DIRECT INVERSION METHOD TO MEASURE THE CONCENTRATION PROFILE OF SUSPENDED PARTICLES USING BACKSCATTERED SOUND

Acoustic backscatter using a single-frequency transducer can be used for field measurement of irregularly shaped, suspended sediment with a given size distribution. It is assumed that the size distribution of the suspended sediment does not change significantly along the sound path. A newly found explicit solution to acoustic backscatter equation is derived. This explicit solution reduces the computation time significantly during the inversion process. The near-field effect can be included in the inverted quantities by calculating the initial condition with the far-field acoustic backscatter equation and under the assumption of uniform near-field concentration. This method proved successful in the laboratory with initial concentrations of sand up to 3.5 g/L. The errors in estimation of concentration that result from errors in the attenuation coefficients or near-field concentration are significantly amplified with range in a uniform concentration field. However, with an exponential concentration profile, which is commonly found in sediment suspension phenomena, the inversion is only weakly sensitive to attenuation coefficients associated with sound absorption by water and sound scattering by suspended particles. Near-field concentration errors result in commensurate errors throughout the range in the exponential profile.