Multifrequency acoustic measurement of concentration and grain size of suspended sand in water

Abstract

The paper describes an ultrasonic backscatter device for measuring vertical profiles of the concentration of suspended sand in water in the presence of air bubbles. The use of six frequencies permits a simultaneous measurement of the concentration of suspended sand, the grain size, and the concentration of air bubbles. For measuring in the near field of a transducer, the sensitivity of an ideal piston-like transducer in the near field is calculated and compared with measurements. A simple, but fast and efficient inversion algorithm to convert the backscattered intensities into sediment concentration and grain size is described. Measurements of known suspensions with concentration and grain size are presented, they exhibit a low error (20% rms) over a grain radius from 40 to 300 μm. In-situ measurements of the concentration of suspended sediment in the surf zone are compared with the results of two optical backscattering sensors (OBS).