Natural gas hydrates form in offshore sediments when certain pressure and temperature conditions are met. Massive hydrate formations should theoretically have anomalously high stiffnesses relative to surrounding ocean sediments due to their ictlike nature. Underwater seismic methods using compression waves have been used to detect the presence of massive hydrate formations from reflections off the interface between the ice and overlying sediment. IN this work, it is proposed to develop a seismic method which uses a dispersive type of interface wave to detect gas hydrated. This method is called the Spectral-Analysis-of-Surface-Waves (SASW) method. Adaptation of the SASW method to offshore use has the potential of mapping the lateral and vertical extents of massive hydrate formations from the shear modulus profiles in the oceanic sediment systems.
The Spectral- Analysis-of-Surface-Waves (SASW) method is a nondestructive seismic method which uses Rayleigh-type waves to measure in situ dispersion characteristics of a soil system. Dispersion occurs in layered system because particle motion decays exponentially with depth and because different wavelength stress material to different depths. Various types of dispersive seismic waves propagate along an interface between two propagation media. Rayleigh waves propagate along an air/solid interface and Scholte waves propagate along a fluid/solid interface. Pseudo-Rayleigh waves are a special case of interface waves which occur when and overlaying low velocity layer influences the transmission of seismic energy in such a way that most of the energy remains in the underlying higher-velocity layer.
Past research indicates that the type of wave propagation along the seafloor is dependent upon the relative stiffness of the ocean floor sediment. According to Roever, et al (1959) and Rauch (1986), a Scholte mode of propagation is expected for “soft” seabed conditions (Vs Seafloor Vp water).
An experimental test facility was constructed to evaluate application of the SASW method underwater. An initial field study was performed on bedrock limestone in which various problems were recognized in applying the SASW method. As a result of this initial work, a second-generation study was conducted utilizing specially made underwater seismic receivers and a water-tight canister to deliver the piezoelectric source to the underwater testing surface. A concrete slab, which modeled “stiff” seabed conditions, was placed in the test facility and underwater SASW measurements were performed. Particular interest was directed toward studying the characteristics of the interface wave propagation. A layer of cemented sand, which modeled “soft” seabed conditions, was placed over the concrete layer and underwater SASW measurements were repeated with the same source/receiver configuration that was used for testing the “stiff” model seabed.