Modeling of sound propagation in sandy ocean sediments

Abstract

A physically accurate model of sound reflection and penetration into ocean sediments is the objective. Sandy sediments are of particular interest because current models are not compatible with acoustic measurements. The discrepancies of the fluid and elastic solid approximations are clearly demonstrable. The Biot theory has the potential to satisfy the objective. Stolls formulation 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 physical constants; the second is less precisely known, and the third is not measurable. An inversion procedure was devised to estimate the immeasurable group from simple acoustic measurements — reflection loss, compressional and shear wave speeds, and attenuations. The imprecisely known group was handled in a probabilistic manner. The inversion results for water-saturated sand, using published laboratory and at-sea measurements, show a definite incompatibility between model and measurements. To find a solution, a couple of hypotheses were considered: (1) the inclusion of some proportion of the pore fluid within the frame and (2) the relaxation of the uniform, elastic frame assumption. The latter proved to be a viable solution. [Work supported by ONR, Ocean Acoustics.]