A Wavelet-based Seismogram Inversion Algorithm for the In Situ Characterization of Nonlinear Soil Behavior |
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Authors: | D Assimaki W Li A Kalos |
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Institution: | (1) School of CEE, Georgia Institute of Technology, Atlanta, GA, USA |
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Abstract: | We present a full waveform inversion algorithm of downhole array seismogram recordings that can be used to estimate the inelastic
soil behavior in situ during earthquake ground motion. For this purpose, we first develop a new hysteretic scheme that improves
upon existing nonlinear site response models by allowing adjustment of the width and length of the hysteresis loop for a relatively
small number of soil parameters. The constitutive law is formulated to approximate the response of saturated cohesive materials,
and does not account for volumetric changes due to shear leading to pore pressure development and potential liquefaction.
We implement the soil model in the forward operator of the inversion, and evaluate the constitutive parameters that maximize
the cross-correlation between site response predictions and observations on ground surface. The objective function is defined
in the wavelet domain, which allows equal weight to be assigned across all frequency bands of the non-stationary signal. We
evaluate the convergence rate and robustness of the proposed scheme for noise-free and noise-contaminated data, and illustrate
good performance of the inversion for signal-to-noise ratios as low as 3. We finally employ the proposed scheme to downhole
array data, and show that results compare very well with published data on generic soil conditions and previous geotechnical
investigation studies at the array site. By assuming a realistic hysteretic model and estimating the constitutive soil parameters,
the proposed inversion accounts for the instantaneous adjustment of soil response to the level and strain and load path during
transient loading, and allows results to be used in predictions of nonlinear site effects during future events. |
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