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.