Based on results from the WARR sounding, a CO ra-
dar survey was conducted at the top of the southeast face
of the abandoned quarry. Figures 10B and D show the
mapped fractures visible along the vertical face and the
CO radar profile, respectively. The roughly 18-m linear
traverse consisted of a source-to-receiver offset of 1.5 m,
shot and group intervals of 15 cm, 120-MHz high-pass and
400-MHz low-pass filter settings, a 16-signal stack, and
AGC.

Several horizontal and inclined reflection events are
evident in this radar profile record. The inclined reflec-
tions dip inward from the north end, with several subparal-
lel linear trends increasing in time to the south. These
inclined reflections are interpreted as a plunging fracture.
This interpretation is supported by observations of an in-
clined fracture dipping at roughly 12° along the vertical
highwall. The depth from the top of the highwall to dis-
crete horizontal fracture traces were tape measured to be
0.7, 1.6, 2.4, 2.96, and 3.6 m. The corresponding depths
to reflections, based on the travel time and computed rock
mass velocity information, are 0,63, 1.4, 2.49, and 3.72 m,
respectively. Although each horizontal fracture could be
identified in the radar record, the calculated depth of each
fracture might be off by 10 to 20 pet. The radar record
also shows a large-amplitude, low-frequency event at about
163 ns, a depth of roughly 4.6 m. This reflection is con-
tinuous and nearly horizontal across the record, and it is
interpreted as the location of the water table. Specifically,
two plausible explanations are offered: First, radar reflec-
tion from the phreatic surface (water table). In this case,
spreading of the radar wavelet occurs in response to atten-
uation of energy due to saturated media. Second, wide
angle reflection from the adjacent quarry pond surface.
Reflections of this type may occur since radar antennas are
not focused.

Spectral analysis of the gabbro WARR sounding reveals
that the radar energy is broadband, existing between to
about 200 MHz (fig. 9). The lower frequency content,

B

compared with the radar surveys conducted at granodiorite
sites, suggests that gabbro is more lossy. Specifically,
more attenuation occurs with materials characteristic of
greater dielectric constants. The most prominent event is
roughly 2-1/2 times greater than any other, and it is asso-
ciated with a relatively low frequency of 10 MHz. This is
interpreted to be a reflection from the water table. The
remaining packet of frequencies are attributed to reflec-
tions from fractures.

Based on the spectral analysis, two processed versions
of this radar profile were created. The first processed
section, shown in figure 10B, represents the results of
implementing a low-pass 15-MHz filter. Here, the single,
high-amplitude, and continuous reflection event is isolated.
The second processed section, shown in figure 10C, repre-
sents the radar section following the application of a high-
pass 150-MHz filter, 40-ns refraction mute, and AGC with
a 20-ns window. Again, the resulting radar section is
enhanced in amplitude and continuity of reflected events,
some not evident on the unprocessed section. Specifically,
it is interesting to note that previously unnoticeable re-
flections now appcar below the water table, Moreover, the
cusps or undulations in the low-frequency event appears
to be attributed to inclined conjugate shear fractures
(fig. 10D).

2 100
E 80] Water table reflection
= 60}} . Fracture
| J reftections
S
& 20)
iS)
Eo 100 200 300 400 500

FREQUENCY, MHz

Figure 9.—Spectral analysis of radar profile of Duluth gabbro
quarry in northeastern Minnesota.


STATION NUMBER E

) 20 40 60 80 100 120
0
aay.
——) 4
6
0 0
S E
x
e
&
! 2a
w
E
50 BS
3
F 4. ¢
¥ a
q a
; a.
i
2 | | |
uy 100
2 () 7
=
T TTT
1
i
Gs saute sth |
LAT Ts
+H rs) NAPE EREL 0 dase
a |
co ff ee
Pi K es ANTS t «
Li Oa a ia 4
FRAT Sa He ,
Canntnn H
100 = : mi 6
0,1 2
Loy
Scale, m

Figure 4.—Vertical analysis of fractures exposed along north face of Charcoal granite quarry No.3. A, Mapped
highwall fracture distribution; B, unprocessed vertical common offset radar profile; C, processed vertical common
offset radar profile.


CONTENTS

Abstract
Introduction .
Radar principles .
Range ..
Resolution
Equipment
Field procedure ...
WARR sounding for system calibration
Profiling technique . ace
Data reduction ... .
Field results and discussion . .
Fracture detection in St. Cloud granodiorite a0
Fracture detection in Duluth gabbro .....
Conclusions
References,

ILLUSTRATIONS

Common offset radar reflection sites in central and northeastern Minnesota
WARR sounding conducted at north face of Charcoal granite quarry No. 3 .
Spectral analysis of WARR sounding conducted at north face of Charcoal granite quarry No. 3
Vertical analysis of fractures exposed along north face of Charcoal granite quarry No. 3 . 2
Horizontal or plan view analysis of fractures from north face of Charcoal granite quarry No. 3 inward ..
WARR sounding conducted at outcrop near Cold Spring, MN
Analysis of fractures in outcrop near Cold Spring, MN .......
WARR sounding conducted at abandoned Duluth gabbro quarry site in northeastern Minnesota
Spectral analysis of radar profile of Duluth gabbro quarry in northeastern Minnesota .

