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.
Analytical discussion of past measurements of acoustic attenuation in mud sediments and of possible future experimental approaches
Attenuation of compressional waves in mud is higher than in sea water, and less than in sandy/silty sediments. For experiments reported in 1964 (Acustica) by Wood and Weston, the Inferred circumstances are a 1 m thick mud slab overlying a gravel bottom, with air above (low tide), and with source and receiver either at the mud-gravel interface or slightly above it. Frequencies from 4 kHz to 72 kHz were transmitted, with signals received at a succession of horizontal distances of up to 50 m. Reported data in decibels versus range show considerable erratic behavior. Attempt is made to explain this behavior using a full-wave analysis of a Pekeris waveguide, with a fluid layer (the mud slab) overlying a (fluid or elastic) half-space, with a series of plausible guesses concerning the properties of the half-space. The conclusion is that the data and the circumstances allowed considerable variability in the attenuation estimates, but those made by Wood and Weston were about as good as could be expected. it is arg...
The Damping of Sound by Frothy Liquids
The fact that a tumbler containing a frothy liquid gives a dull sound when struck is familiar to every one, but I cannot find that any explanation of the rapid damping of the vibrations, which is indicated by the character of the sound, has been published. The converse case, namely, that of waves propagated through a gas in which small solid or liquid spheres are disseminated, has received considerable attention, and the results deduced, which agree with observation, are to the effect that although the presence of small obstacles does cause some damping, it is very small in amount.