Abstract

Multi-frequency acoustic backscatter measurement can be used to observe spatial profiles of suspended sediment. The use of multiple frequencies helps to resolve mean particle size by exploiting the frequency-dependent relationship between scattering intensity and particle size. The AQUAscat acoustic backscatter profiler uses this principle to estimate mean particle size and concentration profiles with spatial resolution between 2.5 mm and 40 mm. The instrument is usually configured to acquire profiles of backscatter amplitude. However, an infrequently used setting on the standard instrument provides direct access to the quadrature sampled backscatter data stream, from which amplitude is normally calculated. By using the in-phase and quadrature sample data, the backscatter phase can also be calculated. The rate of phase change between profiles can then be related to the along-beam velocity of the scatterers. For a vertical beam, in the absence of vertical water motion, this is the settling velocity. This paper evaluates datasets and data processing approaches for estimating particle settling velocity. The experimental data is acquired with various types of laboratory generated suspensions, including glass spheres, sands, and flocculating mud. Besides extracting settling velocity data, the processing aim is to enhance the quality of acoustic backscatter data analysis.