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  • 1
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    Fluctuation of 400-Hz sound intensity in the 2001 ASIAEX South China Sea experiment (2006)
    IEEE
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © IEEE, 2004. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 29 (2004): 1264-1279, doi:10.1109/JOE.2004.836997.
    Description: We present analyses of fluctuations seen in acoustic signals transmitted by two 400-Hz sources moored as part of the ASIAEX 2001 South China Sea (SCS) experiment. One source was near the bottom in 350-m deep water 31.3 km offshore from the receiving array, and the other was near the bottom in 135-m deep water 20.6 km alongshore from the array. Time series of signal intensity measured at individual phones of a 16-element vertical line array are analyzed, as well as time series of intensity averaged over the array. Signals were recorded from 2 May to 17 May 2001. Fluctuations were observed at periods ranging from subtidal (days) to the shortest periods resolved with our signaling (10 s). Short-period fluctuations of depth- and time-averaged intensity have scintillation indexes (computed within 3-h long windows) which peak at values near 0.5 during an interval of numerous high-amplitude internal gravity waves, and which are lower during intervals with fewer internal waves. The decorrelation times of the averaged intensity (energy level) are also closely related to internal wave properties. Scintillation indexes computed for unaveraged pulses arriving at individual phones often exceed unity.
    Description: This work was supported by the U.S. Office of Naval Research.
    Keywords: Acoustic intensity ; Fluctuation ; Underwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 2
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    Acoustic mode radiation from the termination of a truncated nonlinear internal gravity wave duct in a shallow ocean area (2010)
    Acoustical Society of America
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    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2009. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 126 (2009): 1752-1765, doi:10.1121/1.3203268.
    Description: Horizontal ducting of sound between short-wavelength nonlinear internal gravity waves in coastal environments has been reported in many theoretical and experimental studies. Important consequences arising at the open end of an internal wave duct (the termination) are examined in this paper with three-dimensional normal mode theory and parabolic approximation modeling. For an acoustic source located in such a duct and sufficiently far from the termination, some of the propagating sound may exit the duct by penetrating the waves at high grazing angles, but a fair amount of the sound energy is still trapped in the duct and propagates toward the termination. Analysis here shows that the across-duct sound energy distribution at the termination is unique for each acoustic vertical mode, and as a result the sound radiating from the termination of the duct forms horizontal beams that are different for each mode. In addition to narrowband analysis, a broadband simulation is made for water depths of order 80 m and propagation distances of 24 km. Situations occur with one or more modes absent in the radiated field and with mode multipath in the impulse response. These are both consistent with field observations.
    Description: This work was supported under ONR Grant No. N00014-05-1-0482 and the ONR postdoctoral fellowship award, Grant No. N00014-08-1-0204.
    Keywords: Acoustic field ; Acoustic intensity ; Approximation theory ; Parabolic equations ; Underwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 3
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    Acoustic ducting, reflection, refraction, and dispersion by curved nonlinear internal waves in shallow water (2010)
    IEEE Oceanic Engineering Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © IEEE, 2010. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 35 (2010): 12-27, doi:10.1109/JOE.2009.2038512.
    Description: Nonlinear internal waves in shallow water have been shown to be effective ducts of acoustic energy, through theory, numerical modeling, and experiment. To date, most work on such ducting has concentrated on rectilinear internal wave ducts or those with very slight curvature. In this paper, we examine the acoustic effects of significant curvature of these internal waves. (By significant curvature, we mean lateral deviation of the internal wave duct by more than half the spacing between internal waves over an acoustic path, giving a transition from ducting to antiducting.) We develop basic analytical models of these effects, employ fully 3-D numerical models of sound propagation and scattering, and examine simultaneous acoustical and oceanographic data from the 2006 Shallow Water Experiment (SW06). It will be seen that the effects of curvature should be evident in the mode amplitudes and arrival angles, and that observations are consistent with curvature, though with some possible ambiguity with other scattering mechanisms.
    Description: This work was supported by E. Livingston and T. Pawluskiewicz of the U.S. Office of Naval Research (ONR) under Grant N00014–04-1–0146 and the ONR postdoctoral fellowship award Grant N00014-08-1-0204.
    Keywords: Horizontal acoustic ducting ; Horizontal acoustic refraction ; Horizontal acoustic shadowing ; Shallow-water acoustics ; Curved nonlinear internal waves
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 4
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    Observationally constrained modeling of sound in curved ocean internal waves: Examination of deep ducting and surface ducting at short range (2011)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2011. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 130 (2011): 1173-1187, doi:10.1121/1.3605565.