Analysis of fractures at abandoned Duluth gabbro quarry in northeastern Minnesota .. .

Seernaveyne



14

s STATION NUMBER N
te)

2
Zo
= [0 |}°
2:
6
8
300
Gr al 2
ieee
Scale, m

processed (150-MHz high-p:
distribution.

APPROXIMATE DEPTH, m


WwW STATION NUMBER E

) 20 40 60 80 100 120
)
5
10
15
0 me
} Hl E
a 5
Qa
WW
i a
it Wu
150 fi to &
Ht 2
ti x
‘ fe)
| a
| 15 2
oa i <
2 so Lill |
uz 300
2 0
=

150

300

Figure 7.—Analysis of fractures in outcrop near Cold Spring, MN. A, Inferred fracturing; B, common offset radar wig-
gle trace profile; C, common offset radar scan mode profile.


RANGE

The theoretical performance range for a radar wavelet
propagating in granite was established using an clectro-
magnetic forward model. Based on the modeled informa-
tion, and using a dielectric constant of 10, an instru-
ment dynamic range of 70 dB, and a center frequency of
250 MHz, results indicated a reflected radar pulse could
be expected to travel a distance of about 45 m. These
depths of penctration appear adequate for assessing stress-
relief fracturing and blast-induced damage caused by the
mining process. However, it should be noted that highly
conductive media within the propagation path, such as
water or clay, tend to absorb electromagnetic energy and
drastically alter the radar range in rock,

RESOLUTION

Resolution (the ability to detect and define a target)
depends on the transmitted signal character, i.e., amplitude
and wavelength; the diclectric properties of the rock mass
and contrast with the target; and the shape and character
of the target, e.g., depth, shape, and size. Often resolution
is expressed as a ratio of the minimum target dit ion
to wavelength. Since the primary targets of the investi-
gation are fractures, of small aperture in igneous rock, it
is important to maintain a radar wavelet of relatively high-
frequency content. The important question then arises,
What fraction of a wavelength, relative to the frequency
content of the propagating radar wavelet, is sufficient to
detect a fracture? The following relationships assist in the
determination of the appropriate radar frequency band
when the electrical properties of the rock mass are known:

q@

and

2)

where d= wavelength, m,

V, = velocity of a radar pulse propagating
through the rock mass, m/ns,

frequency, MHz,

C = velocity of a radar pulse propagating
through air (0.3 m/ns),

and = dielectric constant of media.

Assuming a diclectric constant of 10 for granite (10),
the wavelength would be on the order of 3.5 cm. Using
the scismic analogy, where minimum resolution is about
1/32 of a wavelength, individual granite fractures (or zones
of fractures) greater than 0.1 cm could be identified based
on an impinging radar 250-MHz wavelet. The overall per-
formance could be improved by (1) increased frequency
content (the antenna used in this study has an output pulse
that contains energy to roughly 500 MHz), (2) a strong
rock mass-to-target dielectric contrast, ¢.g., if fractures
were wet as opposed to air filled, and (3) the use of digital
enhancement during data acquisition, ¢.g., signal stacking,
analog filters, and an automatic gain control (AGC).

EQUIPMENT

The commercially available OYO model 2441,
GEORADAR-1‘ radar system was used for the field in-
vestigations of fractures in igneous rock. The radar system
consisted of a profiling recorder, oscilloscope control unit,
two antennas, and associated cables. The OYO control
unit has a fixed time-domain asymmetric broadband pulse
with a center frequency of 250 MHz. The radar system
can be operated in either a continuous or individual shot
mode. The recorder can be operated in either a scan or
wiggle trace mode. A pair of system-compatible antennas
are shiclded to improve the transmitted signal direction-
ally. This reduces noise in the form of back reflections
from overhead power lines, trees, etc. The system pro-
vides for both direct and time varying gains, and AGC to
amplify the later, weaker signals that achieve greater pen-
etration into the rock.

FIELD PROCEDURE

WARR SOUNDING FOR SYSTEM CALIBRATION

A Wide Angle Refraction and Reflection (WARR)
sounding, analogous to a seismic walk-away noise test, was
conducted to establish an optimum (1) high- and low-pass
filter setting, (2) number of signal stacks, and (3)
transmitting-receiving antenna offset for profiling. The

radar system permits front-end noise reduction and
wavelet shaping through proper analog filter settings. The
high-pass filter is used to compensate for the low-pass
characteristics of the rock mass. The high-pass filter

“Reference to specific products does not imply endorsement by the
US. Bureau of Mines.


DELINEATION OF FRACTURES IN IGNEOUS ROCK MASSES USING
COMMON OFFSET RADAR REFLECTION

By Michael J. Friedel,’ James J. Jessop,' and Richard E. Thill?

