A time-variant impulse response method for acoustic scattering from moving two-dimensional surfaces

Abstract

A time-variant impulse response method is proposed, developed, tested, and shown to provide new insights into different scattering problems involving moving surfaces. The method proposed is general, conceptually straightforward, and can accommodate moving sources and receivers. While the time-variant impulse response method has been developed specifically in this study to extend the capability of the wedge assemblage ͑WA͒ model ͓e.g., R. S. Keiffer and J. C. Novarini, J. Acoust. Soc. Am. 107, 27-39 ͑2000͔͒, the framework can be applied to other scattering models including those based in the frequency domain. Calculations involving moving periodic surfaces and moving receivers are presented and compared with good results to a generalized grating equation and small wave-height approximation perturbation theory. The time-variant impulse response model is also applied to time-evolving sea surfaces, and the previously published results of Pourkaviani and Willemsen ͓J. Acoust. Soc. Am. 90, 426-432 ͑1991͔͒ are confirmed. It is also shown that windward-oriented backscatter geometries can lead to a Doppler spectrum that peaks at higher than expected Doppler shifts.

A wedge diffraction based scattering model for acoustic scattering from rough littoral seafloors

“…Second, the density-contrast wedge diffraction solution can be manipulated using an ad hoc reflection coefficient so that is can predict the diffraction from wedges that have sound speed and density contrast. These results suggest that already proven models like the wedge assemblage method [2][3][4][5][6][7], an approach based on wedge diffraction but previously limited to pressure-release, hard, or density contrast boundaries, can accurately be applied to many rough littoral seafloors.…”

“…Models for acoustic scattering from rough surfaces based on Biot and Tolstoy's (BT) exact wedge diffraction theory [1] have proven accurate and useful in a number of experimental and numerical studies [2][3][4][5][6][7]. Of particular importance to the oceanacoustic modeling community, which deals with surfaces having significant roughness across a large range of scales, is this modeling approaches ability to efficiently compute the impulse response directly in the time domain.…”

“…For a rough 2D surface, discretely sampled and measuring M points on a side, the calculation of a single-scatter approximation to the impulse response is proportional to M 2 . This kind of efficiency has allowed a model based on BT wedge diffraction theory to be used in very demanding simulations, for example, in large Monte Carlo calculations for the average Doppler spectrum of acoustic signals backscattered from time-evolving, two-dimensional sea surfaces [7].…”

Modeling broadband ocean acoustic transmissions with time-varying sea surfaces

“…The goals are somewhat similar to the work of Keiffer et al 7 but the approach is different, and the emphasis is on broadband signals for application to underwater communications, for example. In a ray formulation, the complex pressure field, P͑͒, can be represented as a sum of N arrival amplitudes A n ͑͒ and delays n ͑͒ according to…”

Effect of wind-duration, swell-contamination, and bimodal ocean wave spectra on acoustic doppler

“…The purpose of the this paper is to apply a new time domain scattering model [1] (not limited to small wave heights, small slopes, or plane waves) to developing seas which may be swell-contaminated and which assume a new bimodal ocean wave directionality function [2]. More specifically, a bimodal distribution of the first kind is generated through nonlinear wave-wave interaction in which energy near the spectral peak feeds into both, shorter and longer wavelength components.…”