Geotechnical and Geophysical Site Characterization – Huang & Mayne (eds)
© 2008 Taylor & Francis Group, London, ISBN 978-0-415-46936-4
Site characterization studies of Bangalore using a geophysical method
P. Anbazhagan & T.G. Sitharam
Department Civil Engineering, Indian Institute of Science, Bangalore
ABSTRACT: A number of geophysical methods have been proposed for near-surface site characterization
and measurement of shear wave velocity by using a great variety of testing configurations, processing techniques, and inversion algorithms. In particular, two widely-used techniques are SASW (Spectral Analysis of
Surface Waves) and MASW (Multichannel Analysis of Surface Waves). MASW is increasingly being applied to
earthquake geotechnical engineering for the local site characterization, microzonation and site response studies.
A MASW is a geophysical method, which generates a shear-wave velocity (Vs) profile (i.e., Vs versus depth)
by analyzing Raleigh-type surface waves on a multichannel record. MASW system consisting of 24 channels
Geode seismograph with 24 geophones of 4.5 Hz frequency have been used in this investigation. For the site
characterization program, the MASW field experiments consisting of 58 one-dimensional shear wave velocity
tests and 20 two-dimensional shear wave tests have been carried out. The survey points have been selected in
such a way that the results supposedly represent the whole metropolitan Bangalore having an area of 220 km2 .
The average shear wave velocity of Bangalore soils have been evaluated for depths of 5 m, 10 m, 15 m, 20 m,
25 m and 30 m. The subsoil site classification has been made for seismic local site effect evaluation based on
average shear wave velocity of 30 m depth (Vs30 ) of sites using National Earthquake Hazards Reduction Program (NEHRP) and International Building Code (IBC) classification. Soil average shear wave velocity estimated
based on overburden thickness from the borehole information is also presented. Mapping clearly indicates that
the depth of soil obtained from MASW is closely matching with the soil layers in bore logs. Among total 55
locations of MASW survey carried out, 34 locations were very close to the SPT borehole locations and these
are used to generate correlation between Vs and corrected “N” values. The SPT field “N” values are corrected
by applying the NEHRP recommended corrections.
1
INTRODUCTION
To ascertain the manifestation of earthquake shaking
on the ground surface, velocity parameters at a shallow
level would be of consequence. Shear wave velocity (Vs) is an essential parameter for evaluating the
dynamic properties of soil in the shallow subsurface.
A number of geophysical methods have been proposed
for near-surface characterization and measurement of
shear wave velocity by using a great variety of testing configurations, processing techniques, and inversion algorithms. The most widely-used techniques
are SASW (Spectral Analysis of Surface Waves) and
MASW (Multichannel Analysis of Surface Waves).
SASW method uses the spectral analysis of a surface
wave generated by an impulsive source and recorded
by a pair of receivers. Evaluating and distinguishing
signal from noise with only a pair of receivers by this
method is difficult. Thus to improve inherent difficulties, a new technique incorporating multichannel
analysis of surface waves using active sources, named
as MASW, was developed (Park et al., 1999; Xia et al.,
1999; Xu et al., 2006). The MASW has been found to
be a more efficient method for unraveling the shallow
subsurface properties (Park et al., 1999; Xia et al.,
1999; Zhang et al., 2004). MASW is increasingly
being applied to earthquake geotechnical engineering
for microzonation and site response studies. In particular, MASW is used in geotechnical engineering for the
measurement of shear wave velocity and estimation of
dynamic properties, identification of subsurface material boundaries and spatial variations of shear wave
velocity. MASW is non-intrusive and less time consuming geophysical method. It is a seismic method
that can be used for geotechnical characterization of
near surface materials (Park et al., 1999; Xia et al.,
1999; Miller et al., 1999; Park et al., 2005a; Kanli
et al., 2006). MASW identifies each type of seismic
waves on a multichannel record based on the normal
pattern recognition technique that has been used in
oil exploration for several decades. The identification
leads to an optimum field configuration that assures
the highest signal-to-noise ratio (S/N). Effectiveness
in signal analysis is then further enhanced by diversity
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and flexibility in the data processing phase (Ivanov
et al., 2005). MASW is also used to generate 2-D shear
wave velocity profiles. In this paper the average shear
wave velocity for 5 m, 10 m, 15 m, 20 m, 25 m and 30 m
(Vs30 ) has been evaluated and mapped for Bangalore
covering an area of 220 km2 in Bangalore Municipal
corporation limits. The study has been carried out for
assigning soil classification for seismic local site effect
evaluation.
