ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

1aAO4. Modeling of high-frequency acoustic wave scattering by sediment gas voids.

Dajun Tang

Dept. of Appl. Ocean Phys. Eng., Woods Hole Oceanographic Inst., Woods Hole, MA 02543

Scattering by volumetric inhomogeneities of marine sediments comes in many forms. It is found in a 1993 CBBL-SRP experiment conducted in Eckernfoerde Bay, Germany, that high-frequency backscattering is caused by gas voids buried at about a meter beneath the seafloor [Tang et al., J. Acoust. Soc. Am. (to be published)]. The backscattering strength at 40 kHz is estimated to be -10 dB. Assuming the gas voids do not resonate, a simple scattering model is developed based on the Kirchhoff approximation to calculate the backscattering and bistatic scattering strength. Acoustic ray bending due to the sound-speed discontinuity at the water--bottom interface as well as sediment attenuation are taken into account. We find that at 40 kHz, only those gas voids whose exposed cross section is larger than the acoustic wavelength contribute to backscattering significantly. The gas void distribution is estimated based on data from the few cores obtained in situ. The model results are compared with backscattering data, and it is intended that this model be used to compare with bistatic scattering data in the future. [Work supported by ONR through NRL.]