The influence of large-scale seafloor slope and bottom velocity on low-grazing-angle monostatic acoustic reverberation

Abstract

Analysis of low-grazing-angle monostatic reverberation data acquired at Site A on the Acoustic Reverberation Special Research Program 1993 Acoustics Cruise suggests that scattered signals cannot be quantitatively explained in terms of large-scale slope alone, even though a strong correspondence between high intensity backscatter and seafloor ridges is observed. Numerical modeling using a finite-difference solution to the elastic wave equation shows that large-scale seafloor slope and subseafloor average velocity influence the backscattered intensity levels but that the source of scattering is wavelength-scale heterogeneity. The results of the modeling suggest that the influence of large-scale seafloor slope is strongest at small grazing angles. For example, for a rough seafloor and a source wave field incident at 15° relative to the horizontal, a change in the seafloor slope from 0° to 15° yields a 5-dB change in backscatter intensity, while a change from 15° to 30° in seafloor slope yields only a 1-dB change in backscatter intensity. The influence of average subseafloor velocity is shown to be a bimodal effect. Homogeneous soft bottoms, e.g., sediment, generate dramatically less backscatter than homogeneous hard bottoms, such as basalt.