B41

A fundamental understanding of wave propagation along the soil-water interface of the ocean bottom is developed. Potential for application of the Spectral Analysis of Surface Waves (SASW) technique offshore is theoretically investigated. Theoretical solutions for wave propagation along the interface between fluid and solid media are developed. Both two-dimensional and three-dimensional models are used. Theoretical results are presented in the form of dispersion curves which are the variation surface wave velocity of Rayleigh-type with wavelength (or frequency). Effects of gravity, water depth and wavelength on soil water interface waves are examined for a half-space with shear wave velocities ranging from 200 to 10,000 ft/sec. For shear wave velocities smaller than 5,000 ft/sec, the Scholte wave is found to be the dominant wave propagating along the soil-water interface.

For shear wave velocities greater than 5,000 ft/sec, many acoustic waves propagate along the interface in addition to the Scholte wave for deep water conditions. To verify the two-dimensional solution, comparisons are made between the experimental and theoretical dispersion curves. The experimental results are obtained from the SASW measurements performed on a small testing facility. For cemented sand and limestone bedrock with shear wave velocities of 2,500 and 4730 ft/sec, respectively, the experimental data are in good agreement with the Scholte wave solution. For stiffer materials (i.e. concrete slab with a shear wave velocity of 7,900 ft/sec), the experimental results agree with the fundamental acoustic wave solution. The fundamental acoustic wave is identified among many acoustic waves as the one with the least wave energy.

Two-dimensional theoretical solution are also compared with more rigorous results obtained from three-dimensional solution. For shear wave velocities of 200 ft/sec and 6,000 corresponding to soft seafloor and gas hydrates, respectively, the two-dimensional and three-dimensional dispersion curves are in excellent agreement. For shear wave velocity of 10,000 ft/sec, the three-dimensional results match well the fundamental acoustic wave obtained from two-dimensional solution in deep water.