Geotechnical Site Characterization with 3-D Seismic Waveform Tomography

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Unanticipated site conditions such as highly variable soil and rock layers with embedded low-velocity anomalies (soft soils or voids) cause significant problems during and after construction of foundations. Knowledge of the anomalies is crucial, as the anomalies can cause structural damage or collapse that can result in significant property damage and occasional loss of life. This award supports fundamental research to provide the knowledge needed to quantify embedded anomalies and characterize variable soil/rock layers at high resolutions. Results from this research will accelerate economical and practical implementation of innovative non-destructive testing (NDT) methods for protecting and improving our nation's infrastructure. The developed analysis can be used for material imaging and characterization at various scales from millimeters to hundreds of meters for several engineering applications such as evaluation of geotechnical subsurface site conditions, bridge foundation scour, concrete structural components, and asphalt pavements. The goal of this research is to characterize both shear wave (S-wave) and compression wave (P-wave) velocity profiles of 3-D subsurface structures at meter scales down to 30 meter depth from surface-based seismic wave fields. The subsurface structures consist of soil, rock, and voids filled by air or water. To achieve the goal, the research will include (i) to develop a three-dimensional full waveform inversion (3-D FWI) analysis of seismic wave fields for high-resolution (meter pixel) characterization of subsurface site conditions, and (ii) to verify the 3-D FWI analysis by full scale field experiments. Full seismic surface wave fields will be used to quantify embedded anomalies and characterize variable soil/rock layers, as the propagation properties of seismic waves are modulated by the anomalies and layer interfaces. The seismic wave fields are acquired from geophysical testing using sensors and sources located in uniform 2-D grids on the ground surface, and then inverted for the extraction of 3-D subsurface wave velocity structures. The research will address inherent issues of small scale tomography including dominant Rayleigh wave components, inconsistent wave excitation, strong attenuation, poor a priori information, and highly variable of near surface soil/rock. Using full information contained in the measured waveform data, the FWI analysis is expected to provide more detailed and accurate characterization of materials than methods currently used in practice, which use only portions of the measured data. This will result in new knowledge in the area of engineering geophysics, and it is a critical step toward using an effective geophysical method for site investigations to improve design of foundations and other geotechnical structures.