In Situ quantification of the suspended load of estuarine aggregates from multifrequency acoustic inversions

The study of the suspended particulate matter (SPM) transport is essential to understanding oceans and rivers, for their presence can impact the environment, from marine habitats or water quality degradations to important changes of the seabed morphology. Among a large number of surrogate techniques in traditional water sampling, acoustical methods have the advantage of providing nonintrusive measurements, with high spatial and temporal resolutions. However, the ability of fine-grained sediments to aggregate under the process of flocculation complexifies the interpretation of acoustical measurements. The objective of this paper is to design a simple backscatteringmodel for flocculated sediment suspensions, in order to interpret the information provided by a multifrequency profiler and to retrieve both the concentration and the dominant size of a suspension of flocculated sediments in an estuarine context. In situ granulometry laser data, collected in the Aulne macrotidal estuary (France), showed that over the size distribution observed, a mean porosity of apparent particles in suspension can be used directly as input for model generation. The in situ acoustic signal was concurrently recorded at 0.5, 1, 2, and 4MHz, and then inverted using the nonnegative least squares algorithm after constraining the model with an optimal porosity, allowing for a discrete representation of the mass concentration distributed over several equivalent spherical radii. The inversion results are in good agreement with the in situ mass concentration obtained through in situ water samplings