Estimation of Shear Wave Speed in Ocean-Bottom Sediment Using Electromagnetic Induction Source

An experiment for studying the shear wave speed in ocean-bottom sediment using Scholte pulse waves was conducted on a shelf in the East China Sea. To generate Scholte pulse waves, we developed a new electromagnetic induction source, which was placed at 100-m depth on the seafloor and was remotely controlled on board through a 1000-m-long cable. A Scholte pulse propagating in sandy sediment was then measured using a couple of geophones placed on the seafloor at a distance of 100 m from the source. The dispersion property of the Scholte waves was obtained by applying a multiple filter technique (MFT) to the observed waves. We then obtained the shear wave speed structure, i.e., the depth dependency of the shear wave speed in the sediment, by applying an inversion method based on a genetic algorithm (GA). The estimated speed was about 100 m/s near the bottom and increased with depth. The power was a close match to the estimated depth-speed profile, as reported previously by Hamilton [E. L. Hamilton, Geophysics, vol. 41, pp. 985-996, 1976].