Paul R. Ogushwitz
PRO Scientific Consulting, 303 Center Street, Hackettstown, NJ 07840
Acoustical data from a field survey at a disposal site for dredged materials near Mobile Bay, Alabama, are analyzed. During the survey, two independent acoustical instruments provided measurements of echo strength from vertical plumes of suspended sediments. Particulate concentrations and grain size distributions were determined from in situ water samples. Quantitative analysis of the measurements indicates that the acoustical echoes from the suspended sediments are dominated by Rayleigh backscattering. The best fit to data is achieved when the measured decrease of grain size with decreasing concentration is explicitly accounted for. Attenuation appears to play a significant role at high concentrations (i.e., within the dense part of the plume). For the grain sizes studies here (1--200 m), viscous effects account for at least 90% of the attenuation. When the theoretical estimates are subtracted from the data, the residual echo strength is seen to increase with decreasing concentration; some possible causes of this are beam-to-beam variability, effects of grain shape, and factors related to experimental design. One of the instruments measured echo strength on four beams simultaneously; the average beam-to-beam variability was about 3 dB, corresponding to a precision of about 50% in the measured concentration. [Work supported by U.S. Army, Corps of Engineers, Dredging Research Program.]