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Nov 1981

Volume 70, Issue S1, pp. S1-S109

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back to top Session Q. Underwater Acoustics IV. Fluctuations—I
Invited Papers
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Sound transmission as a probe of ocean internal waves and microstructure (A)

Stanley M. Flatté

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S37-S37 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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The general theory of sound transmission through a fluctuating ocean is reviewed. The connection between sound fluctuations and the underlying ocean‐medium fluctuations is described in terms of a physical picture derived from the path‐integral technique. Quantitative comparison between the theory and several recent ocean‐acoustic experiments is discussed, as are limits of validity and accuracy of the theory in the face of experimental uncertainty in the deterministic sound‐speed profile. Prospects for future use of sound transmission as a probe of ocean statistical processes are summarized.
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Fluctuations of resolved acoustic multipaths at long range in the ocean (A)

John L. Spiesberger and Peter F. Worcester

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S37-S37 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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A phase‐coded signal centered at 220 Hz with 64‐ms resolution was transmitted at 10‐min intervals between an acoustic source moored at 2‐km depth and a bottom‐mounted receiver at ∼3‐km depth and at ∼900‐km range. A variety of fluctuation statistics (pulse and CW) are computed for three resolved multipaths from a 32‐day record. The fluctuations are compared with theoretical predictions based on internal wave scattering. Some measurements agree with theory; measurements that disagree differ by amounts that are statistically significant but not large. Uncertainty of the internal wave energy level could account for some discrepancies. Some theoretical results are very sensitive to small changes of the second derivative of the mean sound speed profile. There are substantial fluctuations at frequencies below those that can be accounted for by internal waves.
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Fluctuations of resolved acoustic multipaths at short range in the ocean (A)

Peter F. Worcester, Gordon O. Williams, and Stanley Flatté

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S37-S38 (1981); (2 pages)

Online Publication Date: 12 Aug 2005

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The fluctuations in acoustic transmission between a broadband source and an autonomous receiver moored somewhat below the sound channel axis at 23‐km range have been examined. The transmitted signal was centered at 2273 Hz. A variety of measures of the observed fluctuations (pulse and cw) for three well‐resolved paths computed from 72 h of data are compared with theoretical predictions based on sound‐speed perturbations due to internal wave vertical displacements. Measurements and theory are found to be consistent for some statistics and inconsistent for others. The theoretical predictions depend sensitively on the mean sound‐speed profile.
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Acoustic tomography applied to imaging the oceanic mesoscale field (A)

Bruce Cornuelle

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S38-S38 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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In the spring of 1981, an acoustic tomography experiment consisting of 4 moorings with low‐frequency (224 Hz) sources and 5 moorings carrying receivers was deployed as an array about 300 km on a side centered at approximately 26 N, 70 W. Subsurface moorings were used in order to minimize motion, so that the acoustic instruments were placed at about 2100‐m depth, well below the sound channel. The array was sited in a region of moderate mesoscale activity, overlapping the MODE region somewhat, and CTD surveys during the experiment showed a cold eddy in the experimental area, with 2‐ to 3‐degree amplitude at 500‐decibars depth. Rays traveling from a source to a receiver within a volume of ocean sample the velocity and sound speed of the water along their propagation paths. The fields of dynamic significance are coupled to the acoustic fluctuations primarily through sound speed changes, so that the ability of a tomographic array to directly measure a given feature is partly limited by the degree to which the feature affects the sound speed. At present, acoustic travel times from source to receiver are the primary data to be used for the estimation of the mesoscale fields. Estimates are produced using both statistical optimal estimation and deterministic linear inversion. These are similar techniques with somewhat different limitations and capabilities. The performance of the inverse methods can be evaluated relative to each other and to the CTD data to yield checks of resolution, accuracy, and noise sensitivity. This array thus provides a theoretical and experimental test of tomography as a system for studying the mesoscale variability.
Contributed Papers
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Theory of propagation in fluctuating oceans using the radiation fransport equation (A)

F. D. Tappert

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S38-S38 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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A new theory of low‐frequency deep ocean propagation has been developed based on the radiation transport for randomly fluctuating oceans. Using the Fokker‐Planck approximation and a novel ray‐bundle approximation, we have derived analytic solutions for the means and spreads of arrivals. The mean arrival is predicted to differ significantly from the unscattered arrival, but the spreads are predicted to be greatly suppressed relative to spreads in an unchanneled medium. Thus the “3/2 law” of Chernov becomes a “1/2 law” due to the sound channel itself. Further, a new Λ parameter has been derived which is nonsingular at caustics and saturates at small values at large range. [Work supported by ONR and NSE.]
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Measurements of transmissions fluctuations for three long ranges in the deep ocean (A)

