PME Previous minedentry
CR Coalrib

Mined coat

Fig. 6 —Complete radar system as it would be applied to a highwall-
mining operation.

First Second
coal-air coal-air
Radar interface interface
antenna
Transmit
antenna Previously
Receive cut entry
antenna
Transmit
Pulse (T,)
(+) .
First Second
S reflection reflection (T,)
©
3
2 —
3
€
<

Time (ns) ——-

(-)

Fig. 7 — Basic principle of the radar-based coal rib-thickness-
measurement method.

orthe two-way travel time should be changed to give the proper coal
rib thickness.

Summary and comments

A ground-penetrating radar (GPR) system has been used
by the USBM to verify the feasibility of the radar technique
for determining coal rib thickness in highwall applications at
a selected field site. A prototype GPR-based apparatus has
been developed and constructed. The accuracy of the coal
thickness measurements will be determined by the dielectric
characteristics of the coal at each location.

For the GPR technique to work properly, the main require-
ments are as follows:

* Accontinuous portion of the coal seam should be relatively
clean (at least the width of the antenna, e.g., 0.6 m (2 ft) or
more) with little or no partings or dirt/clay bands.

* Rib thickness should be between 0.6 to 3 m (2 to 10 ft).

* GPR antenna should be within 5 cm (2 in) of the rib.

The advantages of the GPR system mounted on a mining
machine vs. drilling horizontal holes through coal ribs to
determine rib thickness are:

Drilling is not required.

Roof support is not needed.

Workers are not required to enter the highwall.
Measurements do not require stoppage of production.
Measurements are made in a timely manner.

Consequently, this system can be used to control rib
thickness by providing timely information to an operator or
computer, so that the mining machine heading or lateral
position can be altered while continuing to mine coal.

Other benefits that can be obtained using GPR include

* increased safety for miners;

* increased production rates, i.e., the mining machine
does not have to be stopped in order to obtain a coal
thickness reading — thickness readings are continuous
and in real time;

* less downtime due to rib instabilities, i.e., more con-
stant and substantial coal ribs result in more stable
geological conditions;

* deeper penetration into the highwall, owing to the
greater certainty in knowing and maintaining constant
coal rib thickness;

* increased recovery of reserves resulting in higher prof-
its and less waste; and

* increased reserves by being able to mine previously
unminable seams.

The initial findings in this research have shown consider-
able promise in confirming that radar-based coal rib thick-
ness sensor technology has the potential for working in a
highwall mining environment. It is expected that the end
result of this research will lead to the development of a
tugged, low-cost, radar-based rib thickness monitoring tool
that can be routinely used by highwall mine operators.

Inaddition to measuring ribthickness, USBM personnel hope to.
eventually use this type of radar as a CID technique for measuring
roof and/or floor coal thickness as well. This will require visiting
selected mines having various geologies, so that the equipment can
be adjusted for optimum performance.

References

‘Cook, J.€., 1975, "Radar transparencies of mine and tunnel rocks.” Geophysics, Vol. 40,
No. §, Oct, pp. 865-885,

Mowrey, G.L., 1992, “Promising coal interface detection methods," Mining Engineering,
Vol. 43, No.1, Jan.,.pp. 134-138.

Mowrey, GL, Maksimovic, $.D., and Pazuchanics, M.J., 1992, “Staying in the seam: An
‘overview of coal interface detection and steering,” Engineering and Mining Journal, Vol.
193, No. 8, Aug., pp. 26-29,

Research Program Overview," USBM IC 9227, 15 pp.

‘Sammarco, JJ., 1999, "Field Evaluation of the Modular Azimuth and Positioning System
(MAPS) for a Continuous Mining Machine,” USBM, IC 9354, 14 pp.

‘Schnakenberg, G. H. Jt., 1989, “Computer-Assisted Continuous Coal Mining System —
‘esearch program overview.” USBM IC 9227, 15 pp.


radar-absorbing material and placing Cutting head
it in a metal box, the GPR response is
approximately the same as the GPR

response without the metal box and F

radar-absorbing material.

In all three cases, the radar was
capable of “seeing through” the coal
rib at thicknesses ranging from 0.9 m
3 ft) to more than 3 m (10 ft). -

A

Prototype coal rib-thickness
monitoring apparatus Lk

Highwall miner

Boom,

AT] ‘Antenna cable

To operator's hut

nnn

Based on the results of the tests done at
the Kentucky highwall site, a prototype
apparatus was constructed for highwall
coalrib-thicknessmonitoring applications.

