Ocean Waveguide Research Articles

A two-way coupled mode formalism that satisfies energy conservation for impedance boundaries in underwater acoustics

This paper shows that energy conservation and the derivation of the two-way coupled mode range equations can be extended in three dimensions to complex mode functions and eigenvalues. Furthermore, the energy in the coupled mode formulation is conserved for finite thickness fluid ocean waveguides with a penetrable bottom boundary beneath any range dependence. The derivations rely on completeness and a modified orthonormality statement. The mode coupling coefficients are specified solely and explicitly by the waveguide range dependence. The statement of energy conservation is applied to a numerical coupled mode calculation.

Simultaneous localization of multiple broadband non-impulsive acoustic sources in an ocean waveguide using the array invariant.

The array invariant method, previously derived for instantaneous range and bearing estimation of a single broadband impulsive source in a horizontally stratified ocean waveguide, can be generalized to simultaneously localize multiple uncorrelated broadband noise sources that are not necessarily impulsive in the time domain by introducing temporal pulse compression and an image processing technique similar to the Radon transform. This can be done by estimating the range and bearing of broadband non-impulsive sources from measured beam-time migration lines of modal arrivals along a horizontal array arising from differences in modal group velocity and modal polar angle for each propagating mode. The generalized array invariant approach is used to estimate the range of a vertical source array and vocalizing humpback whales over wide areas from measurements made by a towed horizontal receiver array during the Gulf of Maine 2006 Experiment. The localization results are shown to have roughly 12% root-mean-squared errors from Global Positioning System measured ground truth positions for controlled source transmissions and less than 10% discrepancy from those obtained independently via moving array triangulation for vocalizing humpbacks, respectively.

Estimation of the depth of a stationary sound source in shallow water

The noise immunity of the method for estimating the depth of a stationary sound source in an oceanic waveguide, based on the information about the amplitude ratio of selected wave field modes, is considered. It is shown that the error in recovering the source depth is always limited and tends to a stationary value with an increase in the noise level. The results of a computational experiment with a single detector and a horizontal linear antenna for the low-frequency region are reported. The behavior of the depth estimate in dependence of the mode numbers and detector depth is analyzed for the limiting case of intense noise. The stability of the method with respect to variations in the speed-of-sound profile is demonstrated.

Improved passive bottom-loss estimation below 10 kHz using arrays deployable on autonomous underwater vehicles

Accurate modeling of acoustic propagation in the ocean waveguide is important for SONAR performance prediction, and requires, among other things, characterizing the reflection properties of the bottom. Recent advances in the technology of autonomous underwater vehicles (AUV) make it possible to envision a survey tool for seabed characterization composed of a short array mounted on an AUV. The bottom power reflection coefficient (and the related reflection loss) can be estimated passively by beamforming the naturally occurring marine ambient-noise acoustic field recorded by a vertical line array of hydrophones. However, the reduced array lengths required by AUV deployment can hinder the process, due to the inherently poor angular resolution. In this paper, data from higher frequencies are used to estimate the noise spatial coherence function at a lower frequency for sensor spacing beyond the physical length of the array. This results in higher angular resolution of the bottom loss estimate, while exploiting the large bandwidth available to current acquisition systems more efficiently than beamforming does. The technique, rigorously justified for a halfspace bottom, proves to be effective also on more complex bottom types, both in simulation and on experimental data.

Nonlinear acoustic pulse propagation in ocean waveguides containing an elastic seabed

High intensity underwater sources, such as explosions, generate nonlinear finite-amplitude pulses that behave differently than linear acoustic pulses within shallow water waveguides. The nonlinearity is known to decrease the critical angle for total internal reflection from that of the linear case when the seafloor is approximated as a fluid. However, this result has not been extensively studied for elastic seafloors where shear waves are present. In this work, a time-domain model is developed assuming an isotropic linear elastic bottom, inviscid water column, and allowing for nonlinear advective acceleration and a nonlinear equation of state. The model is numerically implemented using a high-order Godunov scheme and then benchmarked against tank experiment data for the linear case. The nonlinear model is used to study the critical grazing angle for ocean bottoms of varying shear speeds to determine the combined effect of nonlinearity and elasticity on bottom penetration.

