NIOSH-Sponsored Research in Through-the-Earth
 

Communications for Mines: A Status Report
 Michael R. Yenchek, Senior Member, IEEE, Gerald T. Homce, Nicholas W. Damiano, and Justin R. Srednicki

Abstract—This paper presents the results of recent contractual 

research sponsored by the National Institute for Occupational 

Safety and Health that aimed at demonstrating the feasibility of 

through-the-earth (TTE) wireless communication in mining. TTE 

systems, developed by five different contractors, are discussed with 

a focus on technical approach, prototype hardware, and field test 

results. System features include both magnetic and electric field 

sensing, loop and line antennas, digital and analog processing, 

noise filtering and cancelation, and direction finding. The systems 

were demonstrated at commercial mine sites. The results of these 

tests are characterized by transmission range and power levels. 

This paper concludes with a discussion of issues that remain to be 

resolved as TTE communications are implemented. These include 

text versus voice format, acceptable time delays, portability, ease 

of deployment, an interface with existing communications systems, 

permissibility, and the effect of geological variations. 

I. INTRODUCTION

T HE Mine Improvement and New Emergency Response

Act of 2006 (MINER Act) [1] mandated that each underground coal mine in the U.S. develop an emergency response

plan within three years. The plan must provide for two-way

wireless postaccident communications between underground

and surface personnel as well as electronic tracking of underground workers. Communication between rescuers at the surface and miners in underground mines is particularly important

during emergencies including explosions, fires, inundations,

and entrapment from rock falls. In an emergency, conventional

communication systems may be interrupted if sufficient infrastructure is damaged. An alternative is to directly communicate

to the surface through the overburden. Typical high-frequency

wireless technologies cannot penetrate significant distances

through the earth. However, signals at frequencies below

10 kHz can penetrate the earth and offer promising capabilities

to establish through-the-earth (TTE) connections. Proposed

TTE communications applied to mining typically involves

inducing an ultralow frequency (ULF) signal into the earth

through an antenna and receiving that signal through an antenna. The transmission path may be from underground to surface or vice versa. A major advantage of TTE technology is that

it requires much less underground and surface infrastructure

than higher frequency mine communication systems.

Although TTE has long been considered a desirable technology for mine emergencies by safety experts, research conducted by the U.S. Bureau of Mines (USBM) in the 1970s

determined that it would be technically difficult to establish

TTE transmitting capability from inside a coal mine since

the transmitter power must be limited to meet permissibility

requirements [2]. However, recent advances in receiver and

signal processing technology have led to better sensitivity and

offer new possibilities for TTE communication.

In June 2006, the Congress allocated Emergency Supplemental Appropriation funding for improvements in mine emergency

response, including communications and tracking, to the National Institute for Occupational Safety and Health (NIOSH)

Office of Mine Safety and Health Research (OMSHR). The

funds targeted technologies that could be mine-ready in

24–36 months. This led to a series of contracts and grants

including those reviewed in this paper, which were awarded via

Broad Agency Announcements (BAAs) [3], [4].

II. TTE COMMUNICATIONS

Discounting antennas and transceiver hardware, TTE systems have the potential to provide true wireless communications between underground and surface personnel, meeting

the intent of the MINER Act. With less infrastructure than

traditional communication systems require and typically stored

or cached until deployment, TTE systems are more likely to

survive an underground explosion and provide a communications link to the surface for trapped or escaping workers.

These systems typically operate at frequencies less than 4 kHz,

with the electromagnetic signals characterized by wavelengths

measured in thousands of meters. The path for transmitted

signals may be vertical through the overburden or horizontal through the coal seam. The half-duplex systems provide

communications in both directions but only one direction at a

time (not simultaneously). They can provide voice in real time,

voice, or text messages or emit a periodic beacon signal, which

can be sensed on the surface and allows rescuers to estimate

the location of its origin. These modes are characterized by

different signal transmission rates. For example, rates as high
as 2.5 kb/s permit real-time digitized voice, whereas rates as

slow as 10 b/s only allow text at one keystroke per second.

Obviously, real-time voice is desirable, but TTE transmission

is affected, and many times limited, by a number of factors.

