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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. Predicting Site Response. Imaging of Near-Surface Seismic Slowness (Velocity) and Damping Ratios (Q). Image What?. - PowerPoint PPT Presentation

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  • Predicting Site Response

  • Predicting Site ResponseBased on theoretical calculations1-D equivalent linear, non-linear2-D and 3-D non-linearNeeds geotechnical site properties

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

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

    S(z) (compressional-wave slowness)

    (z) (shear-wave damping ratio [Q])

    Image What?Why?Site amplificationSite classification for building codesIdentification 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-depthSlowness is the usual regression coefficient in fits of travel time vs. depthVisual 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 slownessFocus attention on what contributes most to travel time in the layers

  • Imaging SlownessInvasive MethodsActive sourcesPassive sourcesNoninvasive MethodsActive sourcesPassive sources

  • Invasive MethodsActive Sourcessurface sourcedownhole sourcePassive sourcesRecordings 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 RECEIVERSdownhole profilingvelocities from surfacedata gaps filled by average velocityexpensive (requires hole)depth range limited (but good to > 250 m)seismic cone penetrometeradvantages of downholeinexpensivelimited rangenot good for cobbly materials, rock

  • Plotting sideways makes it easier to see slopes changes by viewing obliquely (an exploration geophysics trick) Create a record sectionopposite directions of surface source (red, blue traces)

    Pick arrivals (black)CCOC

  • Finer layering in upper 100m

  • Two models from the same travel time picks.

  • The increased resolution makes little difference in site amplification

  • SUBSURFACE SOURCE --- SUBSURFACE RECEIVERScrossholepoint measurements in depthexpensive (2 holes)velocity not appropriate for site responsesuspension loggerrapid collection of data (no casing required)average velocity over small depth rangescan be used in deep holesexpensive (requires borehole)no way of interpolating across data gaps

  • From GeovisionDownhole source--- P-S suspension 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

  • Noise fluctuations in both S and P logs agree with variations in lithology! (No averaging)

  • Some Strengths of Invasive MethodsDirect measure of velocitySurface 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 MethodsExpensive! (If need to drill hole)Surface source may have difficulties in deep holes, requires cased holes, logging must waitPS 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 MethodsActive Sourcese.g., SASW and MASWPassive sources (usually microtremors)Single stationArrays (e.g., fk, SPAC)Combined activepassive sources

  • Overview of SASW and MASW MethodSpectral-Analysis-of-Surface-Waves (SASW2 receivers); Multichannel Analysis of Surface Waves (MASWmultiple receivers)Noninvasive and NondestructiveBased on Dispersive Characteristics of Rayleigh Waves in a Layered Medium

  • SASW Field ProcedureTransient or Continuous Sources (use several per site)Receiver Geometry Considerations:Near Field Effects Attenuation Expanding Receiver SpreadLateral Variability

    (Brown)

  • SASW & MASW Data Interpretation(Brown)Dispersion curve built from a number of subsets (different source, different receiver spreads)

  • Some Factors That Influence Accuracy of SASW & MASW TestingLateral Variability of SubsurfaceShear-Wave Velocity Gradient and ContrastsValues of Poissons Ratio Assumed in the inversion of the dispersion curvesBackground Information on Site Geology Improves the Models

  • Noninvasive MethodsPassive 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)

  • (Boore & Toksz, 1969)Ellipticity (H/V) as a function of frequency depends on earth structure

  • Noninvasive MethodsPassive 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 MethodsOften 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

  • (Yoon and Rix, 2005)An example from the CCOCWSP experiment (active: f > 4 Hz; passive: f
  • Comparing Different Imaging Results at the Same SiteDirect comparison of slowness profilesSite amplificationFrom empirical prediction equationsTheoreticalFull resonanceSimplified (Square-root impedance)

  • Comparison of slowness profiles:

  • Coyote Creek Blind Interpretation Experiment (Asten and Boore, 2005)CCOC = Coyote Creek Outdoor Classroom

  • The ExperimentMeasurements and interpretations done voluntarily by many groupsInterpretations blind to other resultsInterpretations sent to D. BooreWorkshop held in May, 2004 to compare resultsOpen-File report published in 2005 (containing a summary by Asten & Boore and individual reports from participants)

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

  • 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

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

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

  • 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.

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

  • Summary (short)Many methods available for imaging seismic slownessNoninvasive methods work well, with some suggestions of systematic departures from borehole methodsSeveral 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

    **********************The Spectral analysis of surface waves method is the successor to the steady state Rayleigh wave method developed in the 1950s. Much of the development of the modern SASW method was carried out at UT Austin in the early 1980s.SASW testing is used to obtain a shear wave velocity profileIt is non-invasive and non-destructive - testing is performed on the ground surface and strains are in the elastic rangeInstead of measuring shear wave velocity directly, Rayleigh wave velocities are measured and Vs is inferred.*The general testing setup is shown here. A seismic source generates surface waves, which are monitored by two in-line receivers.Both transient and continuous dynamic sources are used to generate surface waves, with the data usually cleaner from continuous swept-sine sources. A vibroseis truck (slide) was used for the long wavelengths and various hand-held hammers (slide) were used for the short wavelengths. Several factors must be considered in receiver geometry. To avoid near field effects associated with Rayleigh waves and body waves, the distance from the source to the receiver, d1, is at least half of the maximum recorded wavelength. Attenuation reduces signal quality if d1 is greater than 4-10 wavelengths, depending on the source. Therefore, an expanding receiver spread is used, with overlap between the wavelengths recorded in each setup.To minimize lateral variability, forward and reverse profiles are taken, usually with a common centerline. The time records from the two geophones are transformed to the frequency domain to generate the dispersion curve. The most important data are the phase of the cross-power spectrum and the coherence.It is important that the frequency domain calculation be done in the field so that the experiment can be modified as needed.

    *From the unwrapped phase of the cross power spectrum, the Rayleigh wave velocity is calculated, given the frequency and inte

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