ABSTRACT

As part of an investigation aimed at improving the health and safety and competitiveness of the
mining industry, the U.S. Bureau of Mines evaluated the application of common offset radar profiling,
using a 250-MHz ground-penetrating radar (GPR) system, for the detection of fractures in igneous rock.
A series of radar reflection surveys were conducted to detect and delineate the extent of fracturing at
various granodiorite and gabbro quarries and outcrops located in Minnesota. The application of radar
profiling for detecting joints, sheeting fractures, shear zones, and depth to water table was demonstrated
to be feasible with minimal processing. Radar reflection interpretations were verified by visual
inspection of the rock mass and field mapping of local structure. The radar reflection section provides
a simple, rapid, and cost-effective means for mapping of shallow (less than 10 m) small-scale fractures
(greater than 0.25 cm) in igneous rock masses (characteristic of velocities between 0.061 m/ns for
gabbro and 0.125 m/ns for granodiorite). Depth or distance estimates to fractures are within 10 pet of
the actual, and time-shift compensation is necessary only when topographic irregularities exceed 30 cm.

1Geophysicist.
?Supervisory geophysicist.
Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, MN.


UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cm centimeter min minute

dB decibel m/ns meter per nanosecond
GHz gigahertz ns nanosecond

m meter pet percent

MHz _ megahertz Vv volt



FRACTURE DETECTION IN DULUTH GABBRO.

Another sct of GPR radar investigations was conducted
at an abandoned Duluth gabbro quarry site in northeastern
Minnesota, in the SE1/4, NW1/4 of Section 13, T6N,
R1IW. The WARR sounding, conducted along a linear
traverse at the top of a southeast face and 1.7 m above a
prominent horizontal fracture, is shown in figure 8. Re-
fracted air- and ground-coupled energy, a reflection ar-
riving from a horizontal fracture at a zero offset time of
about 56 ns, and a reflection arriving from the water table
at a zero offset time of about 150 ns were apparent in

the sounding, The optimum offset window exists between
traces 7 and 12, giving source-to-receiver distances of 1.05
to 18 m. From this sounding, the velocity for radar
propagation in the gabbro was determined to be roughly
0.061 m/ns, giving a dielectric constant of about 24. This
value could not be substantiated, since there is no mention
of any diek value for gabbro in the literature. The
increase in diclectric property over that derived for grano-
diorite, however, can be attributed to the known existence
of high amounts of disseminated copper and iron sulfides
in gabbro,

ya 5

Ss STATION NUMBER N
0 10 20
0
eeenteHiitt MT]
aaa)

[Water table reflection] table [Water table reflection]

aan”

offset window

150

TIME, ns

300

Dy)
Nis a

re Soe

APPROXIMATE DEPTH, m

cae

Figure 8.—WARR sounding conducted at abandoned Duluth gabbro quarry site in northeastern Minnesota.

Scale, m


INTRODUCTION

Geosensing is an area of considerable interest in the
U.S. Bureau of Mines’ research program. One of the ob-
jectives in ground control research is to detect and
delineate anomalous strata and ground conditions and to
use geosensing for the characterization and classification
of rock masses. In accord with its mission to help resolve
health and safety and productivity problems facing the
mining industry, the Bureau evaluated the applicability of
ground-penetrating radar (GPR) for detecting fractures
and other subsurface geologic structures in igneous rock
masses. The investigations, conducted in cooperation with
the Minnesota Department of Natural Resources and the
Cold Spring Granite Co. as co-interested parties, evaluated
the use of GPR for site characterization and flaw detection
at potential quarry sites.

In the past, GPR has been successfully employed to re-
solve structural features in both soil and bedrock (J-3)3
and to located buried pipes and cables (4). Related inves-
tigations have concentrated on assessing the feasibility of
GPR for detecting coal (5), granular deposits, and sea ice
thickness in permafrost regions (6). Recent studies have
also confirmed the usefulness of GPR for delineating tun-
nels (7-8) and abandoned mines in granitic and limestone
rock masses (9-10). This report discusses results from
GPR field investigations involving the detection of frac-
tures, joints, and the water table in active and potential
granitic quarry sites in central and northeastern Minnesota

(fig. 1).

Abandoned gabbro
quarry *

Active granodiorite

quarry 7

Granodiorite
outcrop

z

e
Minneapolis

Figure 1.—Common offset radar reflection sites in central and
northeastern Minnesota.

RADAR PRINCIPLES

GPR is an electromagnetic technique that responds
to changes in electrical properties in the earth, The trans-
mission, reflection, refraction, and diffraction of an elec-
tromagnetic wave is defined by the Maxwell equations.
These equations describe the electric and magnetic fields
of an electromagnetic wave over a broad range of fre-
quencies in terms of the electrical and magnetic properties
of the material that the wave travels through. In the
electromagnetic spectrum, the frequency range of radar
energy lies between 1 MHz and 1 GHz. A comprehensive
discussion of the principles affecting radar propagation is
given by Bochicchio (17).