The factors that affect signal transmission include the frequency, transmitter power, and the nature of the overburden,

such as its electrical conductivity, its depth, and any geologic

variations that present changes in the overburden electrical

properties. The reception of a transmitted signal is also affected

by the presence of electrical noise, both underground and on

the surface and both natural and man-made. The ability of

the system to communicate is dependent upon the remaining

energy of the transmitted signal, after attenuation through the

earth, being sufficient to overcome the noise at the receiver

location. Finally, the antenna configuration (e.g., loop or linear)

can impact transmission and reception.

It is known that the lower the frequency of a transmitted

signal, the lower the signal attenuation through the earth. However, at very low frequencies, data transmission rates are limited

and only text messages or preprogrammed messages may be

possible. The lowest practical rates generally allow the greatest

depth penetration and are typically used to generate beacon

signals underground. The beacon signals can be detected by

rescuers on the surface at long ranges for which data and voice

transmissions are not possible, and can help determine the

approximate location of the transceiver underground.

Given these limitations, it is desirable to have multiple transmission frequency capability to optimize range and mode for a

given set of geological conditions. Transmission range is proportional to transmitter power but, underground, consideration

must be given to the safe use of electrical equipment in potentially methane-laden atmospheres. Consequently, transmitter

power underground can be limited.

Turning attention to the hardware anticipated for a TTE

communications system, large loop antennas may be used

to transmit and receive small magnetic signals. ferrite core

windings may be employed in receiving antennas to conserve

space for portability. To optimize coupling and, consequently,

signal transmission, both sending and receiving loops should be

oriented in the same direction (see Fig. 1) [5].

Fig. 1. TTE communications system.

Multiple ferrite

windings may be placed in an X–Y–Z configuration for receiving antennas. Antenna outputs can be then vectorally added

to obtain a resultant signal. Underground transceivers can be

used for communication both between surface and underground

and from point-to-point (horizontally) within the mine. The

transceivers may be transportable or in a fixed location, but they

must be limited in power to ensure safe operation in potentially

hazardous atmospheres.

Surface antennas may theoretically consist of large loops

of wires, which are thousands of meters in circumference

and encompass most of the mine. However, from a practical

standpoint, they may be limited in size by the terrain and

by limited access to property above the mine. Alternatively,

smaller antennas on the surface may be placed above stationary

antennas underground or deployed during an emergency. Much

greater power may be used on the surface to transmit a strong

signal to the underground environment, with higher frequencies

and higher data rates for voice transmission. For the greatest

signal, i.e., maximum coupling between antennas, the surface

loop antenna should be above the underground antenna. Signal

reception on the surface will generally decrease if the surface

antenna is not directly above the underground transmitting

antenna or if the separation distance increases.

Projects with five firms were funded through NIOSH to

investigate the feasibility of applying TTE technology to mining. All contracts resulted in the development of prototype or

preprototype systems. System features include both magnetic

and electric field (E-field) sensing, loop and line antennas,

digital and analog processing, noise filtering and cancelation,

and direction finding. The systems were evaluated in tests

at various commercial mine sites. The following summaries

describe principles of operation, hardware, and test results.

Mention of commercial names does not constitute endorsement

by NIOSH.

III. ALERTEK, LLC

The objective of Contract 200-2008-25720 awarded to

Alertek, LLC was to design, build and demonstrate a batterypowered TTE wireless voice communications system for overburdens up to 183 m [6]. The technical approach was to

establish magnetic coupling between separate loop antennas

underground and on the surface. Voice communication is the

key feature of the system. The signal from a handheld microphone is amplified, processed, and entered into a digital signal

processor where it undergoes further conditioning (see Fig. 2).

A single sideband is applied to a magnetic loop antenna. The

magnetic flux linking the receiving antenna generates a voltage

replicating the transmitted voice, along with noise being picked

up at the receiving end. The voice message is extracted, and

the baseband is amplified and applied to a speaker or optional

earphones. The technology does not use speech compression,

avoiding the errors and inherent delays associated with error

correction.

