Initial results from a cartesian three-dimensional parabolic equation acoustical propagation code

A three-dimensional (3D) parabolic equation acoustical propagation code has been developed and run successfully. The code is written in the MATLAB language and runs in the MATLAB environment. The code has been implemented in two versions, applied to (1) Horizontal low-frequency (100 to 500 Hz) propagation through the shallow water waveguide environment; (2) Vertical high-frequency propagation (6 to 15 kHz) to study normal-incidence reflection from the lower side of the ocean surface. The first edition of the code reported on here does not implement refinements that are often found in 2D propagation models, such as allowing density to vary, optimally smoothing soundspeed discontinuities at the water/seabed interface, and allowing an omni-directional source. The code is part of a development effort to test the applicability of 2D (and N by 2D) models, which have more refinements than this model, to the study of fully 3D propagation problems, such as sound transiting steep nonlinear coastal-area internal waves and/or sloping terrain, and to provide a numerical tool when the full 3D solution is needed.Funding was provided by the Office of Naval Research under Contract No. N00014-05-1-0482

Modeling and forecasting ocean acoustic conditions

Author Posting. © The Author, 2017. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 75 (2017): 435–457, doi:10.1357/002224017821836734.Modeling acoustic conditions in an oceanic environment is a multiple-step process. The environmental conditions (features) in the area first must be measured or estimated; relevant features include seabed geometry, seabed composition, and four-dimensionally (4D) variable sound-speed and density variations related to evolving or wave motions. Often the dynamical wave modeling depends on first obtaining correct seabed and mean stratification conditions (for example, nonlinear internal wave modeling). Next, this information must be included in sound propagation modeling. A selection of the many methods and tools available for these tasks are described, with a focus on modeling sounds of 20 to 1000 Hz propagating through water-column features that are time-dependent and variable in three dimensions (i.e., 4D variable). An example of a 3D parabolic equation acoustic calculation shows how variability caused by evolving internal tidal waves affects sound propagation. Different propagation and scattering regimes are discussed, including the theoretically delineated weak scattering and strong scattering regimes, as well as the empirically examined regime found in nonlinear internal waves. The histories and the current state of our oceanographic knowledge (the input to acoustic modeling) and of our ability to effectively model complex acoustic conditions are discussed. Example acoustic simulation applications are also discussed; these are ocean acoustic tomography, coherence prediction, and signal-to-noise ratio prediction. Types of ocean models and acoustic models and how they are interfaced are also examined. These include deterministic, statistical analytic feature models.Funding for this work was provided by the U.S. Office of Naval Research, Ocean Acoustics Program, Grants N-00014-11-1-0701 and N00014-14-1-0223

Acoustic signal and noise changes in the Beaufort Sea Pacific Water duct under anticipated future acidification of Arctic Ocean waters

Author Posting. © Acoustical Society of America, 2017. 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 142 (2017): 1926–1933, doi:10.1121/1.5006184.It is predicted that Arctic Ocean acidity will increase during the next century as a result of carbon dioxide accumulation in the atmosphere and migration into ocean waters. This change has implications for sound transmission because low-pH seawater absorbs less sound than high-pH water. Altered pH will affect sound in the 0.3−10 kHz range if the criterion is met that absorption is the primary cause of attenuation, rather than the alternatives of loss in the ice or seabed. Recent work has exploited sound that meets the criterion, sound trapped in a Beaufort Sea duct composed of Pacific Winter Water underlying Pacific Summer Water. Arctic pH is expected to drop from 8.1 to 7.9 (approximately) over the next 30−50 yr, and effects of this chemical alteration on the intensity levels of this ducted sound, and on noise, are examined here. Sound near 900 Hz is predicted to undergo the greatest change, traveling up to 38% further. At ranges of 100−300 km, sound levels from a source in the duct may increase by 7 dB or more. Noise would also increase, but noise is ducted less efficiently, with the result that 1 kHz noise is predicted to rise approximately 0.5 dB.This work was supported by Office of Naval Research Grant N00014-16-1-2372

