Acoustical Properties of Mud Sediments
Abstract : Goals include the development of models and methods that explain observed material and acoustic properties of different physical types of shallow-ocean mud sediments. Other goals are to assess prior data relating to the acoustic properties of mud and to provide guidance in the development and interpretation of experiments. A related goal is to construct models that will guide inversion techniques for inferring properties of mud sediments, for estimating accuracy of results, and for use in applications. These goals are science relevant to resolving long-term Naval problems of transmission loss and array response in shallow water over mud sediments and of acoustic detection, localization, and classification of objects buried in mud. Develop analytical fundamental-physics models for geoacoustic properties that show their dependence on mud parameters. Properties include material density, porosity, and compressional and shear sound speeds and attenuations, with focus on depth and frequency dependence. Estimate the accuracy of physical mud parameters that is required for acoustic data comparisons and for making predictions in applications. Use modeling results to provide methodologies for inversion methods in order to determine parameters and profiles in mud sediments.
Tentative acoustic wave equation that includes attenuation processes for mud sediments in the ocean
The formulation of Pierce and Carey (JASA-EL 2008) for marine sediments is extended to mud. A revised Mallock-Wood theory includes the electrostatic attraction between clay platelets. The effect is to increase the low frequency sound speed by 1% at high porosities. A wave equation is derived starting from the approximate premise that the pressures in the fluid and solid particles are the same because of the viscosity in the fluid, which tends to drag the clay particles along with the fluid. Relaxation effects are introduced with the assumption that density fluctuations in the fluid have additive contributions from density-like variables that satisfy relaxation equations. The inclusion of the relaxation variables leads to a wave equation with an additional term that is of the form of a frequency-dependent operator times the frequency. Theory leads to integral expressions for attenuation and phase velocity versus frequency that involve a single function of relaxation time. The mechanism for the relaxation p...
Propagation in a liquid layer lying over a liquid half‐space (Pekeris cut)
Sound propagation in a uniform duct can be represented as a sum of normal modes plus a branch line integral(s) contribution. In the case of a homogeneous liquid layer lying over a homogeneous liquid half‐space the complex modes exhibit a complicated migration as frequency is changed. In the case of a Pekeris branch cut this can result in the mode changing from a proper to an improper Riemann sheet or vice versa. This is accompanied by a discontinuous change in the branch line integral. This paper examines the migration of a typical mode and discusses a simple algorithm for calculating the Pekeris branch line integral.