Sediment Acoustics: Wideband Model, Reflection Loss and Ambient Noise Inversion

Abstract

LONG-TERM GOALSPhysically sound models of acoustic interaction with the ocean floor including penetration, reflection and scattering in support of MCM and ASW needs. OBJECTIVES(1) Consolidation of the new BIC08 model of sediment acoustics, its verification in a variety of sediment types, parameter reduction and documentation in preparation for transition. (2) A new model of sediment reflection based on a mixture of models suitable for shallow water sonars for transition to NAVO. (3) Coupling of BIC08 to rough surface scattering models. APPROACH(1) Consolidation of the new BIC08 model: The Biot-Stoll grain contact squirt flow and shear viscous drag (BICSQS) model, which includes squirt flow at the grain-grain contacts (Chotiros, Isakson 2004), combined with improvements in squirt flow modeling, the frame virtual mass extension to account for the random grains rotation (Chotiros, Isakson 2007), and the high-frequency viscous drag correction (Chotiros, Isakson 2008) form the Biot-Stoll contact physics model of 2008 (BIC08). This model contains plausible physical processes and is able to track the measured frequency dependence of sound and shear speed and attenuation in water-saturated sandy sediments. In its present form, it is too unwieldy for practical applications. The approach is to consolidate the input parameters and reduce the parameter count, to continue to test it against experimental measurements and extend it to a wider variety of ocean sediments. In support of the modeling, new experimentation methods are envisioned, including the exploitation of ambient noise for bottom characterization using a compact, partially buried array. Through participation in future shallow-water (SW11) and sediment acoustics experiments (SAXnn), further testing is planned with a wide range of sediment types and over a broadband of frequencies, to extract model parameter values that are typical for specific classes of sediments.(2) A new model of sediment reflection: The reflection measurements from the SAX04 experiment (Isakson, Chotiros, Camin, Piper 2007) indicate that the sediment is often inhomogeneous. Although the SAX04 experiment may be a rather extreme case, due to the passage of a number of nearby storms, the results suggest that a significant proportion of uncertainty in underwater transmission loss and 1