2aUW10. Bottom penetration at subcritical grazing angles by scattering.

Session: Tuesday Morning, May 14

Time: 11:05

Author: Ralph A. Stephen
Location: Woods Hole Oceanograph. Inst., 360 Woods Hole Rd., Woods Hole, MA 02543


The physics of sound propagation and scattering is distinctly different between hard and soft ocean bottoms. A hard bottom is defined as having a shear velocity greater than water velocity; a soft bottom has a shear velocity less than water velocity. Geologically hard seafloors consist of igneous rocks such as gabbros and basalts; soft seafloors consist of sediments such as mud and sand. For a soft bottom with sound incident from above, total internal reflection never occurs. For all grazing angles there is always a Snell's law ray path from compressional energy in the water to shear energy in the bottom. For hard, flat bottoms over homogeneous rock, total internal reflection will occur for grazing angles less than critical. However in this case sound can penetrate the bottom by scattering from surface roughness or volume heterogeneities. Snapshots from time domain modeling by the finite-difference method show that the scattered field consists of wavefronts which are analogous to transmitted compressional and shear body waves. Since the seafloor is rough and laterally heterogeneous over a broad range of length scales, this mechanism could be significant over a broad frequency range. [Work supported by ONR.]

from ASA 131st Meeting, Indianapolis, May 1996