METERS

TIME, ns

1

TTT
2.74
2.44 +40
2.15 HIN
1.85 +30
1.55
1.254 Hy Hq i an HATE 20
0.944
0.65 10
MoO SS (
0.34
9 0
) 15 30 45 60 7.6 \9.0 10
DISTANCE, m Roof line

Figure 15.—600-MHz GPR record at the SRCM with overlay of lithology.


INTRODUCTION

Groundfalls accounted for almost one-half of fatalities in
US. underground coal mines in 1987-94 (1, p. 2; 2). Ad-
ditionally, the average cost per underground accident resulting
in a permanent disability approached $500,000 in 1990 (3). In
underground limestone mines, 11 miners were killed and 9
injured since 1985 in falls of ground (4, p. 2). The risk of
serious injury to a coal miner is greatest at the working face.

When humans are relocated away from the working face, the
traditional methods of mine roof stability monitoring are also
removed. There are many indicators that 8 miner or bolter
operator would use to determine whether a roof is unsound,
Initial flaking or spalling of immediate roof may damage the
machine or indicate imminent larger failure. ‘The sound of
"working" roof is an important indicator. Roof sag or bed
separation over time may indicate instability. "Cutter" roof, a
failure of weak rock due to horizontal stress from weak rock, is

GROUND PENETRATING

GPR is a geophysical technique based on radar technology
and is primarily used to detect shallow subsurface objects, such
as pipes, barrels, saturated zones, water- or air-filled mine
openings, caves, or fault planes (5). New USBM applications for
GPR have been to monitor highwall thickness and the extent of
a burning waste bank (6-8).

An antenna is moved across the surface of interest, and an
electromagnetic pulse is transmitted into the ground at set
intervals (e.g., 30 times per second). The pulse travels through
the subsurface until it encounters an object of dissimilar electrical
properties (e.g., dielectric constant) compared with the host
material. Provided the pulse strikes an object, part of its energy
is reflected back to the surface while the remaining energy
continues to propagate through the subsurface materials. The
reflected energy is detected by a receiving antenna at the surface.
For each transmitted pulse, the resultant reflected signal is
detected, digitized, and displayed and/or plotted as a trace.

The lineup of the disturbance, variations of adjacent traces
while the antenna is being moved over the surface at a uniform
speed, creates an "image of the subsurface" as shown in figure 1.
The depth to the reflecting surface is determined by dividing the
two-way travel time to the object by two times the velocity of the
electromagnetic wave through the ground. Different earth
materials have different electromagnetic wave propagation
velocities, which are determined from the dielectric constant of
each type of material. It is also possible to estimate the velocity
of a material by using an appropriate dielectric constant for that
material if known from previous experience or available from a
dielectric table for geological materials.

“talic numbers in parentheses refer to ilems in the list of references at the
end of this report.

often a precursor to failure. Lithologic parameters, e.g., a
thickening claystone, may cause the roof bolts to anchor in weak
strata. A roof bolt operator can usually detect these lithologic
changes by drill cuttings or vibration changes, Staying in the
coal seam horizon and leaving a specified amount of roof coal
may also be critical to roof control. An operator can often tell
the correct horizon by some marker within the coal seam (i.e.,
parting).

Technology is now available that may be able to enhance the
ability of mines operators to detect roof hazards. One promising
technology is ground penetrating radar (GPR). This report
presents the results of field testing of GPR by the U.S. Bureau
of Mines (USBM) Pittsburgh Research Center for its application
in providing early waming to underground mine workers of
potential roof hazards, thereby improving mine safety.

RADAR (GPR) THEORY

‘The strength of the reflection is influenced by the differences
in dielectric constant at two material boundaries. For example,
the best-quality data are obtained by the reflections of surfaces
or objects that have large material contrasts compared with the
surrounding material and when the dielectric constant of the two
materials is largely different (c.g., water-filled fractures, buried
bartels, water table, concrete foundations, or open mine voids).

ia | | Control

ll

\
L display

/ ry unit

Radar antenna dl
I

YNZ

0 my

Figure 1—GPR ray paths penetrating the ground and producing a wiggle
trace on the control panel.


between the radar record and stratascope log. The location of the
rider seam is important to roof control at this mine. The roof
bolters track the rider position with blind test holes on regular
intervals to avoid anchoring roof bolts into the rider seam.

‘Again, the larger open fractures are more prominent reflecting
surfaces. However, it is clear from the radar record that the
quality of the data is not good enough to be able to distinguish
individual lithologies other than these noted.

