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Dec 1986

Volume 80, Issue S1, pp. S1-S128

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back to top Session Y. Underwater Acoustics IV: Range‐Dependent and Under‐Ice Propagation
Contributed Papers
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Characterization of range‐dependent shallow water waveguides (A)

George V. Frisk, Ferdinand J. Diemer, and Peter H. Dahl

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S52-S52 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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A technique for characterizing range‐dependent shallow water waveguides is described. The method consists of determining the beamformed output of a horizontal array over short apertures for signals due to a cw point source. By modeling the acoustic field locally as a sum of damped normal modes and using Prony's method to perform the beamforming, the local modal structure of the waveguide can be resolved. As a result, the modal composition of the waveguide as a function of range can be determined and interpreted in terms of range‐dependent mode theories (e.g., adiabatic mode theory). In addition to identifying important propagation characteristics such as mode cutoff, the method can be used to determine range‐dependent acoustic properties of the bottom. Examples of the application of the technique to the case of propagation in a wedge‐shaped ocean are presented. [Work supported by ONR.]
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High‐frequency acoustic propagation in a range‐dependent guiding‐to‐antiguiding ocean channel transition (A)

L. B. Felsen and T. Ishihara

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S53-S53 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Variations in the ambient physical parameters in the ocean may be such as to change a depth‐dependent profile with weak range dependence from guiding to antiguiding. This paper examines conversion of an initially well‐guided high‐frequency adiabatic mode into a nonguided sound field after passing through the transitional profile region. A theory is developed which smoothly patches a parabolic equation for the transitional domain onto trapped adiabatic mode fields on the guiding side and onto leaky modes on the nonguiding side. The parabolic equation is implemented numerically where it cannot be reduced to the simpler asymptotic forms. Numerical results for the dominant mode in a model profile confirm the analytical features of the theory, and reveal clearly the phenomenology of initial confinement in a surface duct, with subsequent detachment and “beaming” into deep water after passing through the transition to the antiguiding environment. [Work supported by ONR.]
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VLF pulse propagation in range‐dependent geoacoustic waveguides (A)

R. Stephen and M. Holzrichter

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S53-S53 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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The finite difference method is used to study VLF pulse propagation (peak frequency of 10 Hz) in shallow water waveguides (100‐m water depth) with range‐dependent depth and geoacoustic parameters out to ranges of 5 km. The method solves the full two‐way elastic wave equation in space and time using an explicit scheme based on centred finite differences. Compressional and shear velocity and density can be varied arbitrarily in a two‐dimensional grid. The models represent continental margin environments with both upslope and downslope propagation. Propagation can be studied in range‐time space, in snapshots of the wave field at given instants in time and in frequency wavenumber space. In soft bottom environments where the shear wave velocity of the bottom is less than the compressional wave velocity in the water, energy is continually leaking into converted shear waves in the bottom and there are no “perfectly trapped modes.” However, for sources and receivers near the seafloor, Stonely waves are observed with velocities near the shear wave velocity. For sources over the continental shelf and receivers over the continental slope, the numerical experiments predict two new wave phenomena. Compressional ground wave arrivals are enhanced by focusing at the shelf break and water wave modes do not adapt immediately to changes in water depth. In the latter case, the water wave modes set up in the shallow water waveguide persists almost unaffected out to 2 km beyond the shelf break. Reflections of energy propagating back to the source from the shelf break are also observed.
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Parabolic equation modeling of normal mode propagation in a wedge (A)

Finn B. Jensen and C. T. Tindle

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S53-S53 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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The parabolic equation model has been used to investigate the behavior of the “wedge modes” described in the experimental results of Hobaek, Tindle, and Muir [J. Acoust. Soc. Am. Suppl. 1 78, S70 (1985)] for propagation in a shallow water wedge. The wavefronts of wedge modes are not vertical, but are curved into arcs of circles centered on the wedge apex. Individual wedge modes can be examined by simulating a curved line source. A wedge mode propagates without coupling to other modes. By contrast, if the line source curvature is omitted it is not possible to excite a pure mode, and strong mode interference is observed. As wedge modes propagate upslope through their cutoff depth, their energy is dumped into the bottom as a beam. Any coupling to other modes as a mode passes through cutoff is very small and may be ignored.
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Sound propagation in three dimensions using a new numerical algorithm (A)

D. Lee, P. D. Scully‐Power, G. Botseas, and W. L. Siegmann

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S53-S53 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Recently, a numerical algorithm was developed for solution of a three‐dimensional, wide‐angle parabolic approximation to the Helmholtz equation [D. Lee, Y. Saad, and M. Schultz, Proc. First IMACS Syrup. Comp. Acoust. (to appear)]. The method is both stable and efficient, since each range step requires solving only two tridiagonal systems. Numerical computations have been performed for test examples and model problems, and the accuracy and efficiency of the method have been illustrated. Extensions and improvements in the algorithm are reported here. These include implementations on parallel processors and on supercomputers, and modifications for switching between two‐ and three‐dimensional calculations. The latter feature permits bypassing the full three‐dimensional capabilities of the code where environmental conditions are sufficiently simple. Data requirements for three‐dimensional computations using this new algorithm are discussed, in connection with longer range propagation through models of mesoscale variations such as fronts and eddies. [Work supported by NUSC and ONR.]
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Time series simulation in a sloping bottom environment using ray theory (A)

Evan K. Westwood, H. Hobaek, and C. T. Tindle

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S53-S53 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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A method for simulating time series in shallow water using ray theory has been extended to an isovelocity wedge with a penetrable bottom. Beam displacement at the water‐bottom interface is included, and the resulting caustics are treated with correction factors. Simulated time series compare favorably with time series measured in an experimental, scale model tank using an 80‐kHz pulse as a source signal. Mode extraction from sets of time series at different water depths works well on both simulated and experimental data. Phenomena such as wave front curvature, upslope mode cutoff, and downslope mode capture are investigated using the ray model. [Work supported by Independent Research and Development, Applied Research Laboratories, The University of Texas at Austin.]
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Attenuation of the modes of propagation in an homogeneous floating ice plate (A)

