SPT-Seismic Full Waveform Inversion for Deep Site Characterization

Detailed information of soil/rock properties and associated spatial variability is crucial for the successful design and construction of deep foundations. Traditional invasive methods such as standard penetration test (SPT) and cone penetrometer test (CPT) only sample a small volume of soil and rock material at the testing site. Surface-based geophysical methods can provide overall subsurface conditions but are limited to shallow depths due to reflections of surface source energy from stiffer underlying material. To address this issue, we present a novel SPT-seismic method for deep site characterization. Seismic waves generated by SPT spoon (traditional N-value) at various depths are recorded by a 2D grid of geophones on the ground surface and analyzed by a 3D full-waveform inversion (3D FWI) to extract subsurface material properties. A field experimental trial (soil over rock) was conducted to provide new insights into its utility as a viable geophysical tool for deep site characterization. Not only were 3D S-wave and P-wave velocities of subsurface soil and rock within 9 m around the SPT boring well characterized, but also a deep void (15-m depth) away from the boring was identified within the rock. S-wave velocity values of the final inverted result are also compared with SPT N-values, and a good overall agreement over the whole depth is observed.