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)
120
100
80
60
40
20
0
For
ce,
kN
86420
Frequency, Hz
Conventional Vibroseis Liquidator
1.3 Hz
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 VibroseisReaction Mass
(kg)5900 1680
Stroke (cm) 40 10Peak Force (kN) 89 155
Force at 1Hz (kN)
48 3.3
Isolation Resonance (Hz)
0.3 1.5
Mississippi Embayment Study Area
Measurement Locations
• 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)
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
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
Passive Array
200 m
-1.0x10-3
-0.5
0.0
0.5
1.0
Ve
loci
ty,
in/s
ec
302520151050
Time, minutes
SASW Method
Receiver Located 340 m from Source
-3
-2
-1
0
1
2
3
Pha
se, r
ad
3.02.52.01.51.0
Frequency, Hz
1.0
0.8
0.6
0.4
0.2
0.0
Coh
ere
nce
3.02.52.01.51.0
Frequency, Hz
Sample Data
• 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
dφ
360fVR
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:
fk
2πVR
2-D Passive Array f-k Method
Peak
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:f
2πVR k
kx (rad/m)
ky (
rad/
m)
-0.05 0 0.050.05-0.05
0
0.050.05
Refraction Microtremor (ReMi)
Slowness versus Frequency• Utilizes the slant stack (p- 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”
Measurement Issues
I. SASW phase unwrapping error
II. Fundamental mode inversion error
III. Wavefield assumption in ReMI
Example Dispersion Curve Comparison
800
600
400
200
0
Su
rfa
ce W
ave
Ve
loci
ty,
m/s
1086420
Frequency, Hz
SASW 1D fk (active source) 2D fk (passive source)
6004002000
Wavelength, m
ReMi-high ReMi-low ReMi-mid
II. Fundamental Mode Inversion
Site A : SASW/Effective
Site A : fk/fundamental Site B : fk/fundamental
Site B : SASW/Effective
Site A
Site A
Site B
Site B
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
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