- 68mm

Figure 7.—Stratascope image of large open crack 1.7 m (5.5ft) into the
root at Lake Lynn limestone mine at hole No. 3.
3.0 ] 50
40
}- 30
2
Eas c
oo 2
= -
20
t- 10
i |
0 . i)
0 3.0

Roof line

DISTANCE, m

Figure 8.—GPR record at hole No. 2 at Lake Lynn showing crack
reflections.

Safety Research Coal Mine

‘The response of GPR was also tested in the roof of the USBM
Pittsburgh Research Center's Safety Research Coal Mine
(SRCM) at Bruceton, Allegheny County, PA, in the Pittsburgh
Coalbed. Its roof has been well documented. It typically
consists of 15.2 cm (6 in) of head coal, 0.9-1.5 m (3-5 ft) of
interbedded coals and shales, 1.8-2.4 m (6-8 ft) of sandy shale,
and >0.9 m (>3 ft) of sandstone. This site was chosen because
there are no constraints on access. Additionally, a 5.1-cm (2-in)
corehole was drilled to 4.6 m (15 ft) in the roof to verify radar
records. Figure 12 shows the mine layout and the GPR survey
site, as well as the confirmation corehole. Several different radar
parameters were used to gather data. These include 300-, 500-,
900-MHz antennas, static (stationary) and dynamic (moving)
tests, and expanded scales with reduced ranges. Figure 13 shows
‘the roof lithology at the location of the radar surveys determined
by roof core. Two features are prominent. First is a typical rider
coalbed from 0.3-0.8 m (1-2.5 ft) into the roof. Secondly, a
sandstone bed occurs from 4,0-4.6 m (13-15 ft) up into the roof.
Both of these beds may be expected to be good reflections and
be prominent on the radar record.

‘The 900-MHz antenna generally provides the best resolution,
but sacrifices depth of penetration. It seems best suited to zones
of 0.9-1.2 m (3-4 ft) into the roof (figure 14). The higher
frequency radar detects discontinuities (most likely bedding)
within the strata and generates numerous reflections. These
minor reflections may be mistaken for significant features, like
faults or lithology changes. Additionslly, no prominent
reflections are present to signify the rider coal seam. The rider
seam may be important for strata control for several reasons,
including bolt anchorage and potential head coal.

Figure 15 is a radar record made with a S00-MHz antenna,
which has a range of 3 m (10 ft). Starting at the roof line (0m),
the black band is the initial pulse reflection. Above that, a
reflection band represents the rider bed at 0.3-0.9 m (1-3 ft).
Again, the rider sequence shows as a smeared-out band instead
of the sharply defined boundaries of the rider bed. A low-angle
bedding shear at approximately 2.1 m (7 ft) corresponds to a
weak reflection, but this could be coincidental. Reflections
throughout the record above 0.8 m (2.5 ft) do not represent
significant discontinuities and, therefore, in the absence of
ground truth could be misinterpreted.

Figure 16 is a record generated by a 300-MHz antenna with
arange of 6.1 m (20 ft) into the roof. This antenna theoretically
has the poorest resolution, but has the greatest range. The
overlay shows an offset correlation of the rider bed with the
radar record. At 4-4.6 m (13-15 ft) up into the roof, there is a
sandstone bed. As a reflector this bed should stand out. There
is a good reflector in the general vicinity at 3.8 m (12.5 ft), just
offset from the location of the sandstone.

An explanation for this offset and others is that the
assumption of using one dielectric constant for the entire roof
sequence is not a good one. There are numerous rock types in
the sequence with presumably different dielectric constants. A
more accurate measure of their electric properties may allow
more accurate matching of the lithology to the travel time GPR
record.


electrical conductivity controls the depth of pulse penetration.
‘The lower the conductivity of the material, the deeper the radar
pulse can penetrate. The conductivity is controlled by the water,
mineral, and clay content in the subsurface. The depth of
penetration of the radar pulse also depends on the antenna
frequency. Higher frequency antennas (e.g., 900 MHz) provide
high resolution, but shallow depths of penetration; conversely,
lower frequency antennas (¢.g., 300 MHz) have low resolution,
but can detect significantly deeper targets.

FIELD SITES

Four field sites were chosen to test GPR. A stack of
sandstone slabs from a Pennsylvania quarry was chosen as a
physical model to represent air gaps between roof units. A
limestone mine was chosen because of the relative simplicity of
the roof. At this site, only limestone was present in the
immediate roof. The remaining field sites were two
Pennsylvania coal mines in two different seams representing
common roof conditions.

