Validation of an inversion scheme for shear wave speed using Scholte wave dispersion

Acoustic propagation in littoral waters is greatly affected by seafloor sediment properties. Shear properties of sediments are also directly related to the strength of sediments for geotechnical applications. These factors emphasize the importance of estimating shear wave speeds in semi-consolidated shallow water sediments. One of the most promising approaches to estimate shear speed is to invert the shear speed profile using the dispersion of seismo-acoustic interface (Scholte) waves that travel along the water-sediment boundary. The propagation speed and attenuation of Scholte waves are closely related to the shear wave speed and attenuation over a depth of 1-2 wavelengths into the seabed. Based on this concept, the University of Rhode Island Ocean Engineering Department has developed a geophone-hydrophone system for the measurement of these interface waves, along with an inversion scheme that would invert the dispersion data for sediment shear speed profiles. The objective of this research was to investigate the validity of the system in estimating the shear speed in the sediment by measuring the surface waves at the water-sediment interface. This geophone-hydrophone system was tested at Davisville, RI and the results obtained from this test will be presented. The data collected was processed using an approach similar to the land based Spectral Analysis of Surface Waves (SASW) method to estimate the phase speed dispersion. Phase speed data was then inverted using an inversion scheme based on a genetic algorithm and the Godin-Chapman approach as the forward model. The results are compared to historic estimates of shear speed and recently acquired vibracore data collected at the experimental location to validate the inversion scheme. Results suggest that the inverted profile is consistent with the shear speed profiles related to sand and silt, which is in agreement with the types of sediment described in the boring logs and seen in the core. The values of the shear speed estimated from the inversion scheme are also close to the shear speeds estimated from the bore hole and core data. The overall validity and usefulness of the inversion scheme as a fast remote sensing tool providing a range averaged shear speed profile will be addressed. © 2013 MTS