ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

4pUW12. High-frequency acoustic penetration into seafloor sediments at subcritical grazing angles due to roughness at the water--sediment interface.

Eric I. Thorsos

Appl. Phys. Lab., Univ. of Washington, 1013 NE 40th St., Seattle, WA 98195

Acoustic penetration into sediments at low grazing angles is of interest for buried mine detection. For sandy sediments the critical angle can be in the 25(degrees)--30(degrees) range, which suggests that penetration at lower grazing angles might be quite limited. It will be shown, however, that when the sediment roughness is modeled to be consistent with the occurrence of sand waves, then significant penetration into the sediment can occur at subcritical grazing angles due to scattering at the rough interface. An integral equation method is used to obtain acoustic fields near the surface to illustrate the degree of penetration. A 1-D surface model is used, and the sediment is modeled as a fluid. For a given surface profile, the acoustic penetration is found to increase as the frequency increases. When the frequency is reduced to bring the roughness into the perturbation theory region, acoustic penetration is much reduced. [Work supported by ONR.]