A GPR system can operate in either a monostatic or bi-
static mode. In the former made a single antenna oper-
ates as both the transmitter and receiver of radar energy.
The latter mode requires use of two antennas; one
transmitting a radar source signal and one receiving radar

Italic numbers in parentheses refer to items in the list of references
at the end of this report.

energy. The transmitted radar pulse travels away from the
transmitter until it counters a change in electrical prop-
erties, or electrical impedance. At this point, the energy
is partitioned in one or more of the following modes:
transmitted, reflected, refracted, and diffracted compo-
nents. The time delay of these various waveforms sensed
by the receiving antenna is a function of the (1) prop-
agation velocity of the impinging wavelet through the rock
mass, (2) depth to the impedance contrast, and (3) di
tance between antennas. Since the received radar pulse is
a convoluted representation of one or more of these radar
wave types, the ability to record useful information hinges
upon the detection, resolution, and interpretive capabilities
of the radar system.

The range and detection capabilities of a GPR system
depend on the electrical properties and propagation char-
acteristics of the materials, geologic noise, system char-
acteristics, and enhancement of the data. Since the elec-
trical properties of the rock mass are intrinsic, the
operator must control characteristics of the source and/or
receiver, or of the data display.



SE EEE
RI} 9424 REPORT OF INVESTIGATIONS/1992

Delineation of Fractures in Igneous
Rock Masses Using Common
Offset Radar Reflection

By Michael J. Friedel, James A. Jessop, and
Richard E. Thill

UNITED STATES DEPARTMENT OF THE INTERIOR

BUREAU OF MINES



FIELD RESULTS

FRACTURE DETECTION IN ST. CLOUD
GRANODIORITE

The St. Cloud granodiorite tests were conducted at two
properties owned by Cold Spring Granite Co., St. Cloud,
MN. The first set of investigations occurred at an active
quarry, where operations involve drilling and blasting of St.
Cloud gray granodiorite (also called Charcoal granite)
from the rock mass. The local geologic structure is char-
acterized by faulting and fracturing attributed to tectonic
forces, stress relief from blasting, and uplift. Local faults
are continuous throughout the study area and include a
zone of sheared rock. Fault surfaces are often chloritized

AND DISCUSSION

and contain minor amounts of clay, whereas the stress-
relieved fractures are clean, having apertures ranging from
0.25 cm to several centimeters (14).

The initial set of radar tests was conducted along the
surface directly above the north face highwall at the
Charcoal quarry No. 3 pit. Field investigations began with
a WARR sounding conducted over a prominent horizontal
fracture at a depth of roughly 2.4 m from the surface. The
survey included 60 stations at 15-cm station intervals, with
band-pass filtering between 160 MHz and 400 MHz,
averaging of 16 stacks per trace, AGC, recording time
window of 200 ns, and a 0.2-ns sample interval. The radar
test results show several wave modes in figure 2; a weak

Ww STATION NUMBER E
0 20 40 60
0 0
Air wave
Or
Ce 2
| Wave
I, &
o fee
2 -
5 a
wi

S a
= 100: 6 w
Fracture reflections P. $
4
é
OO:
a
<

10

200 H We

Optimum 0 1 2
offset window |
Scale, m

Figure 2.-WARR sounding conducted at north face of Charcoal granite quarry No. 3.


setting can be used to improve the significance of high-
frequency energy and, hence, the resolution of fractures.
In the presence of random noise, the stacking capability
greatly improves the signal-to-noise (S/N) ratio.

The WARR sounding establishes the characteristic par-
titioning of wave modes, e.g., refractions and reflections,
occurring at the study site. In this procedure, the trans-
mitting antenna is held stationary. The receiving antenna
is then moved to a predetermined distance from the trans-
mitter, the radar wavelet is transmitted, and the convo-
luted wavelet is recorded. Subsequent receiving antenna
stations correspond to a multiple (1, 2, ...N) of the initial
source-to-receiver distance. Ideally, the receiving station
spacing and total number of traces recorded are adjusted
to clearly identify target reflections(s) and refractions yet
minimize the total number of traces recorded.

Refracted wave modes identified in the WARR sound-
ing calibrations permitted an estimate of the depth-
distance relationships from the time-distance profile. The
first arrivals recorded on the test profile were produced by
energy traveling through the air (air wave). Subsequent
arrivals showing linear moveout (time change with dis-
tance) correspond to refracted energy traveling along the
surface of the rock mass (direct wave). The velocity as-
sociated with this direct wave could be used to compute
the depth to reflections associated with fractures. Later
arriving reflections from horizontal fractures display a

coherent hyperbolic moveout. The range of offsets that
allows these target reflections to be observed with max-
imum coherency, i.¢., with minimum interference from
source-generated noise and maximum amplitude, is the
“optimum window,” as described previously for seismic
applications by Hunter (72). Once the range of applicable
source-to-receiver offsets is identified, the spacing between
profile stations can be determined,

PROFILING TECHNIQUE

The GPR bistatic profiling mode requires moving a
source-recciver antenna combination of fixed spacing at
uniform-distance increments along the survey path, The
distance between subsequent profile stations is a function
of the detail in subsurface coverage required. For the
present study, a spacing of about 15 cm was found to be
adequate, but this spacing probably could have been
increased to 30 cm and still have provided sufficient detail
(without losing lateral resolution) to image the fractures.

