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Proceedings of Indian Geotechnical Conference December 15-17,2011, Kochi (Paper No.B-333)
GEOTECHNICAL SUBSURFACE PROFILING USING GPR SPECTRAL SIGNATURES –
A CASE STUDY
N.Muniappan, Research Scholar,Dept. of Civil Engg.,IIT Bombay,Powai,Mumbai–400076,email: [email protected]
A.Hebsur, Research Scholar, Dept. of Civil Engg., IIT Bombay, Powai, Mumbai – 400 076 email: [email protected]
E.P.Rao, Assoc. Professor, Dept. of civil Engg., IIT Bombay, Powai, Mumbai – 400 076 email:[email protected]
G.Venkatachalam, Emeritus Fellow, Dept. of Civil Engg., IIT Bombay, Powai, Mumbai – 400 076 email: [email protected]
ABSTRACT: Geotechnical exploration is site-specific and, hence, necessarily restricted to limited number of significant
locations at any project site. Therefore, it is not uncommon to come across unexpected deviations in subsurface conditions
during execution. Non-invasive investigation can help to foresee such deviations. In recent times, GPR has gained in
popularity as a tool for such investigations. The availability of low frequency GPR antennae has made it possible to achieve
penetrabilities of significance for geotechnical applications. This paper deals with subsurface investigations using multi-
frequency GPR studies at a construction site within IIT Bombay campus prior to foundation excavation for a proposed
building. The study shows how multispectral GPR data collected at 400MHz, 200MHz, 80MHz and 40MHz together bring out
the subsurface layers and help to foresee anomalies.
INTRODUCTION
The possibility of remote subsurface exploration has attracted
the attention of engineers for the past few decades. A lot of
research has gone into developing different methods of
exploration which could render the hidden details of the
underground clearly visible. A recent advancement in this
direction is the Ground Penetrating Radar (GPR), which is
one such method which sends the radar pulses into the
ground and detects the signals backscattered from the
underground dielectrically dissimilar regions and produces an
image of the subsurface [1,2]. GPRs have found wide range
of applications in buried utility detection, where generally, a
tell-tale hyperbolic expression is manifested. But,
geotechnical mapping is more complex [3] and depends upon
central frequencies, polarization, subsurface conditions of
dielectric contrasts and range. The present study shows how
data collected at multiple frequencies bring out the
subsurface layers. GPR surveys were carried out at a
construction site within IIT Bombay campus prior to
foundation excavation for a proposed building. 80MHz and
40MHz data were collected along a line of length 52m
passing through all the bore holes in Point Mode, at
Dielectric constant of 6. Along the same line of traverse,
400MHz and 200MHz data were collected in distance mode
at Dielectric constant of 6. The quality and the quantity of
GPR data differ from one frequency to another. Hence, using
all the multispectral GPR data, a subsurface profile has been
prepared which shows the variations in the subsurface layers.
Already three boreholes were driven and data was available.
The GPR data was validated with these. The significant
advantage was that the details of subsurface layers could be
obtained even where borehole information was not available,
which is otherwise not possible to get from conventional
geotechnical methods. It also brought out the anomalies in a
few locations. Overall, the study revealed the ortance of GPR
in cost-effective geotechnical exploration and being
forewarned.
THE PRESENT STUDY
The present study involves GPR subsurface profiling in a
construction site. Since the main purpose was to detect the
soil/rock profile over the site, it was decided to use 400 MHz,
200 MHz, 80 MHz and 40 MHz antennae, with
penetrabilities ranging from 3m to 15m. The objective of the
study was to examine the viability of using a GPR to get the
subsurface profile and its variability and to validate the same
with available borehole data. In the present case, three
borehole logs (BH1, BH2, BH3) and corresponding
descriptions of layers were available. GPR data was collected
along 2D grids and linear traverses. A single traverse was
taken from borehole location BH3 to borehole location BH1.
The results of the single traverse are presented in this paper.
This was also useful to check the adequacy of the borehole
investigation.
THE APPROACH
Figure 1 shows the site map along with the locations of
boreholes BH1, BH2 and BH3. GPR data was collected along
the line of traverse AB stretching from near BH3 to a point
close to BH1.
Fig. 1 Site map along with the line of traverse
The initial settings are crucial for data collection and they are
selected based on preliminary trials and evaluation of the
preliminary data so as to get good signal amplitudes. Then
the actual data collection is done. Then data is exported to the
A B
C
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N Muniappan, A Hebsur, E P Rao & G Venkatachalam
proprietary software RADAN6.0 for post processing and
interpretation is carried out for extracting information about
the subsurface layers.
The data acquisition settings are given below in Table 1.
Table 1 Initial Settings made during data collection
Antenna used (MHz) 200, 400, 80,
40
Mode of Data Collection: 200, 400
80, 40
Dist. Mode
Point Mode
Mode of Polarization Co-
Polarized
Transmitter & Receiver Spacing (m) 1
Length or Area of survey 50m
Dielectric Constant 6
Range (ns) 370
Scans/s 64
Scans/m 5
Virtical IIR Filter Range (MHz) 60-100
Horizontal IIR Stack
(No. of Scans)
64
DATA PRESENTATION
The data obtained for traverse AB is presented below. For
200MHz and 400 MHz the radargrams after essential
postprocessing such as position correction and horizontal
scaling i.e., stacking of wiggle traces, have been presented in
Figs 2,3. For 80 MHz and 40 MHz, the results are shown in
Figs 4, 5.in the form of wiggle traces at regular intervals of
5m from the point A.
