Application of SASW Method Underwater

Abstract

The spectral analysis of surface waves (SASW) method is a nonintrusive testing technique for determining shear wave velocity (VS) profiles of geotechnical sites using seismic interface waves (also called surface waves). Both small-scale and full-scale experiments have been conducted to develop the SASW technique for use underwater. The small-scale tests were performed to validate theoretical predictions regarding dominant types of seismic waves at the water/substrate interface. The tests clearly show a difference between soft and stiff substrates, where the definition of substrate stiffness is based on the value of the velocity of the Rayleigh-type interface wave (VR) relative to the velocity of compression waves in water (VW). In the simplest case, where the substrate is soft (VR<VW), the dominant wave was found to be the Scholte wave, whose behavior is similar to that of the Rayleigh wave on land. When the substrate is stiff (VR>VW), the dominant wave was found to be a generalized Rayleigh wave, the measurement of which is less straightforward. Two full-scale field trials were conducted on soft seafloors: one in the shallow water of a harbor and another as a ship-based experiment at sea. These trials demonstrated that the SASW method is a viable technique for characterizing underwater geotechnical sites, and that it shows promise for further development for remote operation in deep water.