Since coupling variations exist because of irregular sur-
face conditions, the phase of the transmitted signal varies
on a trace-by-trace basis, and higher order filtering opera-
tions, such as spiking deconvolution, are rendered ineffec-
tive. Hence, only analog, front-end filtering and signal
stacking were used during the data acquisition phase. The
radar display therefore appears analogous to a seismic sig-
nal display for a wiggle trace in the variable area mode.

DATA REDUCTION

The velocity, V,, of a radar pulse propagating through
the rock can be determined by

Vo=X/t, @)
where X = distance between stations, cm,
and t = travel time to refractor, ns.

This value can be combined with the propagation velocity
in air, C, to determine the characteristic rock mass diclec-
tric constant, €,,, using equation 2. The characteristic ve-
locity of radar waves in the rock mass can also be used to
statically correct or time-shift a trace by an amount, t., to
account for changes in elevation. Thus,

t.=-2Z./Vo, @)
where

Z, = depth to datum.

To accurately assess damaged and intact volumes of rock,
it is necessary to convert the time-distance radar section to

a true depth-distance radar section. Here, it is important
to recognize that the travel times to all reflectors have
two-way (down-going and up-going component) times. As-
suming homogeneous rock blocks, the corresponding depth
to a horizontal reflector, z, can be calculated, based on the
common depth point method (13) by

ar ()
ale fd 8)
where t = time to a reflector,
x = distance between source and receiver,
and 1,2 = indices referring to two different an-

tenna separations,

When depth is determined, the average velocity in the rock
mass can be calculated using a delay time for antenna
separation at zero, ic., zero offset given in a WARR
sounding.


DELINEATION OF FRACTURES IN IGNEOUS ROCK MASSES USING
COMMON OFFSET RADAR REFLECTION

By Michael J. Friedel,’ James J. Jessop,' and Richard E. Thill?

ABSTRACT

As part of an investigation aimed at improving the health and safety and competitiveness of the
mining industry, the U.S. Bureau of Mines evaluated the application of common offset radar profiling,
using a 250-MHz ground-penetrating radar (GPR) system, for the detection of fractures in igneous rock.
A series of radar reflection surveys were conducted to detect and delineate the extent of fracturing at
various granodiorite and gabbro quarries and outcrops located in Minnesota. The application of radar
profiling for detecting joints, sheeting fractures, shear zones, and depth to water table was demonstrated
to be feasible with minimal processing. Radar reflection interpretations were verified by visual
inspection of the rock mass and field mapping of local structure. The radar reflection section provides
a simple, rapid, and cost-effective means for mapping of shallow (less than 10 m) small-scale fractures
(greater than 0.25 cm) in igneous rock masses (characteristic of velocities between 0.061 m/ns for
gabbro and 0.125 m/ns for granodiorite). Depth or distance estimates to fractures are within 10 pet of
the actual, and time-shift compensation is necessary only when topographic irregularities exceed 30 cm.

1Geophysicist.
?Supervisory geophysicist.
Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, MN.


CONCLUSIONS

CO radar reflection, using 250-MHz antennas, provides
nontechnical personnel with a relatively simple, rapid,
and cost-effective means for mapping shallow, small-scale
fractures, in igneous rock, at active or abandoned quarries,
and outcrops. The actual time required for a survey is
governed primarily by site specific acquisition param-
eters. Upon establishing the appropriate filter settings,
record length, and source-to-receiver distance, the sur-
vey time is dependent on the number of stacked shots per
measurement and total number of measurements; typically,

a single measurement requires less than 1 min, It should
be noted that the combination of 250-MHz antennas and
velocities characteristic of igneous rocks (0.061 m/ns for
gabbro and 0.125 m/ns for granodiorite) limit rock mass
interrogation to depths greater than about 1.5 m, but less
than about 10 m. Depth or distance to fractures can be
accurately estimated for this range, and time-shift com-
pensation is necessary only when topographic irregularities
exceed 30 cm.

REFERENCES

1. Bjelm, L., S. Follin, and C. Svensson.
Sonkningsmetod. BFR project No. 800141-9, Loden:
(EVTG-3002)/ 1-71/, 1982.

2. Annan, A. P. Radar Sounding in Potash Mines, Saskatchewan,
Canada. Geophysics, v. 53, No. 12, 1988, pp. 1556-1564.

3. Moffat, D. L., and R. J. Puskar. A Subsurface Electromagnetic
Pulse Radar. Geophysics, v. 41, No. 3, 1976, pp. 506-518.

