Computing the far field scattered or radiated by objects inside layered fluid media using approximate Green’s functions

Abstract

A numerically efficient technique is presented for computing the field radiated or scattered from three-dimensional objects embedded within layered acoustic media. The distance between the receivers and the object of interest is supposed to be large compared to the acoustic wavelength. The method requires the pressure and normal particle displacement on the surface of the object or on an arbitrary circumscribing surface, as an input, together with a knowledge of the layered medium Green's functions. The numerical integration of the full wave number spectral representation of the Green's functions is avoided by employing approximate formulas which are available in terms of elementary functions. The pressure and normal particle displacement on the surface of the object of interest, on the other hand, may be known by analytical or numerical means or from experiments. No restrictions are placed on the location of the object, which may lie above, below, or across the interface between the fluid media. The proposed technique is verified through numerical examples, for which the near field pressure and the particle displacement are computed via a finite-element method. The results are compared to validated reference models, which are based on the full wave number spectral integral Green's function.

Low- To Mid-Frequency Scattering From Elastic Objects on a Sand Sea Floor: Simulation of Frequency and Aspect Dependent Structural Echoes

“…The present model is being extended to buried target cases, using Green's functions for sources buried in the sediment. 4 The comparison between this buried target model, a fully 3-D model and experimental data will make it possible to shed more light onto target-surface rescattering effects, in experimental scenarios with buried targets similar to the ones presented here.…”

“…This approximation is consistent with the first order steepest descent evaluation of the wavenumber spectral integral representation of the Green's function for the two layered medium. 4 For each of the two incident field cases, the aspect angle between the source and the target, and the image source and the target respectively, needs to be mapped into the coordinate system of the FE computation, in which the source, or respectively the image source, is contained in the r-z plane, according to Fig. 3(b).…”

Rayleigh scattering of a cylindrical sound wave by an infinite cylinder

“…Note that, under condition (21), only the A ðIÞ 0 and A ðHÞ 0 coefficients differ for the impedance and hard cylinders. The effect of the surface impedance on the scattered wave is solely contained in the A ðIÞ 0 term.…”

“…In underwater acoustics, such problems can arise in dense multi-scatterer configurations, 6 radiation by finite sources, [7][8][9][10] target classification, [11][12][13] source localization, 14 scattering suppression, 15,16 and when calculating the radiation and scattering of sound due to objects located near the ocean surface 17,18 or the seafloor. [19][20][21] No simple analytic solutions are readily available for such problems, and numerical methods have to be used. This is the motivation for the present study.…”

Scattering from inclusions in Marine Sediments: SAX04 Data/Model Comparisons

“…For example, elimination of the first assumption is necessary to take into account scatterers located on the bottom surface or not buried completely but rather exposed in the water. In this case, analytical solutions are complicated, but can be analyzed numerically [Lim et al 1993and Lim 1998, Fawcett 1996, Fawcett and Lim 2003, Zampolli et al 2008. There is also a simpler semi-phenomenological approach, using ray considerations, that can be useful at high enough frequencies.…”