Comparison of acoustic backscattering techniques for suspended sediments investigation

The aim of this paper is to compare different methods for suspended sediment investigation using the backscattering power of acoustic Doppler current profilers (ADCPs). Different concentration-backscattering models and calibration strategies were compared using two frequencies or only one and accounting or not for sound adsorption due to sediments. Two Teledyne RD Instruments (RDI) ADCPs, working at different frequencies (600 and 1200 kHz), were used simultaneously on the same water column to investigate the suspended sediment concentration and grain size distribution at four cross-sections of a lower Paraná River bifurcation near San Martín city (Argentina). In the same campaign, a Sontek 1000 kHz ADCP previously calibrated with water samples was also used. By applying a mixed electric-acoustical backscatter approach, each RDI ADCP was calibrated on an expected mean concentration, and a homogeneous grain size was fixed among the whole measurement field. Concerning the concentration results, the calibration on the mean value was found to be sufficiently reliable when compared to the previously tested and accepted acoustical backscatter method calibrated against an array of field samples that covered the concentration range of extrapolation. The same electric-acoustic approach was applied for the two frequency method. In this case, the backscatter was calibrated against the expected mean grain size and concentration. Preliminary results concerning the concentration and grain size distribution are consistent with the results of other methods. The quantity and texture of suspended sediments of the Paraná River were found to be consistent with instrument limits in terms of backscattering sensitivity. The wash load did not affect the sound propagation, and sound adsorption due to suspended sand did not perturb the sand concentration assessment.