Continuous P-SV coupling and strongly dispersive surface waves in soft marine sediments

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In the upper tens to few hundred meters of clay and silt marine sediments with high porosity, the shear wave velocity is much smaller than the compressional wave velocity. The shear velocity has very large gradients close to the ocean floor leading to strong P-SV coupling in such ‘‘soft’’ sediments. A theory of elastic wave propagation in continuously stratified soft sediments is developed that fully accounts for the coupling. Elastic waves in soft sediments consist of ‘‘fast’’ waves propagating with velocities close to the compressional velocity and ‘‘slow’’ waves propagating with velocities on the order of the shear velocity. For the slow waves, the theory predicts existence of surface waves at the ocean/sediment boundary. An explicit, exact solution is obtained for the surface waves in the case of linear increase of shear rigidity with depth. Asymptotic and perturbation techniques are used to extend the result to more general environments. Velocity of vertically polarized interface waves is sensitive to sediment density and shear rigidity. Theoretical dispersion relations agree well with available experimental data and are shown to lead to a simple and robust inversion of interface waves travel times for shear-velocity profiles in the sediment.