2
STUDY AREA AND MASW
Bangalore city covers an area of over 220 km2 and is
at an average altitude of around 910 m above mean
sea level (MSL). It is situated on a latitude of 12◦ 58
North and longitude of 77◦ 37 East. It is the principal administrative, industrial, commercial, educational
and cultural capital of Karnataka state, in the SouthWestern part of India (Figure 1). There were over 150
lakes, though most of them are dried up due to erosion
and encroachments leaving only 64 at present in an
area of 220 sq km. These tanks were once distributed
throughout the city for better water supply facilities
and are presently in a dried up condition, the residual
silt and silty sand forming thick deposits over which
buildings/structures have been erected. These soil conditions may be susceptible for site amplification during
excitation of seismic waves. Because of density of
population, mushrooming of buildings of all kinds
from mud buildings to reinforced cement concrete
(RCC) framed structures and steel construction and,
improper and low quality construction practice, Bangalore is vulnerable even against average earthquakes
(Sitharam et al., 2006).
Figure 1. Study area with Marked MASW Testing
Locations.
The recent studies by Ganesha Raj and Nijagunappa
(2004), Sitharam et al. (2006) and Sitharam and
Anbazhagan (2007) have suggested that Bangalore be
upgraded from the present seismic zone II (BIS, 2002)
to zone III based on the regional seismotectonic details
and hazard analysis. Hence subsoil classification for
the Bangalore region is important to evaluate seismic
local site effects for an earthquake. From the 3-D subsurface model of geotechnical bore log data developed
by Sitharam et al., (2007), authors have identified that
the overburden thickness of study area varies from 1 m
to about 40 m.
MASW is a geophysical method, which generates a
shear wave velocity (Vs) profile (i.e., Vs versus depth)
by analyzing Raleigh-type surface waves on a multichannel record. A MASW system consisting of a 24
channels Geode seismograph with 24 geophones of
4.5 Hz frequency were used in this investigation. The
seismic waves are created by the impulsive source of 15
pound (sledge hammer) with 300 mm × 300 mm size
hammer plate using ten shots. The recorded Rayleigh
wave is further analyzed using SurfSeis software. SurfSeis is designed to generate Vs data (either in 1-D
or 2-D format) using a simple three-step procedure:
i) preparation of a multichannel record (some times
called a shot gather or a field file), ii) dispersion-curve
analysis, and iii) inversion. MASW has been effectively used with highest signal-to-noise ratio (S/N) of
surface waves. MASW method has been successfully
applied to various types of geotechnical and geophysical projects such as mapping 2-D bedrock surface and
shear modulus of overburden materials (Miller et al.,
1999), generation of shear-wave velocity (Vs) profiles
(Xia et al., 2000), seismic evaluation of pavements
(Ryden et al., 2004), and seismic characterization of
sea-bottom sediments (Park et al., 2005b).
The test locations are selected in such a way that
these supposedly represent the entire city subsurface
information (Figure 1). In total 58 one-dimensional
(1-D) surveys and 20 two-dimensional (2-D) surveys
have been carried out. In about 38 locations MASW
survey point are very close to the SPT borehole locations. Most of the survey locations are selected in flat
ground and also in important places like parks, hospitals, schools and temple yards etc. The optimum field
parameters such as source to first and last receiver,
receiver spacing and spread length of survey lines are
selected in such a way that required depth of information can be obtained. All tests have been carried
out with a geophone interval of 1 m, source has been
kept on both sides of the spread and distance from
source to the first and last receiver were also varied
from 5 m, 10 m and 15 m to avoid the effects of nearfield and far-field. These source distances were helped
to record good signals in very soft, soft and hard soils.
The exploration services section at the Kansas Geological Survey (KGS) has suggested an offset distance
794
for very soft, soft and hard soil as 1 m to 5 m, 5 m to
10 m and 10 m to 15 m respectively (Xu et al., 2006).
3
DATA ANALYSIS AND RESULTS
“Vs”- site classification (Martin, 1994) and IBC code
site classification (IBC-2000).
4 AVERAGE SHEAR WAVE VELOCITY
The generation of a dispersion curve is a critical step
in MASW method. A dispersion curve is generally
displayed as a function of phase velocity versus frequency. Phase velocity can be calculated from the
linear slope of each component on the swept-frequency
record. The lowest analyzable frequency in this dispersion curve is around 4 Hz and A highest frequency
of 75 Hz has been considered. Each dispersion curve
obtained for corresponding locations has a very high
signal to noise ratio of about 80 and above. AVs profile
has been calculated using an iterative inversion process
that requires the dispersion curve developed earlier
as input. A least-squares approach allows automation
of the process (Xia et al., 1999) which is inbuilt in
SurfSeis. Typical 1-D Vs profile is shown in Figure 2.