The system can also send and receive digital text messages by

using phase-shift keying. Incoming text messages are displayed

on a liquid crystal display (LCD). The system also has beacon

signal capability, which is treated similarly to a text message,

except that only a single tone istransmitted and that the received

signal is applied to a speaker.
Fig. 2. Alertek surface transceiver.

The prototype system was first tested at the NIOSH Experimental Coal Mine in Pittsburgh. The system delivered twoway real-time-voice TTE communication between surface and

underground at approximately 30.5 m depth. This was achieved

when the transmitting and receiving loop antennas, oriented

horizontally on the earth, were directly inline vertically, as well

as when the surface transceiver was moved about 91.5 m off the

vertical axis. In a follow-up demonstration, real-time two-way

voice and text communications were successfully established

horizontally between a point outside the drift portal of the

experimental mine and a point inside the mine, approximately

305 m away.

Further tests of the prototype communication system were

conducted at a commercial coal mine in southwestern PA.

The mine depth was approximately 177 m with a surface

transceiver located directly above the underground unit. Twoway communications were established via beacon, text, and

real-time voice at relatively low power levels (antenna currents

were approximately 1.5-A RMS). Electrical noise sources were

nearby both underground and on the surface. The transceiver

on the surface was in proximity to the mine elevator and

power lines approximately 30.5 m away. The underground

transceiver was located near a trolley line with the 122-m loop

antenna encircling a coal pillar. Voice reception without delay at

3150-Hz resonant frequency on the surface was of such quality

that the voice of the person speaking underground could be

identified. Underground voice reception was intermittent, with

the poor performance attributed to a design deficiency in the

underground receiver. The potential impact of this work is that

it offers a real-time voice option for relatively shallow mines.

However, the system is currently not approved as permissible.

IV. E-SPECTRUM TECHNOLOGIES

The goal of Contract 200-2008-26818 with E-Spectrum

Technologies was to analyze an existing ULF TTE system

and to adapt and test it for communication with underground

mine personnel [7]. The basis for the adaptation was an E-field

sensing system used for oil and gas drilling that was modified

to include remote and surface units, appropriate software, and

worker interfaces. System specifications addressed suitable carrier frequencies, bandwidth, data format, modes of operation,

antenna topology, power requirements, and packaging.

A critical element was developing and testing the transmitting and receiving electrical field antennas. Both antennas

require low-resistance earth connections. On the surface, these

antennas can take the form of a wire connection to earth via a

driven-rod ground bed. Fences with metal conductive posts that

are strung with wire conductors were also found to be excellent

transmit and receive antennas when coupled with the prototype system. The most challenging aspect of implementing the

system in a coal mine is providing an E-field antenna for the

underground transceiver unit. One possibility is the installation

of roof bolts with conductive epoxy at regular intervals in the

mine. These roof bolts could be connected with conductive

braid or wire, or with wire roof mesh to further minimize the

system impedance to earth.

E-Spectrum Technologies tested the prototype system (see

Fig. 3) at NIOSH’s Safety Research Coal Mine in Pittsburgh

for underground-to-surface communications.

Fig. 3. E-Spectrum subsurface transceiver shown on surface.

The surface

E-field antennas were composed of two driven-rod ground

beds separated by 30.5 m. The subsurface system relied on

a series of interconnected roof bolts to establish a relatively

low resistance connection to earth. Text communications were

achieved at frequencies of 10 and 22 Hz with an estimated

surface to underground diagonal separation distance of 366 m.

The contractor also successfully achieved two-way text communications (in prior testing) at approximately 610 m using

practical antenna configurations in relatively low-conductivity

limestone rock formations.

Further evaluations demonstrated that the ULF system was

technically feasible for text communications from underground

to the surface diagonally up to 1.6 km in a commercial mine.

