Underwater Acoustic Measurement of Suspended Sediments: The Calibration of a High-Frequency Acoustic Backscatter Meter
Abstract
A shortwave multi-frequency acoustic backscatter meter (ABM) has been developed for measuring profiles of near-bed suspended sediments in shallow marine and fluvial environments. The system comprises of a monostatic, active pulse sonar using three frequencies 5.65, 7.0 and 10.1 MHz, simultaneously and is capable of taking measurements over a range of 1.28 m.
Before its field deployment the ABM was calibrated in a tank, with 40 l of recirculating water, against a wide range of particle sizes including fine-sand and coarse- to medium-silt. The averaged profiles of backscattered pressure revealed that the rate of attenuation increases rapidly with increase in the frequency. Inspection of the raw data indicated that the size of suspended particles determines the amplitude of the backscattered signal more effectively than the quantity.
A model has been developed to convert the measured backscattered pressure into sediment concentration (mg/l). The model takes into account coefficients of attenuation due to water and sediments as well as beam strength and the acoustic cross-section of particles. Results indicate that the model is fairly reliable.
References
Clay, C.S. & Medwin, I.L, 1977. Acoustical Oceanography: Principles and Applications. John Wiley Sons, New York.
Fisher, F.H., and Simmos, V.P., 1977. Sound adsorption in sea water. J. Areous. Soc. Amer. 62, 558-564.
Hanes, D.M. & Vincent, C.E., 1987. Detailed dynamics of nearshore suspended sediment. Coastal Sediments '87, Amer. Soc. Civil Engineers 2, 285-299.
Hay, A., 198.7. On the remote acoustic detection of suspended sediment at long wavelengths. J. Geophys. Recs. 88, 7525-7542.
Hess, F.R. & Bedford, K. W., 1985. Acoustic backscatter system (ABSS): The instrument and some preliminary results. Marine Geology 66, 357-379.
Hunter, W.F., 1970. An introduction to acoustic exploration. In: Underwater acoustics (R.W.B Stephens, ed.). John Wiley & Sons Ltd., London, 91-127.
Libieki, C., Bedford, K.W. & Lynch, J.F., 1989. The interpretation and evaluation of a 3MHz acoustic backscatter device for measuring benthic boundary layer sediment dynamics. J. Acous. Soc. Amer. 85, 1501-1511.
Lynch, J.F., 1985. Theoretical analysis of ABSS data for HEBBLE. Marinca Geology 66, 277-290.
Orr, M.H. & Hess, F.R., 1878. Remote acoustic monitoring of natural suspensate distributions, Active suspensate resuspension and Slope/Shelf water intrusions. J. Geophys. Res. 83, 4062-4068.
Rehman, S.S. & Vincent, C.E., 1988. Acoustic measurement of suspended sediment in the Eastern Irish Sea. Geol. Bull. Univ. Peshawar 21, 105-116.
Sheng, J. & Hay, A.E., 1988. An examination of the spherical scatterer approximation in aqueous suspensions of sand. J. Acous. Soc. Amer. 83, 598-610.
Urick, R.J., 1983, Principles of Underwater Sound (3rd ed.). McGraw-Hill Book Company.
Vincent, C.E. & Green, M.O., 1990. Field measurement of the suspended and concertation profiles, and of the resuspension coefficient g over a ripped bed. J. Geophys. Res. (in press).
Vincent, C.E. Hanes, D., Tamura, T. & Clarke, T.L., 1986. The acoustic measurement of suspended sand in the surfzone Proc. International Conference on MEASURING TECHNIQUES of hydraulic phenomena in offshore coastal and inland waters, London, Paper K3, 443-451.
Vincent, C.E., Young, R.A. & Swift, D.J.P., 1982. On the relationship between bedload and suspended sand transport on the inner shelf, Long Island, New York. J. Geophys. Res. 87, 4163-4170.
Young, R.A., Merril, J.T., Clarke, T.L. & Proni, J.R., 1982. Acoustic profiling of suspended sediments in the marine bottom boundary layer. Geophys. Res. Letters 9, 175-178.
