AN OVERVIEW OF BUREAU OF MINES COMMUNICATIOIJS RESEARCH 

tiarry Dobroski , Jr. 

John Durkin 

U.S. Department of Interior 

Bureau of Mines 

Pittsburgh Research Center 

Pittsburgh, PA 15236 

ABSTRACT 

This paper presents a general overview of the Bureau of Mines' 

programs in the areas of operational and post-disaster communications. 

These programs encompass both hardware devel opments and system eval uations. Topics discussed include a new visual paging systen compatible with existing pager phone lines, a means of modifying commercial ly avail able pager phones to permit selective paging and message 

leaving, a mu1 tiplexed telephone system that uses twisted shielded 

pair and requires no central control, medium frequency hardware 

developments, and a manual for mine communications. 

Research has continued in the area of post-disaster location 

and conimunications. This research is based on seismic and electromagnetic techniques. Each has been extensively studied and field 

tested to better understand its a b i l i t y t o perform effectively. 

A brief description is given of the post-disaster program. Results 

are promising and provide insight into future areas of research. 

INTRODUCTION 

It can hardly be argued that underground mining is a difficult 

complex activity. And although the future holds great promise for 

the coal industry, there will be those who float and those who sink. 

Those who float will be those who can supply a competitive product 

whose production depends on compliance with regulations, labormanagement relationships, good equipment, and good management. 

Good management is critical . It requires timely information 

on all aspects of a working mine, and an effective communication 

system is the key. It is not sufficient for a manager to think he 

knows where his key people are or what the underground situation is. 

To manage, he must know. Good communication permits good top-down 

management and this is often the difference between success and 

failure. 
I. OPERATIONAL COMMUMICATIONS RESEARCH 

Wired Communication S.ystems 

This section describes new developments and research in the area 

of hard-wire comnunications. This includes improvements in standard 

page phone systems, two visual paging systems, and a multiplexed 

telephone system. 

In addition, research is underway in other areas. Fiber optics 

is making significant inroads in commercial telecommunication systems. 

It is being evaluated to ascertain its applicability to underground 

mines. The advantages of large bandwidth, low cable costs, interference-free transmissions, and intrinsic safety are being weighed 

against cable spl icing, tapping, and repair difficulties, higher 

interface costs, and other problems associated with a higher technology. A small fiber optics system is being developed and will be 

installed in the Safety Research Nine at Druceton, Pa. Studies 

continued in the area of existing wire telephone lines. With the 

development of more sophisticated telephone and telemetry systems, 

more information is needed to learn if existing mine lines are sufficient from a bandwidth/loss/Er11 standpoint to support such systems. 

Finally, there always remains the problem that in a nine emergency 

(fire, explosion, etc.) the communication and data lines are first 

to go, often rendering useless the telephone and monitoring systems. 

To get around this problem, studies are underway to learn the advantages and disadvantages of burying such cables. 

Sel ectabl e Paqer Phones 

Standard loud speaking pager phones are an important part of 

mine conmunications. They are inexpensive, versatile, and re1 iable. 

Unfortunately they a1 so have several disadvantages. The user of a 

pager phone never knows if he or she is really getting through on 

the system unless someone answers. Also, each and every phone in 

the mine broadcasts sa page every time. This may be good for general 

nine paging or emergency use, but it unnecessarily distracts workers 

who keep hearing messages not for them. In addition, continuous 

general paging uses up batteries quickly in all the phones. At about 

$5 apiece for pager-phone batteries which nay last only a few months 

under conditions like this, it quickly becomes expensive. 

The Bureau of Nines has addressed this problem by developing 

a small circuit that can be added to any standard pager phone. (See 

figure 1. ) The system features include-- 

(1 ) Any pager phone can be paged selectively from any other 

pager phone. 
(2) Signal ing is via dual tone multiple frequency (DTFIF) techniques. The caller receives a tone feedback signal from 

the called phone indicating that it came on. This provides 

the caller with information regarding his phone, the called 

phone, and the intervening phone line. 