Borelog close to Vs profile shown in Figure 3, the both
results are matches well. Minimum shear wave velocity found in the region is about 100 m/s and maximum
is about 1200 m/s. Generally the trend in the velocity
profiles is increasing with depth, accordingly to the
soil deposits followed by weathered and hard rock in
the study area. The range of shear wave velocity values obtained from each survey line for the different
layers falls within the recommendations of NEHRP
Elastic properties of near-surface materials and their
effects on seismic wave propagation are very important
in earthquake geotechnical engineering, civil engineering and environmental earth science studies. The
seismic site characterization for calculating seismic
hazard is usually carried out based on the near-surface
shear wave velocity values. The average shear wave
velocity for the depth “d” of soil is referred as VH .
The average shear wave velocity up to a depth of H
(VH ) is computed as follows:
Where H =
di = cumulative depth in m.
For 30 m average depth, shear wave velocity is
written as:
where di and vi denote the thickness (in meters) and
shear-wave velocity in m/s (at a shear strain level of
10−5 or less) of the ith formation or layer respectively,
in a total of N layers, existing in the top 30 m. Vs30 is
Shear wave velocity (m/s)
0
100
200
300
400
0
⫺2
⫺4
Depth(m)
⫺6
⫺8
⫺10
⫺12
⫺14
⫺16
⫺18
Figure 2. Typical 1-D shear wave velocity profile.
Figure 3. Borelog close to Vs profile in figure 2.
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Table 1. Typical average shear wave velocity calculation.
13.06N
Average shear wave velocity for
different depth
Depth
(m)
Vs
Soil(m/s) 72 m 5 m 10 m 15 m 20 m 25 m 30 m
−1.22
−2.74
−4.64
−7.02
−10.00
−13.71
−18.36
−24.17
−31.43
−39.29
316
250
255
241
388
355
435
527
424
687
259
265 286
310
338
362
Latitude (Degree North)
13.04N
306
600 m/s
13.02N
500 m/s
13N
400 m/s
12.98N
300 m/s
12.96N
200 m/s
12.94N
100 m/s
12.92N
77.54E 77.56E 77.58E 77.6E 77.62E 77.64E 77.66E
Longitude (Degree East)
Figure 4. Average shear wave velocity for 5 m depth.
5
SHEAR WAVE VELOCITY DISTRIBUTION
IN BANGALORE
The soil overburden thickness and depth of rock
level information are obtained from the Geotechnical Geographical information system (GIS) data base
developed by Sitharam et al. (2007) for Bangalore. The
north western part has lesser overburden thickness.
However, eastern part, central and other areas have
the overburden thickness of 4 m to about 40 m. The
calculated average shear wave velocities are grouped
according to the NEHRP site classes and the related
maps has been generated. For the mapping of shear
wave velocity the Surfer plotting software has been
used. For the interpolation of data gridding method
of minimum curvature has been used. The average
13.06N
13.04N
760 m/s
Latitude (Degree North)
accepted for site classification as per NEHRP classification and also UBC classification (Uniform Building
Code in 1997) (Dobry et al., 2000; Kanli et al., 2006).
In order to figure out the average shear wave velocity distribution in Bangalore, the average velocity has
been calculated using the equation (1) for each location. A simple spread sheet has been generated to carry
out the calculation, as shown in Table 1. TheVs average
has been calculated for every 5 m depth interval up to
a depth of 30 m and also average Vs for the soil overburden has been calculated. Usually, for amplification
and site response study the 30 m average Vs is considered. However, if the rock is found within a depth of
about 30 m, nearer surface shear wave velocity of soil
has to be considered. Otherwise, Vs30 obtained will be
higher due to the velocity of the rock mass. In Bangalore the soil overburden thickness varies from 1 m to
about 40 m. Hence, for overburden soil alone average
Vs has also been calculated based on the soil thickness
corresponding to the location, which is also shown in
column 3 of Table 1.
13.02N
600 m/s
13N
500 m/s
400 m/s
12.98N
300 m/s
12.96N
200 m/s
100 m/s
12.94N
12.92N
77.54E 77.56E 77.58E 77.6E 77.62E 77.64E 77.66E
Longitude (Degree East)
Figure 5. Average shear wave velocity for 10 m depth.
shear wave velocity calculated for 5 m, 10 m, 15 m,
20 m, 25 m and 30 m depths are mapped and shown
in Figures 4, 5, 6, 7, 8 and 9 respectively. From
Figure 4, the average velocity up to a depth of 5 m
covering most of the study area has a velocity range
of 180 m/s to 360 m/s. Few locations in south eastern part and in smaller portion of northwestern part
of Bangalore have the velocity less than 180 m/s indicating soft soil. The depth may also extend beyond
5 m, matching with the rock level shown in Table 1.