Ranges of this magnitude were achievable because the system

operates at such low frequencies. The performance tradeoff

is the low data rate, which limits communications to text

messages. The prototype subsurface transceiver unit was fitted

with a Bluetooth interface that would allow miners underground

to use a handheld unit, such as a cell phone or a personal

digital assistant, as the user interface for text messaging to the

surface. The handheld device can be up to 30.5 m away from

the transceiver.
The system will need to be modified before it can be submitted for approval by MSHA as permissible. This may entail

selection of appropriate explosion-proof housings and design

of antenna barriers. The potential impact of this work is that it

offers an alternative approach (E-field sensing versus traditional

magnetic field sensing) for TTE communication, which holds

promise for long-range communication.

V. LOCKHEED MARTIN MS2

The purpose of Contract 200-2007-22843, awarded to Lockheed Martin, was to develop and demonstrate a two-way

TTE communication system for mines that applies state-ofthe-art communications technologies to overcome subterranean

wireless communication challenges while meeting acceptable

industrial safety standards [8]. In subsequent Contract 200­

2009-32021, Lockheed enhanced key features of the TTE prototype to improve its usability, investigated design changes to

comply with mine safety standards, ruggedized the hardware,

and demonstrated performance at coal mine sites [9].

Lockheed Martin developed a wireless TTE prototype communication system based upon magnetic field generation and

sensing. The system consists of surface and underground

transceivers. The receiving antennas are made up of three

ferrite core windings arranged orthogonally and housed in a

portable case. Variously sized single- and multiple-turn aircore antennas are used for transmission from underground to

surface, including a single loop of wire that can be deployed

around a coal pillar. Voice and text messages are possible at

signal transmission frequencies of 3200 and 330 Hz, respectively. Reception of either channel is automatic regardless of the

transmission frequency selected. The surface and underground

receivers have noise cancelation capability.

The durability and reliability of the original prototype system

were improved by making the hardware resistant to humidity,

mechanical shock, dust, and water (see Fig. 4).

Fig. 4. Lockheed Martin underground prototype.

The power

usage of the amplifier was reduced to increase battery life

while maintaining transmission range, and features were incorporated to enhance usability. A beacon mode was developed.

A collision avoidance protocol was used to minimize message

collisions involving multiple active transceivers, i.e., having the

underground transceiver trying to broadcast when the surface

transceiver is also trying to broadcast. A radio relay capability

allows for interoperability with conventional handheld radios.

The voice message is stored and then transmitted at low frequency through the earth. Another TTE unit receives and stores

the message, after which it is replayed.

The system was tested at the NIOSH Pittsburgh facility and

at several coal mines in southwestern Pennsylvania, northern

West Virginia, and Virginia. Prototype evaluations demonstrated ranges of 472.5 m for text, from the underground mine

to the surface. In these tests, the surface transceiver was directly

above the underground unit with the underground transmitting

antenna operated at relatively low power comparable to intrinsic safety restrictions.

With the prototype operating at higher power levels,

Lockheed demonstrated ranges, from the mine to the surface,

of 305 m for voice and 594 m for low-rate text. Tests have

also been conducted between two distant points, both within

the mine. The maximum range achieved for the through-themine testing was 1494 m for text and 640 m for voice. Greater

ranges are anticipated for the beacon mode signal. A surface

direction finder was also demonstrated to estimate the origin

of the underground beacon signal. The angular direction of

the beacon signal was measured at multiple locations on the

surface, and through triangulation, the approximate location of

the transmitter antenna underground could be determined.

VI. STOLAR, INC.

The objective of Contract 200-2009-32117 with Stolar, Inc.

was to design and build a two-way TTE communication prototype system and then conduct field tests at several underground

mining operations [10]. The system developed employs lowfrequency transceivers, relatively low-power instrumentation,

and simple portable hardware. It was field tested at several hard

rock and coal mining sites. Preliminary testing in a western

U.S. noncoal mine proved that Stolar’s TTE technology could

provide communication through a variety of rock types, with

depths ranging from 213 to 396 m, depending on the location

in the mine. The most recent demonstration achieved twoway text messaging at a vertical range of nearly 244 m in

a commercial coal mine in southwestern Pennsylvania with

signal strength in reserve. The maximum vertical range at this

mine was extrapolated to be nearly 335 m.