Formation of caustic beams in a refractive oceanic waveguide

We consider the patterns in the formation of the spatial interference structure of an acoustic field excited in the near-surface channel of a vertical array consisting of point sources emitting a tonal signal inphase. It is established that when the array aperture is increased to a certain optimal size, only one caustic beam forms in the channel. As the array aperture further increases, the sequential formation of a certain number of beams after the first is observed.

Target strength in oceanic waveguides

The structure of the target strength in oceanic waveguides is analyzed. The role of the target strength is discussed as one of the key parameters for designing and estimating the efficiency of promising underwater observation systems.

Information-theoretic analysis of iterated Bayesian acoustic source localization in a static ocean waveguide.

Fundamental constructs of information theory are applied to quantify the performance of iterated (sequential) Bayesian localization of a time-harmonic source in a range- and time-invariant acoustic waveguide using the segmented Fourier transforms of the received pressure time series. The nonlinear relation, defined by acoustic propagation, between the source location and the received narrowband spectral components is treated as a nonlinear communication channel. The performance analysis includes mismatch between the acoustic channel and the model channel on which the Bayesian inference is based. Source location uncertainty is quantified by the posterior probability density of source location, by the posterior entropy and associated uncertainty area, by the information gain (relative entropy) at each iteration, and by large-ensemble limits of these quantities. A computational example for a vertical receiver array in a shallow-water waveguide is presented with acoustic propagation represented by normal modes and ambient noise represented by a Kuperman-Ingenito model. Performance degradation due to noise-model mismatch is quantified in an example. Potential extensions to uncertain and stochastic environments are discussed.

Connection of sea level variability between the tropical western Pacific and the southern Indian Ocean during recent two decades

Based on the merged satellite altimeter data and in-situ observations, as well as a diagnosis of linear baroclinic Rossby wave solutions, this study analyzed the rapidly rise of sea level/sea surface height (SSH) in the tropical Pacific and Indian Oceans during recent two decades. Results show that the sea level rise signals in the tropical west Pacific and the southeast Indian Ocean are closely linked to each other through the pathways of oceanic waveguide within the Indonesian Seas in the form of thermocline adjustment. The sea level changes in the southeast Indian Ocean are strongly influenced by the low-frequency westward-propagating waves originated in the tropical Pacific, whereas those in the southwest Indian Ocean respond mainly to the local wind forcing. Analyses of the lead-lag correlation further reveal the different origins of interannual and interdecadal variabilities in the tropical Pacific. The interannual wave signals are dominated by the wind variability along the equatorial Pacific, which is associated with the El Nino-Southern Oscillation; whereas the interdecadal signals are driven mainly by the wind curl off the equatorial Pacific, which is closely related to the Pacific Decadal Oscillation.

Multichannel blind deconvolution of sound source of opportunity in ocean waveguide

Signals that travel through an ocean waveguide are typically distorted when they are received by a remote receiver because of interference and distortion arising from multiple propagation paths. This presentation investigates the applicability of a physics-based blind-deconvolution technique proposed by Sabra et al. (JASA 127, EL42, 2010) to sound sources of opportunity such as randomly radiating ships. Using only the noisy recorded signals from an underwater receiver array, this blind deconvolution provides a means to estimate the Green's functions between the source of opportunity and the elements of the receiver array as well as the original signal radiated by the source of opportunity. These estimated Green's functions can then be used for acoustic characterization of the ocean waveguide parameters which is typically done using controlled sources only. We will discuss the performance of the proposed approach using both numerical simulations and at-sea data using discrete shipping events recorded on a vertical array as sources of opportunities.

Laboratory scale measurements of three-dimensional sound propagation in the presence of a tilted penetrable bottom

Results of laboratory-scale measurements of long-range across-slope pulse propagation in three-dimensional wedge-shaped oceanic waveguides with a sandy bottom are reviewed. The experimental data considered were collected during two laboratory-scale experimental campaigns that were led in 2006 and 2007 (after a required calibration phase) in the large indoor shallow-water tank of the LMA-CNRS laboratory in Marseille. Operational frequencies and water depths were chosen to produce a reduced number of propagating modes so as to facilitate the analysis of the received signals. The experimental data contain strong 3-D effects like mode shadow zones and multiple mode arrivals, all consistent with well-known three-dimensional effects described in the literature. The data are compared with numerical solutions obtained using a fully three-dimensional parabolic equation based model. Comparisons are performed in both time domain (received signals at distinct ranges and depths) and frequency domain (TL-versus-range curves at distinct depths for several discrete frequencies). In both cases, the experimental data are in good agreement with the numerical predictions.