The average shear wave velocity for 10 m depth varies
from 180 m/s to 360 m/s (Figure 5). In the 10 m average map, very dense soil/soft rock velocity range of
360 m/s to760 m/s is found in western part of the
study area. In this location, the rock depth is found
within 10 m as seen in Table 1. Figure 6 show that the
area covered has a very dense soil/soft rock, which
is increased when compared to Figure 6. In this map
south eastern part having an average velocity less than
796
13.06N
13.06N
13.04N
13.04N
900 m/s
13.02N
Latitude (Degree North)
Latitude (Degree North)
760 m/s
600 m/s
13N
500 m/s
400 m/s
12.98N
300 m/s
12.96N
200 m/s
760 m/s
600 m/s
13N
500 m/s
12.98N
400 m/s
300 m/s
12.96N
200 m/s
100 m/s
12.94N
13.02N
100 m/s
12.94N
12.92N
12.92N
77.54E 77.56E 77.58E 77.6E 77.62E 77.64E 77.66E
77.54E 77.56E 77.58E 77.6E 77.62E 77.64E 77.66E
Longitude (Degree East)
Longitude (Degree East)
Figure 6. Average shear wave velocity for 15 m depth.
Figure 8. Average shear wave velocity for 25 m depth.
13.06N
13.06N
13.04N
13.04N
1500 m/s
760 m/s
Latitude (Degree North)
Latitude (Degree North)
880 m/s
13.02N
600 m/s
13N
500 m/s
12.98N
400 m/s
300 m/s
12.96N
200 m/s
13.02N
1000 m/s
760 m/s
13N
600 m/s
500 m/s
12.98N
400 m/s
300 m/s
12.96N
100 m/s
12.94N
200 m/s
100 m/s
12.94N
12.92N
77.54E 77.56E 77.58E 77.6E 77.62E 77.64E 77.66E
12.92N
Longitude (Degree East)
77.54E 77.56E 77.58E 77.6E 77.62E 77.64E 77.66E
Longitude (Degree East)
Figure 7. Average shear wave velocity for 20 m depth.
Figure 9. Average shear wave velocity for 30 m depth.
13.06N
13.04N
Latitude (Degree North)
180 m/s, matching with the larger overburden thickness. Similar increased area of higher velocity is found
in average depths of 20 m and 25 m shear wave velocity profiles (Figures 7 and 8). Figure 9 shows the
map of average shear wave velocity for a depth of
30 m. Even though the average shear wave velocity
is calculated for every 5 m depth intervals and up to a
maximum depth of 30 m, these maps do not show the
average shear wave velocity of soil because of the wide
variation in the soil overburden/ rock level. Hence,
the average shear wave velocity of soil has been calculated based on the overburden thickness obtained
from bore holes close to the MASW testing locations.
The average shear wave velocity for soil overburden in
the study area is shown in Figure 10. Figure 10 shows
that most of the study area has medium to dense soil
with a velocity range of 180 m/s to 360 m/s. Based on
the 30 m average shear wave velocity and average soil
overburden velocity maps, major part of Bangalore
BMP (Bangalore Mahanagara Palike) has the shear
600 m/s
13.02N
500 m/s
13N
400 m/s
12.98N
300 m/s
12.96N
200 m/s
12.94N
100 m/s
12.92N
77.54E 77.56E 77.58E 77.6E 77.62E 77.64E 77.66E
Longitude (Degree East)
Figure 10. Average shear wave velocity for only soil overburden.
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wave velocity of 180 m/s to 360 m/s. Hence BMP area
can be classified as “Site class D” as per NEHRP and
IBC recommendation.
6
CONCLUSIONS
In this study MASW one-dimensional survey at 58
locations and two-dimensional surveys at 20 locations
have been carried out in an area of 220 km2 in Bangalore city. The shear wave velocity profiles (Vs versus
depth), spatial variability of shear wave velocity (Vs
versus depth and length) and ground layer anomalies
have been presented. The average shear wave velocity of study area has been estimated for 5 m, 10 m,
15 m, 20 m, 25 m and 30 m depth and presented in
this paper. Also average shear wave velocity for the
soil depth, which is estimated based on overburden
thickness, is presented. Site soil classification has been
carried out by considering the NEHRP and IBC classification. Based on the estimated Vs30 , BMP area can
be classified as “site class D” as per NEHRP and IBC
classification chart.
ACKNOWLEDGEMENT
This work is carried out as part of the project titled
“Seismic Microzonation of Grater Bangalore region”.
Authors thank Seismology division, Department of
Science and Technology, Government of India for
funding the project. (Ref no. DST/23(315)/SU/2002
dated October 2003).
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