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A user's guide has been developed for BOGGART, a model for acoustic backscatter from marine sediments, developed at Applied Research Laboratories, The University of Texas at Austin (ARL:UT), with support from the Naval Research Laboratory at Stennis Space Center (NRL/SSC). The model, named for bottom grain gas and roughness technique, computes the total backscattering strength as an incoherent sum of contributions from three components. They are (1) scattering from sediment grains, (2) scattering from trapped gas bubbles within the sediment, and (3) scattering from interface roughness. The grain scattering is modeled semiempirically, based on the diffusion equation, in lieu of using a theoretical model which is under development. The gas bubble and roughness-scattering components are theoretical. Acoustic propagation in the sediment is modeled by way of the Biot theory for poroelastic media. This report is a user's description for BOGGART, version 3.0.

A model for acoustic scattering from the seabed was previously developed by Boyle and Chotiros (ARL-TR-93-15, 'Bottom Backscatter from Trapped Bubbles' and ARL-TR-94-21, 'Bottom Backscatter from Trapped Bubbles II'). The model treats acoustic scattering as a sum of three independent processes, including scattering from the sediment interface, volume scattering from sediment grains, and volume scattering from trapped gas bubbles that might exist between grains. Recently the gas bubble component of this model has been modified to allow for bistatic scattering and depth dependent sediment gas fractions. The theories behind these modifications are described, and in each case the character of the resulting model studied. jg

A use's guide for ARL:UT's marine sediment acoustic backscatter model has been developed. The model, named BOGGART (Bottom Grain Gas and Roughness Technique), is a three-component model which computes the total backscattering strength as an incoherent sum of contributions from three components. They are (1) scattering from sediment grains, (2) scattering from trapped gas bubbles within the sediment, and (3) scattering from interface roughness. The grain scattering is modeled empirically in lieu of a theoretical model which is under development. The gas bubble and roughness scattering components are theoretical. Acoustic propagation in the sediment is modeled by way of the Biot theory for poroelastic media. This report describes BOGGART's program structure, as well as its input and output file management. It is intended to suffice as a user's description of the model's operation.

A model for difference frequency backscatter from trapped bubbles in sandy sediments is developed. A nonlinear volume scattering coefficient is 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, B. R. Kerman (ed.), 363-373 (1993)) which treats the case of bubbles surrounded by water. The Biot theory is incorporated to model the acoustics of sandy 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., Prepared in cooperation with Texas Univ. at Austin. Applied Research Labs., Rept. no. ARL-TR-95-19.

A model for acoustic backscatter from trapped gas bubbles in sandy sediments was described in 'Bottom Backscatter from Trapped Bubbles'. In that model, trapped bubbles were assumed to scatter as if they were free bubbles in open water. In this report, the effects of bubble confinement in sediment pores on the resonance behavior of the bubble are accounted for. This is done by assigning the pore fluid an effective density that differs from its actual density, accounting for the fact that the fluid is partially confined within pores. The effective density is computed by way of the Biot theory. Two effective densities are specified, one for each of the two compressional waves that the Biot theory predicts. As a result, the medium has two scattering cross sections, which are both included in the resulting expression for scattering strength.

A study was made of acoustic propagation through sandy sediments. It was done in two parts. First an acoustic model for penetration based on the Biot model was considered. The model was used to predict phase velocities, attenuations, reflection coefficients, and pressure amplitudes in the sediment. Second, a preliminary model for acoustic backscatter from a distribution of gas bubbles was developed. The model involves a combination of existing theories for backscattering strengths from individual bubbles with a hypothetical bubble size distribution function which is related to the sediment rain size distribution.

Following recent experimental results at sea (N.P. Chotiros, Proceedings of Ocean '89) that suggest the existence of a previously undetected type of acoustic wave in sandy sediments, an experiment was designed to detect and measure the speed of acoustic waves in an isolated environment. The experiment was conducted in a laboratory tank containing 1 m of unwashed river sediment under a 3 m water column. Observations were made of the travel time and attenuation of a pulse from an acoustic source located above the water-sediment interface to a set of probes below the interface. It was observed that at normal incidence the pulse traveled at about 1675 m/s, while at shallow grazing angles the pulse traveled through the sediment at close to 1200 m/s. An interesting possible explanation exists in the Biot theory for acoustic propagation through fluid-saturated porous media, which predicts a slow acoustic wave in porous materials., Original contains color plates: All DTIC and NTIS reproductions will be in black and white.