(3) The called phone pages in the normal manner, and flashes 

a light for a preset time (usually 20 minutes) or until the 

phone is answered. 

(4) Battery 1 ife is greatly enhanced because only the desired 

phones comes on. 

(5) The "all call" feature of the standard pager phone is 

retained. 

Visual Paging 

A useful complement to audio paging is visual paging. In a 

sense, the selectable pager phone uses visual paging to a limited 

extent by the flashing light on the called phone. In this section, 

visual paging refers to independent systems. 

The advantage of visual paging over audio paging is that if 

no one is in the vicinity of the paging device when a page comes 

in, the message is not lost. Two visual paging systems were 

developed. 

The first developed under contract, is a direct spin-off of 

earlier Bureau work. This system (See figure 2.) used electromechanical, retroreflective disks that can be "set" or "unset" by a 

dispatcher. Each disk carries a number or letter assigned to 

individuals or work functions. lJhen an individual sees his number 

"set," he knows to contact the dispatcher at once or whatever phone 

system the mine uses, to receive a message. The system features 

incl ude-- 

(1) Uses the existing pager phone lines and is completely 

compatible with standard or selective pager phones. 

(2) Patch-panel programmable to permit sing1 e location, 

zone, or minewide paging. 

(3) Disk control via FSK at 14 kHz, 109 baud rate. 

(4) Interface to pocket pagers that permit radio paging to 

roving individuals. 

(5) Uses redundant transmission techniques to assure virtually 

error-free operation (that is, a 12-bit FKS word repeated 

four times). 
(6) Battery 1 ife comparable with pager phones. 

The second visual pager system was developed in-house and is 

shown in figure 3. The system features include-- 

(1) DTMF signaling from a dispatcher's control panel with a 

supervisory tone feedback. 

(2) The use of a 1 iquid crystal display instead of electromechanical disks. 

(3) The prototype system will post three two-digit numbers from 

11 to 99. These three numbers "roll " through the display on 

2-second intervals and have individual (usually 20-minute) 

time-outs. In addition the dispatcher can clear the numbers 

if desired to cancel the page or post ones of higher priority. 

(4) Provisions for remote control of alarm or other functions. 

(5) Power consumption is nominally 0.5 ma at 6 vdc, resulting in 

long battery life. 

Mu1 tip1 exed Tel e~hone System 

A mu1 tiplexed telephone system is being developed that provides 

eight voice-grade channels. The system is based on modern microprocessor techno1 ogy using no central control and twisted shielded 

pair cable. Phones can be attached to the 1 ine at any place, and 

the failure of individual phones does not affect general system 

operation. (See figure 4.) 

System features include-- 

(1) Eight channels available for narrowband FM duplex voice or 

duplex data, FSK, Eel1 spec 103. 

(2) System control via digital signal ing at 30 ktlz. 

Voice and data channels, 320 ktlz to 640 kHz. 

(3) System operation control led directly by the phones, 

requiring no central control or repeaters. 

(4) Full system versatil ity where phones can be located 

anywhere along the line, similar to pager phones. 

(5) Very low power consumption. The system receives power 

from an intrinsically safe power supply. Internal backup 

batteries provide emergency power at the phones. 

(6) Easily installed and maintained cable. Initial plans call 

for No. 14 gauge t~isted shielded cable. This cable offers 

good mechanical strength, electrical characteristics, and 

high immunity to nine electromagnetic interference ( EMI) . Prel iminary tests show 1 i ttle degradation in transniission 

when grounds and terminations are not intact. 
l.Ji re1 ess Systems 

The ultimate communication system for mines is one where any person 

could communicate with any other person in the mine or on the surface 

complex. In reality, this is not achievable because of distances invol ved, power requirements, and other factors. Nevertheless, systems 

have been and are being developed that promise to go far to meet this 

ideal. 

Ul tra-high frequencies (UHF) offer excellent local coverage and 

long distance linear coverage if used with leaky feeder cables. Using 

reflection techniques, it is possible to even do away with the leaky 

feeder cable, if mine entryways are large enough. With modification, 

UHF systems can be interfaced with pager phone lines to permit special 

coverage. 

