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  • Internal waves  (7)
  • In situ oceanic observations
  • IEEE  (7)
  • 1
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    Passive time reversal acoustic communications through shallow-water internal waves (2011)
    IEEE
    Details
    Publication Date: 2022-05-26
    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): 756-765, doi:10.1109/JOE.2010.2060530.
    Description: During a 12-h period in the 2006 Shallow Water Experiment (SW06), binary phase shift keying (BPSK) signals at the carrier frequencies of 813 and 1627 Hz were propagated over a 19.8-km source–receiver range when a packet of strong internal waves passed through the acoustic track. The communication data are analyzed by time reversal processing followed by a single-channel decision feedback equalizer. Two types of internal wave effects are investigated in the context of acoustic communications. One is the rapid channel fluctuation within 90-s data packets. It can be characterized as decreased channel coherence, which was the result of fast sound-speed perturbations during the internal wave passage. We show its effect on the time reversal receiver performance and apply channel tracking in the receiver to counteract such fluctuation. The other one is the long-term (in the scale of hours) performance degradation in the depressed waveguide when the internal waves passed through the acoustic track. Even with channel tracking, the time reversal receiver experiences average 3–4-dB decrease in the output signal-to-noise ratio (SNR). Such long-term performance degradation is explained by the ray approximation in the depressed waveguide.
    Description: This work was supported by the U.S. Office of Naval Research (ONR) Code 322OA under Grants N00014-07-1-0546 and N00014-06-1019.
    Keywords: Acoustic communications ; Decision feedback equalizers ; Internal waves ; Time reversal processing
    Repository Name: Woods Hole Open Access Server
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  • 2
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    Modeling weak fluctuations of undersea telemetry signals (2007)
    IEEE
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    Publication Date: 2022-05-25
    Description: Author Posting. © IEEE, 1991. 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 16 (1991): 3-11, doi:10.1109/48.64880.
    Description: Numerical calculation of acoustic field perturbation expressions can be used to predict fluctuations after propagation through ocean sound-speed structures, but before the onset of multipath. The general form of the expressions for signal spectra or correlation functions allow numerical evaluation for an unlimited quantity of vector wave-number spectral models of refractive index. In order to help define the bounds of applicability of the theory, log-intensity fluctuation variances have been calculated for three major situations: ocean internal waves, ocean turbulence, and continuous strong large-scale turbulence. Propagation through ocean thermocline internal waves, realistically weak thermocline turbulence, and unrealistically strong turbulence show that scintillations of intensity can be predicted and understood to first order up to ranges of tens of kilometers, given the proper transmission geometry. Internal wave effects dominate over any effects from expected microstructure. Nonhorizontal transmission yields small fluctuations, but eventually refractive effects of the sound channel will contribute some additional spatial variability and multipath, complicating the use of the theory. Multipath due to the sound channel can exist at ranges where the random small-scale structures would contribute only small perturbations (no multipath from small structures)
    Description: This work was supported by the Office of Naval Research, Ocean Acoustics Program.
    Keywords: Wave propagation ; Forward scattering ; Internal waves ; Microstructure
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 3
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    Internal tide and nonlinear internal wave behavior at the continental slope in the northern south China Sea (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): 1105-1130, doi:10.1109/JOE.2004.836998.
    Description: A field program to measure acoustic propagation characteristics and physical oceanography was undertaken in April and May 2001 in the northern South China Sea. Fluctuating ocean properties were measured with 21 moorings in water of 350- to 71-m depth near the continental slope. The sea floor at the site is gradually sloped at depths less than 90 m, but the deeper area is steppy, having gradual slopes over large areas that are near critical for diurnal internal waves and steep steps between those areas that account for much of the depth change. Large-amplitude nonlinear internal gravity waves incident on the site from the east were observed to change amplitude, horizontal length scale, and energy when shoaling. Beginning as relatively narrow solitary waves of depression, these waves continued onto the shelf much broadened in horizontal scale, where they were trailed by numerous waves of elevation (alternatively described as oscillations) that first appeared in the continental slope region. Internal gravity waves of both diurnal and semidiurnal tidal frequencies (internal tides) were also observed to propagate into shallow water from deeper water, with the diurnal waves dominating. The internal tides were at times sufficiently nonlinear to break down into bores and groups of high-frequency nonlinear internal waves.
    Description: This work was supported in part by grants from the U.S. Office of Naval Research, Physical Oceanography and Ocean Acoustics Programs, and by the National Science Council of Taiwan.
    Keywords: Baroclinic tides ; Internal waves ; Nonlinear waves
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 4
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    South China Sea internal tide/internal waves-impact on the temporal variability of horizontal array gain at 276 Hz (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): 1292-1307, doi:10.1109/JOE.2004.836794.
    Description: The temporal variability of the spatial coherence of an acoustic signal received on a bottomed horizontal array has been calculated for 276-Hz narrow-band signals. A conventional plane wave beamformer was applied to the received signals. The temporal variability of the array's omnipower, beam power, and array gain are related to variability in the sound-speed field. The spectral characteristics of array omnipower are nonstationary and changed as the spectral characteristics of the temperature field varied. The array omnipower and beam-power variability tracked each other in time and varied by as much as 15 dB over time intervals as short as 7 min. Array gain varied up to 5 dB and usually tracked the omnipower variability. A contiguous 24-h section of data is discussed in detail. This data section is from a time period during which the high-frequency fluid dynamic perturbation of the sound-speed field was of smaller amplitude than other sections of the 16-d data set. Consequently, this section of data sets an upper bound for the realizable array gain. The temporal variability of array gain and spatial coherence at times appears to be correlated with environmental perturbation of the sound-speed field, but are also correlated with changes in the signal-to-noise ratio. The data was acquired during the Office of Naval Research's South China Sea Asian Seas International Acoustics Experiment. The 465-m 32-channel horizontal array was placed on the bottom in 120 m of water at the South China Sea shelf break. The acoustic source was moored in 114 m of water /spl sim/19 km from the receiving array.
