Geophysical Subsurface Characterization to Estimate Top-of-rock Elevation for Deep Foundations in Challenging Geologic Settings: Numerical Investigation of Full Waveform TomographyÂ-Paper

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The study evaluates full waveform inversion (FWI) as a method for estimating bedrock depth where highly variable geology, including karst, weathering, and pinnacled rock, makes deep-foundation design difficult. Conventional drilling and penetration tests provide information only at discrete locations, while seismic refraction (SR) and multichannel analysis of surface waves (MASW) can have limited resolution or produce spatially averaged results.<br /><br />Researchers modeled a 50 m-long, 20 m-deep site with variable soil, weathered rock, and competent bedrock. They compared FWI with SR tomography based on first-arrival times and dispersion-based MASW. FWI uses complete waveforms to build a two-dimensional subsurface model, whereas MASW estimates velocity profiles from surface-wave dispersion and SR relies on refracted-wave arrivals.<br /><br />FWI most accurately reproduced the modeled stiffness and the locations of the weathered-rock transition and competent bedrock, with elevation errors generally around 2–3 m. It nevertheless smoothed sharp, small-scale changes in bedrock topography. SR substantially overestimated stiffness and placed rock too shallow, potentially supporting an inappropriate foundation design. MASW indicated overly soft conditions and did not identify competent bedrock, potentially leading to unnecessarily deep foundations. Its spatial coverage was also limited by the receiver-array geometry.<br /><br />The accuracy advantage came at a substantial cost: FWI required more extensive waveform acquisition and about 50 hours of high-performance computing, compared with roughly five hours for SR or MASW. The authors conclude that FWI may be preferable for challenging, variable bedrock conditions, but emphasize that field studies are needed to assess reliability and address issues such as three-dimensional wave propagation and source characterization.

Keywords

full waveform inversion

bedrock depth estimation

karst geology

weathered rock

pinnacled rock

seismic refraction

MASW

surface wave dispersion

deep foundation design

subsurface imaging