Utilising frequency-time analysis (FTAN) of surface waves for geotechnical and dam investigations

Abstract

The shallow shear-wave velocity structure of the subsurface is a key input for dam engineering, civil construction, and earthquake hazard assessments. Many commercial solutions lack sensitivity in resolving complex shallow Vs structure. Here we develop a pure python approach for frequency-time analysis for shallow active source seismic data, and demonstrate its sensitivity to common depth ranges of geotechnical investigations. We show the approach's effectiveness in isolating the fundamental mode in group-velocity maps of Rayleigh waves, typically acquired in one-component geophone active surveys, and develop guidelines for consistent field survey deployment. We present a number of new case studies, ranging from construction to hydrogeological investigations, demonstrating our approach's ability to resolve complex shallow structures and its high visual correlation with independent geotechnical information, such as logs. Sensitivity kernel calculations show that FTAN has significantly more depth penetration than equivalent active techniques like refraction, at given shot-receiver distances, raising the possibility of processing legacy survey data for greater depth resolution and Vs structure.