Acoustic modeling of sandy ocean sediments

Acoustic interaction with the seafloor is an integral part of underwater sound propagation. The reflection coefficient at the seafloor interface is a significant component of sound propagation loss, especially in coastal areas where the water depth is limited. The reflection coefficient may be used to characterize the properties of the seafloor for many undersea applications. In all cases, the quality of the result depends on the ability to accurately model the acoustics of the seafloor. Sandy sediments are of particular interest because current models are not consistent with measurements, signifying that our understanding of the physical processes is still inadequate. The discrepancies of the fluid and elastic solid approximations are clearly demonstrable. A poro-elastic model, such as Biot´s theory, is more likely to succeed because sandy sediments are porous structures saturated with water. Using laboratory and at-sea measurements, an incompatibility between model and measurements was found. A redefinition of the boundary between frame and fluid was found to provide a plausible explanation. This work is expected to lead to a physically sound model of sediment acoustics, which will be useful in future applications involving acoustic interactions at the seafloor.