The antennas used for this project were rigid air-core loops of

either 1.2- or 1.8-m diameter (see Fig. 5). The antennas can be

used as either vertical or horizontal magnetic dipoles (VMD or

HMD, respectively) lying flat or standing upright, as shown in

Fig. 5. The VMD mode is predicted by the vendor to be optimal

for low-noise environments where TTE signal sensitivity can be

high, permitting the range of signal penetration into the earth

to exceed 60% of the maximum capabilities of the system.

The HMD mode is predicted to be optimal for higher noise

environments since less noise from horizontal power lines can

couple to the vertical antenna. However, the HMD mode has

less signal sensitivity and will reduce the depth of penetration

to less than 70% of the maximum capabilities.

The primary screen menu on the transceiver’s LCD display

allows the user to enter custom text messages or turn on a

location beacon. Another screen menu allows the user to select

from a variety of “predefined” messages. The text messages use

a selected frequency of either 2 or 4 kHz. The default lower
frequency is commonly used for depths of 152 m and deeper.

The higher frequency has less penetrating power in the earth

(higher attenuation rate) and is typically reserved for shortrange applications where background noise at 2 kHz may be

higher than normal. In addition to the text message signals,

each system also possesses a beacon mode, which allows the

transceiver to be used as a continuous transmission source for

detection and location by using a surface gradiometer, which

triangulates the received signal by sensing the difference in the

magnetic field at several locations in space. To help with field

testing and data collection, and to assist with the transceiver

setup at different sites, a diagnostic program was created that

wirelessly links to the individual transceivers for real-time signal analysis, including signal strength, decoding sensitivity and

threshold, and background noise. The contractor initiated the

MSHA approval process for a system featuring an explosionproof enclosure for the transceiver.

Fig. 5. Integrated Stolar surface transceiver with antennas.

VII. ULTRA ELECTRONICS

The goal of Contract 200-2009-31292 with Ultra Electronics

was to fabricate and test a previously designed TTE communication system for use in an underground mine [11]. In

this system, the voice signal to be transmitted is conditioned

and converted to digital form. It is then compressed, using a

vocoder, and packetized. The bits in the packet modulate the

transmitter output using an orthogonal frequency-shift keying

(FSK) approach. The transmit loop antennas consist of flexible

wire of several turns. The receive antennas, internal to the

transceiver enclosures, feature windings on ferrite rods. The

receiver provides selectivity using analog and digital filtering,

demodulates the FSK signal, passes the received “voice” data to

the vocoder for regeneration of the audio, and finally drives the

speaker (or earpiece) with the audio signal. The current system

is powered internally by two 12-V lead-acid batteries. The units

are not approved as permissible at this time.

The transmitter can also accept serial data from an RS-232C

port and text information from an external device (keyboard,

suitably configured terminal device, or personal computer). For

serial data, the magnetoinductive link employs a packeting

approach similar to that for voice mode. The received serial

data are made available to the RS-232C port on the receiver.

The TTE transceiver operates similar to a two-way radio, using

a push-to-talk half-duplex mode. Using the low-bit-rate text

mode, the range into the earth is doubled over that for voice.

Six magnetoinductive TTE communication devices were

built according to a plan approved by NIOSH. The modules

are digital transceivers supporting both voice and text communications. Wideband mode at 4.82 kHz is used for digital

voice and data, whereas the narrow band is for data only. Four

transmit antennas (3.8-, 12.5-, 26-, and 90-m loop diameter)

having different transmission ranges were also included.

The system was evaluated to determine TTE operating range,

electrical noise interference, an interface to other emergency

communication subsystems, and repeater functions, as well as

ease of use and mechanical ruggedness. Preliminary testing

of the equipment in a Pennsylvania coal mine revealed that

narrow-band communication of text was successful, but realtime voice was not achieved. Magnetic signal and noise were

also recorded for later analysis. High attenuation of the magnetic signal and high electrical noise levels in the mine were

determined to be the cause for the reduced performance of

the wideband voice communication. In a follow-up test at the

same Pennsylvania coal mine (see Fig. 6), a larger transmit

antenna was employed to increase signal strength to overcome

the attenuation.

Fig. 6. Transmitting voice to underground with the Ultra system.

