The effect of surface and linear internal waves on higher order acoustic moments in shallow water

The article of record as published may be found at http://dx.doi.org/10.1121/1.4799345Acoustic fields in shallow water have a statistical nature due to complex, time-evolving sound speed fields and scattering from rough boundaries. Previously, coupled-mode transport theory [Raghukumar and Colosi (2012)] was applied to high frequency acoustic fluctuations in an environment typical of the Shallow Water 2006 (SW06) experiment on the New Jersey continental shelf. As a consequence of the strong adiabatic component in SW06 propagation, a hybrid approach was used to calculate mode coherences where mode energies from the Dozier- Tappert approach were combined with adiabatic phase terms. Mode energies, coherences and acoustic intensities were examined and it was found that internal and surface waves preferentially couple low and high modes respectively. Here, we extend that study to include higher moments such as scintillation index and shift focus to modes that are coupled by both internal and surface waves. Oceanographic and sea surface measurements are used to constrain the internal wave and sea surface models. The relative importance of linear internal waves and surface scattering effects are studied using transport theory and Monte Carlo simulations.Office of Naval ResearchNational Academy of Sciences through the National Research Council research associateship progra

High-frequency normal-mode statistics in shallow water: The combined effect of random surface and internal waves

The article of record as published may be located at https://doi.org/10.1121/1.4919358In an earlier article, the statistical properties of mode propagation were studied at a frequency of 1 kHz in a shallow water environment with random sound-speed perturbations from linear internal waves, using a hybrid transport theory and Monte Carlo numerical simulations. Here, the analysis is extended to include the effects of random linear surface waves, in isolation and in combination with internal waves. Mode coupling rates for both surface and internal waves are found to be signif- icant, but strongly dependent on mode number. Mode phase randomization by surface waves is found to be dominated by coupling effects, and therefore a full transport theory treatment of the range evolution of the cross mode coherence matrix is needed. The second-moment of mode ampli- tudes is calculated using transport theory, thereby providing the mean intensity while the fourth- moment is calculated using Monte Carlo simulations, which provides the scintillation index. The transport theory results for second-moment statistics are shown to closely reproduce Monte Carlo simulations. Both surface waves and internal waves strongly influence the acoustic field fluctuations. VC 2015 Acoustical Society of America

A seminal paper linking ocean acoustics and physical oceanography

17 USC 105 interim-entered record; under review.A look back at a historical article that had a significant impact on the science and pradtice of acoustics. Article: Sound propagation through a fluctuating stratified ocean: Theory and observation Author: Walter H. Munk and Fred Zachariasen Publication Date: April 1976 (JASA 59, 818); https://doi.org/10.1121/1.38093

Entropy rate defined by internal wave scattering in long-range propagation

Author Posting. © Acoustical Society of America, 2015. 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 138 (2015): 1353, doi:10.1121/1.4928617.The reduction of information capacity of the ocean sound channel due to scattering by internal waves is a potential problem for acoustic communication, navigation, and remote sensing over long ranges. In spite of recent progress in research on acoustic signal scattering by random internal waves and the fact that random internal waves are ubiquitous in the world oceans, there is no clear understanding of how these waves influence data communication performance. The entropy decrease resulting from scattering by internal waves is an important measure of information loss. Here a rigorous calculation of the entropy is carried out using second moment transport theory equations with random sound-speed perturbations obeying the Garrett–Munk internal-wave model. It is shown that full-wave rate of entropy is of the same order of magnitude as the Kolmogorov–Sinai entropy and Lyapunov exponents for the relevant ray trajectories. The correspondence between full-wave and ray entropies suggests a correspondence between full-wave scattering and ray chaos near statistical saturation. The relatively small level of entropy rate during propagation through the random internal-wave field shows that scattering by internal waves is likely not an essential limitation for data rate and channel capacity.This work was supported in part by Office of Naval Research grant

Observations of upper ocean sound-speed structures in the North Pacific and their effects on long-range acoustic propagation at low and mid-frequencies