    Description: A study of 400 Hz sound focusing and ducting effects in a packet of curved nonlinear internal waves in shallow water is presented. Sound propagation roughly along the crests of the waves is simulated with a three-dimensional parabolic equation computational code, and the results are compared to measured propagation along fixed 3 and 6 km source/receiver paths. The measurements were made on the shelf of the South China Sea northeast of Tung-Sha Island. Construction of the time-varying three-dimensional sound-speed fields used in the modeling simulations was guided by environmental data collected concurrently with the acoustic data. Computed three-dimensional propagation results compare well with field observations. The simulations allow identification of time-dependent sound forward scattering and ducting processes within the curved internal gravity waves. Strong acoustic intensity enhancement was observed during passage of high-amplitude nonlinear waves over the source/receiver paths, and is replicated in the model. The waves were typical of the region (35 m vertical displacement). Two types of ducting are found in the model, which occur asynchronously. One type is three-dimensional modal trapping in deep ducts within the wave crests (shallow thermocline zones). The second type is surface ducting within the wave troughs (deep thermocline zones).
    Description: Grants from the Office of Naval Research funded this work. Use of the vessels Ocean Researcher I and Ocean Researcher II in this experiment was funded by the Taiwan National Science Council.
    Keywords: Acoustic field ; Acoustic focusing ; Acoustic intensity ; Acoustic wave scattering ; Acoustic wave velocity ; Ocean waves ; Oceanographic regions ; Underwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 5
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    Acoustic field coherence in four-dimensionally variable shallow water environments : estimation using co-located horizontal and vertical line arrays (2014)
    Institute of Applied and Computational Mathematics
    Details
    Publication Date: 2022-05-25
    Description: Underwater Acoustic Measurements (UAM) 2nd International Conference & Exhibition on "Underwater Acoustic Measurements: Technologies & Results," 25-29 June 2007, Heraklion, Crete, Greece.
    Description: The implementation of two- and three-dimensional acoustic receiver arrays is challenging in the ocean environment. Fixed geometry and connectivity can only be built and maintained at great expense. However, such ideal arrays can be very powerful for signal detection, classification, and tracking, although many of the signal-processing methods employed are subject to constraints of acoustic field temporal and spatial coherence. Thus, understanding the processes at work determining coherence is essential because system effectiveness may then be predictable from environmental parameter input. To study acoustic fields and coherence over finite aperture, the research community has recently taken steps to enable routine use of co-located horizontal and vertical line arrays, typically arranged in the shape of the Roman letter L (or Greek 􀀪 ), with the horizontal leg on the seafloor. This is a small subset of all possible geometries, but it enables measurements of acoustic field coherence not possible with single line arrays. Here, new L-array measurements made in the SW06 field program are used to measure coherence and test coherence predictions via joint analysis of vertical and horizontal line array receptions. Impulsive mode arrivals (including mode multipath) from fixed sources will be estimated using the vertical array. Signals on the horizontal array, which is neither broadside nor endfire, will be compared with signals synthesized using the mode arrivals to estimate azimuthal decorrelation effects. Results can be compared with coherence estimates from computational and theoretical studies.
    Keywords: Shallow-water acoustics ; Receiver array ; L-array ; Coherence ; Normal modes
    Repository Name: Woods Hole Open Access Server
    Type: Working Paper
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  • 6
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    Theory and observation of anisotropic and episodic internal wave effects on 100-400 Hz sound (2014)
    Institute of Applied and Computational Mathematics
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    Publication Date: 2022-05-26
    Description: Underwater Acoustic Measurements (UAM) 4th International Conference and Exhibition on "Underwater Acoustic Measurements: Technologies & Results." 20-24 June 2011, Kos, Greece
    Description: Propagation of sound through shallow-water internal waves of various types is discussed. The anisotropy of the waves imparts an anisotropy to their effects on sound. The internal waves are of two types: Long-wavelength internal tides and short-wavelength high-frequency waves. On the continental shelf both types of waves tend to move shoreward from deep water (i.e. have anisotropic motion and anisotropic correlation scales). The internal tides are less predictable than the surface tides that generate them. The short-wavelength nonlinear internal waves are also somewhat unpredictable, and also have anisotropic correlation scales, having crests of tens of kilometres in length but wavelengths of order 300 to 1000 m. Coupled-mode propagation dominates across-shelf sound propagation, which in the direction of short internal wave correlation scale. Refracted-mode propagation dominates along-shelf propagation. Data from two sea exercises illustrate the character of the waves and their effects on sound.
    Description: Funding for this work is from the Office of Naval Research, Ocean Acoustics Program.
    Keywords: Internal waves ; Nonlinear internal waves ; Internal tides ; Coherence ; Shallow-water acoustics
    Repository Name: Woods Hole Open Access Server
    Type: Working Paper
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