H. A. DeFerrari and R. I. Davis

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S38-S38 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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Pulse‐like signals were transmitted continuously between a moored source and a fixed bottom‐mounted receiver located 700 m below the deep sound channel axis. The source was deployed at three ranges: 210, 412, and 563 km. A second mooring measured temperature fluctuations caused by internal waves and source mooring motion was also measured. The duration of the transmitted pulse was approximately 9 ms (4 cycles of 459‐Hz carrier). Several distinct RR arrivals have travel time consistent with ray theory predictions. Statistics of fluctuations for the individual paths and the combined multipath are computed and compared with predictions of the theory of scattering from internal waves. Those fluctuations associated with travel time (Phase pulse‐to‐pulse coherence, etc.) are consistent with a priori model predictions. Statistics of intensity fluctuations for all paths are typical of saturated multipath interference, a result difficult to reconcile with model prediction even when ambiguities in the computation of the defraction parameter are taken into account.
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Physical implications of a two‐component model for signal fluctuations in the sea (A)

R. J. Urick

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S38-S38 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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An earlier paper [J. Acoust. Soc. Am. 62, 878 (1977)] described a model for the fluctuation of transmitted sound in the sea, wherein the received signal is postulated to have two components: A steady component plus a random component to such causes as multipath interference, microstructure and scattering. The parameter describing the statistics of the fluctuation was called the randomicity, defined as the ratio of random power to the total power in the received signal. The purpose of the present paper is to point out that the model accounts for two fluctuation characteristics in a simple way. One is the observation that, at short ranges, the square of the coefficient of variation, equivalent to randomicity, increases linearly with range, as power is extracted from the steady component and fed into the random component. The other is that, at long ranges, a slowly varying signal, when momentarily strong, fluctuates less than when it is momentarily weak. In the model, this is the result of adding a constant amount of scattered power to the slowly varying power arriving via multipaths. The model thus has physical implications important to an understanding of sound propagation in the sea.
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Bounds on performance in acoustic tomography (A)

Ronald New, Thomas J. Eisler, and Denise Calderone

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S38-S39 (1981); (2 pages)

Online Publication Date: 12 Aug 2005

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The results of a three‐dimensional ocean experiment in Acoustic Tomography (AT) are, at this time, being processed and reviewed by the originators in the University Consortium [W. Munk and C. Wunsch, Deep‐Sea Res. 26A, 123–161 (1979)]. The hope is that AT will some day soon provide an economical and accurate means to monitor changes in the sound velocity structure over large expanses of the ocean volume. NOAA is attempting to anticipate the engineering implications of operating and maintaining basin‐wide and smaller‐scale AT systems. We are concerned with the likely bounds on performance and, ultimately, will study the relationships between performance and benefits in the civilian sector—in particular, for climate and fishery studies. Estimates of the sound speed distribution are made by inverting travel time relationships. The accuracy and resolution of these estimates can themselves be estimated and, when combined with field data, used to study the performance of a given AT system versus given design parameters. In our present work, these performance relationships are studied and interpreted by means of a well‐known approach of geophysics, the Backus‐Gilbert theory. In this approach, the estimate is interpreted as a weighted local average of the unknown function, with statistical error. An averaging kernel is defined which, when integrated with the actual unknown function, produces a local average; this kernel can, therefore, be interpreted as a property of the system. The width of this function defines resolution length. A trade‐off relation exists between accuracy and resolution length. Examples will be presented for a two‐dimensional problem, a tomographic configuration in a horizontal ocean slice.
Invited Papers
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Estimation of current profiles by inversion of tomographic data (A)

Thomas F. Eisler, Ronald New, and Denise Calderone

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S39-S39 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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Pulse traveltime data contains information in combined and integrated form on both sound velocity and currents. Inversion methods can be generalized so as to yield estimates simultaneously of sound velocity and current vector components as functions of position. We are concerned with establishing bounds on the estimates derived from inversion procedures. One such procedure is described. In this approach, the estimate of one of the unknown functions may be interpreted as a weighted local average of the actual function with contamination by the other unknown functions. Statistical error may also be included in the analysis. The quality of the inversion is determined by the degree to which contamination is minimized; this is a function of the geometry of the tomographic configuration. Examples are presented for a horizontal ocean slice. The inversion may be performed with or without reciprocal transmissions (interchange of sources and receivers); however, their use dramatically improves isolation of sound velocity from the current vector.
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