In this article acoustic scattering by a random rough interface that separates a fluid incident medium from an underlying uniform scattering medium, either fluid or elastic solid, in cases for which the Bragg scale lies within the power-law tail of the roughness spectrum is dealt with. The physical foundation is an inherently reciprocity-preserving, local small-slope theory. A fully bistatic formulation is developed for the scattering strength, together with a robust numerical implementation that allows a wide range of spectral exponent values. The practical result for ocean acoustics is a significantly improved description of the interface component of sea floor scattering. Calculations are presented to demonstrate the advantage of this approach over perturbation theory, and to illustrate its dependence on frequency and environmental parameters as well as its operation in bistatic geometries.

The problem of acoustic scattering from the ocean surface can be solved using the formalism that has been developed to describe the scattering of a wave off a finite object. A refined version of this formalism is used to generate various approximation schemes. The validity of these approximations are investigated and their relative merits are discussed. The scattering cross section is evaluated numerically and it is concluded that the observed cross section for scattering near the ocean surface cannot be explained by ocean surface scattering alone.

Standard Form 298 (Rev. 8-98)Prescribed by ANSI Std. Z39.18Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Approved for public release; distribution is unlimited. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S ACRONYM(S) 9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S REPORT NUMBER(S)Multistatic active system performance can be driven by reverberation from the ocean boundaries and biologics. Providing accurate sonar performance predictions of reverberation, in turn, relies on providing accurate estimates of bistatic scattering strengths. This report presents new three-dimensional models that provide physics-based estimates of the dependence of scattering strength on the incident and scattered grazing angles, the bistatic angle, the acoustic frequency (10 to 10000 Hz), and physical descriptors of the environment (such as bottom properties for the bottom model, wind speed for the surface model, and fish properties for the volume model). The bottom model describes scattering from rough, elastic interfaces, while the surface model describes scattering from both the rough air-sea interface and subsurface bubbles. The volume models describe scattering from dispersed bladdered fish, including boundary-interference effects. For all, parameter studies along with data-model comparisons demonstrate the importance of using physics-based scattering models to describe the complex acoustic interaction processes at the ocean boundaries. These broadband models can enhance sonar performance prediction capabilities through their inclusion as submodels in both active performance/reverberation models

Use of the discrete variable representation in the infinite order sudden approximation for reactive scattering This is the second in a series of articles about the theory of scattering from a rough surface. A symmetric representation of the scattering amplitude and a formal statement of the composite model, both derived in the previous article, are used to approximate the scattering amplitude in terms of the known results for a reference surface. When the reference surface is a plane, the small slope approximation follows, while the traditional composite model is obtained if the curvature of the reference surface is neglected. The medium is assumed to be homogeneous, and the calculation is performed for scalar waves obeying Dirichlet or Neumann boundary conditions, electromagnetic waves obeying perfect conductor boundary conditions and the interface between two media for both types of waves. Finally, the requirement that the formal composite model must reproduce the traditional composite model in the appropriate limit is used to obtain a new approximation to the scattering amplitude. This is done for the Dirichlet, Neumann, and perfect conductor problems. The error goes as the square of the curvature. 986

The Foldy-Wouthuysen transformation can be used to reduce the relativistic Klein-Gordon equation to the nonrelativistic Schrödinger equation. This technique is modified and applied to the problem of wave propagation through media with a range-dependent index of refraction. The forward and backward propagating components of the field are decoupled order-by-order to produce a perturbative expansion of the range-dependent parabolic equation. The result includes energy-conserving correction terms that can be associated with a rapid fluctuation of energy between forward and backward propagating solutions of the Helmholtz equation. The approach selects out physical processes which accumulate over the entire range of propagation, distinguishing them from effects which depend solely on the initial and final values of the index of refraction and its derivatives. It is also shown that the corresponding backscatter mechanism is fundamentally nonperturbative, so that the parabolic equation technique as applied to the problem of propagation through range-dependent media generates an asymptotic expansion of the exact solution. This procedure has been applied to long-distance low-frequency propagation through a sound channel with internal waves. For this application, the expansion parameters are typically very small, so the propagation distances must be very large for the effect to be detectable. ͓S0001-4966͑97͒02203-0͔