The ability to measure near-surface stiffness profiles .of geologic strata underwater is desirable for offshore hydrocarbon exploration and production in addition to numerous geotechnical purposes. A nonintrusive seismic measurement technique called the Spectral-Analysis-of- Surface-Waves (SASW) method is under development for this application. Recently, a suite of experiments was conducted to investigate the dominant seismic wave motion at the surface of a homogeneous material overlaid by water. Measurements were conducted on the surface of a concrete slab as the concrete cured, permitting testing over a wide range of stiffnesses (shear wave velocities from 900 to 8000 ft/s (270 to 2500 m/s)). The concrete was placed in a tank which was repeatedly filled with water and drained so that measurements could be made with and without overlying water. Phase velocities were measured on the surface of the slab using the SASW technique, which is founded on the dispersive nature of surface waves. Average velocities were compared with theoretical predictions of Scholte surface wave velocities and minimum-energy acoustic wave velocities. It was found that measured velocities correspond to the Scholte wave velocity when the material is soft (shear wave velocity less than 4000 ft/s (1200 m/s)) and to the minimum-energy acoustic wave velocity when it is stiff (shear wave velocity more than 5000 ft/s (1500 m/s)). The two trends are joined by a smooth transition zone. In general, the phase velocity measured on the simulated ocean bottom is within a few percent of the Rayleigh wave velocity of the same material with no overlying water.
Knowledge overlaid by water is of the physical properties of earth necessary in geotechnical engineering applications such as structural foundation, dredging, and slope stability evaluation. In offshore hydrocarbon exploration and production, knowledge of the ground conditions is needed to develop platforms, anchors and pipeline routes. Recently, attention has focused on detecting gas hydrates offshore. These ice-like compounds of water and natural gas present potential hazards to exploration and construction when permitted to melt. To aid in gas hydrate detection, a nonintrusive method for material profiling using seismic surface waves, called the Spectral-Analysis-of-Surface-Waves (SASW) method, is currently being investigated.
The SASW method is a technique for measuring shear stiffness profiles in situ using seismic surface waves. This subsurface investigation technique has been used on land in situations including soil and rock sites for building foundations, liquefaction assessments, slope stability investigations, tunneling studies, pavement subgrades, and even structural concrete members (Stokoe and Nazarirm, 1985; Stokoe et al, 1989; Bowen and Stokoe, 1992). An advantage of the SASW method over other geotechnical investigation techniques is that it is performed on the exposed surface of the medium to be evaluated. Therefore, it is convenient, quick, and relatively inexpensive. Compared with seismic body wave measurements, it has the advantage of permitting resolution of softer zones underlying stiff layers using only surface-based measurements.
