Sound Speed and Attenuation in Multiphase Media

Abstract

LONG-TERM GOALSOne research goal developed from conducted shallow water (SW) acoustic transmission experiments in sandy-silty areas [1] revealed a nonlinear power law frequency-dependent attenuation at lower frequencies consistent with results reviewed in [2,3] and the observations by the ONR-HEP program. The Biot Theory [4] predicts that sandy sediment attenuation should have a quadratic dependence, however the nonlinear dependence observed was closer to a 1.8 power law most likely due to modal effects. Thus one long-range goal is to develop a simplified theory of sediment attenuation [5] verified by measurements that can be applied to ocean sediments.A second long-range goal is to better understand that acoustical behavior of individual bubbles and bubbly assemblages in water and water-filled sediments. The focus is on attenuation, dispersion, and bubble response near and about the resonance frequency. OBJECTIVEThe objective of the work was to determine the frequency dependent attenuation and phase speed characteristics of selected sandy and muddy sediments (both water saturated and partially saturated) at the lower frequencies to verify a simplified Biot theory [5] and to provide a theoretical / experimental basis for the water-sediment boundary condition necessary for the accurate prediction of wide band transmission loss in shallow waters. APPROACHThis work was aimed at enhancing our understanding of saturated and partially saturated sandy sediment for frequencies ranging from 100 Hz to 10 kHz. The basic hypothesis is based on the simplified Biot sediment model [5] and the prediction that high permeability sands will have a quadratic frequency dependent attenuation, and that these measure-ments can be described by a Biot time constant. During the past year several major tasks have been performed. Holmes [3,8] has compared this theory to experimental results for which the environmental conditions and the 1