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inapplicable for most surface and bottom scattering measuremcnts, be-cause thc acoustic receiver is invariably in thc ncar-ficld zonę of thc ensonified patch and scattering cross section is defined in the far-field limit. Winebrcnner and Ishimaru (IEEE Trans. Antennas Propag. AP-34, 847-849 (1986)] have shown that the correlation łength of the surface field defines the relevant far-field region for stochastic scattering. This sets a less stringcnt constraint than the far-field critcrion bascd on the ensonified patch size and greatly extends the domain of applicability of the scattering cross-section concept. Using a standard formalism that includes near-field phase terrns. it is shown how the scattering cross section can be used even in the near field of the ensonified patch. The result is a proof of the method commonly used in reverberation calcu-iations: One integrates over the ensonified patch with the appropriate propagation loss factors and bistatic scattering cross section as integra-tion variables. This assumes that the far-field criterion based on the correlation łength is satisfied. (Work supported by ONR.]

1:15

5UW3. Short-range surface and bottom backscattering strength measuremcnts from the CEAREX 89 aretie experiment. Thomas J. Hayward and T. C. Yang (Naval Res. Lab., Codę 5123, Washington, DC 20375)

Short-range SUS reverberation data collected during the CEAREX 89 experiment in the Norwegian/Greenland Sea are analyzed to extract undcr-icc and bottom backscattering strengths from direct-path mea-surements. Because of the mildly bistatic experimental geometry, propagation paths with different grazing angles contribute to the revcrbera-tion at the same time; therefore, a least-squares method is used to recover thc angular dependence of backscattering strengih. Ice backscattering strengths are compared with the results of Milne, while bottom backscattering strengths are compared with results for similar ocean bottoms. [Work supported by ONR arctic Sciences program.)

1:30

5UW4. Sea surface scattering using 2-D perturbation theory. Erie I. Thorsos (Appl. Physics Lab., Univ. of Washington, Seattle, W A 98105)

Comparisons with exact integral equation results for 1-D surfaces h3vc shown that perturbation theory, when carried to fourth order in kh for the scattering cross section, gives accurate predictions at Iow fre-quencies for low-grazing-angle backscattenng from the "classical” sea surface. Here, k is the acoustic wavc number, h is the rms surface hcight, and the classical sea surface assumes linear gravity waves with no bubbles. it will be shown that I-D perturbation theory is accurate for frequcncies up to 800 Hz for a Pierson-Moskowitz surface spectrum with a wind speed of 20 m/s (39 kn). For this example, kh = 1.1. Perturbation theory has now been extended to 2-D surfaces, in other words, to surfaces that vary in two horizontal directions. Similar accu-racy for perturbation theory can be expected for 2-D and 1-D predictions. Examples of 2-D results for scattering strengths will be presented. (Work supported by ONR.]

1:45

5UW5. The efTects on long-range ocean acoustic propagation of step-periodic roughness along the interface of a shear-supporting basement. Stanicy A. Chin-Bing (Naval Oceanographic and Atmospheric Res. Lab., Stennis Space Ctr., MS 39529) and Joseph E. Murphy (Univ. of New Orleans, LA 70148)

I( has been obscrvcd [Evan:» and Gilbert, J. Acoust. Soc. Am. 77, 983-988 (1985)] that one of the effeets of ocean subbottom roughness is to slightly decrease the acoustic propagation in the ocean waveguidc. While this transmission loss is smali, it is cumulative and the net effect over a long rangę can be a significant decrease in the received acoustic signal. To demonstrate this cumulative loss, Evans and Gilbert used a full-wave range-dependent coupled modę model (COUPLE) to account for backscatter from a periodic-step subbottom. COUPLE represented a significant advancement in ocean acoustic propagation modcling sińce it could correctly include thc coupled backscattered acoustic field due to rangę variations in the ocean boundaries. Thcir roughness simulation has been duplicated using our fu!l-wave range-dependent finite-element ocean acoustic models (FOAM, FFRAME, and PE-FFRAME) and also included the effeets of ocean subbottom compression-shear conver-sion using this full-wave range-dependent finite-element ocean seismo-acoustic model (SAFE). Using a cw full-field backscatter method, it has been possible to isolate the various effeets due to the shear and roughness to give insight into the complicated processes that account for cumulative losses over long ranges. (Work supported by ONR/ NOARL]

2:00

5UW6. Acoustic scattering in a three-dimensional oceanie waveguide using boundary integral equation methods. Trevor W. Dawson (Defence Res. Establishment Pacific, F.M.O., Victoria, B.C. V0S 1B0, Canada)

This paper develops in morę detail thc theory for a fully three-dimensional version of a recently published (T. W. Dawson and J. A. Fawcett, J. Acoust. Soc. Am. 57, 111B-1125 (1990)] boundary integral equation method (BIEM) formulation, for the computation of the scattering of underwater sound from compact deformations of an oceanie waveguide’s surfaces. The method allows for three-dimensional sources and boundary deformations in an otherwise uniform waveguide. The techniąue involves only integrations over the compact surface of the wavcguide deformation. The implementation is illustrated for a sca-mountlike deformation in an oceanie waveguide.

2:15

5UW7. Scattering matrix and boundary integral equation methods for long-range propagation in an acoustic waveguide with repeated boundary deformations. Trevor W. Dawson (Defence Res. Establishment Pacific, F.M.O., Victoria, B.C. V0S 1B0, Canada)

Boundary integral equation methods (BIEM) provide an accurate model for predicting the scattering of acoustic radiation from compact deformations of the walls of a waveguide. In numerical implementation, however, the denseness of thc resulting coefficient matrix inhibits the modcling of scattering over very extensive deformations. It is shown in this paper how scattering matrix methods can be combined with BIEM to model long-range propagation in a two-dimensional range-dependent waveguide. The extension to three-dimensional sources is also indicated.

2:30

5UW8. Scattercd field calculations for three-dimensional fluid-elastic interfaces. Kevin LePage and Henrik Schmidt (Dept. of Ocean Eng., MIT, Rm. 5-007, Cambridge. MA 02139)

The calculation of the three-dimensional scattercd field caused by the insonification of two-dimensionally rough interfaces in horizontally stratified elastic media is discussed. In generał, the scattered field may be calculated either deterministically for a realization of surface roughness, or stochastically as scattered covariances for a surface with a specified roughness power spectra. The perturbation method of Kuper-man and Schmidt (J. Acoust. Soc. Am. 86, 1511-1522 (1989)) is used in the fuli three-dimensional formulation to model an isotropically rough elastic icc piąte floating over a fluid half-space. Scattered field realizations are calculated for the point source configuration, cxhibiting the antiplane shear (SH) coupling unique to the 3-D model. The ability of cxpIosive sources in the water column to generale S/ł wavcs in ice has been observed experimentally and it is argued that rough surface scattering is the likely mechanism that facilitates this modę conversion. (Work supported by ONR.]

1941

J. Acoust. Soc. Am., Vol. 89, No. 4, Pt. 2, April 1991

121st Meeting: Acoustical Society of America

1941