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: muniappan@iitb.ac.in

A.Hebsur, Research Scholar, Dept. of Civil Engg., IIT Bombay, Powai, Mumbai – 400 076 email: almelu@iitb.ac.in

E.P.Rao, Assoc. Professor, Dept. of civil Engg., IIT Bombay, Powai, Mumbai – 400 076 email:ceepria@iitb.ac.in

G.Venkatachalam, Emeritus Fellow, Dept. of Civil Engg., IIT Bombay, Powai, Mumbai – 400 076 email: gvee@iitb.ac.in

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

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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.

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