10th Asian Regional Conference of IAEG (2015)

Evaluation of Groundwater Permeation in Fractures around Rock Cavern
using Ground Penetrating Radar
Kazuhiko MASUMOTO(1) and Keisuke KURIHARA(2)(1) Kajima Technical Research Institute, Japan
E-mail:masumotk@kajima.com(2) Kajima Technical Research Institute, Japan
Abstract
The groundwater condition in fractures around the rock cavern is important forgroundwater management in geological disposal of radioactive waste and underground oil /LPG storage project with water containment system or for planning of effective measures
for seepage of mountain tunnel. The GPR (Ground Penetrating Radar) method is unique
technique to survey the water condition in fractures around the rock cavern, indirectly, andwith no disturbance of original ground water condition. The authors conducted the GPR
survey from the surface of the concrete placed on the granitic rock with fractures. In thesurvey, the difference of water condition behind the lining concrete caused the change inthe reflected waveforms and the intense of spectrum due to the results of spectrum analysisof reflected waveforms. The results indicated that the water condition in the rock could be
evaluated using the information of reflected waves of GPR survey.
Keywords: ground penetrating radar, ground water, fractured rock, spectrum analysis
1.

Introduction

In the present study, tests were conducted toverify the survey method using GPR. In these tests,
radar measurements were conducted from the uppersurfaces of concrete that had been placed on the rockwith fractures, assuming a survey from the interior ofunderground lined rock caverns. From the results ofmeasurements of changes in reflected waveforms andtheir spectral analysis during permeation of fresh(tap) water and saline water into the rock, it was
concluded that the survey method using GPR could
be used to evaluate the groundwater permeations in
the rock of concrete backfaces. This paper is a reporton those results.

Fractures developing around rock caverns during
the excavation results in issues related not only to themechanical stability of rock caverns, but also to thegroundwater paths. Specifically, in terms of the safetyassessment of geological disposal of radioactive
waste, the groundwater management of rock caverns
for oil/LPG storage using hydraulic containment
system, or planning of countermeasures for water
seepage of tunnels, etc., it is important to have athorough understanding of the groundwaterconditions around rock caverns. However, in survey
using boreholes, there is a possibility of disturbing
the original hydrological conditions during a survey,
and there are also limits to areal evaluations.
Therefore, the authors have been developing a
non-destructive method that uses GPR (Ground
Penetrating Radar) to make areal evaluations ofgroundwater conditions in fractures around rockcaverns (Masumoto and Kurihara, 2013). So far, inpreliminary experiments using model ground, GRP
has been applied to measure changes in reflected
waveforms and reflection intensity, and the results
indicate that it is possible to evaluate the saturation
conditions of water into fractures and the infiltration
conditions of saline water (Masumoto and Kurihara,2014).

2.

Methodology of the Verification
Test using GPR

As a verification test assuming a survey aroundrock caverns, the granitic rock was cropped out using
the rock splitter and concrete about 30cm thick wereplaced on the surface of rock. Before placement of
concrete, the fracture distribution was observed on
the surface of the test area. After water injection holes
were drilled from the surface of concrete, watercontents of fractures in the rock, and along theboundary between the rock and the concrete, werechanged using water injection holes, and radarmeasurements were taken from the surface of the1

10th Asian Regional Conference of IAEG (2015)

