The seabed as an acoustic mirror for suspended sediment

Abstract

Acoustic backscatter signals from suspended sediment, obtained at Queensland Beach N.S. in 1987, and Stanhope Lane Beach P.E.I. in 1989, are mirrored below the main bottom echo. The aim of this study is to test the hypothesis that the mirrored backscatter signals are the result of the bottom reflected wave scattering from sediment suspended above the seabed, and subsequently re-flecting from the bottom back to the transceiver. This hypothesis is tested by devising an inversion algorithm based on the re-reflection idea, to determine suspended sediment concentration and comparing to results from multifrequency and optical backscatter results. A theoretical analysis of the problem and the outcome of subsequent data inversion are presented. Unlike currently used methods, factors such as beam directivity, system constant, and the sediment backscattering form factor are absent from the equation used to determine suspended sediment concentrations. It is concluded that the bottom reflection plays a key role in the formation of the acoustic mirror image.