Predicting Site Response

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Predicting Site Response
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Predicting Site Response

• Based on theoretical calculations
• 1-D equivalent linear, non-linear
• 2-D and 3-D non-linear
• Needs geotechnical site properties

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Imaging of Near-Surface Seismic Slowness
(Velocity) and Damping Ratios (Q)
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Image What?

• Sß(z) (shear-wave slowness) (1/velocity)
• Sa(z) (compressional-wave slowness)
• ?ß(z) (shear-wave damping ratio Qß)

Why?

• Site amplification
• Site classification for building codes
• Identification of liquefaction and landslide
  potential
• Correlation of various properties (e.g., geologic
  units and Vs)

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Why Slowness?

• Travel time in layers directly proportional to
  slowness travel time fundamental in site
  response (e.g., T 4sh 4travel time)
• Can average slowness from several profiles
  depth-by-depth
• Slowness is the usual regression coefficient in
  fits of travel time vs. depth
• Visual comparisons of slowness profiles more
  meaningful for site response than velocity
  profiles

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Why Show Slowness Rather Than Velocity?
Large apparent differences in velocity in deeper
layers (usually higher velocity) become less
important in plots of slowness
Focus attention on what contributes most to
travel time in the layers
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Imaging Slowness

• Invasive Methods
• Active sources
• Passive sources
• Noninvasive Methods
• Active sources
• Passive sources

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Invasive Methods

• Active Sources
• surface source
• downhole source
• Passive sources
• Recordings of earthquake waves in boreholes---not
  covered in this talk

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Invasive Method Surface Source-- Downhole
Receiver (ssdhr) (receiver can be on SCPT rod)
One receiver moved up or down hole
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SURFACE SOURCE ---SUBSURFACE RECEIVERS

• downhole profiling
• velocities from surface
• data gaps filled by average velocity
• expensive (requires hole)
• depth range limited (but good to gt 250 m)
• seismic cone penetrometer
• advantages of downhole
• inexpensive
• limited range
• not good for cobbly materials, rock

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Create a record sectionopposite directions of
surface source (red, blue traces) Pick arrivals
(black)
Plotting sideways makes it easier to see slopes
changes by viewing obliquely (an exploration
geophysics trick)
CCOC
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Finer layering in upper 100m
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Two models from the same travel time picks.
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The increased resolution makes little difference
in site amplification
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SUBSURFACE SOURCE --- SUBSURFACE RECEIVERS

• crosshole
• point measurements in depth
• expensive (2 holes)
• velocity not appropriate for site response
• suspension logger
• rapid collection of data (no casing required)
• average velocity over small depth ranges
• can be used in deep holes
• expensive (requires borehole)
• no way of interpolating across data gaps

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Downhole source--- P-S suspension logging (aka
PS Log)
Dominant frequency 1000 Hz
From Geovision
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Example from Coyote Creek note 1) overall trend
2) scatter 3) results averaged over various
depth intervals reduces noise
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Noise fluctuations in both S and P logs agree
with variations in lithology! (No averaging)
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Some Strengths of Invasive Methods

• Direct measure of velocity
• Surface source produces a model from the surface,
  with depth intervals of poor or missing data
  replaced by average layer (good for site
  amplification calculations)
• PS suspension logging rapid, can be done soon
  after hole drilled, no casing required, not
  limited in depth range

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Some Weaknesses of Invasive Methods

• Expensive! (If need to drill hole)
• Surface source may have difficulties in deep
  holes, requires cased holes, logging must wait
• PS suspension log does not produce model from the
  surface (but generally gets to within 1 to 2 m),
  and there is no way of interpolating across depth
  intervals with missing data.

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Noninvasive Methods

• Active Sources
• e.g., SASW and MASW
• Passive sources (usually microtremors)
• Single station
• Arrays (e.g., fk, SPAC)
• Combined activepassive sources

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Overview of SASW and MASW Method

• Spectral-Analysis-of-Surface-Waves (SASW2
  receivers) Multichannel Analysis of Surface
  Waves (MASWmultiple receivers)
• Noninvasive and Nondestructive
• Based on Dispersive Characteristics of Rayleigh
  Waves in a Layered Medium

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SASW Field Procedure

• Transient or Continuous Sources (use several per
  site)
• Receiver Geometry Considerations
• Near Field Effects
• Attenuation
• Expanding Receiver Spread
• Lateral Variability

(Brown)
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SASW MASW Data Interpretation
Dispersion curve built from a number of subsets
(different source, different receiver spreads)
(Brown)
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Some Factors That Influence Accuracy of SASW
MASW Testing

• Lateral Variability of Subsurface
• Shear-Wave Velocity Gradient and Contrasts
• Values of Poissons Ratio Assumed in the
  inversion of the dispersion curves
• Background Information on Site Geology Improves
  the Models

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Noninvasive Methods

• Passive sources (usually microtremors)
• Single station (much work has been done on this
  method---e.g., SESAME project. I only mention it
  in passing, using some slides from an ancient
  paper)

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Ellipticity (H/V) as a function of frequency
depends on earth structure
(Boore Toksöz, 1969)
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Noninvasive Methods

• Passive sources (usually microtremors)
• Multiple stations (usually two-dimensional
  arrays)

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The array of stations at WSP used by Hartzell
(Hartzell, 2005)
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Inverting to obtain velocity profile
(Hartzell, 2005)
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Noninvasive Methods

• Often active sources are limited in depth (hard
  to generate low-frequency motions)
• Station spacing used in passive source
  experiments often too large for resolution of
  near-surface slowness
• Solution Combined activepassive sources

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An example from the CCOCWSP experiment (active
f gt 4 Hz passive flt8 Hz)
(Yoon and Rix, 2005)
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Comparing Different Imaging Results at the Same
Site

• Direct comparison of slowness profiles
• Site amplification
• From empirical prediction equations
• Theoretical
• Full resonance
• Simplified (Square-root impedance)

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Comparison of slowness profiles
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Coyote Creek Blind Interpretation Experiment
(Asten and Boore, 2005)
CCOC Coyote Creek Outdoor Classroom
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The Experiment

• Measurements and interpretations done voluntarily
  by many groups
• Interpretations blind to other results
• Interpretations sent to D. Boore
• Workshop held in May, 2004 to compare results
• Open-File report published in 2005 (containing a
  summary by Asten Boore and individual reports
  from participants)

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Active sources at WSP note larger near-surface
smaller deep slownesses than reference for most
methods.
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Passive sources at WSP note larger near-surface
smaller deep slownesses than reference for most
methods. Models extend to greater depth than do
the models from active sources
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Combined active passive sources at WSP note
larger near-surface slownesses than reference
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leading to these small differences in
empirically-based amplifications based on V30
(redactive bluepassive combined)
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Average slownesses tend to converge near 30 m
(coincidence?) with systematic differences
shallower and deeper (both types of source give
larger shallow slowness at 30 m the slowness
from active sources is larger than the reference
and on average is smaller than the reference for
passive sources.
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But larger differences at higher frequencies (up
to 40) (V30 corresponds to 2 Hz)
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Summary (short)

• Many methods available for imaging seismic
  slowness
• Noninvasive methods work well, with some
  suggestions of systematic departures from
  borehole methods
• Several measures of site amplification show
  little sensitivity to the differences in models
  (on the order of factors of 1.4 or less)
• Site amplifications show trends with V30, but
  the remaining scatter in observed ground motions
  is large