Interpretation of Seismic Surface Wave Measurements for Detection of Gas Hydrates Offshore

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Abstract
The Spectral-Analysis-of-Surface-Wave (SASW) technique has been investigated for use offshore to detect the presence of gas hydrates. Experimental and theoretical results are presented to show that, depending on soil stiffness, either Scholte waves, which have propagation velocities less than the compression wave velocity of water, or acoustic waves, which have propagation velocities greater than the compression wave velocity of water, may be generated. The possibility of these two forms of waves being generated exists only when the shear wave velocity of the seafloor exceeds the compression wave velocity of water. In a seafloor containing gas hydrates, the possible generation of acoustic waves during SASW measurements is of considerable interest because the shear wave velocity of gas hydrates (4,000-6,000 fps) is very close to the propagation velocity of water (5,000 fps). Theoretically, a large number of acoustic waves with different propagation velocities may be generated; however, the experimental results suggest that a dominant acoustic wave, corresponding to the acoustic wave with the least energy, is generated. INTRODUCTION The SASW technique is a nonintrusive, nondestructive technique that has been proven References and figures at end of paper successful for detecting stiffness profiles of soil deposits and pavement systems. During the past three years the SASW technique has been investigated for possible use offshore to detect the presence of gas hydrates (Stokoe et al, 1990; Gauer, 1990; Roesset et al 1991; Wright et al, 1991; Sedighi-Manesh, 1991). Theoretical and experimental studies presented in this paper form part of the fundamental understanding that is necessary to apply and interpret the results of underwater SASW measurements. The SASW technique is based on the dispersive nature of surface waves of the Rayleigh type. In the case of surface waves at the ocean-seafloor interface, these waves are normally referred to as "Scholte" waves. Early experimental work by the authors revealed that in certain cases, represented by a very stiff seafloor with shear wave velocities greater than the compression wave velocity of water, waves other than the Scholte wave were sometimes generated and dominated the SASW measurements (Gauer, 1990; Stokoe et al, 1991). The primary objective of the studies presented in this paper have been to better understand these other, non- Scholte waves that may be generated in stiff seafloor deposits. POTENTIAL OF SASW TECHNIOUE FOR USE OFFSHORE In the SASW technique, surface waves predominantly of the Rayleigh-type are generated by a vertically-acting source at the ground surface. The dispersive nature of surface waves forms the fundamental basis of the SASW method for determining the depth profile of stiffness properties of a soil site (Nazarian, 1984; Stokoe et al, 1988). The Rayleigh wave amplitude attenuates exponentially with depth such that particle motion in the deposit becomes negligible at depths greater than approximately one wavelength (Richart et al, 1970). Thus, by examining the velocities of waves of different lengths, different depths of the soil profile can be explored.

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