NZ,

Control cable:

Radar antenna for miner

Fig. 5 — Conceptual diagram of the radar antenna mounted on a highwall miner.

The apparatus consists of the fol-
Jowing basic components:

* A commercially available pulse-based radar system,
which operates in the 100 to 1,000 MHz range, pro-
vides the transmit pulse and the receiver electronics
(e.g., GSSI SIR-10 system or equivalent).

+ Mine Safety and Health Administration- (MSHA-) ap-
proved barrier box, which provides the necessary electrical
isolation between the radar antennas and radar system.

+ MSHA-approved radar transmit and receive antennas,
which are designed to be intrinsically safe for use in a
coal-mine environment.

* A 386-based (or better) personal computer with color
display, which provides the means for performing
signal processing and displaying rib thickness (abso-
lute or relative) continuously, for the guidance of the
mining machine, e.g., rib thickness displayed in real
time (immediately as the mining occurs).

Software that provides the radar signal processing and

display for the highwall machine operator.

‘The antenna assembly, which contains MSHA-approved radar
transmit-and-receive broad-band bow-tie antennas (e.g.,500-MHz),
is mounted on the side of a highwall miner in a protected enclosure
near thecuttinghead, asshowninFig. 5. Anclectrical cableconnects
the antenna to an MSHA-approved barrier box that is located
outside the highwall entry, as shown in Fig. 6. Other components
include the radar-controller system, operator interface (radar key-
board), personal computer and operator display (monitor). The
radar-control system generates the transmit pulses and receives the
return signals. A specially trained operator makes adjustmentstothe
radar system (including scan rate, scan range, time and amplitude
resolution and receiver amplifier gains) to get the best radar signal
possible. Then, the highwall machine operator periodically views a
computer display monitor that provides the highwall coal rib
thickness in feet and inches (¢.g., the thickness of the coal that exists
between the entry that is being cut and the previously cut entry).

‘The radar-control system sends out a radar pulse through
the barrier box to the transmit antenna. A portion of this pulse
then leaves the antenna and propagates through the air to the
surface of the coal rib. Some of the transmitted radar energy
is reflected at this initial coal-air boundary and is detected by
the receive antenna as shownin Fig. 7. Another portion of this
radar pulse passes through the entire coal rib and a second
reflection is produced at the second coal-air interface. This
second reflection is also detected by the receive antenna,
although it is at a reduced amplitude, depending upon the
thickness and dielectric properties of the coal and other

geological materials present in the coal rib.

The computer program reads a series of radar scans and
averages the scans into a composite scan, which helps to
reduce noise spikes and other signal anomalies. Each scan
consists of the transmit pulse and one or more reflected
pulses. Using the composite scan, the program determines
the maximum peak associated with the transmit pulse and
assigns this point the time of “Tp.” which represents the
starting time. After a preset delay after the transmit pulse, the
program searches for the first peak above a predetermined
amplitude threshold level and computes the time difference
between the two peaks. The program then uses the two-way
travel time for radar waves through coal (e.g., 14.8 ns/m (4.5,
ns/ft)) to compute a coal rib-thickness value. The operator
has the option of using preset (default) values (e.g., 30 scans
per second, 14.8 ns/m (4.5 ns/ft), etc.) or selecting other
values, The operator can also put boundaries on the data, ¢.g..
where “T,” begins, when to start to look for a reflection peak
and what level of threshold is needed for best results.

The optimum procedure in using this apparatus consists of
installing the radar antenna on the highwall mining machine as near
as practical to the cutting head. Another important consideration is
topositiontheantennaasclosetothe coal ribas possibleto maximize
the amountof energy thatcan be transferred intothe coal rib. A third
consideration is to have the antenna mounted on the mining
machine, so that the antenna is positioned in the vicinity of the
cleanest coal in the seam.

