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J Acoust Soc Am 2010 Apr;127(4):2169-81

Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093, USA.

Deep acoustic shadow-zone arrivals observed in the late 1990s in the North Pacific Ocean reveal significant acoustic energy penetrating the geometric shadow. Comparisons of acoustic data obtained from vertical line arrays deployed in conjunction with 250-Hz acoustic sources at ranges of 500 and 1000 km from June to November 2004 in the North Pacific, with simulations incorporating scattering consistent with the Garrett-Munk internal-wave spectrum, are able to describe both the energy contained in and vertical extent of deep shadow-zone arrivals. Incoherent monthly averages of acoustic timefronts indicate that lower cusps associated with acoustic rays with shallow upper turning points (UTPs), where sound-speed structure is most variable and seasonally dependent, deepen from June to October as the summer thermocline develops. Surface-reflected rays, or those with near-surface UTPs, exhibit less scattering due to internal waves than in later months when the UTP deepens. Data collected in November exhibit dramatically more vertical extension than previous months. The depth to which timefronts extend is a complex combination of deterministic changes in the depths of the lower cusps as the range-average profiles evolve with seasonal change and of the amount of scattering, which depends on the mean vertical gradients at the depths of the UTPs.

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http://dx.doi.org/10.1121/1.3292948 | DOI Listing |

April 2010

J Acoust Soc Am 2009 Sep;126(3):1069-83

Department of Oceanography, Naval Postgraduate School, Monterey, CA 93943, USA.

Observations of scattering of low-frequency sound in the ocean have focused largely on effects at long ranges, involving multiple scattering events. Fluctuations due to one and two scattering events are analyzed here, using 75-Hz broadband signals transmitted in the eastern North Pacific Ocean. The experimental geometry gives two purely refracted arrivals. The temporal and vertical scales of phase and intensity fluctuations for these two ray paths are compared with predictions based on the weak fluctuation theory of Munk and Zachariasen, which assumes internal-wave-induced sound-speed perturbations [J. Acoust. Soc. Am. 59, 818-838 (1976)]. The comparisons show that weak fluctuation theory describes the frequency and vertical-wave-number spectra of phase and intensity for the two paths reasonably well. The comparisons also show that a resonance condition exists between the local acoustic ray and the internal-wave field, as predicted by Munk and Zachariasen, such that only internal waves whose crests are parallel to the local ray path contribute to acoustic scattering. This effect leads to filtering of the acoustic spectra relative to the internal-wave spectra, such that steep rays do not acquire scattering contributions due to low-frequency internal waves.

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http://dx.doi.org/10.1121/1.3177259 | DOI Listing |

September 2009

J Acoust Soc Am 2009 Sep;126(3):1026-35

Department of Oceanography, Naval Postgraduate School, Monterey, CA 93943, USA.

In this paper Creamer's [(1996). J. Acoust. Soc. Am. 99, 2825-2838] transport equation for the mode amplitude coherence matrix resulting from coupled mode propagation through random fields of internal waves is examined in more detail. It is shown that the mode energy equations are approximately independent of the cross mode coherences, and that cross mode coherences and mode energy can evolve over very similar range scales. The decay of cross mode coherence depends on the relative mode phase randomization caused by coupling and adiabatic effects, each of which can be quantified by the theory. This behavior has a dramatic effect on the acoustic field second moments like mean intensity. Comparing estimates of the coherence matrix and mean intensity from Monte Carlo simulation, and the transport equations, good agreement is demonstrated for a 100-Hz deep-water example.

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http://dx.doi.org/10.1121/1.3158818 | DOI Listing |

September 2009

J Acoust Soc Am 2009 Aug;126(2):599-606

Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1542, USA.

Receptions, from a ship-suspended source (in the band 50-100 Hz) to an ocean bottom seismometer (about 5000 m depth) and the deepest element on a vertical hydrophone array (about 750 m above the seafloor) that were acquired on the 2004 Long-Range Ocean Acoustic Propagation Experiment in the North Pacific Ocean, are described. The ranges varied from 50 to 3200 km. In addition to predicted ocean acoustic arrivals and deep shadow zone arrivals (leaking below turning points), "deep seafloor arrivals," that are dominant on the seafloor geophone but are absent or very weak on the hydrophone array, are observed. These deep seafloor arrivals are an unexplained set of arrivals in ocean acoustics possibly associated with seafloor interface waves.

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http://dx.doi.org/10.1121/1.3158826 | DOI Listing |

August 2009

J Acoust Soc Am 2009 Apr;125(4):1919-29

St Petersburg State Marine Technical University, St Petersburg, Russia.

