Backscattering from sediments: Comparison of scattering strengths from a simple sand and from a sand/mud environment

Abstract

Two sediment acoustics experiments were carried out within 1 km of each other off the coast of Florida, the first in 1999 and the second in 2004. Though close in space the sediment was very different due to the effects of Hurricane Ivan in 2004. Hurricane Ivan resulted in the deposition of mud patches on top of the medium sand. Subsequent weather events deposited sand on top of these patches. The backscattering at each site will be presented for frequencies from 20 to 1000 kHz and grazing angles from 20 to 45 deg. At angles below the critical angle (30 deg) and frequencies below 150 kHz the backscattering measured in the two experiments is consistent given the difference in the sediment roughness spectra. Above the critical angle, however, the backscattering measured in the 2004 experiment is several dB higher than in the 1999 experiment and increases immediately above the critical angle instead of decreasing. This difference is attributed to an increase in sediment volume scattering in 2004. From 150 to 1000 kHz the data from both years indicate a change in scattering mechanism below the critical angle to a mechanism differing from sediment interface roughness scattering. [Work supported by ONR.]