Summary
Based on the seismic wave velocities reported for hydrated, a typical gas hydrate formation, if massive, should represent the stiffest layer in the soil profile of an offshore stratigraphic sequence. In addition, free gas may be encountered in areas where gas exists in hydrate form. Both the presence of free gas and the interbedding of softer and stiffer layers can cause resolution problems with seismic methods which employ compression waves as the primary means of sampling. Therefore, the overall objective of this work is to investigate the Spectral-Analysis-of-Surface-Waves (SASW) method offshore. The SASW method was selected for this study because it is not hindered by some of the problems with using compression waves and because it shows excellent potential for detecting the presence of gas hydrates by identifying distinct changes in the shear stiffness profile of the seafloor.
The Spectral-Analysis-of-Surface-Waves (SASW) method is a nonintrusive seismic method which has been successfully used on land to determine shear modulus profiles of geotechnical sites. The SASW method employs seismic waves of the Rayleigh type which are mainly influenced by the shear stiffness of the material through which the wave propagates. The dispersive nature of Rayleigh waves is the basis for the SASW testing method. Dispersion refers to the variation of wave phase velocity with wavelength (of frequency); as wavelengths increase, particle motion extends to greater depths. Therefore, different wavelength sample different depths. This characteristic of Rayleigh waves allows one to use waves over a range of wavelengths to assess material properties over a range of depths even though all testing is performed on the top of the geotechnical deposit.
Application of the SASW method to detect hydrate formations in oceanic sediments is being investigated both analytically and experimentally. The focus of the experimental study, which is the thrust of this study, is to examine the influence of water on the propagation velocity of interface waves of the Rayleigh type. The experimental data will be used to verify analytical results from a comparison study by Sedighi, (1991) in which both plane-wave and three-dimensional models of fluid-sediment systems are being developed.
To perform the experimental studies, a small-scale facility has been constructed at the Balcones Research Center of The University of Texas at Austin. With this facility, on is capable of testing a variety of geotechnical materials with different stiffnesses in systems with varying water depths.
Experimental results are reported for three different geotechnical systems. Half-space representation of each surface layer was accomplished by restricting the wavelengths of the input seismic waves such that no waves sampled deeper than the layer thickness. The materials ranged in shear stiffness from very stiff (concrete) to medium stiff (limestone) to “soft” (where “soft” is actually a lightly cemented sand). In each case, tests were initially conducted dry (with no water) and then with various depths of water. The water depths were scaled to wavelength in an effort match the analytical results.
Related Publications: Stokoe, K.H., II, Gauer, R.C. and Bay, J.A., “Experimental Investigation of Seismic Surface Waves in the Seafloor,” Proceedings, Shear Waves in Marine Sediments, La Apezia, Italy, 1990, pp. 51-58.
