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. This makes rubber a material of interest for coating and cloaking underwater devices and vehicles. Both fluid and elastic rubber partial wave series models are tested, using experimentally measured longitudinal and shear speeds, attenuation, and rubber density. A finite element model for the shell is also developed. Comparison of the models and experiments highlights the importance of the waveguide path to the overall scattering. Estimates for the group and phase velocities of the lowest order propagating mode in the shell are determined through waveguide normal mode analysis and Sommerfeld-Watson theory, and are shown to give good agreement with experiments in predicting the time of arrival of the waveguide path.

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. Interference and resonance structure present in the frequency response of the targets is identified and discussed, and the bistatic spectra for a variety of elastic sphere materials are presented. A thorough understanding of the complicated angle and frequency dependence of the scattering from simple elastic targets is helpful for interpretation of backscattering data from targets at or near an interface, or for scattering data taken by moving automated underwater vehicles, acoustic arrays, or other forms of data collection involving bistatic scattering.

A numerical approach to solving for Green’s functions allows far-field scattering to be solved within variable seafloor environments. The approach is applicable for both seafloor and buried target scattering problems. A robust near-field model is required for near-field solution evaluation and Green’s function determination. In this work, a 3-D finite element model is used. Far-field scattering results are determined through numerical integration of the Helmholtz-Kirchhoff surface integral within the near-field of the target or interface. This approach has previously been shown to give accurate results within flat sediment interface environments; now the technique is extended to more complex interface structures, in which the benefit of the numerical Green’s function technique is most apparent. For flat interface environments, analytic Green’s functions are known, making numerical approaches redundant. For more complex environments, exact solutions are difficult to come by, and analytic approximations grow in error with increasing environmental complexity. Furthermore, the numerical approach requires only a single field measurement to be made within the near-field model, making the approach computationally efficient and feasible for a wide variety of scattering problems. [Work supported by ONR, Ocean Acoustics.]

Geoacoustic inversion of transmission loss measurements for determining sediment properties greatly reduces the time and cost needed to characterize the seafloor compared to cores or other in situ measurements. This study sought to determine the efficacy of a Bayesian inversion scheme to determine sediment parameters from transmission loss using simulated data, created using a hybrid finite element/ray acoustic model. For simplicity, the sediment was described as a fluid. A reduced order inversion parameter set was chosen to limit computational time and parameter intercorrelation, as well as to limit the search space to what is believed to be the most contributive parameters in the high frequency regime. This set includes sound speed, density, attenuation, and the von Karman spectral strength and exponent to describe interface roughness. The inversion scheme yielded marginal posterior probability distributions for each of the parameters, as well as information about parameter resolvability and covariance. [Work supported by the Office of Naval Research, Task Force Ocean.]

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. Reflections from the interface are modeled through use of an image source, positioned symmetrically relative to the real source. Results are compared to experimentally obtained backscattering records from elastic spheres at an air-water interface, as well as to an exact partial wave series for a half exposed sphere. These comparisons highlight the omission of Franz-type reflections from consideration within the KA, and the consequences of this omission are discussed. The results can be extended to boundary conditions beyond the ideal free surface limit, and are applicable to the problem of scattering by underwater objects partially buried in sand.