“The highwall miner, aftermining the firstentry into the highwall
without radar guidance, is aligned parallel to the first entry. The
highwall miner then begins cutting into the second entry, again
without radar guidance, until the radar antennais inside the highwall
atleast 3 m (10 ft). The operator then adjusts the radar controller to
obtain a good radar reflection signal from the other side of the coal
rib. A good radar reflection signal is needed for this method to work
successfully. Typical radar parameters would be 16-bit data, 512
data points per scan and 30 scans per second. Assuming a two-way
travel time of 14.8 ns/m (4.5 ns/fi) through coal and having a 1.5-m
(5-ft) rib, the total travel time should be on the order of 22.5 ns. The
operator would then use a personal computer to begin reading the
radar dataand computing coal rib thickness. Ifthe computed coal rib
thickness is acceptable, the operator can begin to perform normal
highwall mining activities. Ifit is not acceptable, the operator must
determine if the radar signal looks normal, based on previous
experience. If it does not look normal, the operator should try
readjusting theradar fora goodreturn reflectionin theexpected time
window (€ g.,atapproximately20to25ns fora 1.5-m(5-ft)coal rib).
Ifthe signal appears acceptable and is arriving at the expected time,
but the coal rib thickness, as reported by the radar, is definitely
wrong, then either the amplitude threshold may not be properly set



A radar-based highwall rib-thickness

monitoring system

G.L. Mowrey, C.W. Ganoe and W.D. Monaghan

Abstract — In addition to developing various types of
coal-interface detection systems, the US Bureau of Mines is
actively involved in developing a coal rib-thickness monitor-
ing system for highwall mining applications. One particular
system that shows promise uses a ground-penetrating radar
system to detect the coal-air boundaries of coal ribs. Based
on preliminary laboratory and field tests, results indicate
that this method can be applied successfully in highwall
mines to maintain a constant rib thickness. The coal seam
should be relatively clean in the vicinity of the antenna (e.g.,
have minimal partings or dirt bands) for best results.

Introduction

The US Bureau of Mines (USBM) is developing technol-
ogy for mining machines that will be capable of tracking the
boundaries of a coal seam and extracting only the desired
material (Schnakenberg, 1989). One critical element associ-
ated with providing an automatic seam-tracking control for
the mining machine is a coal-rock interface detector (CID)
system. This CID system must be capable of measuring one
or more distinguishing geophysical properties of coal and the
adjacent formation and then must be able to make intelligent
decisions such as identifying the type of geological material
being cut, locating where the coal-rock interface is and/or
determining the thickness of the coal remaining below the
roof or above the floor.

Due to recent advances in sensor technology and
computer-processing capability, the USBM has been inves-
tigating several CID concepts that can be applied to both
current and future mining systems. The basic concepts pres-
ently being investigated include natural gamma radiation
(NGR), thermal infrared (TIR), ground-penctrating radar
(GPR), mining-machine vibration and video cameras
(Mowrey, 1992; Mowrey et al., 1992). Other concepts being
pursued include motor current/power, displacement rate,
hydraulic pressure and/or flow rate, and total cutting energy.
Mining systems that incorporate one or more appropriate
CID-sensor technologies should be able to extract a higher-
quality product more easily, ata faster rate and with increased
safety. CID sensors could be used on existing continuous-
mining machines, longwall shearers, highwall miners, bucket
excavators and on any other type of surface or underground
mining machines.

In conjunction with this CID research, the USBM is using
GPR to develop a coal rib-thickness monitoring system,
specifically designed for highwall mining applications.

Highwall mining operations

Highwall operators have been looking for a convenient,
cost-effective method for measuring and maintaining a con-
stant coal rib thickness. For example, for any given geologi-
cal conditions, there is an optimum coal rib thickness that is
wide enough to provide adequate support of the ground, yet
does not waste coal. Highwall operators desire to maintain

this optimum thickness throughout the depth of the entry. If
it becomes too thin, there is a high risk of ground fall, which
could result in having to dig or pull out buried equipment and
even in injuries or fatalities. If it becomes too thick, perma-
nently unrecoverable coal is left behind.

The highwall practice now in use is to take accurate
surveys and align the mining machine properly with respect
to these measurements. Current technology limits the depth
of penetration of the cut. Thais is because of inaccuracies in
setting the direction of the equipment and in the ability to
measure and control the direction of the mining machine
(e.g., cuts may intersect, causing an area of unsupported roof
that may collapse onto the equipment), which results in less
coal being recovered. In addition, both the coal geology and
operating characteristics of the mining machine will affect
how well it will track the previous cut (despite how accu-
rately it is initially aligned). One method being evaluated by
the USBM and some mining companies is the use of a highly
accurate inertial-guidance system developed for military
applications (Sammarco, 1993). This technology, based upon
ting-laser gyroscope (RLG) technology, has an azimuth
accuracy of about 0.028°, which translates to about 30 cm in
uncertainty at a distance of 300 m. Some success has been
achieved by using the RLG in either of two ways:

* toset the initial azimuth of the machine equal to that of
the prior entry and ensuring the machine heading
remains at that value during the first few fect of the cut;
or

* to monitor and correct mining machine-direction
throughout the length of the cut.