4. Morey, R.M. Continuous Subsurface Pulse Radar. Paper in Pro-
ceedings of Conference on Subsurface Exploration for Underground
Excavation and Heavy Construction. ASCE (Henniker, NH), 1974,
pp. 3-99.

5. Cook, J. C. A Study of Radar Exploration of Coal Beds (Tele-
dyne Geotech), BuMines OFR-S-72, 1971, pp. 5-72

6. Pilon, J. A. Applications of Ground Probing Radar in Permafrost
Regions to Granular Deposits, Pipeline Right-of-Way and Frozen Core
Dams. Abstract in Expanded Abstracts. Soc. Explor. Geophys., 1987,
pp. 73-75.

7. Butler, D, L, Evaluation of Geophysical Methods for Cavity De-
tection at the WES Cavity Test Facility. U.S. Army Eng. Waterways
Exp. St, Tech. Rep. GL-804, 1980, 125 pp.

Georadar Som Under-
LUTUDG/

8 Rubin, L. A., J. C. Fowler, and W. L. Still, New Sensing System
for Pre-Excavation Subsurface Investigation for Tunnels in Rock Masses
(final rep., contract DOR-FH-11-8602). U.S. Dep. Transp., 1976, 12 pp.

9. Friedel, M. J., J. A. Jessop, R. E. Thill, and D. L. Veith. Elec-
tromagnetic Investigation of Abandoned Mines in the Galena, KS, Area.
BuMines RI 9303, 1990, 20 pp.

10. Ulriksen, C. P. Application of Impulse Radar to Civil Engineer-
ing. Ph.D. Thesis, Lund University of Technology, 1982, 175 pp.

11. Bochicchio, R. Computerized Geometric Correction of Ground
Penetrating Radar Images. M.S. Thesis, CO Sch. Mines, Lakewood, CO,
1988, 91 pp.

12. Hunter, J, Short Note On Shallow Seismic Reflection Mapping.
Geophysics, v. 49, No. 8, 1984, p. 1381.

13, Daniels, J.J. Fundamentals of Ground Penetrating Radar. Paper
in Proceedings of Symposium on the Application of Geophysics to Engi-
neering and Environmental Problems. 1989, pp. 62-142.

14, Shurr, G. Private communication, 1990; available from M. J.
Friedel, BuMines, Minneapolis, MN,

15. Friedel, M. J. BOMSPS: Bureau of Mines Signal Processing
Software—Concepts, Expressions, and Tutorial. BuMines IC 9242, 1990,
47 pp.



Library of Congress Cataloging in Publication Data:

Friedel, Michael J.

Delineation of fractures in igneous rock masses using common offset radar
reflection / by Michael J. Friedel, James A. Jessop, and Richard E. Thill.

p. cm. — (Report of investigations; 9424)

Includes bibliographical references (p. 15).

Supt. of Docs. no.: I 28.23:9424.

1. Ground control (Mining). 2. Rock mechanics. 3. Water table. 4. Ground
penetrating radar. 5. Rocks, Igneous—Minnesota. I. Jessop, James A. II. Thill,

Richard E. III. Title. IV. Series: Report of investigations (United States. Bureau
of Mines); 9424.

TN23.U43 [TN288] 622 s—dc20 [622.28] 92-5713. CIP



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= 200
Wu
2 0
=

100

‘3
200

(0) 1 2

Scale, m

Figure 5.—Horizontal or plan view analysis of fractures from north face of Charcoal granite quarry No. 3 in-
ward. A, Mapped and inferred vertical fractures; B, unprocessed horizontal common offset radar profile; C, proc-
essed horizontal common offset radar profile.


10

Ww STATION NUMBER
ie} 20 40
0
On
8,
t Wave
Fracture reflection
2 |
;
w 15
S 10
=
300 Lf!

Optimum
offset window

Air Wave;

60

One te 2
Lt

Scale, m

Figure 6.—WARR sounding conducted at outcrop near Cold Spring, MN.

10

15

APPROXIMATE DEPTH, m


Report of Investigations 9424

Delineation of Fractures in Igneous
Rock Masses Using Common
Offset Radar Reflection

By Michael J. Friedel, James A. Jessop, and
Richard E. Thill

UNITED STATES DEPARTMENT OF THE INTERIOR
Manuel Lujan, Jr., Secretary

BUREAU OF MINES
T S Ary, Director


Mission: As the Nation's principal conservation
agency, the Department of the Interior has respon-
sibility for most of our nationally-owned public
lands and natural and cultural resources. This
includes fostering wise use of our land and water
resources, protecting our fish and wildlife, pre-
serving the environmental and cultural values of
our national parks and historical places, and pro-
viding for the enjoyment of life through outdoor
recreation. The Department assesses our energy
and mineral resources and works to assure that
their development is in the best interests of all
our people. The Department also promotes the
goals of the Take Pride in America campaign by
encouraging stewardship and citizen responsibil-
ity for the public lands and promoting citizen par-
ticipation in their care. The Department also has
a major responsibility for American Indian reser-
vation communities and for people who live in
Island Territories under U.S. Administration


air wave, indicating the effectiveness of shielding, and a
prominent, coherent direct wave. The refraction, showing
linear moveout, is attributed to radar energy traveling
along the surface of the rock mass. The computed velocity
associated with radar energy propagating through the
granodiorite was determined to be roughly 0.128 m/ns.
Substituting this value into equation 2 gives a dielectric
constant of 5.5. These values agree with those cited in the
literature (10).

