Validation and verification of three-dimensional finite element target scattering models and Green's functions.

A configurable three-dimensional (3D) finite element framework is used for modeling acoustic scattering from various elastic targets on seabed and for the numerical determination of Green's functions used in far-field target scattering prediction. The 3D selection is chosen for possible inclusion of complex seafloor geometries, such as rough, rippled, cluttered, or variably layered seabeds, in which two-dimensional modeling approaches cannot capture the full acoustic interaction with the target and the seabed. Model results are verified and validated against analytic scattering theories and measured results from the 2013 Target and Reverberation Experiment (TREX) and the 2017 CLUTTEREX experiment.

Method of numerical Green's function determination for far-field scattering solutions from objects at a water-sediment interface.

A method is presented for numerically determining Green's functions for the purpose of calculating the far-field scattering from objects resting on or buried within the seafloor. To obtain the far-field scattering, initial evaluation of the three-dimensional near-field solution is required, through finite element analysis or other means. The Green's function and its spatial derivatives are then numerically evaluated for input into the Helmholtz-Kirchhoff integral, yielding the far-field scattering solution.

Observation and modeling of acoustic scattering from a rubber spherical shell.

Acoustic backscattering from a rubber spherical shell in water is observed to contain a delayed enhancement, demonstrated to be associated with a waveguide path along the shell. This path is somewhat analogous to that of the Lamb wave observed on metallic shells. Rubber is a unique material because of its subsonic sound speed relative to water, and because shear coupling is often small enough to be neglected in typical models, making it fluid-like.

Spectral analysis of bistatic scattering from underwater elastic cylinders and spheres.

Far field sound scattering from underwater elastic spheres and finite cylinders is considered over the full range of scattering angles. Three models for the frequency response of the scattered field are evaluated: a hybrid finite element/propagation simulation for a finite cylinder with broadside illumination, an approximate solution for the finite cylinder, and the exact solution for a sphere. The cylinder models are shown to give comparable results, attesting to the strength of the finite cylinder approximate solution.

Kirchhoff approximation for backscattering from a partially exposed rigid sphere at a flat interface.

The Kirchhoff approximation (KA) is used to model backscatter of sound from a partially exposed, rigid sphere at a flat free interface of two homogenous media. Scattered wavefields are calculated through numerical integration on the sphere of the Kirchhoff integral, requiring detailed knowledge of the illuminated region for each scattering path. This approach avoids amplitude discontinuities resulting from geometric transitions in the number of reflected rays.