for classified documents, follow the procedures in DoD Manual 5200.22-M, Industrial Security Manual. For unclassified, limited distribution documents, destroy by any method that will prevent disclosure of contents or reconstruction of the document.

A model for difference frequency backscatter from trapped bubbles in sandy sediments was developed. A nonlinear volume scattering coefficient was computed via a technique similar to that of Ostrovsky and Sutin [“Nonlinear sound scattering from subsurface bubble layers,” in Natural Physical Sources of Underwater Sound, edited by B. R. Kerman (Kluwer, Dordrecht, 1993), pp. 363–373], which treats the case of bubbles surrounded by water. Biot’s poroelastic theory is incorporated to model the acoustics of the sediment. Biot fast and slow waves are included by modeling the pore fluid as a superposition of two acoustic fluids with effective densities that differ from the pore fluid’s actual density and account for its confinement within sediment pores. The principle of acoustic reciprocity is employed to develop an expression for the backscattering strength. Model behavior is consistent with expectations, based on the known behavior of bubbles in simpler fluid media.

A model for acoustic backscatter from trapped bubbles in sandy sediments was developed. The model combines a Biot acoustic penetration model with a resonance scattering mechanism from trapped bubbles. The bubble size distribution is assumed to mirror the size distribution of the fluid pores that exist between sand grains. An estimate of the pore size distribution is constructed from the grain size distribution, based on the known pore structure between dense random packings of hard spheres. The principle of acoustic reciprocity is employed to compute backscattered acoustic pressure in terms of the incident pressure and the scattering cross section of a bubble distribution. The model is applied to data from experiments recently taken at sea. It is concluded that trapped gas is often a likely cause of observed backscatter from sandy sediments. Very small amounts of gas appear to be sufficient to produce significant backscatter. ¸

Recent theoretical results in Compressive Sensing (CS) show that sparse (or compressible) signals can be accurately reconstructed from a reduced set of linear measurements in the form of projections onto random vectors. The associated reconstruction consists of a nonlinear optimization that requires knowledge of the actual projection vectors. This work demonstrates that random time samples of a data stream could be used to identify certain signal features, even when no time reference is available. Since random sampling suppresses aliasing, a small (sub-Nyquist) set of samples can represent high-bandwidth signals. Simulations were carried out to explore the utility of such a procedure for detecting and classifying signals of interest.

It is well known that muddy sediments often contain significant amounts of gas of biological origin. Since the scattering cross section of a gas bubble in water is typically 1000 times its geometric cross section, it is reasonable that an acoustic backscatter model intended to work over muddy sediment should contain a bubble resonance scattering component. In this paper a heuristic model is presented, based on scattering from a distribution of suspended bubbles in mud. For acoustic propagation purposes, the mud is treated as a viscous fluid. Since estimates of bubble size distributions in mud are currently unavailable, a bubble distribution similar in shape to that observed in the water column is assumed. The model's only free parameter is the gas fraction, which can be varied to fit the model to observed data. Small amounts of gas appear to be sufficient to produce observed levels of backscatter. For a homogeneous bubble distribution, the model can be inverted to give an estimate of the gas bubble size distribution from measured backscatter data. A discussion of depth dependent bubble distributions is included. The simplest case is that of a homogeneous bubble half-space buried underneath a finite gasless layer. The thickness of the layer appears to affect the grazing angle dependence of the backscatter significantly. ¸