In addition, a noise-canceling comb filter for

60-Hz harmonics was implemented to reduce the impact of

the in-band noise. Successful real-time voice reception on

the surface from an underground transmitter, as well as lowspeed text communication, was achieved vertically through

more than 183 m of overburden. The ability of the triaxial

magnetometer to directionally detect underground transmitted

signals was demonstrated for possible application in locating

trapped miners.

VIII. SUMMARY AND FUTURE CHALLENGES

The feasibility of two-way TTE communications in underground mines has been demonstrated in five recent

NIOSH-sponsored research contracts. These projects involved

development of prototype units that were evaluated at commercial mines. The systems used varied approaches to establish

communication through the earth. Most featured magnetic field
sensing, with one design based upon E-field propagation. Both

digital and analog signal processing techniques were employed.

Loop or line antennas were used for transmission. Noise filters

or noise cancelation techniques were found to be necessary

for signal reception. Underground-to-surface communication,

as well as underground point-to-point communication for voice

and text, was demonstrated at ranges that might be applicable

for some mines. Range generally increased with power levels.

Further evaluations are needed to gauge ranges for systems

operating within MSHA permissibility limits underground.

Some systems allowed for estimation of the location of an

underground transceiver through the reception and analysis of

a beacon signal on the surface. In some contracts, prototype

hardware was delivered to NIOSH.

Through these BAAs, NIOSH provided companies an opportunity to demonstrate and develop technology solutions that

can be applied to solve mine safety and health problems.

There was considerable research conducted by the USBM that

documented the challenges to TTE communication. Based on

their understanding of these challenges, most vendors were

initially very confident that they could readily achieve depths

in excess of 305 m for voice and up to 610 m for text (see

Table I).

TABLE I
 

SUMMARY TABLE OF TTE PROTOTYPE TESTS
 

In all cases, the manufacturers found the challenges

to be much more difficult than they had expected. There are

several possible explanations. First, the digital signal processing and noise cancelation techniques used for radio, radar,

and sonar applications were not designed to be effective on

the type of signals and noise experienced at these ultralow

frequencies. Second, past USBM research was conducted in the

1970s and 1980s, when equipment typically operated at lower

utilization voltages and power than at present. Today’s equipment would be expected to generate a different electromagnetic

noise profile. Finally, as a result of continued development, the

noise levels due to above-ground transmission lines and other

man-made sources most likely have increased substantially

in the last 30 to 40 years. Whatever the reason, one thing

is clear: Additional research will be required to identify the

types and sizes of coal mines in which TTE solutions can

work and to understand the performance limitation of these

systems.

Further, before this technology can be implemented by the

mining industry, there are still many questions that remain to be

answered. These relate to communication format, time delays,

portability, deployment, noise characterization, interaction with

other systems, and permissibility. These issues will be briefly

detailed here.

One question revolves around the choice of communication

format, specifically, text versus voice. From a technological

standpoint, the signal range for text can be greater than for

voice. Text can be transmitted at lower bit rates and lower

frequencies, with preprogrammed canned messages having the

greatest potential range. Text also has the advantage of being a familiar and well-accepted communication format for a

workforce becoming more comfortable with computers. There

are concerns over the ability to text under stressful adverse

emergency conditions, particularly when smoke hinders visibility. Those same conditions, however, may also hinder voice

communications when self-contained breathing apparatus are

in use. The solution may lie in software that can convert voice

input into text, and/or text into synthesized voice.

Another question focuses on what is an acceptable time delay

for a message. Real-time communication without functional

delays, as with conventional telephones, is a goal that may not

be practically achievable or necessary. It is conceivable that a

message can be transmitted and received, and a response is

made over several minutes without adversely affecting rescue

or escape. As with message format, message delay times and

range are impacted by bit rate.