Medi urn frequency (MF) promi ses excel 1 ent who1 e mi ne coverage a1 so. 

The advantage of MF is that it can use existing nine wiring to extend 

its range. Much basic research has already been done in underground 

!IF propagations with more planned. Hardware is being built that will 

demonstrate the characteristics of MF under mi newide operational 

conditions. 

UHF Systems 

UHF systems have been avai lable to the underground r,~ining industry 

for many years and have met with various degrees of success. !!lost use 

a leaky feeder cable deployed throughout the nine. This cable permits 

radio-to-radio communications as long as both radios are within range 

of the cable. 

UtiF systems need not be confined to such cables to achieve minewide coverage. Bureau of Mines research in the area has shown that 

reflective techniques can also be eliiployed if the mine entries are 

large enough. Although research in this particular area has been 

confined to a room and pillar noncoal mine, the appl ication to 

coal mines is obvious. (See figure 5.) 

With this system, nonradiating coaxial cable is installed 

in various mine areas with antennas mounted in key crosscuts. 

l.lhen so deployed, the antennas radiate and receive signals for 

several thousand of feet in four directions. Vehicles and personnel 

situated in these prime entries are therefore in voice communication 

with the rest of the UIiF system. 

Reflection techniques are used whenever coverage is desired in 

secondary areas. The situation is analogous to reflecting light 

around a corner with a mirror. Since the entryways are large, 

the reflectors and antennas present no obstacles to personnel or 

machines. The system has given excellent results under the following conditions: 
(1 ) Repeater power 10 w; vehicular power, 10 w; personnel 

radio power, 1 w. 

(2) Entry size, 30 ft high by 30 ft wide. (Limestone Fline) 

(3) Reflector size, 4 ft by 4 ft. 

A UHF Extender System 

No matter how well a UHF system (leaky feeder, reflector, etc.) 

may be configured for a mine, situations always exist where additional 

coverage is sometimes needed. An example might be a temporary work 

area in a remote section of the mine that is rarely used. It is 

usually not practical to try to cover the area by extending the 

leaky feeder cable or using reflectors. However, running phone 

lines is rarely a problem. Because of this, the Bureau has developed 

a simple UHF extender system using phone 1 ines. (See figure 6.) 

The system is designed around commercially available 8, 3, or .7 

w UHF base station radios operating at 451 MHz. In operation, UHF 

portable radios need only be in range of one such base. Nhen the 

base reserves a transmission, a keying signal and audio are impressed 

on a twisted wire pair line where a second base station at the other 

end receives and retransmits. Therefore, the system offers area 

coverage from area 1 to area 2, and vice versa. It can be easily 

seen that if one of the areas overlap that of a primary area such as 

a leaky feeder cable, communication between the two systems is 

achieved. It can further be seen that any two-wire phone line 

can be used, even one containing pager phones. 

Medi um Frequency (MF) Research 

Over the past several years, the Bureau has been involved in 

MF research in coal mines. (I)* Numerous in-mine tests have been 

conducted to learn the characteristics of MF in various geographical 

and geological locations in the major coalfields, in areas of 

nines that contain many conductors, and in areas that do not. 

Simple theoretical models were formulated (2) that characterize 

MF propagation and predict the most favorabie frequencies and 

maximum commur~ i cat ion ranges. 

For conductor-free areas, a three-layer model was developed and 

verified. The propagation mode take% the form of a para1 lel plane 

(0, 0) TEM mode with f vertical and H horizontal. The model assumes 

the coal seam is bounded above and below by rock of higher conductivity 

than the coal. The model fits actual data best between 200 to 1,000 

ktlz and shows a wide band at 300-700 ktiz where range is greatest. 

* llumbers underscore in parentheses refer to items in the list of 

references at the end of this paper. 
Certain coal seams exhibit unfavorable characteristics where the 

operating range in conductor-free areas is only about 250 ft. In other 

more favorable coal seams, the range is up to 7 ,200 ft. If conductors intersect the areas, range is great7y enhanced. 

