Acoustic Techniques for Underwater Sediment Transport Studies

The application of acoustic_techniques in marine sediment transport research is an area of expanding interest, as the movement of sediments is of concern to sedimentologists, hydraulic engineers, oceanographers.and many other disciplines.Acoustic techniques are being applied to measure water velocity profiles using acoustic doppler devices.Acoustic backscattering and attenuation measurements have been used to monitor suspended sediments.The movement of sandbanks is being measured using acoustic bedform monitors, and the detection of the surficial movement of bed-load sand transport by acoustic impedance changes near the bed surface has been applied.The present paper describes the development of a nonintrusive, rapidly responding, continuously recording acoustic system to obtain field measurements of:-(i) the initiation or sediment bed-load transport, (ii) the mass of moving material, (iii) the size distribution of the mobile particles.The technique utilises the acoustic self generated noise (SGN) arising from the interparticle collisions of mobile sedimentary material.A laboratory and marine study of this sediment noise with a theoretical description of its probable origin is investigated.Proceedings of The Institute of Acoustics ACOUSTIC TECHNIQUES FOR UNDERWATER SEDIMENT THANSPGRT STUDIES measurements can be obtained without significantly disturbing the flow , structure or the state of the seabed, conditions difficult to achieve using conventional techniques.The application of this method is investigated in the present paper.A number of exploratory field experiments have been conducted by previous investigators '[10-16].However, due to a number of reasons only limited success was achieved.Theslow progress has been due in part to insufficient knowledge of the spectral structure, signal level, and physical factors which govern the SSH.Furthermore, no theoretical description of the origins of the acoustic noise had been attempted and this lack of understanding has impeded developments.The deficiency of information has led to ineffectual acoustic systems being designed, which have usually been tested without adequate independent assessment of bedload transport, and therelbre the validity of this approach had not been examined in the field with any degreeof exactitude.Some limited laboratory studies have been conducted [14,17,181 to provide information on the SEN, and these have indicated that mass transport may be linearly related to acoustic intensity, and the SGN spectrum should be a function of particle size, however, no successful marine application of these studies has been published.