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Dec 1986

Volume 80, Issue S1, pp. S1-S128

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back to top Session FF. Underwater Acoustics V: Propagation Modeling
Contributed Papers
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Normal mode propagation in a commercial catfish pond (A)

Lambert E. Murray and Kenneth E. Gilbert

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S63-S63 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Acoustic propagation measurements were made in a commercial catfish pond in the frequency range 200–2000 Hz. A low‐frequency cutoff was observed at 600 Hz. Above the cutoff frequency, modal propagation was observed. At 1000 Hz, measurements of propagation loss versus range indicated the presence of a single propagating mode. A second mode appeared at 1400 Hz and became stronger with increasing frequency. Measurements at a fixed range of the pressure field as a function of depth also indicated a single mode at 1000 Hz and showed a second mode as the frequency increased. Comparisons of the data with preliminary theoretical predictions will be presented.
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Use of SALT tables for rapid calculation of sound angle, level, and travel time (A)

Homer P. Bucker

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S63-S63 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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There are cases where a very quick calculation of sound propagation is required. One example is a real‐time torpedo simulator where only a fraction of a second is available to determine sonar returns. Another example is a bistatic reverberation model where thousands of scattering elements must be included in a realistic model. A SALT table is a list of sound angle, level, and travel time listed as a function of range from the source or receiver. The number of tables used depends upon available computer storage and time available to access the data in the tables. This paper will discuss two algorithms for calculating SALT tables. In one method, the table is defined by the number of bottom reflections or ray turnunders. In the second method, the tables are numbered according to order of calculation. Several examples will be given showing reasonable agreement between calculations using SALT tables and those using more exact methods.
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Long‐range arrival structure using timefront analysis (A)

Gary E. J. Bold, Theodore G. Birdsall, and Kurt Metzger, Jr.

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S63-S63 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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If the classical differential equations defining rays are rewritten in terms of travel time instead of ray path distance, it is straightforward to trace families of rays from a point with time as the independent variable. Points on rays having the same time coordinate define a timefront surface, thus designated to emphasize its high‐frequency nature. Timefronts are spherical at short ranges, slowly evolving into an accordian structure. The shape of the timefront changes relatively slower over time scales of seconds, enabling a physical picture of long distance arrival times to be seen by inspection. A simple algorithm for deducing accurate travel times of eigenrays to a given receiver steps the timefront iteratively until its branches cross the receiver coordinates. Several features characteristic of long distance arrival structure are demonstrated. Numerical results from typical profiles, some including surface ducts, are presented.
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Inversion of data for propagation over graveled surfaces (A)

R. J. Lucas and V. Twersky

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S64-S64 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Data obtained by Medwin and D'Spain [J. Acoust. Soc. Am. 79, 657 (1986)] for low‐frequency, near‐grazing propagation over gravel‐covered rigid planes are inverted by applying the procedure developed earlier [Lucas and Twersky, J. Acoust. Soc. Am. Suppl. 1 79, S68 (1986)] for regularly shaped protuberances. The development is based on analytical results for a point source irradiating an embossed plane, and on the uniform asymptotic representation of the coherent field as a Sommerfeld type wave system in terms of the complex error function complement Q [J. Acoust. Soc. Am. Suppl. 1 74, S122 (1983); 76, 1847 (1984)]. Initial estimates of unknown parameters are obtained by working with elementary approximations of the Q integral, and then using the complete integral for refinements and final computations. The corresponding graphical results display the primary data trends for the magnitudes of the normalized fields and for the incremental dispersion (phase velocity) versus frequency at different ranges. The procedure delineates the roles of interference and damping on the elementary wave components of the Sommerfeld type system.
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Finite‐element analysis of marine sediment acoustics (A)

Siamak Hassenzadeh, Yu‐chiung Teng, and John T. Kuo

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S64-S64 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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The propagation of acoustic waves in a shallow water environment is controlled by the dynamic response of sediments, which are generally composite (multiphase) systems, as well as the complex geological structure that results from depositional processes. To analyze the propagation of low‐frequency acoustic waves in such media, the finite‐element‐Biot model has been developed which incorporates the Biot's theory into the finite‐element method. The Biot's theory governing the dynamic behavior of fluid‐filled porous materials provides an adequate description of the effects of the intrinsic properties of the sediments, and the finite‐element method has the flexibility to model large‐scale inhomogeneities that are commonly encountered in marine environments. The effects of such parameters as porosity, lithology, lithification, and dissipation due to the relative motion of a viscous fluid are examined numerically. It is found that the modification of acoustic waves is a result of the intrinsic properties of fluid‐saturated porous sediments as well as the interactions between the fast and the slow compressional waves and energy partitioning at interfaces between dissimilar fluid‐filled sediments.
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Parabolic decomposition of the separable Helmholtz equation (A)

David H. Wood

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S64-S64 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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The solution of the separable Helmholtz equation is represented as the convolution of solutions of two related (formally) parabolic partial differential equations. Separable means that the square of the index of refraction is a function of depth plus a function of range and that the depth of the ocean is assumed to be constant. This representation decomposes the separable Helmholtz equation into two parabolic partial differential equations. One equation is the usual Fock‐Tappert parabolic equation for sound propagation. The solutions of the second parabolic equation may be regarded as candidate kernels of special integral transformations, or transmutations. All of these equations have many solutions, of course, so initial and/or boundary conditions are presented that will determine solutions of the parabolic equations so that their convolution will give the Green's function for the Helmholz equation. Examples of such transmutation representations will be presented.
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