Since I.iF propagation is greatly influenced by the electrical 

properties of the surroundings, it is necessary to investigate this 

propagation in noncoal nines also. Such a neasurement program will 

soon be underway. Coal is found in sedimentary deposits where 

various layers have different electrical properties. Other ~~inerals 

are often found in bulk deposits. Therefore, the l1F propagation 

in such nines, especially in nonconductor areas, is expected to 

be quite different. 

bne of the problems of evaluating PIF on a systems basis is 

that no comclercially available hardware exists with the proper 

operational features. The small portable units of low power or 

modified carrier-current equipment that can be purchased, are of 

little value in a systems evaluation. 

To solve this problem, the Bureau is developing !,IF hardware 

suitable for systems evaluation. It consists of portable and 

vehicular units, base stations, and repeaters. Prototype hardware 

is expected around mid 1981. Figure 7 shows a typical system 

confi$uration. 

11. PUST DISASTER TECI-INIQUES 

Since the earliest days of underground mining, miners have had 

to face the possibility of major disasters. These disasters usually 

take the form of fires or explosions, although flooding and massive 

roof falls \lave also taken nany lives. Frollowing any such disaster, 

two fundamental questions exist: "1s anyone alive?" and if so, 

"Uhere are they? Addressing this problem, government agencies and 

the mining industry have encouraged and f'inanced the development of 

equipment and rescue teams. 

Since the early 1970'~~ theEureau of Elines has conducted research 

in hardware and techniques to locate and cotmunicate t~ith trapped 

~liners. This research was recommended by the Piational Academy of 

Engineers (NAE) (3) and fa1 1 s into two general categories: Seismic 

and electromagnetTc (Dl). This section discusses the latest developments in the area of Ell detection techniques. 

The Gasic System 

The basic EN system developed several years ago is based on a 

narrowband voice frequency (VF) up1 ink signal ing transceiver that 

uses power from a standard cap lamp battery. (See figure 8.) A 

low duty cycle of the transmitted signal assures that operation is 

possible for 3 days on a fully charged new battery or about 1 day 

on an average 2-year old battery that had already been used 8 hours 

for the light. 
The underground unit consists of a VF transmitter and baseband 

receiver referred to as the underground "transceiver." Surface receivers were developed that could be carried by he1 icopter to permi t 

rapid large area searches, or by personnel on foot, to do accurate 

locations. Large surface baseband transmitters were developed that 

permitted the surface personnel to communicate with the trapped underground personnel, once the location was made. In operation, the 

trapped miner would deploy the underground transceiver, which would 

signal through the earth. Once detected, the surface crew would talk 

to the miner via VF to learn of mine conditions and give instructions. 

The surface crew would ask yes-no type questions that could be answered 

by simply keying a button on the transceiver. 

In order to obtain the necessary confidence that the system would 

operate in typical coal mines in different geographical and geological 

areas, a rigorous field test program was conducted from 1976 to 1978. 

Although the basic EM system has many advantages, it also has 

several shortcomings. Because location is based on characteristics 

in the horizontal magnetic field, it provides no information about 

the depth of the underground transceiver. This is generally no problem in coal mines where only a single level is being mined. However, 

in mu1 ti level mining common in metal and nonmetal mining, this could 

create problems. In these cases, depth information is required. A 

second shortcoming is that the accuracy of the location is a function 

of how level the underground antenna is deployed. An antenna on an angle 

produces a displaced field that affects accuracy. The third shortcoming is based on the fact that signal detection and location depends 

on the ski1 1 of a human operator. Some search personnel can detect and 

locate weak signals much better than others. These shortcomings 

prompted research in developing better systems. 

The desirable characteristics of improved systems fol low: 

(1 ) Full automation, except for deployment. 

(2) A reduction of ground effects and depth considerations. 

(3) El imination of antenna orientation effects on location accuracy. 

(4) Simple inexpensive underground hardware. 

