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Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Ground Penetrating Radar for Highwall Guidance

Objective

To provide a safe, practical, cost-effective method for measuring

coal rib thickness during highwall mining.

Background

U.S. highwall operators have been seeking a method to maintain an

optimum coal rib thickness throughout the entire depth of the entry,

particularly in mines where the depth of penetration can be up to 400 m.

In Appalachian coal mines, the typical coal rib thickness is 1 to 2 m,

depending on the mechanical properties of the particular coal seam being

mined. If the rib becomes too thin, the hazard of ground fall increases

and with it the resulting risk of burying the mining equipment. Freeing

a buried machine is an expensive process, places workers at risk, and

produces no coal. Conversely, if the rib becomes too thick, the excess

coal left behind is permanently unrecoverable and makes the rib thin for

the next hole. Although the current typical alignment procedures utilize

precision surveys, geological and mechanical forces during mining

produce uncertainties in position nearly as large as the rib thickness itself

at depths of 300 to 400 m despite the accuracy of the initial alignment.

Approach

The National Institute for Occupational Safety and Health (NIOSH),

Pittsburgh Research Center, is evaluating a new method wherein a

ground penetrating radar (GPR) is mounted on the mining machine to

measure the rib thickness to the adjacent hole as cutting progresses.

The advantage of GPR is that it uses electromagnetic pulses to measure

the coal rib thickness directly. The GPR works best in clean coal

seams; coal seams with significant partings or dirt bands tend to disrupt

the radar signal so that the radar echo from the coal-air boundary is too

weak or indistinct to be usable.

The success of a GPR system at a given location primarily depends

on the electrical conductivity and dielectric constant of the geological

formations. The electrical conductivity of coal is generally quite low

(e.g., 10 S/m), which allows the radar signals to readily propagate -5

through the coal. The dielectric constant of coal is typically 4-5; in

contrast, air has a dielectric constant of 1. Consequently, there is a very

distinct interface at coal/air (or air/coal) boundaries. The side of a

highwall rib is fairly planar, is perpendicular to the direction of the radar,

and thereby becomes a relatively easy target for the GPR. It should be

noted that coal is an anisotropic (nonuniform) material, which requires

that the GPR antenna be optimally oriented in such a way for maximum

transmit and receive signals. This is best done by slowly rotating the antenna through a 90° arc about the transmitting direction and noting the

angle that maximizes the radar signal.

How It Works

The basic system consists of a radar control unit and an antenna,

a PC to process the data, a Mine Safety and Health Administrationapproved barrier box for intrinsic safety, and a remote-controlled

hydraulic- or electrical-powered arm attached to a highwall miner (figure

1). The antenna needs to be housed in a metal box for protection during

mining operations. The remote-controlled arm will keep the antenna

retracted until coal rib thickness measurements are needed, at which time

the operator would activate the arm to press the antenna "gently" against

the coal rib to reduce the air gap between the antenna and the coal. The

ability of the GPR to penetrate coal depends on the frequency of the

radar, which must be selected accordingly. For highwall applications

requiring a rib thickness on the order of 1 to 3 m, a 400- or 500-MHz

antenna would probably be the most suitable. A 900-MHz antenna

would be recommended for thinner coal ribs between 0.5 and 1.5 m.

Figure 2 is a schematic of a typical highwall mining scenario using this

radar system to measure coal rib thickness.

Project personnel have developed in-house software that runs on

a laptop PC, which simplifies the normally cumbersome interpretation

of the radar data. The PC reads and analyzes the
Figure 1.—Radar antenna mounted on highwall miner.

Figure 2.—Example of typical highwall mining scenario

using radar system.

received signals in real time, then provides the machine operator with a

number corresponding to the rib thickness, as well as a tricolor display

(green 
 "OK", amber 
 "caution", red 
 "danger") of the computed rib

thickness.

Results

This GPR system was taken to a highwall mining site in Kentucky

to evaluate the feasibility of using GPR to measure coal rib thickness

and to obtain information regarding the depth of penetration and the

effect of airgap on the GPR response. A 500-MHz antenna was used

and found to be capable of measuring coal rib thickness from 0.9 m to

more than 3 m. The best results were obtained with the antenna being

no more than 5 cm from the coal surface. The antenna was positioned

near the top of the coal seam because the cleanest coal was located

there.

The operator prepared two highwall entries, each approximately

15 m long, 3.5 m wide, and 1 m high. The first entry was driven straight

into the highwall. The second entry was driven in about 3 m from the

first and was deliberately angled in such a way to come to within 0.9 m

of the end of the first entry. Several tests were then run by moving the

antenna continuously from the entrance to the end of the entry and then

back to the entrance, with each test having a different airgap (0 cm,

2 cm, and 4 cm). In all cases, the radar was capable of "seeing through"

the coal rib at thicknesses ranging from 0.9 m to more than 3 m;

however, the best response was when the antenna was physically

touching the rib (0 cm - no airgap).

Patent Status

A patent of this total system has been granted (U.S. patent

No. 5,500,649, "Method and Apparatus for Monitoring the Thickness

of a Coal Rib During Rib Formation"); it is available for field testing

and/or licensing.

For More Information

To obtain a free copy of a technical paper on this topic or for

additional information, contact Gary L. Mowrey, Ph.D., National

Institute for Occupational Safety and Health (NIOSH), Pittsburgh

Research Center, Cochrans Mill Rd., P.O. Box 18070, Pittsburgh, PA

15236-0070, phone (412) 892-6594, fax (412) 892-6764,

e-mail: gdm6@cdc.gov

Mention of any company name or product does not constitute

endorsement by the National Institute for Occupational Safety and

Health.

To receive additional information about mining issues or other

occupational safety and health problems, call 1-800-35-NIOSH

(1-800-356-4674), or visit the NIOSH Home Page on the World

Wide Web at http://www.cdc.gov/niosh/homepage.html

As of October 1996, the safety and health research functions of

the former U.S. Bureau of Mines are located in the National

Institute for Occupational Safety and Health (NIOSH).