Collier Stone Quarry

‘The Collier Stone Quarry in Allegheny County, PA, is mining
the Morgantown sandstone primarily for building facing and
decorative pieces. A stack of seven cut slabs (259 by 160 by 6.4
cm (102 by 63 by 2.5 in)), with slabs separated by 6.4-cm (2.5-
in) blocks, was used to model a sequence of coal mine roof
members (figure 3). The separation between the slabs was
intended to simulate bed separation and air gaps. Although
separation in an actual roof sequence rarely exceeds 1.27 cm (0.5
in) without feilure, the larger gaps were intended to
accommodate a metal target inserted between the slabs. Figure
3 shows the stack of sandstone slabs with the 900-MHz antenna
attached to the control unit recording data from the top of the
sandstone stack. Radar data were recorded as the target was
inserted between the slabs beginning at the bottom of the stack

Figure 3—-Collecting GPa data from a 900-MHz antenna through a stack of
‘sandstone slabs,

and ending at the top. Figure 4 is the resulting radar record.
Overlain on the record is the "known" sandstone stack
configuration. In this instance, the antenna remains stationary on.
the top of the stack. The resulting radar record is a time record
on the horizontal axis and not a measure of horizontal change in
material properties as in a traverse.

‘The overlay was constructed by scaling the total height of the
stack (84 cm (33 in)) to match the total height range of the GPR
survey (12 ns). The top of the stack is visually picked on the
GPR record by the first reflection. Targets are identified by
disturbances in the record, which represent reflections of the
pulse. Whereas the general target locations are easily identified
as significant disruptions in reflecting boundaries on the radar
record, it is difficult to delineate the individual slab (6.4 cm
(2.5 in) thick) and air gap (5.7 cm (2.25 in) thick) boundaries.
The record shows numerous "false" slab boundaries and gives
the appearance of numerous stacked slabs. This is probably duc,
again, to interference of reflected energy from the top and
bottom of slabs. This problem of complicated geometry will be
scen again in the coal roof test sites. Part of the problem lies in
the interpretation criteria, When using the dynamic record, the
eye is drawn to arcas of high- and low-signal amplitude.
Postprocessing can eliminate many extraneous interfaces. The
interfaces between air and sandstone slab arc not clearly
delineated. Without the benefit of the known stack boundaries,
it would be difficult to accurately recreate the stack. It can also
be seen that all radar data are garbled below the target. This may
be explained by transmission interference and reverberations of
energy that have passed through the target. The remaining
energy is reflecting back to the surface, but is also reflecting back
off of the target bottom. As mentioned before, it is the contrast
in dielectric constant between components that makes a good
reflecting surface. Conducting materials present an effective
barrier to radar transmission (i.¢., water-bearing zones, pipes).
For this reason, the metal foil of the target was easily seen. The
metal target also formed an effective barrier to radar transmission
to reflecting surfaces below it. That is why the reflecting
surfaces below the target are obscured. Although the contrast
between sandstone (5 ns/0.3 m (1 ft)) and air (2 ns/0.3 m (1 ft))
is great, the complex reflecting patterns obscure the reflecting
surface. The sandstone stack is a complicated series of reflecting
surfaces that is not easily delineated by GPR.

Lake Lynn Limestone Mine

To avoid the complex interface problems experienced at the
stone stack at the Collier Stone Quarry, a test site was selected in
the roof of the USBM's Lake Lynn underground limestone mine
near Fairchance, Fayette County, PA. The mine is located
geologically in the Greenbrier limestone formation and consists
of mainly limestone with interbedded shales. The mine roof is
one single member, a pure limestone up to the limit of the radar
penetration of 6.1 m (20 ft). Figure 5 shows the location of two
GPR survey lines run across the roof, Four roof holes, which


Orientation of the boundaries with respect to the plane of the
antenna also affects the reflected signal strength; boundaries that
are perpendicular to the pulse direction produce the strongest
reflections.

Several equipment parameters also influence data quality.
‘The frequency of the transmitted wave influences both the depth
of penetration and resolution of the pulse energy, which in
turn influences the reflected record. For example, a

“higher-frequency" antenna (e.g., 900 MHz) has three times the
resolution, but one-third the depth of penetration, compared with
a "lower-frequency” antenna (300 MHz). Other factors that
influence data quality include amount of contact with (or air-gap
distance from) the surface, electrical interference from cultural
noise sources (e.g., buried pipes, cables, trenching, power lines,
radio/TV transmitters, etc.), and maintaining a uniform speed
when moving the antenna along a straight line.