Fig. 2 Postprocessed radargram for 200MHz
INTERPRETATION
200 MHz and 400MHz antennae are sensitive to details at
shallow depths. Frequencies of 80 MHz and 40 MHz are
sensitive upto depths of about 12 to 15m. Hence,
interpretation upto 12 to 14m is possible when multi-
frequency studies are used.
Fig. 3 Postprocessed radargram for 400 MHz
The following points have been considered for interpretation:
1. The tonal variations with depth in the radargram
2. The amplitude variations as depicted by the wiggle
traces
3. The presence of extremely low amplitude zone of
black colour in the radargram
4. The bright coloured very high amplitude zone at
depths of 7m and beyond
All signals are complex quantities. The real values of
amplitudes are more easily understood and are usually
interpreted visually as indicated below, while the phase
components or their frequency equivalents also contain useful
information.
Visual Interpretation
Some obvious observations based on the radargrams are: The
200 and 400 MHz antennae show the presence of a uniform
layer upto a depth of 0.5m followed by another more
reflecting material upto about 2.5 m. However, 80 MHz and
40 MHz data show that the second layer is extending beyond
2.5m, possibly upto about 7m. Beyond 7m, 80MHz loses the
signal strength and is not sensitive enough for subsurface
profiling, but 40MHz data has penetrated to a maximum of
15m and there is a uniform layer of strongly reflecting
material from 7 to 15 m.
A closer study shows that there is a small zone of non-
reflecting layer at around 5 to 7m. This is typical of a highly
weathered material in which signals are completely
diffracted.
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Geotechnical subsurface profiling using GPR spectral signatures- A case study
Digital Interpretation
However, a more appropriate method of delineation of the
layers would be through digital techniques of signal
processing. In this case, the Hilbert Transform is useful
because it helps to separate the real and the imaginary parts
and hence, the magnitude and the phase and frequency
components as shown in Figs 6, 7. The layers of soil and rock
are clearly brought out in these figures, especially in the
phase components.
(a) Trace at 5m (b) Trace at 10m
(c) Trace at 15m (d) Trace at 20m
(e) Trace at 25m (f) Trace at 30m
(g) Trace at 35m (h) Trace at 40m
(i) Trace at 45m (j) Trace at 50m
Fig. 4 Wiggle traces from 80 MHz data at 5m interval
(a) Trace at 5m (b) Trace at 10m
(c) Trace at 15m (d) Trace at 20m
(e) Trace at 25m (f) Trace at 30m
(g) Trace at 35m (h) Trace at 40m
159
N Muniappan, A Hebsur, E P Rao & G Venkatachalam
(i) Trace at 45m (j) Trace at 50m
Fig. 5 Wiggle traces from 40 MHz data at 5m interval
(a) Magnitude (b) Phase and (c) Frequency
Fig. 6 HT components for 80MHz data
(a) Magnitude (b) Phase and (c) Frequency
Fig. 7 HT components for 40MHz data
VALIDATION OF GPR RESULTS
The GPR results are compared in Fig. 8 with the available
borehole data [4]. The zone of no reflection in the radargram
can be identified as highly weathered Basalt. This, in turn,
serves as the basis for interpreting the layers above as fill
material and soil. Similarly, the underlying strongly reflecting
layer can be interpreted as relatively intact unweathered rock.
The 40 MHz data also shows that this layer extends beyond
the depth of exploration.
(a)BH1 (b) BH2 (c) BH3
Fig. 8 Borehole logs
ANOMALY DETECTION
Figure 9(a) shows the 2D 400 MHz profile at location C (see
Figure 1). The radargram clearly shows that there is rock
even at 0.5 m depth not far from BH 2, which the GPR could
pick up successfully. This is borne out by the photograph
shown in Fig. 9(b), which was taken after excavation for
foundation. Extrapolation from BH 2 would have been
grossly in error.
(a) (b)
Fig. 9. Anomaly at location C
CONCLUSIONS
The following Conclusions are drawn from the present study.
1. The subsurface profile obtained from GPR is in
close agreement with that obtained from boreholes.
Hence, GPR profiles can be used for locations
within the site which are far from the boreholes and
for detecting anomalies, if any.
2. The amplitudes and phase components of the
complex signals need to be taken into account for
interpretation.
3. The 400 MHz and 40 MHz antennae together can
give the profiles upto a depth of about 15m.
4. The fairly uniform radargrams over the entire site
indicate the homogeneity in the medium and the
adequacy of having only three boreholes.
REFERENCES
1. Roberts, R.L., Daniels, J.J., and Peters, L., Jr. (1992)
Improved GPR Interpretation from Analysis of Buried
Target Polarization Properties, 5th EEGS Symposium on the Application of Geophysics to Engineering and
Environmental Problems, Oakbrook, Il, 597-611.
2. Jol, M. H. (2009). “Ground Penetrating Radar: Theory
and Applications”, Elsevier Science, 1st Ed., Killington,
UK.
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Geotechnical subsurface profiling using GPR spectral signatures- A case study
(http://www.blazelabs.com/pics/polarization.gif)
3. Hebsur, A., Muniappan, N., Divya priya, B., and
Prashanth, G., (2010) Subsurface geotechnical profiling
using co-polarized GPR, Proceedings on Civil
Infrastructure Development, Pune, India, 118-122.
4. Hegde, R.A. (2010). Geotechnical investigation report
for the proposed transit workshop at IIT Bombay.
ACKNOWLEDGEMENT
The Authors are grateful to IIT Bombay authorities for the
permission to carry out the GPR studies at the site and for
using the Geotechnical investigation data.
161