Electromagnetic Deep Mine Transmitter 

Thru-the-earth detection and location systems based on VF permit 

the use of simple hardware and detection techniques. But because of the 

earth attenuation, it is necessary to go lower in frequency as depth 

increases. One goal is to develop a system useful to 3,000 ft, a range 

useful for coal mines for decades to come. To achieve this goal, subELF frequencies (10 Hz) are necessary. 
One fundamental problem of transmitting EM through the earth at 

these frequencies is that on the surface, the received signal energy 

is inversely proportional to the sixth power of depth. Hence it 

is about 1,000 times more difficult to receive signals from a 3,000 

ft mine than a 1,000 ft mine, other factors being equal. The problem is compounded by the 1 imi ted amount of energy avai lab1 e to the 

trapped miner (the cap lamp battery) and by electromagnetic interference (EI.11) at the surface. A system is presently being developed 

to solve these problems. 

It consists of a cot~tinuous wave (CW) transmitter (10 Hz) deployed 

underground similar to the previous system. On the surface three 3-axis 

antennas are deployed to receive the signal, and a fourth antenna, 

remotely deployed, is used for atmospheric ErlI cancel 1 ation. A1 1 

antennas are connected to a central processor via hard wire or radio 

1 inks. 

Because of the very weak signals, several minutes of CW integration 

are necessary. The ELF used permits free space propagation calculations. 

Hence, if the precise location of the surface antennas are known, vector 

analysi s of the el ectromagnetic f iel d produced by the underground transmitter is sufficient to calculate the location of the source. 

Adaptive Transmitter System 

A novel approach to extend the detection range of trapped miner 

transtilitters is based on adaptive retransmission. This approach 

combines adaptive transmission lengths, automatic feedback, and signals 

having large time-bandwidth product and ideal autocorrelation. 

In operation, the trapped miner transmitter sends out 34 sec 

of signal followed by a 6-sec pause. The signal is received by a 

receiver/repeater that uses coherent integration in matched filters. 

The receiver integrates the signal until a threshold is reached. 

Depending on signal strength this threshold can be reached with the 

first 34-sec "block" of signal from the transmitter, or it may require up to 16 blocks from the transmitter. When the threshold is 

reached, the receiver/repeater (a far more powerful transmitting device than the underground transmi tter) tr-ansmi ts a signal back underground indicating that sufficient signal was received and detection 

is acknowledged. 

If the receiver/retransmitter is located on the surface, a threeaxis receiving antenna system can be used for vector analysis of the 

signal and hence location. An a1 ternative would be for the trapped nliner to encode position information in a second series of transmissions. 
Through-the-earth Voice Communications 

The basic trapped miner transceiver described previously has voice 

downlink reception capabilities. This is possible because the surface 

rescue team have large amounts of power at their disposal, usually 

engine-generator sets and can therefore use powerful baseband amplifiers 

and large loops to communicate with the miner using sheer power. A base 

band magnetic moment of 1 o5 amp turns M2 is not uncommon. But the 

miner trapped underground is limited in power, the only source being 

the cap lamp battery. It is for this reason that uplink code-keying, 

yes-no type answers to down1 ink voice questions is used. 

To advance research in the area of voice communications, a 

theoretical study is underway to study performance levels of analog 

and digital voice conmunications. This is being done by computer 

modeling, considering earth models and EMI. The expected speech int e l l i g i b i l i t y can then be predicted. Field tests are planned using 

different communication systems and prerecorded phonetically balanced 

word lists. Later, a listening panel will determine the intelligibility of the received signal which will verify computer rtiodels. 

PUBL ICATIOIJS 

A vast amount of research has been done over the years on mine 

comunications by the Uureau and private industry. This research 

is usually in the form of hundreds of highly technical engineering 

reports, useful only to experts in the fie1.d. In addition spec 

sheets, new product brochures, etc., from industry add to this wealth 

of knowledge. Unfortunately, information in this form is not very 

useful to people in the mining industry who must specify, identify, 

and ultimately live with systems that may actually not be what they 

need. A general educational overview is not available. The Bureau 

has addressed this problem by developing a series of publications that 

will assist mine managers, mining engineers, etc., in understanding 

mine communications and monitoring. 