Localization of a sound source in oceanic waveguides

A method is developed for passive acoustic diagnostics of a mobile sound source in an oceanic waveguide, based on information on the frequency shifts of the maxima of the wave field and mode eigen-functions. An algorithm is formulated for solving the inverse problem. A numerical experiment demonstrates the possibilities of this method for reconstructing the trajectory, velocity, and depth of a source.

Matched field signal processing in underwater sound channels (Review)

The state of the art of matched field hydroacoustic signal processing is described from the viewpoint of estimating the signal parameters in adaptive antenna arrays. The focus is on methods for solving the problem of source localization in an oceanic waveguide under mismatching effects of different nature, caused by disagreement between the received acoustic field and its model. Different approaches to increase the stability of the algorithms for source position estimation are discussed, which allows an increase in their efficiency in natural conditions.

Equatorial Pacific Easterly Wind Surges and the Onset of La Niña Events*

Abstract The processes responsible for the onset of La Niña events have not received the same attention as those responsible for the onset of El Niño events, for which westerly wind events (WWEs) in the tropical Pacific have been identified as important contributors. Results here show that synoptic-scale surface easterly wind surges (EWSs) play an important role in the onset of La Niña events, akin to the role of WWEs in the onset of El Niño events. It is found that EWSs are a substantial component of zonal wind stress variance along the equatorial Pacific. Using reanalysis wind stress fields, validated against buoy measurements, 340 EWS events are identified between 1986 and 2012. Their distributions in space, time, and El Niño–Southern Oscillation (ENSO) state are described. About 150 EWSs occur during ENSO-neutral conditions, during the months associated with La Niña initiation and growth (April–December). Composites of changes in sea surface temperature anomaly (SSTA) following these ~150 events show statistically significant cooling (0.1°–0.4°C) along the oceanic waveguide that persists for 2–3 months following the EWSs. Experiments with EWS forcing of an ocean general circulation model show SSTA patterns like those in the observations. It is suggested that EWSs play an important role in the onset of La Niña waveguide surface cooling and deserve additional study.

Reverberation and biological clutter in continental shelf waveguides

Seafloor reverberation in continental shelf waveguides is the primary limiting factor in active sensing of biological clutter in the ocean for noise unlimited scenarios. The detection range of clutter is determined by the ratio of the intensity of scattered returns from clutter versus the seafloor in a resolution cell of an active sensing system. We have developed a Rayleigh-Born volume scattering model for seafloor scattering in an ocean waveguide. The model has been tested with data collected from a number of Ocean Acoustic Waveguide Remote Sensing (OAWRS) experiments in distinct US Northeast coast continental shelf environments, and has shown to provide accurate estimates of seafloor reverberation over wide areas for various source frequencies. We estimate scattered returns from fish clutter by combining ocean-acoustic waveguide propagation modeling that has been calibrated in a variety of continental shelf environments for OAWRS applications with a model for fish target strength. Our modeling of seafloor reverberation and scattered returns from fish clutter is able to explain and elucidate OAWRS measurements along the US Northeast coast.

Three-dimensional acoustic propagation effect in subaqueous sand dune field

Very large subaqueous sand dunes are discovered on the upper continental slope of the northern South China Sea in water depth of 160–600 m, which composed of fine to medium sand. The amplitude and the crest-to-crest wavelength of sand dunes are about 5–15 m and 200–400 m, respectively. This topographic feature could causes strong acoustic scattering, mode coupling, and out-of- plane propagation effects, which consequently result in sound energy redistribution within ocean waveguide. This research focus on the three-dimensional propagation effects (e.g., horizontal refraction) induced by the sand dunes in the South China Sea, which are expected as the angle of propagation relative to the bedform crests decreases. The three-dimensional propagation effects are studied by numerical modeling and model-data comparison. For numerical modeling, the in-situ topographic data of subaqueous sand dune and sound speed profiles were inputted to calculate the acoustic fields, which were further decomposed into mode fields to show the modal horizontal refraction effects. The modeling results were manifested by data observations. [This work is sponsored by the Ministry of Science and Technology of Taiwan.]

