Abstract:

Recent experiments at sea and in the laboratory indicate anomalously high levels of acoustic penetration into sandy marine sediments at subcritical grazing angles, which are not accounted for via current viscoelastic acoustic models for sediments. Current research is directed toward a possible physical explanation. One possibility [Chotiros, J. Acoust. Soc. Am. 97(1), 199--214 (1995)] is that the sediment acts as a Biot poroelastic medium, supporting Biot slow waves with sufficient amplitude to account for the observed penetration. Another [Thorsos et al., SACLANT Conf. Proc. CP-45, 563--569 (1997)] is that seabed interface roughness scatters a significant amount of sound into the sediment. These two mechanisms differ in their ray geometries for penetrating sound. In order to determine the relative importance of the two mechanisms, a tank experiment was performed to determine acoustic ray paths into a sandy bottom. It involved a narrow beam acoustic source directed at hydrophones in the sediment column. Initial results appear to be consistent with Biot slow-wave penetration. A description of the experimental work will be included, along with a discussion of preliminary results.