Internal wave effects on acoustic propagation

Underwater Acoustics Measurements (UAM) 1st International Conference “Underwater Acoustic Measurements: Technologies & Results,” 28 June- 1 July 2005, Heraklion, Crete, Greece,Internal gravity wave induced acoustic fluctuations are reviewed. It is well known that internal waves cause temporal and spatial variability above and beyond that caused by mesoscale and larger ocean heterogeneity. Increasingly detailed work over many decades has shown that deep-ocean internal waves, which have often been parameterized using the Garrett-Munk spectrum as a guide, are responsible for rapid acoustic field variability at all propagation ranges. Numerous experiments have shown that various sections of wavefronts from impulsive sources have fluctuation qualities well-described by theory, simulation, or both. In contrast, fluctuations in shallow water experiments, although known to be consistent with those expected from internal waves via theoretical and simulation arguments, are incompletely described by theories for a number of reasons. These reasons include nonstationary, inhomogeneous or anisotropic wave environments, unknown geoacoustic properties, and rapidly changing background currents, all of which prevent detailed comparison of observation and prediction. At this time, many different shallow-water internal wave scenarios give rise to similar field fluctuations, within reasonable confidence intervals for the predictions. This may simplify order-of magnitude fluctuation prediction, while simultaneously making inversion and highly-detailed prediction problematic.This work was supported by the Office of Naval Research

Theory and observation of anisotropic and episodic internal wave effects on 100-400 Hz sound

Underwater Acoustic Measurements (UAM) 4th International Conference and Exhibition on "Underwater Acoustic Measurements: Technologies & Results." 20-24 June 2011, Kos, GreecePropagation 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.Funding for this work is from the Office of Naval Research, Ocean Acoustics Program

Temporal and cross-range coherence of sound traveling through shallow-water nonlinear internal wave packets

Author Posting. © Acoustical Society of America, 2006. 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 119 (2006): 3717-3725, doi:10.1121/1.2200699.Expressions governing coherence scales of sound passing through a moving packet of nonlinear internal waves in a continental shelf environment are presented. The expressions describe the temporal coherence scale at a point, and the horizontal coherence scale in a plane transverse to the acoustic path, respectively. Factors in the expressions are the wave packet propagation speed, wave packet propagation direction, the fractional distance from the packet to the source, and the spatial scale S of packet displacement required to cause acoustic field decorrelation. The scale S is determined by the details of coupled mode propagation within the packet and the waveguide. Here, S is evaluated as a function of frequency for one environment, providing numerical values for the coherence scales of this environment. Coherence scales derived from numerical simulation of coupled mode acoustic propagation through moving wave packets substantiate the expressions.This work was funded by grants from the Ocean Acoustics Program of the U.S. Office of Naval Research

Modeling weak fluctuations of undersea telemetry signals

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.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)This work was supported by the Office of Naval Research, Ocean Acoustics Program

Acoustic mode coupling by nonlinear internal wave packets in a shelfbreak front area

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.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.This work was supported by the Office of Naval Research Grants N00014-99-1-2074 and N00014-01-1-0772

Analysis of finite-duration wide-band frequency sweep signals for ocean tomography

Author Posting. © IEEE, 1993. 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 18 (1993): 87-94, doi:10.1109/48.219528.A group of amplitude and frequency modulated signals which generate narrow synthesized pulses are described. The pulse-compression properties of these signals should approach those of maximal (M) sequence phase-modulated signals now commonly used in ocean experiments. These amplitude-tapered linear frequency-sweep (chirp) type signals should be accurately reproducible with most acoustic sources since they have controllable limited-bandwidth frequency content and differentiable phase. The Doppler response of the signals is calculated using a wideband approach, where the frequency shift from relative motion is not constant throughout the waveform. The resultant Doppler effect on the matched-filter output is a function of the signal duration. The signals are suitable for use with tunable resonant transducers, and have adequate Doppler response for use with Lagrangian ocean drifter

Smoothly modulated frequency-bounded impulse signals for tomography

A group of amplitude and frequency modulated signals which generate narrow synthesized pulses and which also have smoothly varying phase are described. The frequency-sweep (chirp) signals have exactly-defined frequency content and differentiable phase. These signals can be used with efficient resonant transducers, if the resonant frequency is adjustable, and they have adequate Doppler response for use with drifting apparatus.Funding was provided by the Office of Naval Research under Contract No. N00014-87-K-0017