The first, entitled "Underground Mine Communications, Control, 

and Monitoring" has reduced the research to understandable terms, 

presents system operational features, lists suppliers, etc. Ir! 

general, it gives an overview that makes the selection of a system 

to meet actual mine needs more likely. This publication will be 

available in 1981. 

A series of application guidelines specific to certain areas, 

have a1 so been developed. The first, "Technical Guide1 ines for 

Installing, Maintaining, and Inspecting Underground Telephone 

Systems" is available upon request. A second entitled "Guidelines 

for Install ing and Upgrading Trolley Carrier Phone Systems" will 

be available in 1981. A third will address radio systems and should 

be available in 1982. 
Research has continued in developing improved telephone and wireless 

comunication systems for mines. Fiber optics, selectable pager phones, 

visual pagers, and advanced telephone systens will play even larger 

roles in mine operations. The improvements in operational and managerial 

control are niany. 

Post-disaster systens are also being developed that show great 

promise. Since the trends in energy development are toward coal, 

this means more miners underground in deeper, gassier nines. The 

potential for disasters can only increase, in spite of stricter 

enforcements and better equipment. The post-disaster program 

is devoted to reducing the risks to the smallest point possible. 

1 . Col 1 ins Radio Company, Propagation of Ebl Signal s in Underground Mines, 

H0366028, OFK No. 136-7G*; El ectromagnetic Propagation in Low Coal 

at Medium Frequencies, t10377053. 

2. Little, Arthur O. Inc., Modeling and Data Analysis of 50 to 5,000 ktlz 

Radio Nave Propagation in Coal Mines, H0346045. OFR 20-76. 

3. National Academy of Engineering, Mine Rescue and Survival , S190606. 

OFR 4-70, NTIS /.lo. PU 191 b91.** 

4. blestinghouse, Trapped Iliner Location and Communication System, 

OFR 41-74 (4 volumes) ; NTIS PB 235 G05/AS; 235 606/AS; 235 607/AS 

235 608/AS. Also, Penn State University, Theoretical Investigation 

of Sei smic Waves Generated in Coal Mines, G0155044. 

* Open File Reports (OFR's) are available for refrence only at the following 

Bureau of Plines 1 i braries: Denver, Colo. ; Twin Cities, Minn. ; Bruceton 

and Pittsburgh, Pa.; Spokane, Wash.; and the National Library of Natural 

Resources, U.S. Department of the Interior, Mashington, D.C. 

** Elational Technical Information Service (NTIS) items are available from 

National Technical Information Service, Springfield, Virginia 22161. 

The PB number should be given when ordering. Prices for publications 

are $3 for microfiche and range from $4 to $16.50 depending on the 

number of pages for printed copy. 

Contact the author of this paper regarding all other reports listed. 
r DTMF Encoder 1 fj (optional location) J 

\ 

L-A 

Selective 

Supervisory 

acknowledge sign: ) 

1. Selectable Pager Phone System 

I Phone line 

1 Disc control 

signals and 

radio voice 

Radio page 1 ~~e-a~~ena - transmitter 

Voice , page receiver 

2. Disk-type Visual Pager System 
Dispatcher's 

Display 

unit 

Supervisory 

acknowledge C signal 

Phone line 

Selective 

control 

signal I 

3. Liquid Crystal Visual Pager System 

8 - Chanr~el / phones 

- 2 - 

Shielded twisted pair 

telephone line 

4. 8-Channel Mu1 ti pl exed Telephone Sys tern 
Feed Direct 

cable -, 

I 

* 

signals 

' Reflector 

signals 

5. UHF Reflection System 

Leaky feeder ---- 

p?o!eli!e~$Tl radio 1 

Audio and 

keying signals 

6. UHF Phone1 i ne Extensi on Sys tern 
Existing mine wiring network 

Base 

stat~on 

7. Whole Mine Medium Frequency Configuration 

Receiver 

w Transceiver 

8. Basic EM Trapped Miner System