Analysis of acoustic measurements of suspended sediments

Estuarine measurements of suspended sediments over sand waves have been taken with the objective of clarifying the role of turbulence in suspension transport, sand waves development, and migration. To discern the mechanisms of transport, observations of the turbulent flow were taken and records of the response of the suspended sediment concentration were obtained using acoustic backscattering (ABS). As an intermediate goal of the overall objective an analysis of the ABS data has been conducted with the aim of assessing and quantifying the acoustic technique. Recent theoretical descriptions of the interaction of sound with suspensions are presented, and predictions are compared with a laboratory study on suspensions of spheres and noncohesive sediments. The laboratory data show the theoretical approach to be broadly correct, and it has been implemented to interpret the estuarine observations. Acoustic estimates of the suspended sediment concentration have been compared with pumped sample data and shown to be similar. Comparison of the laboratory and estuarine estimates for the form function and total scattering cross section for the estuarine sediments is presented, and some unresolved differences are considered. The effects of the sediment attenuation and the random amplitude of the backscattered signal are also highlighted, and their implications regarding acoustic estimates of the suspended sediment concentration are addressed. Finally, after assessing the accuracy of the acoustic approach, high-resolution estimates of the suspended concentration are presented with some speculation on the origins of the concentration variability observed.

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