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Nov 1981

Volume 70, Issue S1, pp. S1-S109

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back to top Session Y. Underwater Acoustics V: Fluctuations II
Contributed Papers
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Supersaturation of fluctuations of waves propagating in a random medium (A)

Alan R. Wenzel

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S54-S54 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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A theoretical analysis of the incoherent (or fluctuating) component of the wave field radiated by a point source in a one‐dimensional random medium is presented. The approach is based on a perturbation technique which is similar to the Rytov method, and which is referred to here as the quasi‐Rytov method. The analysis includes both multiple forward‐scatter and multiple backscatter effects. An approximate expression for the second moment of the incoherent wave as a function of propagation range is obtained for the case in which the wavelength is much less than the correlation length of the medium. This expression shows that the fluctuations of the wave increase relatively rapidly with propagation range until a maximum value (the supersaturation value) is reached, after which they decrease relatively slowly with further increase in range. These results are in qualitative agreement with observations of intensity fluctuations of optical waves propagating in the lower atmosphere. [Work sponsored by NORDA.]
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Normal mode propagation through sound speed fluctuations (A)

Robert A. Koch

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S54-S54 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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A systematic mode coupling theory is described. The theory, besides treating the coupled mode problem for slow variation in the gross features of sound speed profile and waveguide boundaries, allows small sound speed profile fluctuations on short scale‐lengths, i.e., for scales on the order of an acoustic wavelength. The theory shows energy to be conserved. A statistical treatment of the sound speed fluctuations, when appropriate, may be applied and is illustrated with a simple example which demonstrates the effect of the sound speed fluctuations on the sound field phase content. [Work sponsored by Naval Ocean Research and Development Activity.]
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A coupled‐mode model for spatial coherence of bottom‐interacting energy (A)

Lewis B. Dozier

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S54-S54 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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In many low‐frequency situations a significant amount of energy is returned from the ocean bottom. The spatial coherence of this energy then determines whether it is useful for signal processing systems. In this paper we address the issue of coherence loss due to range‐dependent variations in sediment properties; in particular, sediment sound speed variations are found to drive the problem. Since such data are sparse, and since it is the scale lengths of the variations which are important, we model the sound speed variations as a random process. We then extend a coupled‐mode theory of stochastic wave propagation previously developed for random internal waves in the water column by Dozier and Tappert [J. Acoust. Soc. Am. 63, 353–365 (1978) and 64, 533–547 (1978)] to include spatial coherence (and transmission loss) in this case. Asymptotic infinite‐range limits, derived from a limiting shape of the modal energy distribution, are also given. [Work supported by NORDA Code 500.]
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A mean multipath intensity relation for sound propagation through a random ocean front (A)

Jerome A. Neubert

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S54-S54 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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By considering the stochastic nature of phase fluctuations in the ocean, the conventional ray theory intensity relation was extended in an earlier paper [J. A. Neubert, J. Acoust. Soc. Am. 51, 310–322 (1972)] to permit consideration of partial coherence in multipath problems. Although this relation worked well in the open ocean [J. A. Neubert, J. Acoust. Soc. Am. 62, 326–334 (1977)], it proves to be incomplete for sound propagation through a random ocean front. By considering also the amplitude fluctuations, a mean multipath intensity relation (as well as its standard deviation σ1) is found that takes into consideration the strong horizontal sound‐speed gradients that occur in certain important ocean frontal regions.
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Fluctuation time series and spectra of signals received on bottom‐mounted hydrophones (A)

Ronald L. Dicus and Thomas Hayward

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S54-S54 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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Measurements of the log‐intensity signal received on bottom‐mounted hydrophones at a depth of approximately 3500 m were examined as fluctuation time series. A 14‐Hz cw source was towed at a depth of 135 m along broadside radial and transverse arc courses at ranges from 500 to 1000 km. Deep nulls (10–15 dB) in the time series characteristic of radial source tows were also observed in the are tows. Averaged fluctuation spectra (resolution = 0.23 h−1) were characterized by two peaks (0.23 and 0.69 h−1) for the are spectrum and by a single peak at a frequency midway between the arc peaks for the radial spectrum. The peaks shifted upwards in frequency by a factor of 2 for the arc spectrum and by a factor of 1.5 for the radial spectrum as the source distance decreased from 1000 to 700 kin. Possible effects of environmental cylindrical inhomogeneity, including interaction with a laterally heterogeneous bottom, are discussed with the aid of parabolic equation algorithm computations.
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Incoherence in the propagation of sound near the ground (A)

G. A. Daigle, J. E. Piercy, and T. F. W. Embleton

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S54-S55 (1981); (2 pages)

