Laboratory and marine measurements on the acoustic detection of sediment transport

This paper describes a series of measurements taken both in the laboratory and at sea, to examine the feasibility of monitoring marine gravel transport, by measuring the acoustic self-generated noise (SGN) produced by the interparticle collisions of bedload material. This study is a continuation of work previously reported on the SGN of agitated artificial sediments. Laboratory measurements are presented on the relationship between the SGN spectrum and particle size for naturally occurring gravels undergoing interparticle collisions. The dependence of the acoustic intensity level upon the mass of agitated material is also studied. Attempts are made to utilize the acoustic spectrum to obtain information on the particle size distribution for poorly sorted gravel samples. Using the laboratory results, a marine system was developed consisting of two hydrophones to monitor the SGN, an underwater TV camera to independently assess bedload transport, and electromagnetic current meters to obtain information on the flow condition. The instrumentation package was deployed on a seabed composed of mobile gravels. The results of an intercomparison between the acoustic and visual data are presented. Also, some discussion on the relationship between flow conditions and gravel movement is pursued. Attempts are also made to obtain the size distribution of the mobile material by an examination of the SGN spectrum taken at sea.