The scattering of a low?angle pulse beam from seafloor volume heterogeneities
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A numerical scattering chamber is used to study the scattering of low-grazing-angle geoacoustic energy from a flat, basaltic seafloor with underlying volume heterogeneities of various length scales (0.1 less than or equal to ka less than or equal to 4, where k is the wave number and a is the length scale of the heterogeneity). The two-dimensional scattered field is computed by the finite-difference method for an incident Gaussian pulse beam at 15 degrees grazing angle (total internal reflection). For a deterministic model the scattering function is not monotonic with respect to angle, such as predicted by Lambert’s law. Peaks in the scattering function (up to 10 dB above background scattering levels) occur at angles that correspond to forward and backward compressional and shear head wave propagation. The scattered field is generated by primary diffractions directly into the water column, secondary diffractions from compressional and shear body waves and interference waves, and tertiary diffractions from interface waves that have converted into shear body waves. For the models studied, monostatic backscatter levels are largest for ka of about 1. The levels are -40 dB for no gradient in the mean subbottom velocity profile and about -48 dB for a gradient in the mean profile.