Peter J. Stein

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S53-S53 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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The characteristic equation for plane‐wave propagation in a homogeneous floating ice plate was solved numerically to determine the phase speeds and attenuations of the first‐ and second‐order modes. While discussion of the phase speeds of the first‐order modes constitutes a review, the discussion of the second‐order modes, along with the attenuation characteristics of the modes when ice absorption is introduced, gives new insight into which modes might be observed in Arctic pack ice. Only the flexural and longitudinal waves, which exist below a frequency‐ice thickness product of 300 Hz‐m, propagate with losses less than 0.1 dB/m in ice less than 3 m thick. This is important to the study of noise from nearby ice events. Results of using a nearby explosive charge to measure the ice longitudinal wave speed and attenuation are given. The ice loss in the 40‐Hz region was found to be approximately a factor of two higher than expected from current empirical absorption values. This may be important to understanding losses from ice interaction in long range propagation. [Work supported by ONR.]
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On the controversial leaky Rayleigh wave at a water/ice interface (A)

Jacques R. Chamuel

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S54-S54 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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To the author's knowledge, no experimental data have been published to date isolating and demonstrating the existence of the controversial leaky Rayleigh wave originating at a water/ice interface. In this paper, ultrasonic laboratory results are presented demonstrating the existence of a leaky Rayleigh wave at a water/ice interface not meeting Brower's (1979) existence condition. The measured phase velocity of transient leaky Rayleigh waves is about 8% higher than the free Rayleigh wave velocity from the air/ice interface. The refracted shear wave amplitude is very small compared to the leaky Rayleigh wave. The received signal is dominated by the leaky Rayleigh wave when the receiver is simultaneously close to the source and at a distance of about one Scholte wavelength from the interface. Substantial energy is associated with the leaky Rayleigh wave indicating that it may be important to account for the contribution of the leaky Rayleigh wave in the problem of scattering from rough water/ice interfaces. The leaky Rayleigh wave is very susceptible to the presence of surface cracks in the ice. The reported findings provide physical insights into arctic acoustics and may contribute to the interpretation of arctic acoustic data. [Work supported by ONR.]
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The influence of thermohaline steps on under‐ice acoustic propagation: A simulation study (A)

Stanley A. Chin‐Bing

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S54-S54 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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A simulation study of the influence of thermohaline steps on under‐ice acoustic propagation has been made using a surface representation of hard layered ice with a water‐to‐ice transition region and a “stair‐step” sound velocity profile (SVP) in the water. The ice surface was modeled using parallel layers of varying thicknesses, each layer being homogeneous in density, compressional velocity and attenuation, and shear velocity and attenuation. A water‐to‐ice transition region [S. A. Chin‐Bing, J. Acoust. Soc. Am. Suppl. 1 78, S57 (1985)] was included to allow a gradual transition from the water to the multilayered ice. The water region contained the stair‐step SVP that is often found in regions containing thermohaline steps [S. A. Chin‐Bing and D. B. King, J. Acoust. Soc. Am. Suppl. 1 76, S84 (1984)]. Results indicate that the transmission loss structure is affected by both the stair‐step SVP and the water‐to‐ice transition region.
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Seismic propagation velocity measurements in arctic sea ice (A)

J. M. Ozard and G. H. Brooke

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S54-S54 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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It has been predicted that the seismic propagation velocities of sea ice affect the propagation of sound in ice covered arctic waters. Since very little data are available on propagation velocities, a series of in situ measurements of propagation velocities and associated density, temperature, and salinity profiles have been made in sea ice. Seismic energy from predominantly shear or compressional wave sources was propagated over ranges of a few hundred meters to two three‐component geophone arrays. Propagation paths in smooth and in slightly rough annual ice were selected. Plate, flexural, and shear wave arrivals were clearly identified from their polarizations and particle velocities. A reduction in plate and flexural wave velocities was observed in the rough annual ice compared to the smoth annual ice.
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A range‐dependent normal mode model with full mode coupling (A)

E. Richard Robinson and David H. Wood

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S54-S54 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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A preliminary version of a range‐dependent transmission loss model is presented. It assumes that the ocean is piecewise constant in range. We use full mode matching at the interfaces to compute the acoustic field. Our approach is similar to that used by Evans and Gilbert [“Acoustic propagation in a refracting ocean waveguide with an irregular interface,” Comp. Math. Appls. 11, 795–805 (1985)] in that we also expand the desired normal modes as a weighted sum of depth dependent basis functions. As is well known, this “Galerkin method” leads to a linear algebraic problem for the unknown weights. However, our approach is different because we insist on using other basis functions that lead to structured algebraic problems, where fast techniques are available. The future development of a fast version of this model will exploit this special algebraic structure. We implement within the generic sonar model because this allows for considerable flexibility in updating developments and the choice of supporting submodels.
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Limitations of sound propagation in the ocean: The curtain effect (A)

D. G. Browing, J. J. Hanrahan, R. J. Christian, and R. H. Mellen

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S54-S54 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Although initially very high, the rate of spreading loss decreases rapidly with range, while the rate of attenuation remains constant for a given frequency. At increasing ranges the two loss curves cross, with attenuation becoming the dominate mechanism. This results in a “curtain effect” due to rapidly increasing propagation loss. Examples are given of convergence zones obtainable as a function of frequency for various oceans and of the transition between near range and distant ambient noise. [Work supported by NUSC.]
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