Fig. 2 Fracture distribution of test site
Fig. 1 Schematic diagram of the test
concrete under different water contents in the rock ofconcrete backfaces. Fig. 1 shows a schematic diagramof the test and a photo of the testing site. In order tomeasure changes in reflected waveforms during
permeation of groundwater into fractures in the rock,
Fracture”2” was selected as the target for water
injection from three fractures (“1” to “3”) with arelatively continuous NE strike, which wereidentified from the results of geological survey. Then
5 water injection holes of 1m in depth were drilled tointersect with Fracture”2” (Fig. 2). Fracture”2” is 75
degrees dip toward the north and has openings inplaces, some of which are soft sections of a few
centimeters in width. Fractures”1” and “3” include awhite vein, and they dip toward the north at angles of
90 degrees and 60 degrees, respectively. Concrete
was placed horizontally within a 3.0m by 3.5m areaon the rock with these fractures. Because the rocksurface was uneven, there was a difference inconcrete thickness of maximum 45cm, minimum
10cm, as shown in the contours of Fig. 3. The
groundwater in the water injection hole was notobserved, therefore, the groundwater level was atleast 1m below the rock surface.
In the verification tests, an 800MHz pulse radar
was used to take profile measurements with three
2m-long measuring lines as shown in Fig. 3.
Measurements were conducted under three different
conditions: (1) original condition, (2) condition underwhich all water injection holes were completely filledwith fresh water, and (3) condition under which thewater in all injection holes was replaced with saline
water (electrical conductivity 1.7S/m). At replacingwith saline water, saline water was injected after all
injection holes were dried up using storage pump. For
the profile measurements, stacking were performed
about 200 times at each receiver/transmitter location,and measurements were taken at a 2.5cm interval.
The procedure of the verification tests is shown in
Fig.4.

Fig. 3 Profile measuring lines andconcrete thickness distribution(1) original condition- Profile measurements(2) filled with fresh water- Profile measurements
Fixed point measurements(3) replaced with saline water- Profile measurements- Common Midpoint measurements

Fig. 4 Procedure of verification test

Fig. 5 CMP measurement lines andmeasurement point during injection
In addition to the above tests,
1) CMP (Common Midpoint) measurements werecarried out to evaluate the thickness of the concrete,and2) Fixed point measurements were taken at one point
in order to monitor changes in reflected waveforms
during the injection of saline water into the injection
holes.
With the CMP measuring lines shown in Fig. 5, a
50 to 500MHz continuous wave radar was used totake measurements at a 10cm interval while2

10th Asian Regional Conference of IAEG (2015)

expanding the offset distance from 60cm to 200cm.
At the position shown in Fig. 5, an 800MHz pulse
radar was used to take fixed point measurements tocontinuously monitor the permeating process of
injected saline water from the B’ hole to the B hole.
The electromagnetic reflected waveforms were
measured at 1-minute intervals after saline water
injection. It should be noted that an electricalconductivity meter was set at the bottom of the B holein order to measure break-through process of thesaline water resulting from changes in electricalconductivity.
3.

Test Results

3.1 Results of CMP measurements
Fig. 6 shows the results of CMP measurements.
The reflected waveform was detected later than the
direct wave that was at the velocity of light C =3.0e+8m/sec. The travel time where the direct wave
was intersected with the offset distance of 0 cm wasset as 0 in two-way travel time. When this reflected
waveform was approximated to the theoretical
hyperbola curve, it could be calculated that thereflection boundary was at a depth of 29cm, and the
propagation velocity of the electromagnetic wave tothe reflection surface was 1.2e+8m/sec. The depth of
the reflection boundary appeared to be slightly
shallower than the 36cm thickness of the concrete in
the center of the CMP measuring line. The calculated
velocity of 1.2e+8m/sec corresponded to a dielectric
constant of 6.3, which roughly coincided with thedielectric constant of 4 to 10 of the concrete in a dry
condition (Sato et al., 2001). The result that thecommon reflection boundary of the CMP
measurements was at a position slightly shallower
than the boundary between the concrete and the rock,indicates that the saline water that permeated into theboundary infiltrated into the interior of the concrete,
forming a saturated capillary zone in the concrete,
and that waves reflected from the upper surface of thesaturated capillary zone may have been captured by
the CPM measurements. In addition, from the
propagation velocity obtained in these CMP
measurements, the waves reflected from the saline
water that had permeated into the boundary between
the concrete and the rock was appeared at around 5ns
in the two-way travel time at the profile
measurements.

Fig. 6 Results of CMP measurements

electric conductivity（mS/m）
電気伝導度（mS/m）

2500

2000

B’ hole (injection)

1500

1000

B hole (observation)
500

0

0

10
20
30
elapsing time （min）

40

Fig. 7 Change of EC after injection of saline water
two-way travel time
（ns）
( )
0

5

10

15

elapsing time

Before injection
1 min. later
2 min.
3 min.
4 min.
5 min.
6 min.
7 min.
8 min.
9 min.
10min.
11min.
12min.
13min.
14min.
15min.
16min.
17min.
18min.
19min.

