Acoustic scattering in a homogeneous waveguide

The relative contributions of various physical factors to producing non-Rayleigh distributions of echo magnitudes in a waveguide are examined. Factors that are considered include (1) a stochastic, range-dependent sound-speed profile, (2) a directional acoustic source, (3) a variable scattering response, and (4) an extended scattering volume. A two-way parabolic equation model, coupled with a stochastic internal wave model, produces realistic simulations of acoustic propagation through a complex oceanic sound speed field. Simulations are conducted for a single frequency (3 kHz), monostatic sonar with a narrow beam (5 À3 dB beam width). The randomization of the waveguide, range of propagation, directionality of the sonar, and spatial extent of the scatterers each contribute to the degree to which the echo statistics are non-Rayleigh. Of critical importance are the deterministic and stochastic processes that induce multipath and drive the one-way acoustic pressure field to saturation (i.e., complex-Gaussian statistics). In this limit predictable statistics of echo envelopes are obtained at all ranges. A computationally low-budget phasor summation can successfully predict the probability density functions when the beam pattern and number of scatterers ensonified are known quantities. [http://dx

Section: Introductionmentioning

confidence: 99%

Echo statistics of individual and aggregations of scatterers in the water column of a random, oceanic waveguide

Jones

Colosi

Stanton

2014

“…The scattering formulation presented here is somewhat similar to that of Sammelmann and Hackman. 10 However, here it is in an operational form that is more straightforward to use, for example, this new formulation allowed for a simple and elegant derivation of the optical theorem, which provided significant insights in the physics of scattering in a bounded medium.…”

Section: Discussionmentioning

confidence: 98%

“…In the case of a layered medium, this result, apart from differences in notation, coincides, formally, with that obtained by Sammelmann and Hackman. 10 However, the formulation presented here in terms of the waveguide Green's function with an explicitly operational procedure to obtain the proper multipole expansion seems to be more general, although equivalent to other formulations that start with an explicit expansion of the acoustic field in cylindrical coordinates and then carry out a projection into spherical harmonics. 4,10 .…”

Section: ͑2͒mentioning

confidence: 97%

“…If one examines previous formulations of this problem, one notices that a great deal of effort is involved in computing transformations from cylindrical to spherical coordinates and basis functions. 4,10 While those calculations are unavoidable when one has to carry out computations they are not necessary when obtaining the basic equations as demonstrated in this article and in Refs. 2 and 3.…”

Section: Introductionmentioning

confidence: 93%

See 1 more Smart Citation

The partial-wave expansion for scattering in waveguides

Dacol

Roy

2006

The scattering of acoustic waves by objects located inside a waveguide is discussed, assuming that the scattering amplitude for the object in an extended uniform medium is known. The scattering process is described by using an expansion of the scattering amplitude in terms of spherical harmonics. An appropriate multipole decomposition of the waveguide Green's function is developed and the effective scattering amplitude in the waveguide is obtained. An important property of the effective scattering amplitude, the generalized optical theorem, is obtained and its implications for scattering in a waveguide are discussed. The scattering problem is formulated entirely and explicitly in terms of the waveguide Green's functions, which makes this approach very flexible in regard to the choice of the incident field. It also establishes the connection between propagation and scattering and allows for the independent computation of the propagation and scattering aspects of the problem. This is the main advantage of using the scattering amplitude in an extended uniform medium as an input. The connection of this work with previous work in scattering in waveguides is discussed.

“…9,10 The work to follow was concentrated on the salient features of the scattering in the vicinity of medium boundaries [11][12][13] and formed the basis for understanding the physics of acoustic interaction with objects that are either suspended in shallow water or buried.…”

Section: Introductionmentioning

confidence: 99%

Subcritical scattering from buried elastic shells

Lucifredi

Schmidt

2006

Buried objects have been largely undetectable by traditional high-frequency sonars due to their insignificant bottom penetration. Further, even a high grazing angle sonar approach is vastly limited by the coverage rate dictated by the finite water depth, making the detection and classification of buried objects using low frequency, subcritical sonar an interesting alternative. On the other hand, such a concept would require classification clues different from the traditional high-resolution imaging and shadows to maintain low false alarm rates. A potential alternative, even for buried targets, is classification based on the acoustic signatures of man-made elastic targets. However, the elastic responses of buried and proud targets are significantly different. The objective of this work is to identify, analyze, and explain some of the effects of the sediment and the proximity of the seabed interface on the scattering of sound from completely and partially buried elastic shells. The analysis was performed using focused array processing of data from the GOATS98 experiment carried out jointly by MIT and SACLANTCEN, and a new hybrid modeling capability combining a virtual source-or wave-field superposition-approach with an exact spectral integral representation of the Green's functions for a stratified ocean waveguide, incorporating all multiple scattering between the object and the seabed. Among the principal results is the demonstration of the significant role of structural circumferential waves in converting incident, evanescent waves into backscattered body waves, emanating to the receivers at supercritical grazing angles, in effect making the target appear closer to the sonar than predicted by traditional ray theory.