Analytical and Experimental Studies of Surface Wave Measurements To Detect Gas Hydrates Offshore

Abstract

ABSTRACT Analytical and experimental studies of the Spectral-Analysisof- Surface-Wave (SASW) technique for offshore geotechnical investigations are presented. Effects of water depth, wavelength and gravity on soil-water interface waves are examined; appropriate wavelengths for SASW testing are suggested. The finite compressibility of water is shown to be important for stiff seafloor materials. INTRODUCTION The Spectral-Analysis-of-Surface-Waves (SASW) technique is a nondestructive seismic technique that employs surface waves of the Rayleigh type to determine layer thicknesses and stiffnesses of subsurface profiles. During the past several years this technique has been developed and successfully used on land to determine stiffness profiles of highway pavements and soil deposits (for instance; Nazarian and Stokoe, 1986, and Stokoe et al, 1988). The success of the technique on land has suggested that the technique may also be applicable to evaluating subsurface stiffness profiles offshore. Specifically, the technique shows promise for possible use offshore to detect nonintrusively the presence of gas hydrates. The presence of gas hydrates can result in stiffness profiles consisting of stiffer materials (hydrates) overlying or sandwiched between softer soil sediments. In such cases the SASW technique is especially promising because it relies on the propagation of Rayleigh-type surface waves which are dominated by the shear stiffness of the material skeleton rather than conventional acoustic methods which are based on compression wave measurements. To extend the SASW technique from on-land to offshore sites, it is necessary to consider the overlying water. Both analytical and experimental studies are currently being conducted to examine the influence of water. This paper concentrates on building the theoretical framework for SASW testing offshore. The analytical work has been performed to extend the solution for surface wave propagation along the seafloor by including the effects of water and gravity over a wide range of seafloor stiffnesses and wavelengths. To study the analytical results, comparisons are made with small-scale experimental tests. SASW TECHNIOUE ON LAND The SASW technique is based on measurement of the dispersive nature of surface waves propagating in a horizontally layered system. Surface wave propagation is characterized by a dispersion curve, a curve which relates phase velocities to the wavelengths or frequencies. A typical dispersion curve, measured at an ancient lake bed near Fucino, Italy (Stokoe and Rix, 1988), is shown in Fig. 1. Each dispersion curve reflects the subsurface layering and stiffness profile for a given site, each site having its own distinct dispersion curve coupled with the testing process. Equipment and experimental techniques for measuring dispersion curves to determine subsurface stiffness profiles are described by Nazarian and Stokoe (1986) and Stokoe er a1 (1988). A stiffness profile for a site is determined from the measured dispersion curve using a forward modelling process in which theoretical dispersion curves are compared with the measured curve. The thicknesses and stiffnesses of subsurface layers are assumed, and a theoretical dispersion curve is computed. The theoretical curve is then compared with the measured curve. This process is repeated until the assumed subsurface layering and stiffness profile produce a dispersion curve that is in good agreement with the measured curve.