Geophysical Site Investigation
(Seismic methods)
Amit Prashant
Indian Institute of Technology Gandhinagar
Short Course on
Geotechnical Aspects of Earthquake Engineering
04 – 08 March, 2013
Seismic Waves

Energy travelling through earth layers
Record
2
1
Types of Waves

Body waves
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Through interior of earth
Surface waves

Travelling along the earth surface
– like on water ripples
3
Body Waves

Compression wave



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Compression and Expansion –
Volume change
Velocity-Moisture
content dependent
Fastest body wave
Shear wave


Do not travel through fluids
No Volume change
4
2
Surface Waves

Rayleigh wave


Amplitude decreases
exponentially with
depth.
Love wave

Faster than Rayleigh
waves
5
Body
waves
Seismic
Waves
Surface
waves
6
3
Wave Velocities

P-wave velocity – Vp

Shear Wave velocity – Vs
Vp > Vs
7
Soil Properties from Wave Velocity

2
Shear Modulus G   .Vs
Density of soil



2
Constrained Modulus, M   .V p
Young’s Modulus, E 
Poisson’s Ratio,  
Vs2  3V p2  4Vs2 
V p2  Vs2
V p2  2Vs2
2 V p2  Vs2 
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4
Typical Values of Wave Velocities
Material
Air
Water
Petroleum
Steel
Concrete
Granite
Basalt
Sandstone
Limestone
Sand (Unsaturated)
Sand (Saturated)
Clay
Glacial Till (Saturated)
P-wave Velocity (m/s)
332
1400-1500
1300-1400
6100
3600
5500-5900
6400
1400-4300
5900-6100
200-1000
800-2200
1000-2500
1500-2500
S-wave Velocity (m/s)
3500
2000
2800-3000
3200
700-2800
2800-3000
80-400
320-880
400-1000
600-1000
9
Wave propagation and Soil
Properties - Need
, G, 
, G, 
, G, 
, G, 
, G, 
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5
Wave Velocities in Geomaterials
11
Preparation for Investigation

Always visit site first. AVOID SURPRISES
Database Information
 Maps


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Topographical, Air photos, Geological maps (bedrock and
surficial geology), Soil survey maps, Oil company logs
Water Well Logs
Previous Reports


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Internal studies, Old reports, Previous consultant’s reports
Local practice, foundation types for similar structures nearby
Representative samples, nearby boreholes
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6
Steps in Investigation

Reconnaissance : Ground surface profile, Rock outcrop, Locality
and constraints, Utilities, Interviewing residents, etc.

Method selection : Technical, Cost considerations

Designing the survey : Configurations to serve the objective

Date Acquisition

Data Processing : Signal processing, Modeling

Interpretation : Soil properties with depth
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Seismic System
Source
Computer
Receiver
Seismic waves
14
7
Seismic Measurement-Systems
1. Geophone
2. Cable
3. Hammer (Source)
4. Processing and
Control Unit
15
Geophone
16
8
Seismic Source
Hammer
Vibroseis
Betsy Gun
Dynamite
Air Guns
(In water)
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Seismic Source
18
9
Seismic Methods

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Seismic Reflection Method
Seismic Refraction Method
Cross-Hole Test
Down Hole Test & Up-Hole Test
Spectral Analysis of Surface Wave (SASW)
Multichannel Analysis of Surface Waves
(MASW) method
Bender Element Test in Laboratory
19
Waves from point source
20
10
Snell’s Law
Critical Angle
of Refraction
V 
A  sin 1  1 
 V2 
21
Seismic Refraction Method
Depths less than ~ 30
m
Cost Effective as
compared to
Reflection method
(<3to5 times)
Used for
computation of layer
thickness of soil
http://www.geologicresources.com/seismic_refraction_method.html
22
11
Measurement at a Geophone
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Shot Record – uniform deposit
24
12
Shot Record – real deposit
Time (s)
Source
25
Two Layer System
h
L
V1ti
2
V 
A  sin 1  1 
 V2 
h  L.cos A
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26
13
Multi-Layer System
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27
Seismic Reflection Method
Depths greater than
~15 m
Particularly suited to
marine applications (e.g.
lakes, rivers, oceans, etc.)
The inability of water to
transmit shear waves
makes collection of high
quality reflection data
possible even at very
shallow depths that
would be impractical to
impossible on land.
28
14
Differences in Seismic Reflection and
Seismic Refraction Method
Seismic Reflection uses field equipment
similar to seismic refraction, but field
and data processing procedures are
employed to maximize the energy
reflected along near vertical ray paths
by subsurface density contrasts.
http://www.enviroscan.com/html/seismic_refraction_versus_refl.html
Seismic Refraction involves measuring
the travel time of the component of
seismic energy which travels down to the
top of rock (or other distinct density
contrast), is refracted along the top of
rock, and returns to the surface.
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Cross-Hole Test
Sensors are placed at one
elevation in one or more
boring. Source is triggered
in another boring at the
same elevation.
S wave travels horizontally
from source to receiving
hole, and the arrivals of S
waves are noted
Shear wave velocity (Vs) is
calculated by dividing the
distance between the bore
holes and the travel time.
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15
Cross-Hole Test
31
geo.cv.nctu.edu.tw/EngGeo/download/D4428D4428M.pdf
Cross-Hole Record
http://www.structuremag.org/article.aspx?articleID=994
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16
Down Hole Test
Sensors are placed at
various depths in the
boring. Source is located
above the receivers, at
the ground surface.
Only one bore hole is
required.
A source rich in S wave
should be used (P wave
travels faster than S wave)
Up-Hole method: source of energy is deep in boring and
the receiver is at the ground surface
http://www.geophysics.co.uk/mets3.html
33
Seismic Cone Penetration Test (SCPT)
A Down-Hole Test
Seismic cone is pushed
into the ground
Shear wave is generated
at the top and the time
required for the shear
wave to reach the
seismometer in the cone
is measured
Computer in the SCPT
rig collects and processes
all the data & shear
wave velocity is
measured
34
http://geoprobe.com/how-seismic-cone-penetration-equipment-works
17
Down-Hole Test Record
SPT
Velocity (m/s)
Time (s)
35
http://www.belirti.com/english/downhole.htm
SASW Test
36
18
Surface wave dispersion
Approximate distribution of vertical motion in particles with
depth for two surface waves of different wavelengths
37
Sensor Array: Midpoint Array
CL
Source
S
Near
Receiver
d
Far Receiver
38
19
Dispersion Curve
39
Inversion Analysis for
Interpretation of data
The shear wave velocity profile that generates a dispersion curve that
most closely matches the field dispersion curve is then presented as the
shear wave velocity profile for the site.
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20
Bender Elements – Piezoelectric Sensors




Bender element is a 2-layer system
of the piezoelectric sensors.
Piezoelectric sensors change their
dimensions when electrically
charged by a voltage. They can
generate electric charge when
mechanically stressed by a force.
The element produces curvature,
when one layer expands and the
other layer contracts.
With alternating electric charge, the
element can vibrate and work as a
wave generator.
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Source
Receiver
Receiver
Source
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21
Bender Element System (BES)
Sensors
Signal
43
http://www.sciencedirect.com/science/article/pii/S0267726104001563
BES Measurements
P-Wave velocity:
Vp 
L  2l
t p
S-Wave velocity:
Vs 
L  2l
ts
L = Distance between source and receiver element
l = Length of the element
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22
Damping Ratio using Half-Power Method
By varying the frequency with constant input
voltage amplitude

f 2  f1
2 fm
Or, sometimes it is preferred to use
f 22  f12

4 f m2
45
Thank You
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