Browsing by Subject "elastic waves"
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Item The Evolution Of Sediment Acoustic Models(2012-11) Chotiros, N. P.; Isakson, M. J.; Chotiros, Nicholas P.; Isakson, Marcia J.Sediment acoustic models contain two connected components, the geo-physical description of the sediment and the model of acoustic processes. Geo-physical descriptors are used in the classification of sediments, and they are based on grain size, density and other physical descriptors. Acoustic sediment models were initially fluid approximations that were simple to implement. As the need for accuracy increased, the fluid model was extended to stratified fluid and visco-elastic models. The latter, with five frequency-independent parameters, appeared to be consistent with sediment acoustic data up to the 1980s. More recent experimental data have revealed discrepancies in the frequency-dependence of attenuation and sound speed, particularly in the case of sandy sediments, which cover a large fraction of the continental shelves. Broad-band acoustic measurements of wave speeds and attenuations are more consistent with a poro-elastic model, consisting of Biot's theory with extensions to account for the physics of granular media. Aside from terminology, there is a fundamental difference between viscoelastic and poro-elastic theories. The former is based on two types of waves, a compressional wave and a shear wave, while the latter has an additional compressional wave, often called the Biot wave. There are currently two approaches to the development of sediment acoustic models: (a) visco-elastic models with frequency dependent parameters that mimic the observed behavior, and (b) poro-elastic models that reflect the physical processes. It is shown that (a) would be a significant improvement over existing models, but (b) is the preferred solution.Item Seabed Attenuation In Water Saturated Sands On New Jersey Continental Shelf In 50-3000 Hz Band(2010-09) Knobles, D. P.; Knobles, D. P.The Shallow Water 2006 ocean acoustics experiment on the New Jersey continental shelf was designed in part to determine the frequency dependence of sound speed and attenuation for a sandy sediment in the 50-3000 Hz band. Two acoustic arrays in L-geometries were positioned about 25 km apart on a ridge of coarse sand. Segments of narrowband and impulsive acoustic data were analyzed for the information they contain on the seabed geoacoustic structure and the frequency dependence of the top sediment layer attenuation. The results clearly demonstrate a nonlinear frequency dependence of the absorption.