| The acoustic backscatter system provides a nonintrusive method for measuring of profiles of suspended sediment size and concentrations. Therefore, an inversion model is required to convert the recorded series of intensities into parameters of sediment suspension. The measurement precision highly depends on the choice of the inversion model. In this work the high-pass backscattering model is modified to allow accurate inversion for both sediment sizes and concentrations. The modified correction function with a single parameter is introduced for the form function and normalized cross-section. The parameters can be found from the calibration of the system in the chamber with generated homogeneous suspension. To account for the difference in shape and mineralogy, a set of two parameters can be determined for a particular sediment. Their use greatly improves the accuracy of mean sediment size measurements obtained with the three-frequency acoustic backscatter system.; The inversion model was applied to data obtained in the chamber with settling mixture of two sediment samples. The results were compared to the predictions by the 1D advection-diffusion model. Good agreement was shown for concentrations and sediment size measurements, with the error for sediment size approximation lying within the standard deviation of size distribution. The accuracy of the concentration measurements was different among the transducer frequencies, and was found to depend on the sensitivity of the transducer to the particular sediment size. The sediment sizes were estimated by minimizing the variance between concentrations, determined from different frequencies, and found to depend on the accuracy of estimated concentrations. For correction of recorded voltage data with applied time-varying gain, the system functions were found to be better approximated by second order polynomials.; A multiple reflection model was developed based on the multiple layer approximation. The numerical simulation with the initial uniform concentration profile of 3g/l shows that the effect in intensities from the multiple reflections can be up to 14%. Also, a bottom echo removal algorithm was developed in order to eliminate the effect of high intensity scattering on the concentration measurements close to the bottom. |