FIELD TEST OF GPR

The purpose of the field studies was to determine whether ~

GPR could identify potentially hazardous roof situations. The
sensing goals for GPR are to—

1. Identify bed separations 6.4 mm (0.25 in) and larger.
Bedding separation is a precursor to bed "unraveling," which
may result in a roof fall.

2. Determine if the common coal measure roof interfaces
(shale/coal, sandstone/shale, sandy shale/claystone, etc.) would
be distinguishable with GPR. A continuous roof profile would
be invaluable in assessing roof hazards.

3. Identify large structures in the roof that may indicate
hazards (e.g., sandstone channels, faults, shears, fault gouge
zones, open fracture zones).

4. Concentrate on identifying roof hazards within the bolted
interval (2.4 m (<8 ft)), but also up to 6.1 m (20 ft) into the
roof,

To set up the most practical test of GPR data quality, it was
decided to—

1. Use earth materials.
2. Utilize a physical model to eliminate complex geology.
3. Conduct underground experiments under similar

conditions in which the instrument would be working.

DESCRIPTION OF THE INSTRUMENTATION

Following is a brief description of the radar system used in
the study.

The SIR-2 is a lightweight, portable, general-purpose radar
system (figure 2) (5). An electromagnetic transmit pulse
generated by the system is injected into the subsurface via an
antenna that is moved along the surface at a uniform speed and
direction. The transmitted energy of the pulse is radiated in an
elliptical conical pattern roughly 90° front to back and 60° side
to side. The transmit pulse encounters different materials in the
subsurface, each of which has different diclectrical properties.
Wherever there is a change in the subsurface material and
dielectric constant, a portion of the pulse energy is reflected back
to the surface and is detected by the receiving antenna. This
reflected pulse provides information regarding two-way travel
time and attenuation characteristics (signal strength) associated

with the subsurface materials, The received retum pulse is then
processed by the control unit in the radar system, and the data are
displayed on a monitor and/or stored on an internal hard disk.
The output display can be (1) a single wiggle trace (analogous to
an oscilloscope trace), (2) a waterfall plot of these wiggle traces,
or (3) a multicolored line scan in which reflected signal
amplitudes are represented by various colors according to a user-
selected color look-up table. The data can also be printed via an
extemal printer.

‘The two-way travel time is determined by measuring the time
interval between the start of the transmit pulse and the start of the
received reflected signal. The amplitude of the reflected signal
is influenced by the size and geometry of the target, the signal
attenuation characteristics of the geological materials, and the
total distance that the pulse has to travel.

How well GPR works depends on two electrical properties of
the geological materials under investigation: dielectric constant
(relative dielectric permittivity) and electrical conductivity. The
diclectric constant affects the velocity of propagation of the radar
pulse. The values of the dielectric constant range from 1 for air
(fastest propagation) to 81 for water (slowest propagation). The
greater the difference in dielectric constant between two
materials, the stronger the reflected pulse energy becomes. The

Antenna

ee ee

Control Unit

Figure 2.—SIR-2 GPR system.


roof bolting, but the radar record indicates a slab anywhere from
0.6-2.4 m (2-8 ft) thick. Roof bolts would have to be at least
2.4 m (8 ft) long to consider the worst case. By contrast, an
2.4-m (8-ft) bolt could be excessive if the slab was 1.2 m (4 ft)
thick.

Scale, m

io) 60

Li.
Legend

‘4 GPR Survey
@3 Test hole

—4— Support wall

3 Sinkhole

Figure &—GPR survey lines and strataecope holes at Lake Lynn limestone
mine.

Legend

+H Crack

~«. Shear

@@ Open crack

RoxCoal Coal Mine

Because the GPR system is intended to be used to sense
potential coal mine roof hazards, a survey was conducted on an
actual coal mine roof, A site was chosen in a section that was
being driven in RoxCoal Coal Mine, Somerset County, PA, The
section was planned as a future pillar section, and instruments
were being placed in the roof to monitor roof bolt performance.
The section was chosen because it was the area with the most
roof damage. It was hoped that this damage would show up on
the GPR record. Figure 10 shows the damage to the roof, mostly
in the form of cutter roof, and the location of the GPR survey
lines and the stratascope hole used for verification. A total of 12
GPR traverse lines were run: 6 used the 500-MHz antenna; 7
used the 300-MHz antenna.