Propagation of energy along the sound channel axis cannot be formally described in terms of geometrical acoustics due to repeated cusped caustics along the axis. In neighborhoods of these cusped caustics, a very complicated interference pattern is observed. Neighborhoods of interference grow with range and overlap at long ranges. This results in the formation of a complex interference wave--the axial wave--that propagates along the sound channel axis like a wave belonging to a crescendo of near-axial arrivals. The principal properties of this wave are calculated for the actual space-time configuration realized during a 2004 long-range propagation experiment conducted in the North Pacific. The experiment used M-sequences at 68.2 and 75 Hz, transmitter depths from 350 to 800 m, and ranges from 50 to 3200 km. Calculations show that the axial wave would be detectable for an optimal geometry-both transmitter and receiver at the sound channel axis--for a "smooth" range-dependent sound speed field. The addition of sound speed perturbations--induced here by simulated internal waves--randomizes the acoustic field to the extent that the axial wave becomes undetectable. These results should be typical for mid-latitude oceans with similar curvatures about the sound speed minimum.

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http://dx.doi.org/10.1121/1.3082112 | DOI Listing |

April 2009

J Acoust Soc Am 2008 Sep;124(3):1452-64

Department of Oceanography, Naval Postgraduate School Monterey, California 93943, USA.

While many results have been intuited from numerical simulation studies, the precise connections between shallow-water acoustic variability and the space-time scales of nonlinear internal waves (NLIWs) as well as the background environmental conditions have not been clearly established analytically. Two-dimensional coupled mode propagation through NLIWs is examined using a perturbation series solution in which each order n is associated with nth-order multiple scattering. Importantly, the perturbation solution gives resonance conditions that pick out specific NLIW scales that cause coupling, and seabed attenuation is demonstrated to broaden these resonances, fundamentally changing the coupling behavior at low frequency. Sound-speed inhomogeneities caused by internal solitary waves (ISWs) are primarily considered and the dependence of mode coupling on ISW amplitude, range width, depth structure, location relative to the source, and packet characteristics are delineated as a function of acoustic frequency. In addition, it is seen that significant energy transfer to modes with initially low or zero energy involves at least a second order scattering process. Under moderate scattering conditions, comparisons of first order, single scattering theoretical predictions to direct numerical simulation demonstrate the accuracy of the approach for acoustic frequencies upto 400 Hz and for single as well as multiple ISW wave packets.

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http://dx.doi.org/10.1121/1.2956471 | DOI Listing |

September 2008

J Acoust Soc Am 2004 Dec;116(6):3515-22

Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA.

Utilizing a simple model in which the acoustic wave function is a sum of independent Gaussian wave packets, the relative intensity variance or scintillation index (SI) is analytically calculated. The model has an unspecified probability density function (PDF) for wave packet amplitudes and Gaussian PDFs for travel-time-induced and non-travel-time-induced phase shifts; amplitudes and both phase shifts are assumed to be mutually uncorrelated. It is shown that a proper treatment of the mean field is required to obtain the saturation value, SI = 1, in the limit of a large number of interfering wave packets. The analytic formulas for SI allow identification of important wave packet parameters in the approach to saturation. Criteria are identified for both broadband and narrow-band cases for which the approach to saturation is from above and below 1. It is demonstrated that the broadband approach to saturation is much slower than the narrow-band cases, since wave packets separated in time by more than an inverse bandwidth do not strongly contribute to interference. This effect is quantified by the time-bandwidth product. The model is also used to obtain an analytic expression for pulse time spread; it is shown that multipath conditions which favor a rapid approach to saturation do not favor large pulse spread.

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http://dx.doi.org/10.1121/1.1798356 | DOI Listing |

December 2004

J Acoust Soc Am 2003 Sep;114(3):1226-42

Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, USA.

A ray-based wave-field description is employed in the interpretation of broadband basin-scale acoustic propagation measurements obtained during the Acoustic Thermometry of Ocean Climate program's 1994 Acoustic Engineering Test. Acoustic observables of interest are wavefront time spread, probability density function (PDF) of intensity, vertical extension of acoustic energy in the reception finale, and the transition region between temporally resolved and unresolved wavefronts. Ray-based numerical simulation results that include both mesoscale and internal-wave-induced sound-speed perturbations are shown to be consistent with measurements of all the aforementioned observables, even though the underlying ray trajectories are predominantly chaotic, that is, exponentially sensitive to initial and environmental conditions. Much of the analysis exploits results that relate to the subject of ray chaos; these results follow from the Hamiltonian structure of the ray equations. Further, it is shown that the collection of the many eigenrays that form one of the resolved arrivals is nonlocal, both spatially and as a function of launch angle, which places severe restrictions on theories that are based on a perturbation expansion about a background ray.

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http://dx.doi.org/10.1121/1.1600724 | DOI Listing |

September 2003

J Acoust Soc Am 2003 May;113(5):2533-47

Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, USA.

Recent results relating to ray dynamics in ocean acoustics are reviewed. Attention is focused on long-range propagation in deep ocean environments. For this class of problems, the ray equations may be simplified by making use of a one-way formulation in which the range variable appears as the independent (timelike) variable. Topics discussed include integrable and nonintegrable ray systems, action-angle variables, nonlinear resonances and the KAM theorem, ray chaos, Lyapunov exponents, predictability, nondegeneracy violation, ray intensity statistics, semiclassical breakdown, wave chaos, and the connection between ray chaos and mode coupling. The Hamiltonian structure of the ray equations plays an important role in all of these topics.

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http://dx.doi.org/10.1121/1.1563670 | DOI Listing |

May 2003

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