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1
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
2
Record
2
Types of Waves
Body waves
Through interior of earth
Surface waves
Travelling along the earth surface – like on water ripples
3
Body Waves
Compression wave Compression and Expansion –
Volume change
Velocity-Moisture content dependent
Fastest body wave
Shear wave Do not travel through fluids
No Volume change
4
3
Surface Waves
Rayleigh wave Amplitude decreases
exponentially with depth.
Love wave Faster than Rayleigh
waves
5
Seismic Waves
6
Body waves
Surface waves
4
Wave Velocities
P-wave velocity – Vp
Shear Wave velocity – Vs
Vp > Vs
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Soil Properties from Wave Velocity
Shear Modulus
Constrained Modulus,
Young’s Modulus,
Poisson’s Ratio,
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2. sG V
Density of soil
2. pM V
2 2 2
2 2
3 4s p s
p s
V V VE
V V
2 2
2 2
2
2
p s
p s
V V
V V
5
Material P-wave Velocity (m/s) S-wave Velocity (m/s)
Air 332
Water 1400-1500
Petroleum 1300-1400
Steel 6100 3500
Concrete 3600 2000
Granite 5500-5900 2800-3000
Basalt 6400 3200
Sandstone 1400-4300 700-2800
Limestone 5900-6100 2800-3000
Sand (Unsaturated) 200-1000 80-400
Sand (Saturated) 800-2200 320-880
Clay 1000-2500 400-1000
Glacial Till (Saturated) 1500-2500 600-1000
Typical Values of Wave Velocities
9
Wave propagation and Soil Properties - Need
10
, G,
, G,
, G,
, G,
, G,
6
Wave Velocities in Geomaterials
11
Preparation for Investigation
Always visit site first. AVOID SURPRISES
Database Information
Maps Topographical, Air photos, Geological maps (bedrock and
surficial geology), Soil survey maps, Oil company logs
Water Well Logs
Previous Reports Internal studies, Old reports, Previous consultant’s reports
Local practice, foundation types for similar structures nearby
Representative samples, nearby boreholes
12
7
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
Steps in Investigation
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Seismic System
Source Receiver
Computer
Seismic waves
14
8
15
1. Geophone
2. Cable
3. Hammer (Source)
4. Processing and Control Unit
Seismic Measurement-Systems
Geophone
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9
Seismic Source
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Hammer
Air Guns (In water)
Vibroseis
Dynamite
Betsy Gun
Seismic Source
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10
Seismic Methods
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
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Waves from point source
20
11
Snell’s Law
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Critical Angle of Refraction
1 1
2
sinV
AV
Seismic Refraction Method
http://www.geologicresources.com/seismic_refraction_method.html
Depths less than ~ 30 m Cost Effective as compared to Reflection method (<3to5 times) Used for computation of layer thickness of soil
22
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Measurement at a Geophone
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Shot Record – uniform deposit
24
13
Shot Record – real deposit
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Tim
e (
s)
Source
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Two Layer System
1
1 1
2
2
sin
.cos
iV tL
VA
V
h L A
h
26
14
27 27
Multi-Layer System
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.
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15
Differences in Seismic Reflection and Seismic Refraction Method
http://www.enviroscan.com/html/seismic_refraction_versus_refl.html
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.
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. 30
16
Cross-Hole Test
31 geo.cv.nctu.edu.tw/EngGeo/download/D4428D4428M.pdf
Cross-Hole Record
32 http://www.structuremag.org/article.aspx?articleID=994
17
Down Hole Test
http://www.geophysics.co.uk/mets3.html
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 33
Seismic Cone Penetration Test (SCPT) A Down-Hole Test
http://geoprobe.com/how-seismic-cone-penetration-equipment-works
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
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18
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SPT Velocity (m/s) Time (s)
http://www.belirti.com/english/downhole.htm
Down-Hole Test Record
SASW Test
36
19
Approximate distribution of vertical motion in particles with depth for two surface waves of different wavelengths
37
Surface wave dispersion
38
C L Source
Near Receiver
Far Receiver
d S
Sensor Array: Midpoint Array
20
39
Dispersion Curve
Inversion Analysis for Interpretation of data
40
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.
21
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.
41
42
Source
Receiver
Receiver
Source
22
Bender Element System (BES)
43
Signal
Sensors
http://www.sciencedirect.com/science/article/pii/S0267726104001563
BES Measurements
2p
p
L lV
t
P-Wave velocity:
2s
s
L lV
t
S-Wave velocity:
L = Distance between source and receiver element
l = Length of the element 44
23
Damping Ratio using Half-Power Method
2 2
2 1
24 m
f f
f
By varying the frequency with constant input voltage amplitude
2 1
2 m
f f
f
Or, sometimes it is preferred to use
45
46
Thank You