The best data were obtained from location 4 using the 300-
MHZ antenna. Figure 11 shows a thick reflecting layer from 0.7-
1.4 m (2.3-4.5 ft) up into the roof. Another prominent reflecting
layer occurs from 3.2-3.5 m (10.5-11.5 ft). Verification of the
roof features was obtained from a stratascope log of a nearby test
hole, which was overlain on the radar data. There are a number
of cracks logged, as noted up to 3.7 m (12 ft) in the test hole.
From 0.8-1.1 m (2.5-3.5 ft), there are several large open cracks
in the bolted roof. One crack has water leaking from it. It
appears that the broad radar band is a reflection surface for this
fracture zone. Again, these bands are broad and "smeared" out
and represent only the same "zone" as the fractures. Other
fractures at approximately 1.8, 2.1, and 2.4 m (6, 7, and 8 ft) are
not represented as reflection surfaces. The band on the radar
record at 3.2-3.5 m (10.5-11.5 ft) is the reflection surface of the
rider seam above the main seam. There is good correlation

#2 E55 Limestone #3 #4

C > 73 "CO 7.5 "o>
4.0 mt Lot PyToy
3.0 m4 3.0m4 3.0 m4

J 1 76-35 mm
1.5m 1.5 m4 > 1.5 mothe

] —

Roof Roof — Roof

Figure 6.—Statascope logs of bed separations in the roof at Lake Lynn limestone mine.


RI

9625 REPORT OF INVESTIGATIONS/1996

Using Ground Penetrating Radar for Roof
Hazard Detection in Underground Mines

By Gregory M. Molinda, William D. Monaghan, Gary L. Mowrey
and George F. Persetic

UNITED STATES DEPARTMENT OF ENERGY

PITTSBURGH RESEARCH CENTER



96
~~ Coalbed
I-75
eae ea
2.47 :

®
4 i—
I-50 wh
Cracks =
Fé

-25

‘
Oo O06 1.2
DISTANCE, m

1.8 24 3.0 3.7

Figure 11.—GPR record of immediate roof at location 4 in
RoxCoal Coal Mine with lithology and roof cracks from
stratascope hole overprinted.

7

o0
U0

7
|
|

E

O00
Arar

a

Legend
4 GPR Survey
© Test hole 5.6-m high

° 30 2

Scale, m a
Mine entrance

Figure 12.—GPR survey line and corehole In immediate roof
at the Safety Research Coal Mine (SRCM).

Legend
WE Coal

METERS

Sandstone
E=3 Shale
[= Black shale

| Shear

EXX9 Missing

Figure 13.—Lithology In the immediate roof at the SRCM.


METERS

oO
©

ee

a HN
if i 1 I 1 i}
0.6 0.9 1.2 1.5 1.8 2.1 2.4
DISTANCE, m
Figure 9—GPR record at hole No. $ at Lake Lynn,
Key
©-~Radar survey
ete Cutter roof
—t_ shear
Scale, m
0 1.5 3.0
oar)

Figure 10.—GPR survey lines and roof damage at RoxCoal Coal Mine.

-40

TIME, ns


GPR
Antenna

METERS

Key

at Air space
iY Sandstone

TIME, ns

[zz] Metal targets

| Ground

Figure 4.—Radar record of Collier Stone Quarry sandstone stack with actual slab locations drawn as overlays.

were used for instruments that measure strata movement, are also
shown. Three of these holes were logged with a stratascope to
verify any roof features such as shears, joints, or bed separations.

Figure 6 shows the roof features in the logs of stratascope
holes Nos. 2-4, There was no access to hole No. 1 because of
ground deformation instrumentation. A prominent fracture
shows up in all three holes at 1.5-1.7 m (5.0-5.5 ft). This fracture
is a large open break in hole No. 3, with broken rock and water
leaking out. Figure 7 is a stratascope photograph of the feature.
The structure is a bed separation with a joint abutting and
stopping. Small rock fragments are also present. Other smaller
fractures occur from the roof up to 1.7 m (5.5 ft), as detected by
stratascope. There are no fractures detected above 1.7 m (5.5 ft).
The large fracture at 1.7 m (5.5 ft) depth appears to be an
extensional break duc to roof sag, and the fracture appears on the
radar record at hole No. 2 at approximately 1.5-2.4 m (5-8 ft)

(figure 8). The data were taken with a 500-MHz antenna with a
range of 3 m (10 ft) into the roof. Figure 9 is the GPR record for
the survey at hole No. 3. The fracture at 1.7 m (5.5 ft) appears
28 zone covering approximately 15-30 ns (0.6-1.5 m (2-5 ft)).
No other features appear clearly on the record.

