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

2. 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.

3. 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

4. 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.
Liquidator
Vibroseis
Reaction Mass (kg)
5900
1680
Stroke (cm) 40 10
Peak Force (kN) 89
155
Force at 1Hz (kN) 48
3.3
Isolation Resonance (Hz)
0.3
1.5

5. 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

6. General Soil Conditions over Study Depth
General Site Geology
General Soil Conditions over Study Depth
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
Alluvium or Loess
Eocene Deposits
Memphis Sand

7. 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

8. 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

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

10. Field Testing Arrangement
Passive Array
200 m

11. SASW Method Sample Data Receiver Located 340 m from Source
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

12. Active Source f-k Method
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:
Sample Data

13. 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

14. 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

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

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

17. I. SASW Phase Unwrapping Issue
200 m spacing

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

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

20. Shear Wave Velocity Profiles
Site A
Site B

21. Simulated fk and Modal Dispersion
Site A
Site B

22. Soil Profiles at Sites A and B
Site A
Site B

23. Simulated f-k: Synthetic Profile 1

24. Simulated f-k: Synthetic Profile 2

25. Simulated f-k: Synthetic Profile 3

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

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

28. Passive Wavefield Characteristics
Freq=3.5 Hz
Freq=1.6 Hz

29. 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.

30. Acknowledgements This work was supported by:
(1) grant No 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