3.2 Results of the fixed point measurements
Fig. 7 shows the changes of the electricalconductivity in the B hole after injection of saline
water into the B’ hole. At 7 minutes after thecommencement of injection, the saline water brokethrough to the B hole, and 35 minutes later theelectrical conductivity in the B hole has reached
1.7S/m, the electrical conductivity of the injected

Fig. 8 Results of fixed point measurements(Difference of amplitude from standard waveform)310th Asian Regional Conference of IAEG (2015)

saline water.
Fig. 8 shows the measurement results of thereflected waves at the fixed point of this permeation
process. The figure shows the results of calculating
the differences in amplitude values of the waveform
after the elapsing time from the standard waveform,
which was the waveform just before the injection ofsaline water. From 5 minutes after injection, thedifferential amplitude of the direct waves at the 2 to
3ns two-way travel time began to increase, and thedifferential amplitude at the 5 to 6ns two-way travel
time also increased. Because it appears from the CMP
measurements that the reflected waves resulting fromthe permeation of the saline water into the boundary
between the concrete and the rock could be observedat around 5ns, the increase in the differential
amplitude indicates an increase in the reflection
intensity due to the permeation of saline water into
the boundary. From these results, it could beconcluded that the main permeation depth was theboundary between the concrete and the rock. In
addition, the commencement of the increase indifferential amplitude from about 5 minutes, before
the break through into the B hole at 7 minutes,
indicates that the saline water arrived directly under
the position of the fixed point measurements after
about 5 minutes had elapsed. Therefore, it was foundthat the permeation velocity of the saline could be
estimated by changes in the reflected waveform.

3.3 Results of the verification testsa) Evaluation by differential profile
The profile measurements were carried out along
three measurement lines under each condition. Fig. 9shows the results of the profile measurements at
measuring lines AB and BC at replacement withsaline water. The reflection from the boundary
between the concrete and the rock, as estimated fromthe CMP measurement results, should have been
observed near 5ns in the two-way travel time, but theamplitude of the direct waves was too large for thisreflection to be confirmed. In addition, there was noclear reflection from the fractures. Therefore, theprofile under conditions of injection of fresh water
was set as the standard profile, and the differential
profile was calculated under the saline water
replacement conditions. Fig. 10 shows the profile
results obtained after differential processing. In thisfigure, a waveform of the large differential amplitude
is seen at around 5 to 6ns, and it could be determinedthat this was reflection from the boundary between
the concrete and the rock. The appearance of changes
in the amplitude due to the differential processing
indicates that the injected saline water permeatedalong this boundary. At the same time, reflected
waves can be seen around the location corresponding
to Fractures”2” to “3”, even at a depth below theboundary. This indicates that the injected saline water
permeated into the fractures through the boundary
and/or the fracture network.

(a) Measuring Line AB(b) Measuring Line BC
Fig. 9 Results of profile measurements at replacement with saline water(a) Measuring Line AB(b) Measuring Line BC
Fig. 10 Profile results after differential processing

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10th Asian Regional Conference of IAEG (2015)b) Evaluation by spectral analysis
Regarding the reflected electromagnetic waves,the reflection coefficient R of media 1 and 2 can beapproximated as follows when the conductivity islow:dominant frequency （MHｚ）

 1‐  2R
1   2350(1)Line AB
300

250

before salin water

fracture"2"

after saline water：decrease of frequency

200
0

0.5

1
location （ｍ）

1.5

2

1.5

2

350

4.

