Measuring suspended sediment concentrations using acoustic backscatter devices

Abstract

In recent years the requirement to develop a technique for measuring suspended sediment concentration profiles, with similar temporal and spatial resolution to that of electromagnetic current meter measurements of the flow, has led to the development of an acoustic approach. Acoustic backscatter devices have been constructed and deployed in marine environments and measurements of backscattered signal profiles obtained. However, the translation from acoustic returns to suspended load has been an empirical procedure resulting in some uncertainty regarding the accuracy and reliability of the conversion process. To address this problem a series of laboratory measurements of acoustic backscattering from marine suspensions were conducted using two acoustic backscatter instruments operating at 3.0 and 5.65 MHz. A simple semi-empirical backscattering theory has been developed to predict the form of the scattered signal and comparisons have been made of the concentrations predicted from the theory against concentration measurements made in laboratory tanks. The results show that the main features of the backscattering signal can be explained and accurate concentration estimates can be made from the acoustic returns. It is concluded that acoustic backscatter techniques are potentially powerful tools for examining, at high resolution, the structure of suspended sediment concentration profiles near the seabed.