Energy conservation via coupled modes in waveguides with an impedance boundary condition

A statement of energy conservation for a coupled mode formulation with real mode functions and eigenvalues has been demonstrated to be consistent with the statement of conservation derivable from the Helmholtz equation. The restriction to real mode functions and eigenvalues precludes coupled mode descriptions with waveguide absorption or untrapped modes. The demonstration, along with the derivation of the coupled mode range equation, relies on orthonormality in terms of a product of two modal depth functions integrated to infinite depth. This paper shows that energy conservation and the derivation of the coupled mode range equation can be extended to complex mode functions and eigenvalues, and that energy is conserved for ocean waveguides with a penetrable bottom boundary at a finite depth beneath any range dependence. For this, the penetrable bottom boundary is specified by an impedance condition for the mode functions. The new derivations rely on completeness and a modified orthonormality statement. Mode coupling is driven solely by waveguide range dependence. Thus, the form of the range equation and the values of the coupling coefficients are unaffected by a finite depth waveguide. Applications of energy conservation to examine the accuracy of a numerical coupled mode calculation are presented.

Simultaneous localization of multiple vocalizing humpback whale calls in an ocean waveguide with a single horizontal array using the array invariant

The array invariant method, previously derived for instantaneous range and bearing estimation of a broadband impulsive source in a horizontally stratified ocean waveguide [Lee and Makris, J. Acoust. Soc. Am. 119, 336–351 (2006)], is generalized to instantaneously and simultaneously localize multiple uncorrelated broadband noise sources that not necessarily impulsive in the time domain. In an ideal Pekeris waveguide, we theoretically show that source range and bearing can be instantaneously obtained from beam-time migration lines measured with a horizontal array through range and bearing dependent differences that arise between modal group speeds along the array. We also show that this theory is approximately valid in a horizontally stratified ocean waveguide. A transform, similar to the Radon transform, is employed to enable simultaneous localization of multiple uncorrelated broadband noise sources without ambiguity using the array invariant method. The method is now applied to humpback whale vocalization data from the Gulf of Maine 2006 Experiment for humpback whale ranges up to tens of kilometers, where it is shown that accurate bearing and range estimation of multiple vocalizing humpback whales can be simultaneously made with little computational effort.

Estimating relative abundance of singing humpback whales in Los Cabos, México, using diffuse ambient noise

Previous research has speculated that diffuse ambient noise levels can be used to estimate relative cetacean abundance in certain locations when baleen whale vocal activity dominates the soundscape (Au et al., 2000; Mellinger et al., 2009). During the 2013 and 2014 humpback whale breeding seasons off Los Cabos, Mexico, visual point and line transects were conducted alongside two bottom-mounted acoustic deployments. As theorized, preliminary analysis of ambient noise between 100 and 1,000 Hz is dominated by humpback whale song. It also displays a diel cycle similar to that found in the West Indies, Australia, and Hawai’i, whereby peak levels occur near midnight and troughs occur soon after sunrise (Au et al., 2000; McCauley et al., 1996). Depending upon site and year, the median band-integrated levels fluctuated between 7 and 16 dB re 1 uPa when sampled in one hour increments. This presentation uses analytical models of wind-generated noise in an ocean waveguide to analyze potential relationships between singing whale density and diffuse ambient noise levels. It explores whether various diel cycle strengths (peak-to-peak measurements and Fourier analysis) correspond with trends observed from concurrent visual censuses. [Work sponsored by the Ocean Foundation.]

Backscattering from an obstacle immersed in an oceanic waveguide covered with ice

The presentation describes the theory and implementation issues of modeling of the backscattering from an obstacle immersed in a homogeneous, range-independent waveguide covered with ice. An obstacle is assumed to be spherical, rigid or fluid body. A bottom of the waveguide and an ice cover are fluid, attenuating half-space. The properties of an ice cover and a scatterer may coincide. To calculate the scattering coefficients of a sphere [R. M. Hackman et al., J. Acoust. Soc. Am. 84, 1813–1825 (1988)], the normal mode evaluation is applied. A number of normal modes forming the backscattering field is determined by a given directivity of the source. The obtained analytical expression for the backscattered field is applied to evaluate its dependence on source frequency, depth of a water layer, bottom and ice properties, and distance between the source and obstacle. Two cases are analyzed and compared: when the upper boundary of a waveguide is sound-soft and when a water layer is covered with ice. Computational results are obtained in a wide frequency range 8–12 kHz for conditions of a shallow water testing area. [Work supported by Russian Ministry of Educ. and Sci., Grant 02.G25.31.0058.]