Deep Shear Wave Velocity Profiling in the Mississippi Embayment Using The NEES Field Shaker

Deep Shear Wave Velocity Profiling in the Mississippi Embayment Using The NEES Field Shaker Brent L. Rosenblad Jianhua Li University of Missouri - Columbia

Motivation • Shear wave velocity profiles are critical input parameters in geotechnical earthquake analysis • Many seismically vulnerable sites in U.S. and worldwide are located on deep soil deposits that are generally not well characterized. • There is need to characterize soil profiles to greater depths than conventional 30 m profiles • Active source studies limited to depths of tens of m • Passive source increasing used for deeper studies • With the advent of low frequency NEES field vibrator, a comprehensive comparison study of active and passive methods for deep Vs profiling (200 m and greater) is possible.

Objective • Present some of the results from extensive field studies of active and passive surface wave methods performed in the Mississippi Embayment using the NEES equipment • Highlight some limitations of common methods

Low-Frequency Shaker (Liquidator) Custom built field shaker designed to address the problem of exciting energy in the frequency band of 5 Hz to less than 0.5 Hz.

Mississippi Embayment Study Area • Many shallow Vs profiles (50 m or less) • Very limited information about deeper deposits • Objective was to determine profiles to 200 to 300 m depth • Measurements performed at 11 sites (mostly CERI seismic stations) Measurement Locations

General Site Geology • Alluvium (lowlands) and Loess (uplands) • Vs~150 to 250 m/s • thickness=10 to 60 m • Silts and Clays (Eocene) • Vs~350 to 450 m/s • thickness=30 to 130 m • Memphis Sand • Vs~600 to 800 m/s • thickness=200 m+ • . . . • Paleozoic Dolomite Depth of 500 to 900 m General Soil Conditions over Study Depth Alluvium or Loess Eocene Deposits Memphis Sand

Surface Wave Methods Active Source • Spectral-Analysis-of-Surface-Waves (SASW) method – 2 channel approach • Multi-channel method using f-k processing Passive Source • 2-D circular array and f-k processing • Refraction Microtremor (ReMi) - passive energy with linear array

Steps in Surface Wave Analysis • Data Collection • Sensor, # sensor, array configuration, frequencies, time or frequency domain, source, source offset etc … • Data Processing • Developing dispersion curve relating surface wave velocity versus frequency or wavelength • Phase unwrapping (SASW), multi-channel transformations • Forward Modeling/Inversion • Match a theoretical dispersion curve to the measured experimental dispersion curve • Two approaches to forward modeling • Modal dispersion curves (typical use fundamental mode) • Effective velocity dispersion curve

Field Testing Arrangement Array 1 (L>150 m) Array 2 (L>300 m)

Field Testing Arrangement Passive Array 200 m

SASW Method • Uses the phase difference recorded between a pairs of receivers with receiver spacing, d, to determine the effective surface wave velocity. • The phase velocity at a given frequency, f, is calculated from the unwrapped phase difference, f, and receiver spacing, d, using: • Procedure is repeated for multiple pairs of receivers to develop a dispersion curve for the site Sample Data Receiver Located 340 m from Source

Active Source f-k Method Sample Data • One of several wavefield transformation methods • Uses a multi-channel receiver array • For each frequency, trial wavenumbers are used to shift and sum the response from all receiver pairs • The phase velocity is calculated from the wavenumber with the maximum power using:

2-D Passive Array f-k Method Sample Data • Similar to 1-D approach but utilizes a 2-D array (typically circular) because the location of source is not known • For each frequency, trial kX and kY values (velocity and direction) are used to shift and sum the response from all receiver pairs • The phase velocity is calculated from the wavenumber with the maximum power using: Peak

Refraction Microtremor (ReMi) • Utilizes the slant stack (p-t)algorithm to develop a frequency-slowness relationship • A spectral ratio is calculated from the power at a each frequency-slowness point normalized by the average power at that frequency • Based on assumption that energy impinges on array from all directions • Identifies likely phase velocity values: peak, and middle “slope” Slowness versus Frequency

Measurement Issues • SASW phase unwrapping error • Fundamental mode inversion error • Wavefield assumption in ReMI

Example Dispersion Curve Comparison ReMi-high ReMi-low ReMi-mid

I. SASW Phase Unwrapping Issue 200 m spacing

I. SASW Phase Unwrapping Issue 200 m spacing 1 2 1 2

II. Fundamental Mode Inversion Site A : fk/fundamental Site B : fk/fundamental Site B Site A Site A : SASW/Effective Site B : SASW/Effective Site A Site B

Shear Wave Velocity Profiles Site A Site B

Simulated fk and Modal Dispersion Site A Site B

Soil Profiles at Sites A and B Site A Site B

Simulated f-k: Synthetic Profile 1

III. Passive Wavefield Assumption Example 2-D ReMi vs. Active Dispersion Comparison

III. Passive Wavefield Assumption Example 2-D ReMi vs. Active Dispersion Comparison to 200 m

Passive Wavefield Characteristics Freq=3.5 Hz Freq=1.6 Hz

Summary • Higher mode transformations at low frequencies can cause errors with: • SASW phase unwrapping • Fundamental mode inversion methods • Need for multi-channel, effective-mode inversion methods • ReMi wavefield assumption may not be valid at low frequencies.

Acknowledgements This work was supported by: (1) grant No. 0530140 from the National Science Foundation as part of the Network for Earthquake Engineering Simulation (NEES) program, (2) USGS Award 06-HQGR0131. The authors also thank personnel from : • Center for Earthquake Research and Information (CERI) at University of Memphis for assistance in accessing the field sites. • Prof. Van Arsdale at University of Memphis • Personnel from NEES at Utexas field site

Deep Shear Wave Velocity Profiling in the Mississippi Embayment Using The NEES Field Shaker