MASW

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Multichannel Analysis of Surface Waves (MASW)

• Short Course
• October, 2000

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Instructor

• Education
• 1984, Seoul National University (B.S.)
• 1988, Ohio University (M.S.)
• 1995, The University of Kansas (Ph.D.)
• Expertise
• Seismic Technique Development for Near-Surface
  Application
• Swept Impact Seismic Technique (SIST)
• MASW
• Software Development
• WinSeis
• SurfSeis

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Short Course (Schedule)

• Introduction to MASW (Friday, 10/13)
• Field Survey I (Saturday, 10/14)
• Field Survey II (Monday, 10/16)
• Use of SurfSeis (Tuesday, 10/17)

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Introduction to MASW

• CONTENTS
• Geophysical Methods for Near-Surface
  Investigation
• Surface-Wave Method
• SASW and MASW
• Higher Modes (Overtones)
• Overall Procedure of MASW
• Case Studies with MASW

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Geophysical Methodsfor Near-Surface Investigation

• Seismic
• Resistivity
• Electromagnetic (EM)
• Ground Penetrating Radar (GPR)
• Potential Field (e.g., Gravity Survey)

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Seismic Methods

• Refraction Method
• Reflection Method
• Surface-Wave Method
• Joint (RefractionSurface Wave) Method

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Seismic Method (Refraction)

• Most Widely Used Method for Engineering
  Applications
• Reconnaissance Method
• (I.e., inappropriate as Exploration Method)
• Problem with Velocity Reversal
• Tomographic Approach-Most Advanced

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Seismic Method (Reflection)

• High-Resolution Imaging
• Most Difficult Method
• Weak Signal
• Complicated Processing
• Limitation with Wavelength
• Future - Ultra High Frequency (UHF) Method

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Seismic Method (Surface Wave)

• Simplest and Easiest Method for Engineering
  Applications
• Reconnaissance and Exploration
• Both S-Wave and P-Wave Velocity
• Widest Application

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Surface-Wave Method

• CONTENTS
• Surface Waves
• MASW
• MASW and SASW

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Surface Waves

• Ground Roll
• Large Amplitude
• Dispersion Property
• Near-Surface

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Dispersion of Surface Waves
Dispersion Curve
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Dispersion and Attenuation
Earth Model
Dispersion Curve
Attenuation Curve
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Multichannel Analysis of Surface Waves (MASW)

• Pattern Recognition
• Fast Survey
• Various Multichannel Processing Methods

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Higher Modes (Overtones) of Surface Waves

• Wave Equation Predicts
• Multiple Velocities for One Frequency
• Energy and Velocity changes with Layer Model,
  Source and Receiver Parameters

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Layer Model and Higher Modes
Layer Model
Amplitudes
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Offset and Higher Modes
River-Bottom Hydrophone Data (Fraser River, B.C.)
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Dispersion Curves Fraser River, B.C.
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Higher Modes Why Excluded?

• Not Significant Energy?
• Not Easy to Detect?

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MASW and SASW

• Multichannel Analysis of Surface Waves (MASW)
• -Pattern Recognition
• -Diverse Processing
• -P- and S-Wave Velocities
• -1-D and 2-D Profiling
• Spectral Analysis of Surface Waves (SASW)
• -Two Receivers (Q/C Impossible)
• -Repeated Measurements (Time
• and Labor Intensive)
• -Dispersion Curves Speculative
• -S-Wave Velocity only
• -1-D

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MASW and SASW
SASW
MASW

• Offset Dependent Dispersion Curve
• Risk of Error
• Many Measurements

• Pattern Recognition
• Accurate Dispersion Curves
• Higher Modes

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Procedure of MASW(3 Steps)
Acquisition
Dispersion Analysis
Inversion
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MASW - Applications
1-D S-Wave Profile
2-D S-Wave Map
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MASW - Applications
2-D 3-D Vp, Vs, and Poisson's Ratio Map
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MASW - Field Procedure

• Seismic Source
• Receivers
• Seismograph
• Offsets (Source Offset and Receiver Spacing)
• Field Notes

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MASW - Seismic Source

• Hammer
• Near-Surface Material
• The Heavier, The Deeper
• Strong Source Low Frequency
• Weak Source High Frequency
• Investigation Depth (Z)
• Z lt 5 m ( lt 2 Kg)
• Z lt 10 m (lt 5 Kg)
• Z gt 10 m (10 Kg)

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MASW - Seismic Receivers

• Low-Frequency Geophones
• Near-Surface Material
• The Lower Frequency, The Deeper
• Investigation Depth (Z)
• Z lt 5 m ( 4.5 Hz, 10 Hz, 20 Hz)
• Z lt 10 m (4.5 Hz, 10 Hz)
• Z gt 10 m (4.5 Hz)

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MASW - Seismograph

• Minimum 12 Channel
• Low Dynamic Range OK
• Preparation of (at least) 24 traces by Walkaway
  Format
• Absolutely No Low-Cut Filter

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MASW - Offsets

• Source Offset
• Approximately 1/2 Max. Depth of Investigation
• Receiver Spacing
• The Softer Material, The Narrower
• Usual Soil Sites (0.5 m)
• Hard Surface (1 m)
• Noise Analysis

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MASW - Field Notes

• Source
• Sampling Interval (e.g., 1 ms)
• Recording Time (e.g., 1000 ms)
• Number of Channels (e.g, 12)
• Geophone (e.g., 4.5 Hz)
• Field Map
• Each Shot
• File Name (e.g., 1001.dat)
• Location of Source and Receiver (e.g., Station
  Number)
• Source Offset (e.g., 5 m)
• Receiver Spacing (e.g., 0.5 m)