic drawing of Fig. 47. Loopas in/by that point. By using a series of transducers at an air back can also be implemented within the mine by routing the 

split to monitor methane, oxygen, carbon monoxide, and tem- return cable through another Part of the mine. 
4 8 PROCEEDINGS OF THE IEEE, VOL. 66, NO. 1, JANUARY 1978 

In the area of phone system reliability, work by Long et al. 

[92] has resulted in the development of techniques and hardware so that mine maintenance personnel can assess the performance of mine phone systems. 

Also to be included in the area of reliability is that of safety 

-obvious concern has been raised about the safety of RF systems and the use of electrical blasting caps. The standard for 

aboveground protection has been IME Pamphlet 20 [93]. Because of the differences in underground operations, a study 

was conducted by Thompson [94] to assess the hazard. The 

results show that with radiated powers of less than 1 W there is 

no hazard. Additional related studies are presently underway. 

d) Remote control: 

i. Stationary devices: There are many rules and regulations [3] that require monitoring, such as ventilation fans; and 

there are other situations which, in addition to the environmental monitoring discussed above, it is advantageous to monitor. Additionally, in many circumstances not only is monitoring desirable, but so is control of the device monitoredobvious situations are pumps, circuit breakers, etc. Presently 

those monitoring and control schemes are implemented via 

carrier over mine phone or power lines or via tone codes over 

braided telephone lines on the surface. In conjunction with 

the environmental monitoring work [go], automatic control 

of ventilation regulations are being evaluated. 

ii. Machinery: The implementation of umbilical cord 

control for continuous miners has been quite successful in the 

United States, and radio remote control is gaining rapid acceptance [95] . Remote control for continuous miners is available 

from all of the major mining machine manufacturers. Remote 

control and automation will continue to grow in acceptance, 

but the changes must be evolutionary rather than revolutionary. In both the U.S. and Britain, major programs have been 

implemented to expedite the development of these systems for 

coal production. 

In Europe there has been considerable attention to radio 

control, primarily of longwall shearers, and to cable-operated 

control for transport of men and materials [96]. 

As the use of radio remote control expands, extreme care 

must be exercised that false starts (or stops) of equipment are 

not caused by stray signals from similar controllers in other 

parts of the mine. 

V. CONCLUSIONS 

The problems of underground mine communications can be 

solved; this paper has presented a variety of concepts that have 

been developed and experimentally evaluated in mines. No 

single concept provides a universal solution, but hybrid scheme 

systems which address all problems can be, and are being, 

implemented. 

The principal short-term effort needs to be in the development and/or modification of hardware that is small enough 

and rugged enough to suit the underground mine requirements. 

General surface specifications would include environmental, 

shock and vibration, and intrinsic safety. The latter requirements vary from country to country, but there is a trend toward consolidation in the European community. - 

In terms of the application of this technology to other industries, while the majority of these discussions covered coal 

mines, applicability to noncoal mines is obvious; additionally 

underground tunneling and public works projects should benefit, but the mqjor benefit will come to the industry for which 

this work has been directed-mining. 

The authors wish to thank numerous people who made contributions in the development of this manuscript and in the 

administration or implementation of the work included, in 

particular H. Dobroski, H. K. Sacks, M. Stock, and M. Miller 

of the: Bureau of Mines, and J. R. Wait of the U.S. Department 

of Colmmerce. 

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