The article of record as published may be found at http://dx.doi.org/10.1121/10.0002174Three 1000-km long, high resolution conductivity, temperature, depth sections in the North Pacific Ocean obtained by the ship towed vehicle SeaSoar are analyzed to quantify 2005 March/April upper-ocean sound-speed structure and determine the effects on low to mid-frequency transmission loss (TL) through numerical simulation. The observations reveal a variable mixed layer acoustic duct (MLAD) with a mean sonic layer depth of 91-m, and an even higher variability, 80-m-average-thickness transition layer connecting the mixed layer (ML) with the main ther- mocline. The sound-speed structure is hypothesized to be associated with thermohaline processes such as air-sea fluxes, eddies, submesoscale, fronts, internal waves, turbulence, and spice, but the analysis does not isolate these factors. Upper-ocean variability is quantified using observables of layer depth, ML gradient, and sound speed to compute low order moments, probability density functions, horizontal wavenumber spectra, and empirical orthogo- nal function decomposition. Coupled mode acoustic propagation simulations at 400 and 1000 Hz were carried out using the sound-speed observations from the upper 400-m appended to climatology, which reveal propagation phys- ics associated with diffraction, random media effects, and deterministic feature scattering. Statistics of TL reveal important energy transfers between the MLAD and the deep sound channel.This work was supported by the Office of Naval Research (ONR) code 32 Ocean Acoustics section, as well as support from the ONR Task Force Ocean initiative.This work was supported by the Office of Naval Research (ONR) code 32 Ocean Acoustics section, as well as support from the ONR Task Force Ocean initiative

Basin-scale internal-wave tomography using a large-aperture vertical array

abstract: Basin-scrde internfl wave tomography has the potential to be a very important oceanographic tool to discover the sources and sinks of internal wave energy. B~in-scde acoustic transmissions are ided for internfl wave tomography since the long-range flows for the bufld-up of the large internal wave acoustic effects. Use of a vertical receiving array simp~fies the tmk of acoustic normal mode detection, wavefront identification in terms of specific geometrical optics ray paths and their use enhances the number of acoustic observable that can be used in an internfl wave inversion. Using wavefront fluctuations inference of the range-average internal wave energy as a function of depth forms the b=is of a hnear inverse problem, yet inversions for other internal wave parameters hke power law spectral slope and vertical mode number bandwidth are more complex nordinear inverses. Inversions using acoustic normti mode observable can only be done using Monte Carlo methods

An Analysis of Long-Range Acoustic Propagation Fluctuations and Upper Ocean Sound Speed Variability

N00014-01-1-031

Echo statistics of individual and aggregations of scatterers in the water column of a random, oceanic waveguide

Author Posting. © Acoustical Society of America, 2014. 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 136 (2014): 90, doi:10.1121/1.4881925.The relative contributions of various physical factors to producing non-Rayleigh distributions of echo magnitudes in a waveguide are examined. Factors that are considered include (1) a stochastic, range-dependent sound-speed profile, (2) a directional acoustic source, (3) a variable scattering response, and (4) an extended scattering volume. A two-way parabolic equation model, coupled with a stochastic internal wave model, produces realistic simulations of acoustic propagation through a complex oceanic sound speed field. Simulations are conducted for a single frequency (3 kHz), monostatic sonar with a narrow beam (5° −3 dB beam width). The randomization of the waveguide, range of propagation, directionality of the sonar, and spatial extent of the scatterers each contribute to the degree to which the echo statistics are non-Rayleigh. Of critical importance are the deterministic and stochastic processes that induce multipath and drive the one-way acoustic pressure field to saturation (i.e., complex-Gaussian statistics). In this limit predictable statistics of echo envelopes are obtained at all ranges. A computationally low-budget phasor summation can successfully predict the probability density functions when the beam pattern and number of scatterers ensonified are known quantities.This research was funded, in part, by the Office of Naval Research Grant Nos. N00014-10-1-0127 and N00014-11-1-0241 and the Oceanographer of the U.S. Navy

Entropy and scintillation analysis of acoustical beam propagation through ocean internal waves

Author Posting. © Acoustical Society of America, 2005. 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 117 (2005): 1611-1623, doi:10.1121/1.1854571.Parabolic equation numerical simulations of waveguide acoustical beam propagation in an ocean of Garrett–Munk internal waves are used to examine the range evolution of beam properties such as beamwidth (both spectral and spatial), Shannon entropy, and scintillation index, as a function of beam angle. Simulations are carried out at 250- and 125-Hz acoustic frequencies. The ray trajectories associated with these beams are predominantly chaotic or exponentially sensitive to initial conditions and/or medium perturbations. At long range near saturation, the finite-frequency beams show a constant rate of change of Shannon entropy with range, independent of acoustic frequency. This full-wave rate of entropy is of the same order of magnitude as the average rate of entropy for the ray trajectories associated with this beam. Finite-range Lyapunov exponents provide the estimates of ray entropy rate or Kolmogorov–Siani entropy. The correspondence between full-wave and ray entropies suggests a full-wave manifestation of ray chaos, but only once statistical saturation is obtained. In spite of this correspondence, the simulated acoustical beams expand diffusively not exponentially (or explosively)