Further questions revolve around the hardware and its deployment. It could be stationary while in use, or it may need

to be transportable. It may be used while workers are awaiting rescue or during their escape. Some systems may feature

components in heavy explosion-proof enclosures that could

only be moved by equipment. Underground antennas could be

predeployed by integration into the design of refuge chambers

or be carried and deployed as needed by an escaping miner who

might also have donned a self-contained self-rescuer. On the

surface, antennas may be permanently deployed over dedicated

hardened transceivers installed underground or may be installed

following an emergency on the surface above a rescue chamber.
It is difficult to predict what mine communications or infrastructure may survive a catastrophic explosion. One possible

TTE system configuration that could enhance survivability

would employ hardened TTE transceivers and antennas installed periodically along escape routes underground and linked

to permanent surface installations. Escaping workers could access the distributed transceivers on their way out, either directly

or via a handheld radio interface. TTE systems could relay

messages underground from trapped miners to rescue teams

or could interface with other existing systems such as a leaky

feeder, medium frequency, or wireless mesh. Signal relaying

and interfacing mandate that message collision avoidance features be incorporated.

There are many sources of electrical noise underground that

may interfere with the reception of a low-frequency signal.

These include motors, motor starters, and transformers. However, TTE systems would most typically be used following an

incident that requires escape or rescue. Under these conditions,

the mine power would often be deenergized, and most lowfrequency underground noise sources would be quieted. On the

surface, atmospheric conditions and electrical installations may

generate noise. However, during rescue operations other noise

sources such as from drilling, vehicles, and the media might

be present on the surface. The signal captured on the surface

must not be swamped by background noise; therefore, noise

reduction is important. With conventional techniques, a 20-dB

reduction in noise is achievable. Further improvements may be

gained with advanced digital filtering. For communication in

deeper mines, such reductions will likely be necessary.

Prior to commercialization, systems must be approved as

permissible for use in potentially hazardous atmospheres underground. Traditional methods to approach permissibility generally negatively affect TTE performance or system features. For

example, one approach to permissibility is to locate hardware

in explosion-proof enclosures but that tends to limit portability.

An alternative approach is to use intrinsically safe underground

components, but the associated energy limitations often limit

range, and in some cases, may make designs impractical for

any but the shallowest of mines.

Finally, it is known that the geology of the earth transmission

path poses further challenges for the designer and user of

TTE communication systems. The electrical conductance of the

overburden has a great impact on the efficiency of transmission

and can vary by orders of magnitude from mine to mine.

Signal transmission with minimal loss is most easily achieved

through materials with relatively low conductivity, such as

air, dry limestone, or sandstone. Higher conductivity materials

such as salt or coal can impede transmission. Maximum TTE

system ranges can be simplistically extrapolated assuming a

homogeneous overburden with uniform conductivity. However,

aquifers, salt deposits, and mined-out seams may adversely

affect transmission. There could be significant signal reflections

at the air–earth interfaces that might necessitate burying or

embedding both the surface and underground antennas. Underground, for a point-to-point transmission, multiple entries, solid

coal blocks, roof mesh, and gob could significantly alter the

range. NIOSH OMSHR plans to characterize the TTE path loss

by determining the factors that most affect signal transmission

and reception for TTE systems, and to investigate methods

that optimize system performance. This will be accomplished

through modeling with validation at commercial mine sites, but

it may still require that transmission measurements be made at

each TTE installation.

ACKNOWLEDGMENT

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

Health (NIOSH). In addition, citations to Web sites external to

NIOSH do not constitute NIOSH endorsement of the sponsoring organizations or their programs or products. Furthermore,

NIOSH is not responsible for the content of these Web sites.

All Web addresses referenced in this document were accessible

as of the publication date. The findings and conclusions in this

paper are those of the authors and do not necessarily represent

the views of the National Institute for Occupational Safety and

Health.

The prototypes identified in this paper were not all tested

under similar conditions, and the maximum distances reported,

in some cases, were influenced by test site conditions in addition to system performance. As a result, any information

provided should not be interpreted as an objective system comparison. NIOSH does not endorse any specific manufacturer or

technology.

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28.23:8711.

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requestsforproposals.htm

[4] NIOSH,	 Requests for Proposals and Broad Agency Announcements,

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requestsforproposals.htm

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[Online]. Available: http://www.cdc.gov/niosh/mining/topics/electrical/

commtrackingtutorial/commtrackingtutorial.htm

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