Head waves, diving waves, and interface waves at the seafloor

Abstract

Brekhovskikh (1960) summarizes the system of waves that arises from reflection and refraction of spherical waves at the interface between homogeneous solid half-spaces. By eliminating the shear wave potential in one half-space, the system for fluid–solid half-spaces like the seafloor is obtained. There are two cases: one where the shear speed in the bottom is less than the compressional speed in the fluid (soft sediments), and the other where the shear speed in the bottom is greater than the compressional speed in the fluid (hard volcanic basement). This model is the basis for defining interface phenomena such as evanescent waves, head waves, pseudo-Rayleigh waves, and Stoneley/Scholte waves. If a positive gradient is introduced into the compressional and shear sound speeds in the bottom, one obtains diving waves and interference head waves (Cerveny and Ravindra, 1971). There are two types of interface waves: pseudo-Rayleigh waves that are evanescent in the bottom but propagate in the water, and Stoneley/Scholte waves that are evanescent in both media. In multi-interface models there are of course normal modes. In actual seafloors, low speed layers, sound and shear speed gradients, and interface and volume lateral heterogeneities affect the characteristics of propagation and scattering. [Work supported by ONR.]