The accuracy of these procedures has not yet been fully
determined. Disadvantages of the RLG are its cost (greater
than $100,000), an initialization process that requires an
accurate surveying of the RLG coordinates, a periodic re-
quirement (for full accuracy) of a few seconds of non-
production activity every minute or so to allow the RLG to
rereference itself and that the RLG does not produce a direct
measurement of rib thickness. Ignoring the initialization
procedures reduces the RLG accuracy. If the requirement of
periodic referencing is ignored, the accuracy of the RLG will
degrade to a drift of 0.015 to 0.020 m/hr.

Asa possible alternative to the RLG, the USBM is concur-
rently investigating the application of ground-penetrating
radar (GPR), which is capable of seeing through the coal
seam and measuring the coal rib thickness to the previously
cutentry. The advantages of GPR are that it directly measures

G.L. Mowrey, C.W. Ganoe and W.D. Monaghan are electronics
engineer, electronics technician and electrical engineer, respec-
tively, with the US Bureau of Mines Pitssburgh Research Center,
Pittsburgh, PA. SME Preprint 95-163, SME Annual Meeting, March
6-9, 1995, Denver, CO. Manuscript Feb., 1995. Discussion of this
peer-reviewed and approved paper's invited and must be submitted,
in duplicate, prior to Oct. 31, 1996.


the coal rib thickness between the mining machine and the
previously cutentry and that the current cost is about one-fourth.
that of an RLG system. The main disadvantage of the GPR is
that some coal seams are “dirty”, i.e., that there are partings
and dirt bands in the coal that can attenuate the GPR signals,
so that the reflected echo signals are too small and/or noisy to
be used to determine coal rib thickness.

Ground-penetrating radar system

Background. The idea that radar could be used to monitor
the thickness of coal ribs was based on research done by Cook
(1975). Cook found that clean coal is relatively transparent to
ground-penetrating radar (GPR), The success of a radar
system at a given location primarily depends upon two
electrical parameters of the material being tested, namely,
electrical conductivity and dielectric constant.

Electrical conductivity is the ability of a material to
conduct electrical current, and it determines the depth of
penetration of the radar signal, i.c., greater conductivity
results in lesser depth of penetration, Conductivity, ex-
pressed in Siemens per meter (S/m), can vary greatly for
geological materials and is primarily governed by the water
content and the amount of dissolved salts, as well as the
density and temperature of the material.

The dielectric constant ¢, is a dimensionless measure of
the capacity of a material to store a charge when an electrical
field is applied, and it can range between one (air) and 81
(water). The €, ranges of materials found in coal mines are as
follows: coal, 4 to 5; saturated silt or clay, 8 to 12; dry sand,
4 to 6; water-saturated sand, 30; and limestone, 7 to 9. It is
important to note that radar signals are reflected by changes
inthe dielectric constant in its path (¢.g., by different geologi-
cal boundaries). Larger changes in the dielectric constant
over shorter distances result in larger reflections. Reflections
are even greater if the boundary is nearly planar and perpen-
dicular to the direction of the radar. The coal-air boundary of
the coal rib provides an excellent condition for radar reflec-
tions. In addition, layered geological formations, typically
found in coal-bearing strata, lend themselves to being de-
tected and measured by radar techniques. Hence, aradar-based
system could also be used to measure thickness of coal left on
the roof or floor, if mining practices required that to be done.

Knowing the dielectric constant of the material (e.g., coal)
will provide velocity and travel time information. Con-
versely, by measuring the travel time and measuring the
distance between the antenna and the boundary of interest,
the dielectric constant of the material can be determined. For
‘example, the velocity of the radar signal through a particular
geological material is related to the dielectric constant:

Vn =O! a)

in m/sec, where

V.q = radar signal velocity in the material (m/sec);

c = speed of light in a vacuum (3 & 108 m/sec); and
€, = dielectric constant of the material.

The travel time slowness (reciprocal of velocity) is also
related to the dielectric constant as follows

TT =3.33x+pe, 2)
where TT = travel time slowness (in ns/m).

The two-way travel time is what is actually seen ona radar

Fig. 1 — Diagram of prepared highwall field site for radar testing,

plot, i.e., outgoing travel time plus return signal travel time
Two-way travel time per meter of depth is

2x3.33x+ge, (3)
in ns/m.

