A comparison of two inversion methods for determination of sand concentration and size from multifrequency acoustic backscatter

Abstract

RASTRAN (Remote Acoustic Sediment TRANsport measurement) System 1 has been used in laboratory trials and deployed in nearshore locations as part of several field experiments. The system operates at three frequencies, 1, 2.25 and 5 MHz, over a range of approximately 1 m, with resolution of about 2 cm. A central concern has been the probable errors in suspended sediment concentration and size derived from the backscatter data. An inversion algorithm based on the differences in backscattered pressure amplitude at the three frequencies has been developed. Laboratory experiments, measuring multifrequency backscatter from a turbulent sediment-carrying jet, have been used to calibrate the system and to test the inversion algorithm. Concentration and sizes inverted from field and laboratory data are compared with results from a previously developed algorithm based on signal ratios. Inverted concentrations are also compared with Optical Backscatter Sensor (OBS) data collected during the field experiment. This latest inversion scheme is less sensitive to errors arising from low signal levels, increasing the size/concentration measurement range to regions of lower concentration. The concentration results agree well with the independent OBS data.