Because the roof consists only of limestone, there is none of
the bedding interference as seen with the sandstone air stack at
the Collier Stone Quarry. The fracture appears to have made the
reflection on the record, but it appears as a multiple reflection
and not the clean interface seen in figure 7. This is probably due
to the fragmented nature of the bed separation and joint
intersection. Better resolution of this large feature is necessary,
but this "recognition" is useful. From a data-quality standpoint,
the contrast in material properties from a fracture in a single
media is a good situation for radar reflection interpretation. A
possible remediation for this 1.5-m (S-ft) thick slab would be


USING GROUND PENETRATING RADAR FOR ROOF HAZARD
DETECTION IN UNDERGROUND MINES

By Gregory M. Molinda, William D. Monaghan,’ Gary L. Mowrey’ and George F. Persetic®

ABSTRACT

Ground penetrating radar (GPR) has been investigated by the U.S. Bureau of Mines Pittsburgh Research
Center for its potential to determine roof hazards in underground mines. GPR surveys were conducted at four
field sites with accompanying ground truth to determine the value of GPR for roof hazard detection. The
resolution of the current system allows detection of gross roof fractures (>6.4 mm (>0.25 in) zone) or rider beds
in coal measure roof. Data quality is not yet sufficient to detect small bed separations or subtle lithologic changes
in the roof. Differences in data quality are discussed, as well as suggestions for collecting improved data.

‘Geologist.

*Blectrical engineer.

*Physical science technician,

Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh PA.


rm to
20 Jor ck shale
an
roe
2 UU mn DSN
zi cee |,

Figure 16.—S00-MHz GPR record at the SRCM with overlay of iithology.


14

CONCLUSIONS

GPR surveys were conducted at four field sites to determine
the value of GPR for detection of roof separations, defects, or
lithologies that might lead to hazardous mine roof failures. All
surveys were supported with roof cores or stratascope holes for
confirmation of radar results. With the present system (SIR-2),
the most successful survey results occurred at the Lake Lynn
limestone mine and RoxCoal Coal Mine. At Lake Lynn, a
relatively large (1.27 cm (0.5 in)) bed separation zone was
detected by GPR at 1.5-2.4 m (5-8 ft) up in the roof. This was
confirmed in three local stratascope holes. At Roxcosl Coal
Mine, several cracks were detected at 0.6-0.9 m (2-3 ft) into the
roof. Also, at 3.4 m (11 ft) into the roof a 30-cm (12-in) rider
coalbed was indicated. This was also confirmed by stratascope.

Although success was reported in these instances, there were
several ambiguous or unsuccessful cases in the form of missed
‘cracks or lithologies. At the SRCM, a rider coalbed (0.6 m (2 ft)
thick) and sandstone bed (0.6 m (2 ft) thick) were vaguely
detected by GPR. At the Collier Stone Quarry, the interfaces
between stone slabs were detected, but sharp boundaries were

difficult to discem. Several instances of "false" bedding were
interpreted from the data (e.g., RoxCoal Coal Mine).

Three areas govern the usefulness of GPR for mine roof
hazard detection. First, the quality of the data collection is
determined by the instrument components, the type of radar
pulse used, and signal enhancement (gain, filtering). Second,
postprocessing enhancement of the radar record can highlight
significant reflectors. Third, interpretation of the GPR record
will assign physical structure (e.g., lithology, bed separation,
etc.) to the reflections. All of these areas should be addressed as
variables for study, but interpretation remains one of the most
difficult. Perhaps the most productive direction for GPR
development may be in calibrating the instrumentation on
well-defined, well-documented structures (e.g., sandstone
channels, bed separations, coalbed/rock interfaces). By looking
for a limited number of specific structures, interpretation of GPR
records will become more manageable.

With future development, GPR appears to have potential
application as a fast, noninvasive, high-coverage roof hazard
detection system.

REFERENCES

1, Peters, R. H. (comp.) Improving Safety at Small Underground Mines.
Proceedings: Bureau of Mines Technology Transfer Seminar. USBM Spec.
Publ. 18-94, 1994, 174 pp.

2, MSHA REAP Siatistcs, July 6, 1995, Available from Tony Turyn, Roof
‘Bvaluation - Accident Prevention (REAP) Coordinator, MSHA, 4015 Wilson
Bivd., Arlington, VA 22203.

3, Chi D.N.H,, and B, Hamilton. Trends in Mining Accidents and Their
‘Costs (1975-1989). Part 1: Breakdown by Mining Areas. Presented at the 19th
International Symposium on the Application of Computer Methods in Mineral
Industries, PA State Univ., University Park, PA, Apr. 14-18, 1986, 26 pp.
Available from G. M. Molinda, Pittsburgh Research Center, Pittsburgh, PA.