Another consideration is the effect of air gap that exists
between the radar antenna and the formation. Ideally, the
antenna should be placed as close as possible to the formation
being tested for optimum coupling and to minimize energy
losses due to significant dielectric-property contrasts be-
tween air (€, = 1) and various geological materials (e, = 4 to
6). The further the antenna is away from the formation, the
greater the amount of signal attenuation; generally, the spac-
ing should be no more than 10 cm (4 in) away from the
formation.

System description. A commercially available
pulse-based radar system, the SIR-10, made by Geophysical
Survey Systems, Inc. (GSSI),! can provide instant continu
ous-profile radar records (in either color or black and white)
that indicate the location and depth of strata and buried
objects (buried up to more than 6 m (20 ft), depending upon
the antenna frequency and target size and geometry) within
the subsurface of soil, sand, concrete, asphalt, rock, water, ice
and other materials, Profile data are displayed on an EGA
(640 x 350 pixels) color monitor in either a color line-scan or
a black-and-white wiggle-plot format. Scan rates can be set
from 3.2 to 256 scans per second with either 128, 256, 512 or
1,024 samples per scan. Signal amplitudes can be set at either
8-bit or 16-bit resolution, Either manual receiver-gain adjust-
ment or an automatic gain can be used, with up to eight
individually adjustable gain points. Collected radar data can
be stored on a 2.3-GB internal cassette-tape drive.

The USBM currently has radar antennas that operate at
300, 500, 900 and 1,000 MHz. Depth of penetration is
primarily dependent upon the antenna frequency (i.e., as the
frequency increases, the depth of penetration decreases). For
example, the S00-MHz antenna has been found to have a
range of approximately 0.5 to 5 m(1.5to 15 fi) incoal and the
900-MHz antenna has a range of approximately 0.2 to 1.5m
(8 in. to 5 fl) in coal,

"Reference to specific products does not imply endorsement by the USBM.


(TINE - nanosuconds)

Fig. 2 — Wiggle-line radar plot of Test 1 (no air gap between antenna and coal rib),

book

(TIME ~ nanoseconds)

Fig. 3—Wiggle-line radar plot of Test 2 (10-cm (4-in.) air gap between antenna and coal rib).

(TIME ~ nanoseconds)

Fig. 4 — Wiggle-line radar plot of Test 3 (5-cm (2-in.) air gap between antenna and coal rib).

GPR test at highwall site

The SIR-10 system was taken to a
highwall mining site in Kentucky in
March 1993 for the purpose of evaluat
ing the feasibility of using GPR to
measure coal rib thickness and to ob-
tain information regarding the depth of
penetration and the effect of air gap on
GPR response. A 500-MHz antenna
was used and was found to be capable
of measuring coal rib thickness from
0.9 m (3 ft) to more than 3 m (10 ft).
The best results were obtained with the
antenna being at no more than 5 cm (2
in) from the coal surface. The antenna
was positioned near the top of the coal
seam, because the cleanest coal was
located there.

The highwall mine operator pre-
pared two highwall entries, each being
approximately 15 m (50 ft) in length,
3.5m (11.5 ft) in width and 1 m (3.2 ft)
high (Fig. 1). The firstentry was driven
straight into the highwall. The second
entry was driven in about 3 m (10 ft)
from the first and was deliberately
angled in such a way as to come to
within approximately 0.9 m (3 ft) of
the end of the first entry. The overbur-
den consisted primarily of shale and
was approximately 15 m (50 ft) thick.

USBM personnel ran several basic
radar tests. The first test consisted of
placing the 500-MHz antenna directly
against the rib wall in the angled entry
and moving it continuously from the
entrance to the end of this entry and
back to the entrance. A wiggle plot of
the entire test is shown in Fig. 2. For
the next test, a 10-cm (4-in) piece of
Styrofoam was placed in front of the
antenna, and the antenna was continu-
ously moved from the entrance to the
end and back to the entrance (Fig. 3).A
third test consisted of replacing the
10-cm (4-in) piece of Styrofoam with
a 5-cm (2-in) piece of Styrofoam and
rerunning the same test a third time
(Fig. 4).

The amplifier gain settings were
adjusted for optimum performance for
the first test and were kept at the same
values for the remaining tests. Also,
due to the difficult operating condi-
tions at the site and inclement weather,
USBM personnel decided to run the
antenna without its surrounding metal
shield and a radar-absorbing material,
which would be needed when it was
mounted on a mining machine. Labo-
ratory and underground GPR tests at
the USBM have shown that, by sur-
rounding the 500-MHz antenna with 5
to 10 cm (2 to 4 in) of the