dominant frequency (MHz)where  1 ,  2 : dielectric constants (F/m) of
medium 1 and medium 2, respectively.
On the other hand, when the conductivity is high,the reflection coefficient of the normal incidence isexpressed as a function of angular frequency of theelectromagnetic wave, magnetic permeability (H/m)
and conductivity (S/m) of medium 1 and medium 2.
Here, the influence on reflection of differences in
conductivity cannot be ignored. For example, when ahigh-conductivity medium which contains saline
water is the reflection boundary, it indicates that thehigh-frequency component of the reflected wave isdecreasing (Onishi et al., 2004). From this theory, it
can be estimated that the amplitude of reflected
waves in the area where saline water permeates
greatly attenuates in the high-frequency bands.
Therefore, spectral analysis was applied to confirm
the area where the dominant frequency decreased,
and to evaluate the saline water permeation.
Fig. 11 shows the results of spectral analysis
conducted in the 4ns to 20ns range for the waves after
injection with fresh water and after its replacementwith saline water for each measuring line, and thedominant frequencies that were derived for eachreflected waveform. Due to the specification of thepulse radar used in the test, the dominant frequency
for each waveform was shown to be around 300MHz,
and the area where the frequency decreased aftersaline water replacement was confirmed. A
noticeable decrease was detected at the A hole, butsuch a large decrease was not found at the location
around Fracture 2. From these results, the decrease infrequency was the effect of reflection not from thefracture, but from the concrete and rock boundary.
Fig. 12 shows the location where the frequency
decreased along the measuring lines. In this figure,the areas with thick concrete, that is, the places with alow rock surface, corresponded to the areas offrequency decrease, and it can be concluded that thesaline water injected from the injection holepermeated by passing through the trough of the rocksurface.

Line BC
300

250

fracture"2"

before salin water
after saline water200

dominant frequency （MHｚ）

0

0.5

1
location （ｍ）

350

Line CA
300

250

before salin water
after saline water

A hole

200
0

0.5

1
location （ｍ）

1.5

2

Fig. 11 Results of spectral analysis

Fig. 12 Location of decrease of dominantfrequency and permeation paths

groundwater permeation in the rock of lining
backfaces. As a verification test, fresh (tap) water andsaline water were permeated into fractured granite,and concrete of about 30cm thick was placed on the
granite, then 800MHz pulse radar was used to takeprofile measurements from the concrete surface at a
2.5cm interval. Furthermore, CMP measurements

Conclusions

This study was conducted as a groundwater
survey in fractures around rock caverns to investigate
the practicality of methods for using GPR to evaluate510th Asian Regional Conference of IAEG (2015)

were also carried out, and fixed point measurements
were taken during the injection of the saline water. As
a result, it could be indicated to estimate thepermeation velocity and permeation paths of saline
water in fractures and concrete backfaces, in a
non-destructive way, based on spectral analysis andchanges in the reflected waveforms of GPR.
This method, such as differential profiles andspectral analysis, may make it possible to conduct
non-destructive monitoring of changes in saturated
and non-saturated conditions of groundwater, and of
permeation behavior as a tracer of liquids having
different conductivities. In the next step, the authors
plan to apply this method to monitor the submerge
process of unsaturated zones around rock caverns, tomeasure the break-through process in tracer tests
2-dimensionally, and to monitor the permeation ofgrouting materials in grouting work.
Acknowledgements
We are grateful to Professor T. Tokunaga(University of Tokyo) for useful advices. We also
would like to thank Dr. K. Onishi (Akita University)
for his fruitful discussions.
References
Masumoto, K. and Kurihara, K. (2013) : Evaluationof groundwater flow in a fractured rock mass using
ground penetrating radar, Proc. JSEG, pp. 57-58 (inJapanese).
Masumoto, K. and Kurihara, K. (2014) :Experimental study using artificial fractures forapplication of ground penetrating radar to survey
groundwater flow in a fractured rock mass, Journalof the Japan Society of Engineering Geology，
Vol.55，No.1，pp.17-27 (in Japanese).
Onishi, K., Rokugawa, S. and Katoh, T. (2004) :Estimationofsalineregionsusing
ground-penetrating radar, Inter. Conf. on Ground
Penetrating Radar, 10, pp.509-512.
Sato, M., Uno, T. and Kim Jung-Ho (2001) :Subsurface
measurement
technology
usingelectromagnetic waves, Center for Northeast AsianStudies, Tohoku University, pp.44 (in Japanese).

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