A. S. Schaafsma
Delft Hydraulics, P.O. Box 1277, 26090 MH Delft, The Netherlands
The in situ measurement of the size distribution and the concentration of suspended sediment, by means of the technique of acoustic attenuation spectroscopy in the frequency range of 1--100 MHz, is discussed. Results of laboratory experiments in well-defined conditions are presented to illustrate the possibilities to develop a general analysis method, for a priori unknown particle size distributions. For spherical scatterers, a satisfactory phyiscal interpretation of the attenuation spectra can be given [A. E. Hay and A. S. Schaafsma, J. Acoust. Soc. Am. 85, 1124--1138 (1989)]. For nonspherical scatterers, e.g., sediment particles, one possible approach is a purely empirical one, which, however, involves the need of extensive calibrations [A. S. Schaafsma, in Ultrasonics International 89, Conference Proceedings (Butterworth, London, 1989), pp. 388--393]. Nevertheless, the results indicate that a size resolution of about a factor of 2 can be obtained, for average sediment sizes in the range of 5--500 (mu)m. As a further development along these lines, it is shown that a semi-empirical method for irregularly shaped (sediment) particles, involving less extensive calibrations, gives promising (preliminary) results.