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Nov 1975

Volume 58, Issue S1, pp. S2-S132

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back to top Session HH. Underwater Acoustics V: Rough Surface Scatter and Transmission
Invited Papers
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Acoustic scattering in the ocean (A)

B. G. Hurdle, K. D. Flowers, and J. A. DeSanto

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S66-S66 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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A review of acoustic scattering from the surface, bottom, and volume of the ocean is given. Scattering problems in underwater acoustics are delineated and discussed relative to present scientific capabilities. Areas of deficiency are cited.
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Multiple scattering from a random interface (A)

John A. DeSanto

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S66-S67 (1975); (2 pages)

Online Publication Date: 11 Aug 2005

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The problem of scattering sound from a random interface separating two fluids with different densities and sound speeds is considered. The interface height is Gaussian distributed. The mathematical approach is the construction of the Green's function G for the problem by applying Green's theorem in both fluids to G and a single free space Green's function. Mixed surface‐volume integral equations are derived in coordinate space using continuity conditions on G and its normal derivative. In Fourier transform space, these equations can be combined to yield a single three‐dimensional integral equation for a singularity‐free function Γ linearly related to G. Using cluster decomposition techniques yields the Dyson integral equation for the coherent part (mean, first moment) of Γ and the Bethe‐Salpeter integral equation for its second moment. The Born terms and kernels of both these equations can be represented as infinite series of Feynman‐like diagrams. The first diagram corresponds to a Kirchhoff approximation. Using it in the Dyson equation yields a one‐dimensional integral equation which can be solved numerically. This multiple‐scattering‐Kirchhoff solution, in the case of a Neumann boundary, yields more coherent specular intensity for large values of the Rayleigh roughness parameter than the single‐scattering‐Kirchhoff approximation, and can be used to account for diverse experimental data. This as well as further results are discussed, the main point being the necessity of taking multiple scattering into account for problems of this type.
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Backscatter of an impulsive wave at a rough surface (A)

C. S. Clay

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S67-S67 (1975); (1 page) | Cited 1 time

Online Publication Date: 11 Aug 2005

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An extension of Eckart's scattering theory that includes sphericity of the wave front (Fresnel approximation) shows agreement with experimental data within a few percent. These laboratory experiments are made with long ping and approximately meet the continuous wave condition of the theory. In applying the theory to the interpretation of side scan sonor data, I decided that the c.w. assumption didn't apply because the insonified area is not insonified at the same time. Solutions for an impulsive point source and receiver at the same location are a good approximation to the problem. Trorey [Geophys. 25, 762 (1970)] and Hilterman [Geophys. 25, 1020 (1970)] give expressions for the boundary wave scattered at a segment of a plane. We use these expressions in two ways: We calculate the spectra of signals scattered from different sizes of planes. We use segments of planes to model rough surfaces. The theoretical signal wave forms and amplitudes agree with experimental measurements. [Research supported in part by NSF grant GA 39889.]
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Scattering of acoustic waves by randomly rough surfaces (A)

P. J. Welton

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S67-S67 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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The scattering of acoustic waves by randomly rough surfaces has been investigated using an integral equation approach similar to the Helmholtz integral formula. For the calculation of the mean scattered intensity to remain analytically tractable, numerous approximations must be introduced. The five important quantities most often approximated in the scattering integrals are (1) the surface slopes, (2) the phase function, (3) the source directivity function, (4) the probability density function of surface heights, and (5) the surface autocorrelation function. The results to be discussed in this paper are based on an exact representation of the surface slopes, a Fresnel phase approximation valid for specular, forward, and backward scattering, a Gaussian source directivity approximation, both Gaussian and non‐Gaussian surface statistics, and a general representation of the autocorrelation function. Several new results have been obtained and these provide considerable insight into the significance and consequences of the various approximations used in scattering calculations.
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Broad‐band pulse techniques in model scattering research (A)

J. G. Zornig

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S67-S67 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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A number of statistical properties of sound scattered from wind driven water surfaces have been studied using a broad‐band preemphasized probing pulse technique. These studies, conducted in a small scale model tank, have shed light on cross frequency and spatial correlations of the scattered field and on the fading behavior of received signals. The pulse‐forming technique [Zornig and McDonald, J. Acoust. Soc. Am. 55, 1205–1211 (1974)] has been refined and applied to additional measurements of forward scattered fields. Using surface data obtained with a high‐resolution waveheight probe comparison has been made with theoretical predictions [McDonald and Tuteur, J. Acoust. Soc. Am. 57, 1025–1029 (1975)] showing good agreement. Construction of a large computer controlled acoustic goniometer has made it possible to conduct high‐resolution spatial surveys of scattered field statistics as a means of validating theory. [Work supported by ONR.]
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Theory of reflection and transmission at rough boundaries with application to sound propagation in the ocean (A)

W. A. Kuperman

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S67-S67 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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The interaction of an acoustic field with a randomly rough two‐fluid interface is studied. A perturbation method is used to replace the flat interface boundary conditions with a new set of boundary conditions on the mean interface. These new conditions are valid for surfaces characterized by arbitrary correlation functions and are therefore not restricted to the Kirchhoff regime. Using these boundary conditions plane wave reflection and transmission coefficients are derived. The pressure release case is extracted by letting the density of the second medium vanish. It is shown that in the Kirchhoff approximation the reflection and transmission coefficients reduce to those obtained earlier by Eckhart, Clay, Medwin, and Hagy. These same boundary conditions are also used to solve the wave equation for a point source placed in an ocean duct with rough boundaries. Using a normal‐mode solution, the effect is to add an imaginary part to the modal wavenumbers which can be interpreted as a modal attenuation coefficient. Calculated results are presented for the single interface and for realistic ocean ducts where attenuation in the bottom sediment is also included.
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Underwater acoustic scattering from the random, moving ocean surface (A)

David Middleton

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S68-S68 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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A new approach to scattering from the random, moving surface of the ocean is described, which combines the “classical” or continuum scatter theories with the more recently developed FOM (Faure‐Ol'shevskii‐Middleton) theory, based on random “specular” point scatterers, obeying Poisson laws. Among the new features of the theory are (1) its ability to deliver exact, statistical solutions to the governing Langevin equations of propagation in their interactions with the scattering interface; (2) the explict inclusion of the dynamic Doppler (omitted in the classical treatments), which produces nonstationary random frequency modulations of the incident signal, in addition to the more familiar, stationary phase modulation arising from the random surface positions; (3) a theoretical and experimental technique for determining when one or the other of the continuum or specular point scattering mechanisms is preemptively predominant (for second‐order statistics); (4) a second‐order, four‐parameter characterization of underwater acoustic channels; and (5) among others, various experiments to obtain point and directional surface wave spectra (acoustic Crombie‐Barrick experiments). The general approach is developed and illustrated by recent results of the author [“Characterization of Active Underwater Acoustic Channels,” Rep. ARL‐TR‐74‐61, Applied Research Laboratories, University of Texas at Austin (Dec. 1974)]. [Work supported primarily by the Naval Sea Command.]
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