Experimental and theoretical studies in support of implementing the Spectral-Analysis-of-Surface-Waves (SASW) method offshore

An experimental study of underwater surface wave testing was undertaken to study the implementation of the Spectral-Analysis-of-Surface-Waves (SASW) method offshore. The SASW method is a nondestructive and nonintrusive method for evaluating the shear wave velocity profile of geotechnical materials, structural elements, and construction materials. The SASW methodology involves exciting motions on the exposed surface of a solid, and measuring the phase velocity of the surface wave as it propagates past receivers also located on the surface. On land the surface wave is called a Rayleigh wave, and underwater the surface wave is called a Scholte wave. Shear wave velocity profiles are determined by matching theoretical results to the experimental measurements. The objectives of this research were to: (1) study and understand factors affecting the application of the SASW methodology offshore, and (2) develop equipment for implementing the SASW method offshore.; Experiments were performed at several underwater sites, primarily in the ocean, using a variety of sources, receivers, and testing configurations. Body wave velocities from crosshole and downhole tests or soil boring information were used to compare with the results from the SASW testing. Theoretical results were developed to supplement and verify experimental observations. Experiments performed with an explosive source demonstrated the potential for interference and misinterpretation due to acoustic wave motions in the water column, motions not typically modeled in the theoretical solution. Near-surface testing, performed in shallow water using small impactive sources, demonstrated a lower level of near-field interference in the underwater testing as compared to on land. Field implementation of a small, remotely-operated impactive source was effective for near-surface shear wave velocity profiling offshore. Small-scale tank testing demonstrated the potential use of hydrophones to detect and measure the phase velocity of the underwater interface wave. These results were consistent with tests performed using a hammer source and geophone receivers.; The results from these tests should lead to the development of a robust system for performing SASW testing underwater.