Acoustic Modeling of Sandy Ocean Sediments

A0121 Nicholas P. Chotiros Applied Research Laboratories, The University of Texas at Austin

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. The first difficulty lies

in the relatively large number of input parameters. Stoll's

formulation of the Biot model requires 13 input parameters, which may

be divided into three groups according to the accuracy with which they

are known. The first group consists of tabulated or accurately

measurable physical constants; the second consists of flow related

parameters that are measurable but at a lower precision; and the third

contains frame related parameters that are not directly measurable.

Using published laboratory and at-sea measurements, the parameter

values in the third group were obtained by inversion, and as a result,

it became clear that there is a second difficulty: An incompatibility

between model and measurements. It was not possible to adjust the

model to satisfy all of the measurements. To bridge the gap between

model and measurements, a couple of hypotheses were considered:(1) re-

evaluating the boundary between frame and fluid, and (2)the relaxation

of the uniform, elastic frame assumption. There are plausible reasons

supporting both hypotheses and both appear to be viable solutions.

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. [Work supported by the Office

of Naval Research, Ocean Acoustics]