Apparent reflections from fractures occur at zero offset
travel times greater than about 38 ns, corresponding to
depths in excess of about 2.4 m. Any attempt to image
fractures at lesser depths would be masked by the strong
air- and ground-coupled radar energy. The most distant
reflection, having very weak amplitude, occurs at a travel
time of roughly 162 ns. This gives a depth of penetration
of roughly 10 m. The optimum offset window was deter-
mined to exist between traces 4 and 10, where maximum
coherency occurred for the majority of reflections arising
from a horizontal and inclined set of fractures. Since the
first trace is offset by 0.6 m, the closest distance to which
the antennas could be placed, the source-to-receiver offset
range is 1.2 to 2.1 m.

A spectral analysis of trace 15 from the north face
WARR sounding indicates that broadband energy exists to
275 MHz with a primary frequency band between 50 and
200 MHz (fig. 3). Based on the spectral analysis of radar
records, it is clear that sufficient radar energy can be
propagated to depths on the order of 10 m, with sufficient
resolving power to detect both horizontal and/or inclined
fractures in granodiorite.

Figure 44 gives the approximate locations of fractures
mapped at the radar study site, along the north face of
the Charcoal granite quarry No. 3. The unprocessed (raw)

AMPLITUDE, V

RELATIVE AMPLITUDE
&

(0) 100 200 300

FREQUENCY, MHz

400 500

Figure 3.—Spectral analysis of WARR sounding conducted at
north face of Charcoal granite quarry No. 3.

vertical common offset (CO) radar profile recorded at
120 stations along the north face is shown in figure 4B.
The CO profile was conducted using the acquisition pa-
rameters suggested by the WARR survey with a source-to-
receiver offset of 18 m. The uniformity in refracted
energy occurring at a time between 0 and 38 ns, repre-
sented by the three dark continuous bands crossing the
record, is typical of profiling along a flat-lying surface.

In general, when the ground surface is essentially flat
and uniform, static corrections are not required for an ac-
curate interpretation of the radar data. The slight change
in elevation of roughly 15 cm occurring between stations
95 and 113, however, does affect the radar profile. Re-
fracted energy is delayed in this zone, resulting in what
appears to be a sag in the radar record. Clearly, topo-
graphic discrepancies in excess of 30 cm would require
time-shift compensation to accurately portray subsurface
features. Since most active quarrying operations work sys-
tematically with flat-lying benches, the potential for this
sort of problem is reduced. When assessing prospective
quarry outcrops over irregular topography, however, an
elevation survey should be conducted to enable a time-shift
correction to the radar data.

Several prominent reflections could be observed in the
north face profile (fig. 4B). Between stations 0 and 45, a
pair of inclined reflections occur; their travel times de-
crease eastward from roughly 60 ns. These reflections cor-
relate with two shear fractures, dipping roughly 50° to the
west, as observed from the quarry wall. Between stations
50 and 75, reflections at about 50 to 55 ns define a hori-
zontal wedge-shaped block at depths of 3.2 and 3.5 m, re-
spectively. The upper reflection coincides with a contin-
uous fracture that dips slightly to the east and then flattens
out near the end of the profile. The reflection amplitude
associated with this fracture becomes strongest between
stations 80 and 100 at a depth of about 3 m. As the frac-
ture continues eastward it again has strong amplitudes
between stations 115 and 120.

To enhance the interpretation of GPR record, the ana-
log records were digitized and subsequently processed
using BOMSPS (15). Figure 4C represents the CO radar
section shown in figure 4B, following the implementation
of refraction muting (0 to 42 ns), high-pass filter
(150 MHz) with a cosine taper (24 dB per octave), and an
AGC using a 25-ns window. Digital processing sharply
defined reflection peaks and extended the coherency of
reflectors over greater distances.

Figure SA gives the approximate locations of vertical
fractures occurring at various distances from the north face
of the Charcoal granite quarry No. 3. the raw horizontal-
slice CO radar profile, shown in figure 5B, was produced
by traversing the front of the north face in Charcoal
granite quarry No. 3, using the same acquisition param-
eters outlined above. This radar profile produced reflec-
tions sensitive to vertical joints and fractures, with the
vertical face aligned at time zero. Refracted energy


traveling along the surface of the rock mass appears as
three coherent black lines crossing the radar section be-
tween 0 and 25 ns. Beyond about 25 ns, reflections occur,
indicating the presence of vertical fractures detected
inward from the quarry wall into the rock mass.