    Description: This work was supported by the Office of Naval Research.
    Keywords: Array signal gain ; Coherence ; Internal waves ; Narrow-band conventional beamforming ; Shallow-water arrays
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 5
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    Shelf-edge frontal structure in the central East China Sea and its impact on low-frequency acoustic propagation (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): 1011-1031, doi:10.1109/JOE.2004.840842.
    Description: Two field programs, both parts of the Asian Seas International Acoustics Experiment (ASIAEX), were carried out in the central East China Sea (28 to 30 N, 126 30 to 128 E) during April 2000 and June 2001. The goal of these programs was to study the interactions between the shelf edge environment and acoustic propagation at a wide range of frequencies and spatial scales. The low-frequency across-slope propagation was studied using a synthesis of data collected during both years including conductivity- temperature-depth (CTD) and mooring data from 2000, and XBT, thermistor chain, and wide-band source data from 2001. The water column variability during both years was dominated by the Kuroshio Current flowing from southwest to northeast over the continental slope. The barotropic tide was a mixed diurnal/semidiurnal tide with moderate amplitude compared to other parts of the Yellow and East China Sea. A large amplitude semidiurnal internal tide was also a prominent feature of the data during both years. Bursts of high-frequency internal waves were often observed, but these took the form of internal solitons only once, when a rapid off-shelf excursion of the Kuroshio coincided with the ebbing tide. Two case studies in the acoustic transmission loss (TL) over the continental shelf and slope were performed. First, anchor station data obtained during 2000 were used to study how a Kuroshio warm filament on the shelf induced variance in the transmission loss (TL) along the seafloor in the NW quadrant of the study region. The corresponding modeled single-frequency TL structure explained the significant fine-scale variability in time primarily by the changes in the multipath/multimode interference pattern. The interference was quite sensitive to small changes in the phase differences between individual paths/modes induced by the evolution of the warm filament. Second, the across-slope sound speed sections from 2001 were used to explain the observed phenomenon of abrupt signal attenuation as the transmission range lengthened seaward across the continental shelf and slope. This abrupt signal degradation was caused by the Kuroshio frontal gradients that produced an increasingly downward-refracting sound-speed field seaward from the shelf break. This abrupt signal dropout was explained using normal mode theory and was predictable and source depth dependent. For a source located above the turning depth of the highest-order shelf-trapped mode, none of the propagating modes on the shelf were excited, causing total signal extinction on the shelf.
    Keywords: Environmental acoustics ; Internal waves ; Kuroshio Current ; Shelf circulation
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 6
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    Research highlights from the Asian Seas International Acoustics Experiment in the South China Sea (2006)
    IEEE
    Details
    Publication Date: 2022-05-26
    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): 1067-1074, doi:10.1109/JOE.2005.843162.
    Description: The Asian Seas International Acoustics Experiment (ASIAEX) included two major field programs, one in the South China Sea (SCS) and the other in the East China Sea (ECS). This paper summarizes results from the work conducted during April and May 2000 and 2001 over the continental shelf and slope in the northeastern South China Sea, just east of Dongsha Island (Pratis Reef). The primary emphasis of the field program was on water-column variability and its impact on acoustic propagation loss. The reader is steered to the appropriate paper within this Special Issue when more information on a specific topic is desired.
    Description: The planning, execution, and analysis of this work was funded by the U.S. Office of Naval Research Ocean Acoustics and Physical Oceanography Programs. Significant support was also made by the National Science Council of Taiwan.
    Keywords: Acoustic propagation ; Asian Seas International Acoustics Experiment (ASIAEX) ; Internal waves ; Ocean currents ; South China Sea
    Repository Name: Woods Hole Open Access Server
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  • 7
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    Acoustic mode coupling by nonlinear internal wave packets in a shelfbreak front area (2005)
    IEEE
    Details
    Publication Date: 2022-05-26
    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): 118-125, doi:10.1109/JOE.2003.822975.
    Description: A computational case study of coupled-mode 400-Hz acoustic propagation over the distance 27 km on the continental shelf is presented. The mode coupling reported here is caused by lateral gradients of sound-speed within packets of nonlinear internal waves, often referred to as solitary wave packets. In a waveguide having unequal attenuation of modes, directional exchange of energy between low- and high-loss modes, via mode coupling, can become time dependent by the movement of waves and can cause temporally variable loss of acoustic energy into the bottom. Here, that bottom interaction effect is shown to be sensitive to stratification conditions, which determine waveguide properties and, in turn, determine modal attenuation coefficients. In particular, time-dependent energy loss due to the presence of moving internal wave packets is compared for waveguides with and without a frontal feature similar to that found at the shelfbreak south of New England. The mean and variability of acoustic energy level 27 km distant from a source are shown to be altered in a first order way by the presence of the frontal feature. The effects of the front are also shown to be functions of source depth.
    Description: This work was supported by the Office of Naval Research Grants N00014-99-1-2074 and N00014-01-1-0772.
    Keywords: Continental shelf ; Internal waves ; Mode coupling ; Shallow water ; Shelfbreak front ; Solitary waves ; Sound propagation
    Repository Name: Woods Hole Open Access Server
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