Experimental studies were pursued to investigate application of the Spectral-Analysis-of-Surface Waves (SASW) method to measure in situ stiffness profiles of geotechnical sites underwater. The SASW method, originally developed for use on land, involves generating and monitoring seismic interface waves at the ground surface, analyzing the differential response between receivers in the frequency domain, and inverting the results using an elastic, horizontally layered earth model.
This research involved small-scale testing to identify dominant wave motion at the solid-water interface, and two full-scale field trials. In the small-scale tests, interface waves were measured on a bed of cemented sand, a slab of concrete during and after curing, and a layered system of cemented sand over concrete. In each case, measurements were performed with and without overlying water. For confirmation, shear and compression wave velocities were also measured using crosshole, surface refraction, and laboratory resonant column methods. Results confirmed that while different types of waves dominate the vertical particle motion at the interface, depending on surface stiffness, the velocity of the dominant wave is always close to the Rayleigh wave velocity of the same material without overlying water; and therefore the general approach used in the SASW method to determine stiffness profiles on land is applicable offshore.
One small-scale test in a separate facility and two full-scale experiments demonstrated the SASW method in layered systems underwater. The small-scale test took place in a tank containing a bed of loose sand. Crosshole body wave velocities were also measured. One full-scale test took place in an ocean harbor under 5 meters of water, using a fixed platform. Shear wave velocities were also measured using a seismic cone penetrometer. The final test was performed on a sand deposit at sea under 27 meters of water. The results of these ship-based measurements were compared to high-resolution sonar data. All three experiments showed good agreement among complementary techniques, supplied data sets of validation of analytical models, and provided insight into development of equipment for deep water application of the SASW method.