Strong reflections between stations 0 and 100 appear to
define parallel sets of near-vertical joints or fractures
that trend in a northwest direction from the quarry face
(fig. SB). These fractures could not be observed from the
surface, i.e., top of the highwall, but agree with structural
features previously mapped; the fractures are attributed to
a local shear zone that trends N70W (14). The reflections
with the greatest amplitude, between stations 0 and 10 at
travel times between 50 and 75 ns, are interpreted as the
back side of a block that terminated laterally at station 10.
The lateral extent of this block is inferred from the dif-
fractions that cross the parallel set of reflections. Two
coherent shallow reflectors subparallel to the face exist
along the western third of the record. The reflection oc-
curring between stations 95 and 115 at a travel time of
43 ns corresponded to a visible fracture trace at a distance
of about 1.5 m from the quarry face. The deeper reflec-
tion occurring between stations 65 and 115 at a travel time
of 61 ns was also believed to be a near-vertical subparallel
to the face at a distance of roughly 2 m from the face.
The presence of this fracture could not be verified, how-
ever, because of the presence of a soil cover. The six
vertical fractures begin at stations 20, 30, 70, 77, 85, and
92, and trend in a northeasterly direction inward from the
face. Visual inspection of the highwall face revealed a
fracture trace for each, confirming the radar interpretation.

Using the same parameters, the digitally processed ra-
dar record, shown in figure 5C, enhanced reflection peaks
and extended the coherency of reflectors over distances
throughout the profile (43 ns). It is interesting to note the
continuity of a horizontal reflection extending from east to
west. Only a portion of this reflection was present (be-
tween stations 100 and 110) in the previous unprocessed
section. The presence of this vertical fracture was con-
firmed by trace observation at the horizontal surface of the
highwall. The digital processing enhanced the radar data
such that this vertical fracture, subparallel to the highwall
face, could be traced along its extent. Additionally, the
locations of fractures intercepting the vertical face (at
stations 20, 30, 70, 77, 85, and 92) can be more easily lo-
cated where the horizontal continuity is broken. Previous-
ly, the fracture occurring at station 20 was not apparent.

A second site investigation using radar was conducted
at the Cold Spring Granite Co. Meridian property outcrop,
near Cold Spring, MN. This test site was chosen to eval-
uate rock mass integrity for use as a potential dimen-
sion stone quarry site. A linear north-south traverse was

conducted along the axis of the Charcoal granite outcrop.
Measurement stations were at 15-cm intervals.

A WARR sounding was conducted that indicated the
presence of several wave modes (fig. 6); a weak air wave
at stations beyond 40 beginning at a time of roughly 75 ns,
and coherent ground-coupled energy at all stations. Based
on the slope of the air-rock refractor, the computed radar
velocity for this site was determined to be roughly
0.125 m/ns. Substituting this value into equation 2 gives
a dielectric constant of 5.8, which is consistent with the
previous site. Also present in the WARR sounding are
strong reflectors, between stations 1 and 35, occurring at

‘a zero offset time between about 93 and 110 ns. This cor-

responded to a depth of roughly 5.8 to 6.9 m. Note that
the moveout, ic., change in travel time with distance,
changes character; the moveout originally displays hyper-
bolic trajectory at near offsets and becomes linear at
increasing distances. This behavior represents a change
from reflected to refracted radar energy. Since high-
amplitude and coherent reflections could be observed
between all traces, any corresponding source-to-receiver
offset could have been used. For this case, a source-to-
receiver offset of 1.8 m was again chosen for profiling.

The CO radar profile resulting from the survey con-
ducted at the Meridian outcrop is shown in figure 7B, The
acquisition parameters are the same as those previously
mentioned, except that the time window was extended to
300 ns. The primary event appears as a sequence of three
reflection segments increasing in time from 53 ns at the
east end to about 100 ns at the west end, corresponding to
depths of roughly 3.3 and 6.3 m, respectively. This is
believed to be sheeting—a stress relief fracture (fig. 7A)
that developed as a result of glacial unloading. The num-
erous short-duration events extending away from the pri-
mary reflections along the lower side are interpreted to be
diffractions, These diffractions occur in response to point
sources, or in this case, discontinuous fractures occurring
in this zone.

A second traverse, using the conventional scan mode,
was conducted along the same survey line for comparison
to the wiggle trace. The radar scan mode record obtained
over the same profile segment is shown in figure 7C.
While the fracture reflection and diffractions also appear
in this record, they could easily have been overlooked
without knowing their existence a priori. The additional
primary features (greater than 150 ns) that appear as
large, dark, continuous horizontal bands represent coher-
ent noise. Although the scan mode can be done in about
half the time it takes to do the wiggle trace CO mode, the
noisy records hinder a rapid and meaningful interpretation.
Hence, these results suggest that radar investigations in
igneous rocks be conducted using the wiggle trace mode.