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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

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Record

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Types of Waves

Body waves

Through interior of earth

Surface waves

Travelling along the earth surface – like on water ripples

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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

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3

Surface Waves

Rayleigh wave Amplitude decreases

exponentially with depth.

Love wave Faster than Rayleigh

waves

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Seismic Waves

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Body waves

Surface waves

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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

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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

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Wave propagation and Soil Properties - Need

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, G,

, G,

, G,

, G,

, G,

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Wave Velocities in Geomaterials

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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

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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

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1. Geophone

2. Cable

3. Hammer (Source)

4. Processing and Control Unit

Seismic Measurement-Systems

Geophone

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Seismic Source

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Hammer

Air Guns (In water)

Vibroseis

Dynamite

Betsy Gun

Seismic Source

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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

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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

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Measurement at a Geophone

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Shot Record – uniform deposit

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Shot Record – real deposit

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Tim

e (

s)

Source

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Two Layer System

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1 1

2

2

sin

.cos

iV tL

VA

V

h L A

h

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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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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

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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

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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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SPT Velocity (m/s) Time (s)

http://www.belirti.com/english/downhole.htm

Down-Hole Test Record

SASW Test

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Approximate distribution of vertical motion in particles with depth for two surface waves of different wavelengths

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Surface wave dispersion

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C L Source

Near Receiver

Far Receiver

d S

Sensor Array: Midpoint Array

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Dispersion Curve

Inversion Analysis for Interpretation of data

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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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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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Bender Element System (BES)

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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

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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

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Thank You