Online Publication Date: 12 Aug 2005

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Coherent propagation is normally assumed when predicting levels of community noise. However, atmospheric turbulence reduces both longitudinal (along the propagation path) and lateral coherence (along the wavefront) progressively with distance from the source. We have measured both coherences for pure tones between 500 to 4000 Hz during propagation over ranges up to 100 m from a point source on the ground, with microphone heights up to 6 m. Preliminary analysis of results indicates that: (1) the longitudinal coherence length is similar to the saturation distance for fluctuations of amplitude, (2) the lateral coherence drops asymptotically with increasing distance along the wavefront to the longitudinal value for a given range, and (3) the lateral coherence length for the incoherent wave is approximately 1 m (a figure also representative for the correlation distance of fluctuations of acoustic refractive index), and is relatively independent of frequency, provided the microphone is not too close to the ground. These results are compatible with some theories of propagation in a turbulent medium. The results indicate that the “incoherent wave,” which is commonly ignored, is often a more important carrier of acoustic energy outdoors than the coherent wave to which normal acoustical theory applies.
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Analysis of the sensitivity of 2‐1000 Hz shallow water acoustic/seismic propagation to geophysical and oceanographic parameters (A)

E. G. McLeroy

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S55-S55 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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An extensive acoustic/seismic propagation experiment has been conducted in shallow water off Panama City, Florida. Using explosives as sources, signals received at hydrophones and geophones were measured. In addition to water column measurements, detailed measurements of the bottom sediments and sub‐bottom geology were made using reflection and refraction profiling, cores and grab samples. The experiment provided simultaneously both propagation and environmental data sufficient to permit an analysis of the sensitivity of the propagation to each of those environmental variables and parameters that could be measured either directly or indirectly. This analysis has been carried out, using some 15 environmental variables and parameters, for both the water wave and arrivals traveling in the sea bottom. The sensitivity on these environmental variables is found to be frequency dependent and also to depend on the type of wave constituting a given arrival. Multiple regression model techniques used in the analysis yielded quantitative measures of each variables contribution to the model.
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Generating random sound speed fields in water using heating or cooling (A)

Coleman Levenson and Joe W. Posey

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S55-S55 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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Both heating and cooling techniques have been developed for generating random sound speed fields along a ten meter acoustic propagation path in a 14 m tank having a 3.7 × 3.7 m cross section. A 1.5‐MHz cw signal was propagated along this path for the purpose of studying high‐frequency acoustic signal fluctuations in a random medium. Previous investigators of acoustic fluctuations have produced random sound speed distributions by locating heating elements beneath the propagation path. Localized heating of water generates bubbles due to degassification, and it has been suggested that observed fluctuations may be due to resonant scattering by these rising microbubbles [J. A. Neubert and J. L. Lumley, J. Acoust. Soc. Am. 64, 1148–1158 (1978)]. Preliminary qualitative results indicate that statistically similar random sound speed fields generated by heating from below or by cooling from above produce similar acoustic fluctuations.
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Simulation of propagation in fluctuating oceans using the radiation transport equation (A)

D. S. Ko and F. D. Tappert

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S55-S55 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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We have developed a radiation transport code, based on the Monte Carlo numerical method, which applies to low‐frequency deep ocean propagation with random oceanic fluctuations. Refraction in the sound channel is done by means of a bi‐cubic spline interpolation in the angle‐depth coordinates. Scattering is accomplished by appropriately sampling several scattering kernels including those given by Wilson and Tappert [J. Acoust. Soc. Am. 66, 256 (1979)]. Numerical experiments show that the mean positions of arrivals are significantly displaced, but that spreading about the mean is suppressed due to the anisotropy of the fluctuations and especially due to the presence of the sound channel itself. [Work supported by ONR.]
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Dispersive properties of a fluctuating parabolic waveguide (A)

Bruce J. Bates and Susan M. Bates

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S55-S55 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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The acoustic field of the fluctuating parabolic waveguide was expressed as an eigenmode expansion of the Green function. The field was assumed to be negligibly influenced by the boundary conditions and the source/observer locations were on the waveguide axis. Variations in pressure, arrival time, and angle for each mode, m, and source frequency, ω, were examined for fluctuations in axis velocity, C0, and waveguide curvature parameter, a. The results showed the measurable quantities to be approximately ten orders of magnitude more sensitive to fluctuations in the waveguide curvature parameter than to fluctuations in axis velocity. The arrival time asymptotic behavior was tA/t0  ∼  O(exp(−ωm2/4ω2)) for large source frequencies, ω ≫ ωm  =  (2m + 1)aC02, and fluctuations in the waveguide curvature parameter resulted in negligible variation in arrival time. For source frequencies near the mode‐dependent cutoff frequency, ωm, arrival times showed extreme sensitivity to fluctuations in the waveguide curvature parameter. Mode‐dependent arrival angles were least sensitive to fluctuations in the curvature parameter near grazing angles of π/4 radians. Fluctuations in pressure exhibited similar sensitivity to the waveguide parameters and intrinsic high‐order moments.
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