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...
Observations of nearshore infragravity waves: Seaward and shoreward propagating components
[1] The variation of seaward and shoreward infragravity energy fluxes across the shoaling and surf zones of a gently sloping sandy beach is estimated from field observations and related to forcing by groups of sea and swell, dissipation, and shoreline reflection. Data from collocated pressure and velocity sensors deployed between 1 and 6 m water depth are combined, using the assumption of cross-shore propagation, to decompose the infragravity wave field into shoreward and seaward propagating components. Seaward of the surf zone, shoreward propagating infragravity waves are amplified by nonlinear interactions with groups of sea and swell, and the shoreward infragravity energy flux increases in the onshore direction. In the surf zone, nonlinear phase coupling between infragravity waves and groups of sea and swell decreases, as does the shoreward infragravity energy flux, consistent with the cessation of nonlinear forcing and the increased importance of infragravity wave dissipation. Seaward propagating infragravity waves are not phase coupled to incident wave groups, and their energy levels suggest strong infragravity wave reflection near the shoreline. The cross-shore variation of the seaward energy flux is weaker than that of the shoreward flux, resulting in cross-shore variation of the squared infragravity reflection coefficient (ratio of seaward to shoreward energy flux) between about 0.4 and 1.5.