Predicting Site Response

Predicting Site Response

• Based on theoretical calculations– 1-D equivalent linear, non-linear– 2-D and 3-D non-linear

• Needs geotechnical site properties

Imaging of Near-Surface Seismic Slowness (Velocity) and Damping and Damping

Ratios (Q)Ratios (Q)

• Sβ(z) (shear-wave slowness) (=1/velocity)

• Sα(z) (compressional-wave slowness)

• ξβ(z) (shear-wave damping ratio [Qβ])

Image What?Image What?

Why?Why?

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

Why Slowness?

• Travel time in layers directly proportional to slowness; travel time fundamental in site response (e.g., T = 4*s*h = 4*travel 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

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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Shear-Wave Slowness (sec/km)

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SASW TestingDownhole Seismic

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Imaging Slowness

• Invasive Methods– Active sources– Passive sources

• Noninvasive Methods– Active sources– Passive sources

Invasive Methods

• Active Sources– surface source– downhole source

• Passive sources– Recordings of earthquake

waves in boreholes---not covered in this talk

Invasive Method

Surface Source--Downhole Receiver(ssdhr)

(receiver can be on SCPTrod)

One receiver moved up or down hole

SURFACE SOURCE ---SUBSURFACE RECEIVERS

• downhole profiling– velocities from surface– data gaps filled by average velocity– expensive (requires hole)– depth range limited (but good to > 250 m)

• seismic cone penetrometer– advantages of downhole– inexpensive– limited range– not good for cobbly materials, rock

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Plotting sideways makes it easier to see slopes changes by viewing obliquely (an exploration geophysics trick)

Create a record section—opposite directions of surface source (red, blue traces)

Pick arrivals (black)

CCOC

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Finer layering in upper 100m

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CCOC: S-Wave SlownessGibbs (Vs(30) = 232 m/s)More detail (Vs(30) = 235 m/s)

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Two models from the same travel time picks.

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vertical incidence,density=2 gm/cc, Q= 25, and ahalfspace withV=1200 m/s anddensity = 2.4 gm/ccat 234 m depth.

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The increased resolution makes little difference in site amplification

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

Cable Head

Head Reducer

Upper Geophone

Lower Geophone

Filter Tube

Source

Source Driver

Weight

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7-Conductor cable

Diskettewith Data

OYO PS-160Logger/Recorder

Overall Length ~ 25 ft

From Geovision

Downhole source--- P-S suspensionDownhole source--- P-S suspension logging (aka “PS Log”)logging (aka “PS Log”)

Dominant frequency = 1000 Hz

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)

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

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.

Noninvasive Methods

• Active Sources– e.g., SASW and MASW

• Passive sources (usually microtremors)– Single station– Arrays (e.g., fk, SPAC)

• Combined active—passive sources

Overview of SASW and MASW Method

• Spectral-Analysis-of-Surface-Waves (SASW—2 receivers); Multichannel Analysis of Surface Waves (MASW—multiple receivers)

• Noninvasive and Nondestructive

• Based on Dispersive Characteristics of Rayleigh Waves in a Layered Medium

SASW Field Procedure• Transient or

Continuous Sources (use several per site)

• Receiver Geometry Considerations:– Near Field Effects

– Attenuation

– Expanding Receiver Spread

– Lateral Variability

(Brown)

SASW & MASW Data Interpretation

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Experimental DataTheoretical Dispersion Curve

Rinaldi Receiving Station

(Brown)

Dispersion curve built from a number of subsets (different source, different receiver spreads)

Some Factors That Influence Accuracy of SASW & MASW Testing

• Lateral Variability of Subsurface• Shear-Wave Velocity Gradient and

Contrasts• Values of Poisson’s Ratio Assumed in

the inversion of the dispersion curves• Background Information on Site

Geology Improves the Models

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 ancientancient paper)

(Boore & Toksöz, 1969)

Ellipticity (H/V) as a function of frequency depends on earth structure

Noninvasive Methods

• Passive sources (usually microtremors)– Multiple stations (usually two-

dimensional arrays)

(Hartzell, 2005)

The array of stations at WSP used by Hartzell

(Hartzell, 2005)

Inverting to obtain velocity profile

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 active—passive sources

(Yoon and Rix, 2005)

An example from the CCOC—WSP experiment (active: f > 4 Hz; passive: f<8 Hz)

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)

Comparison of slowness profiles:

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Garner ValleyPS Log APS Log BSASW TestingDownhole Seismic

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Coyote Creek Blind Interpretation Experiment (Asten and Boore, 2005)

CCOC = Coyote Creek Outdoor Classroom

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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WSP: Active Sources

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Shear WaveReference modelReflection (Williams)SASW (Bay, forward)SASW (Stokoe, avg lb, ub)SASW (Kayen, Wave-Eq)MASW (Stephenson)

WSP: Active Sources

Active sources at WSP: note larger near-surface & smaller deep slownesses than reference for most methods.

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Shear WaveReference modelSPAC (Asten, pkdec2)SPAC (Hartzell)H/V (Lang, Oct04)Remi (Stephenson, mar05)Remi (Louie)

WSP: Passive Sources

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Shear WaveReference modelSPAC (Asten, pkdec2)SPAC (Hartzell)H/V (Lang, Oct04)Remi (Stephenson, mar05)Remi (Louie)

WSP: Passive Sources

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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Shear WaveReference modelMASW+MAM (Hayashi)MASW+MAM (Rix)

WSP: Active + Passive Sources

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Shear WaveReference modelMASW+MAM (Hayashi)MASW+MAM (Rix)

WSP: Active + Passive Sources

Combined active & passive sources at WSP: note larger near-surface slownesses than reference

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Red: Active Sources; Blue: Passive & Combined SourcesSASW, CCOC (Stokoe, Cl1)SASW, CCOC (Stokoe, Cl2 avg)Reflection, WSP (Williams)SASW, WSP (Kayen)MASW, WSP (Stephenson)SASW, WSP (Stokoe, avg)MASW+MAM, WSP (Hayashi)MASW+FK, WSP (Rix)H/V, WSP (Lang, oct04)SPAC, WSP (Asten, pkdec2)SPAC, WSP (Hartzell)ReMi (Stephenson, mar05)ReMi (Louie)

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leading to these small differences in empirically-based amplifications based on V30 (red=active; blue=passive & combined)

Average slownesses tend to converge near 30 mconverge 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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Passive & Combined Sources (WSP)reference modelSPAC, WSP (Asten, pkdec2)H/V, WSP (Lang, oct04)SPAC, WSP (Hartzell)ReMi, WSP (Stephenson_mar05)ReMi, WSP (Louie)MASW+MAM, WSP (Hayashi)MASW+FK, WSP (Rix)

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But larger differenceslarger differences at higher frequenciesat higher frequencies (up to 40%) (V30 corresponds to ~ 2 Hz)

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