4. lannacchione, A. T., T. P. Mucho, and L. J. Prosser. Ground Control
Problems in the Underground Limestone Industry. Presented at the Proceedings
of the Environment, Safety, and Health Forum (National Stone Asso.),
Nashville, TN, Oct. 22-24, 1995, and the Pennsylvania Ground Coatrol

Seminar, Somerset, PA, Nov. 1, 1995, 16 pp. Available from A. T.
Tannacchione, Pittsburgh Research Center, Pittsburgh, PA.

5. Daniels, J. J., and R. L. Roberts. Ground Penetrating Radar for
Geotechnical Applications in Geophysical Characterization of Sites (ed. by
R. D. Woods). Balkema, 1994, pp. 1-13.

6. Subsurface Interface Radar Systems. Training Notes. Geophysical
‘Survey Systems, Inc., v. 1.1, Mar. 1991, 27 pp.

7. Mowrey, G. L., C. W. Garal, and W. D. Monaghan. A Radar-Basod
Highwall Rib Thickness Monitoring System (Denver, CO). SME preprint No.
95-163, 1995, 8 pp.

8 Mowrey, G.L. Ground Penetrating Radar and Infrared Thermography
Tests at a Burning Coal Waste Bank. Paper in Proceedings of the National
‘Assoc. of Abandoned Mine Lands Program, 17th Annual Conf,, Freach Lick,
IN, Oct. 18, 1995, pp. 87-105.



CONTENTS

Applicability of GPR to coal mine roof hazard detection .

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Ground penetrating radar (GPR) theory
Field test of GPR
Description of the instrumentation
Field sites ............

. Lithology in the immediate roof at the SRCM ...........0006
}. 900-MHz GPR record at the SRCM with overlay of lithology
. 500-MHz GPR record at the SRCM with overlay of lithology ..
._ 300-MHz GPR record at the SRCM with overlay of lithology ..

Collier Stone Quarry .
Lake Lynn Limestone Mine .
RoxCoal Coal Mine......
Safety Research Coal Mine

ILLUSTRATIONS

GPR ray paths penetrating the ground and producing a wiggle trace on the control panel .
SIR-2 GPR system 1.0... 6.ceceeececceeceeceeseeeeeseececeeeeeeas
Collecting GPR data from a 900-MHz antenna through a stack of sandstone slabs
GPR record of Collier Stone Quarry sandstone stack with actual slab locations drawn as overlays
GPR survey lines and stratascope holes at Lake Lynn limestone mine ....
Stratascope logs of bed separations in the roof at Lake Lynn limestone mine
Stratascope image of large open crack 1.7 m (5.5 ft) into the roof at Lake Lynn limestone mine at hole No. 3 .
GPR record at hole No. 2 at Lake Lynn showing crack reflections .
GPR record at hole No. 3 at Lake Lynn ............
GPR survey lines and roof damage at RoxCoal Coal Mine .
GPR record of immediate roof at location 4 in RoxCoal Coal Mine with lithology and roof cracks from satasope
hole overprinted .
GPR survey line and corehole in immediate roof at the Safety Research Coal Mine (SRCM) .

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UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cm centimeter MHz megahertz
ft foot mm __ millimeter
in inch ms nanosecond
m meter ° degree

Reference to specific products does not imply endorsement by the U.S. Bureau of Mines


Report of Investigations 9625

Using Ground Penetrating Radar for Roof
Hazard Detection in Underground Mines

By Gregory M. Molinda, William D. Monaghan, Gary L. Mowrey
and George F. Persetic

This work was performed by the U.S. Bureau of Mines prior to
transferring to the Department of Energy on April 4, 1996

UNITED STATES DEPARTMENT OF ENERGY

PITTSBURGH RESEARCH CENTER


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13

APPLICABILITY OF GPR TO COAL MINE ROOF HAZARD DETECTION

As mentioned earlier, the ideal hazard detection system would
be able to—

1, Detect significant lithologic changes;

2. Detect bed separations of at least 6.4 mm (0.25 in) and
more;

3, Detect large structural discontinuities such as faults,
shears, and paleochannels.

