Site Characterization with 3-D Full Seismic Waveform Tomography

A new three-dimensional full waveform inversion (3D FWI) method is presented for geotechnical site characterization. The method is based on a solution of 3D elastic wave equations for forward modeling to simulate wave propagation, and a cross-adjoint gradient approach for model updating to extract material property. The staggered-grid finite-difference technique is used to solve the wave equations, together with implementation of the perfectly matched layer condition for boundary truncation. The gradient is calculated from the forward and backward wavefields. Reversed-in-time displacement residuals are induced as multiple sources at all receiver locations for simulation of the backward wavefield. Seismic wavefields are acquired from geophysical testing using sensors and sources located in uniform 2D grids on the ground surface, and then inverted for extraction of 3D subsurface wave velocity structures. The capability of the presented FWI method is tested on both synthetic and field experimental datasets. The inversion results from synthetic data show the 3D FWI ability in characterizing laterally variable low- and high-velocity layers. Field experimental data were collected using 96 receivers and a propelled energy generator (PEG) to generate seismic wave energy. The field data result shows that the waveform analysis was able to delineate variable subsurface soil layers. The seismic inversion results are generally consistent with invasive standard penetration test (SPT) N-values, including identification of a low-velocity zone.