Acoustic measurements of suspended sediments in turbulent currents and comparison with in-situ samples

Increasingly in recent years the application of acoustic backscattering to the quantitative measurement of suspended sediment particle size and concentration at sea has gained acceptance. A number of works describing the interaction of sound with suspensions have been published, and the scattering properties of suspended sediments formulated. However, there have been relatively few experiments conducted in the marine environment, which have attempted to assess the accuracy of the acoustic measurements by direct comparison with in-situ samples, taken simultaneously with the acoustic observations. The purpose of the present work is to report on such an experiment, and to evaluate the accuracy of the acoustic technique. To this end multifrequency acoustic measurements of suspended sediment profiles were collected in an estuarine environment, subject to strong turbulent tidal currents, which generated high concentrations of suspended sediments. To obtain the sediment parameters from the acoustic data an inversion needs to be applied, and this inversion is examined here in some detail, particularly for the case when sediment attenuation is substantial. To assess the sediment parameters derived from the acoustic inversion, mean acoustic estimates of particle radius and concentration are compared with the benchmark of in-situ pumped sampling. In addition to the analysis of the mean data, high-resolution images of the suspension dynamics have been generated, and the validity of these observations appraised by evaluating the internal consistency of the multifrequency results. (C) 1997 Acoustical Society of America.