In-Situ Measurement of Stiffness Profiles in Ocean Bottom Materials Using the SASW Method

Abstract

ABSTRACT The Spectral-Analysis-of-Surface-Waves (SASW) method is a nonintrusive seismic method which has been used successfully on land to determine shear modulus profiles of geotechnical sites. Application of the SASW method offshore is being investigated both analytically and experimentally. Of particular interest is the possibility of detecting the presence of gas hydrates within about the upper 200 ft of the seafloor by distinct changes in the shear stiffness profile. Initial studies show the potential usefulness of the method, particularly in deep water. INTRODUCTION Gas hydrates are known to occur in the upper several hundred feet of the seafloor in water depths greater than about 1500 ft. In terms of exploration and construction, gas hydrates are undesirable because of their unstable nature. Accordingly, it is important that these materials be detected at an early stage in the design and location of offshore oil production facilities. Seismic methods are particularly attractive for this purpose. Among the various seismic methods, one of the most promising is the Spectral-Analysis-of-Surface Waves (SASW) method as discussed herein. Typical stratigraphic sequences where gas hydrates may be encountered are expected to contain stiffer layers of material sandwiched between softer strata. It is also expected that the gas hydrate will typically represent the stiffest layer in the soil profile and that 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. The SASW method, on the other hand, employs surface waves of the Rayleigh type. Rayleigh waves are dominated by the shear stiffness of the material and can be used to sample softer materials underlying stiffer layers. Because of these two characteristics, the SASW method eliminates many of the problems for gas hydrate detection associated with other seismic methods. In addition, the SASW method is nonintrusive which makes its use on the seafloor especially attractive. During the past several years, the SASW method has been extensively used on land to determine shear modulus profiles of geotechnical sites (Stoke and Nazarian, 1985; and Stoke et ai, 1988) and Young's modulus profiles of pavement sites (Stoke and Nazarian, 1986; and Rix and Stoke, 1989). In the case of land-based applications, the exposed material surface is overlaid by air, which has negligible density and is highly compressible relative to the underlying geologic or man-made materials. In the case of offshore applications, the surface of the deposit will be overlaid with water, which will be much less compressible than air and have a density of approximately one-half the density of the underlying materials. As a result, the presence of water needs to be considered when adapting the onshore SASW method to seafloor exploration. The studies presented in this paper were undertaken to determine the potential impact of water over the soil on the propagation along the soil-water interface of surface waves of the Rayleigh type.