‘The GPR system that was tested is best suited to detect large
horizontal bed separations, or by projection, large structures in
general. For example, at the Lake Lynn limestone mine, the
large bedding separation and fracture zone (1.27 cm (0.5 in)
thick) at 1.7 m (5.5 ft) into the roof was easily seen (although as
a zone, not a clean fracture). This fracture was also a water-
bearing feature, which may have helped enhance the contrast of
this feature from the surrounding limestone. The fact that the
roof was a single medium (limestone) also allowed the
discontinuity to stand out in contrast on the radar record.
Conventional coal mine roof strata monitoring typically
measures dilation or strata separation in one-tenths of an inch at
up to 20 locations vertically in the roof. Better GPR resolutions
are needed to match this precision. The tendency for GPR to
represent a discrete bed separation as a zone also obscures the
precise location of the feature. Closely spaced separations are
smeared into a zone instead of clearly defined by the GPR
system used in the investigation.

Paleochannels are large structures that can seriously degrade
the strength of roof strata. These lenticular features can occur
abruptly at high angles or gradually at low angles. Often, they
are filled with sandstone, which would appear to be a high-
contrast material, but just as often they are filled with siltstone or
shale, which would not be in great contrast with normal roof
rock. The sandstone bed at approximately 4.3 m (14 ft) up in the
roof at the SRCM was detected, but its location indicated by its
reflection was 15.2-30.0 cm (6-12 in) lower than actual location.
Under these conditions, it is questionable whether an acceptable
percentage of paleochannels would be successfully predicted.

Unfavorable lithologic changes in the roof can present roof
hazards (e.g,, thick riders or claystones, thinning stiff members).
At the RoxCoal site, the rider coalbed was correctly detected by
GPR at its approximate location, but its thickness and precise
location (relative to bolt horizon) could not be measured because
of the quality of the data. It appears that coal/rock interfaces are
in sufficient dielectric contrast to be detected by GPR, but
tock/rock interfaces (¢.g., shale/siltstone) do not have dielectric
constants of sufficient contrast to make good reflective
interfaces. Thus, with the quality of data currently generated,
complete lithologic tracking will be difficult.

One possible approach to strata sensing with GPR might be
to "train" the software to recognize a potentially hazardous mine
roof situation and alert the machine/operator when such a
condition is met. With this approach, it would be unnecessary to

understand the geologic associations, but only to "recognize" one
of perhaps several undesirable GPR records. For example, a
hypothetical hazard No. 1 is a situation where a bed separation
of greater than 6.4 mm (0.25 in) occurs anywhere within the first
2.4 m (8 ft) of roof. In the event of this occurrence, a warming
(audible or visual) would be sounded. To calibrate the waming
system, a GPR record must be generated somewhere in the mine
where this situation is known to exist (via instrumentation or
optically through a stratascope log). This "type" record would
exist in software memory, and any time a match is detected in
normal monitoring, a hazard alert would be given. A library of
such records could be compiled for each individual mine. This
approach would be very specific to each mine and would require
a calibration of any sensing package at the outset of application.

Presently, there are a few barriers to a GPR application for
roof hazard sensing. Interpretation of the records currently relies
on visual picks of either the lineup of high-amplitude peaks of
parallel traces or the identification of unusual parts of the
waveform of individual traces. This interpretation needs to be
‘standardized to a point where recognition of significant events is
triggered by a "threshold" amplitude. This is standard practice
in the interpretation of downhole geophysical logs. For example,
coalbeds are identified when the density log goes lower than a
certain threshold. This objective methodology would take some
of the guesswork out of interpretation.

Perhaps more important than interpretation, the quality of data
being generated needs to be improved. Material contrasts
determine reflective horizons. The material properties of the
rocks cannot be changed. The character of the source radar
energy may need to be changed. Additionally, the way reflected
energy is recorded and presented could be enhanced to highlight
subtle material property changes. Current "postprocessing" can
enhance records, but this practice needs to be standardized. The
GPR records from the four field sites all showed lineups of
variable area traces, which amounted to “false bedding.” Even
when it was known that bedding was nonexistent (e.g,, limestone
at Lake Lynn), these reflectors were recorded. This false
bedding confuses the interpretation and detracts from "real"
reflective surfaces. Perhaps a high/low pass filter could remove
some of this reflected energy and record only significant events.

Another improvement may be the use of more accurate
material property values, such as dielectric constant. The
dielectric constants recommended are, for the most part, taken
from soils instead of hard rocks. The variation in velocities
within the common coal measure rocks may well be significant.
‘The improved accuracy from using velocities from samples from
actual roofs may make the interpretations much better. The only
way to currently determine if this is a significant variable is to
test representative coal measure roof rocks in situ and on cores
in the laboratory. Future radar advances promise an in situ
determination of dielectric constant by moving the radar antenna
toward the roof and a frequency survey over a broad range at
each step.