Ocean Waveguide Research Articles

Features of the Invariant of the Spatial–Frequency Interference Structure of Acoustic Fields in Oceanic Waveguides

The paper considers the regular patterns that can be manifested in the behavior of the invariant of the spatial–frequency interference structure of an acoustic field in oceanic waveguides that are homogeneous and inhomogeneous along a track. Using the WKB and adiabatic approximations, an analytic expression is obtained for the invariant that, when certain conditions are fulfilled, reduces to the well-known classical expression independent of the mode numbers, their parity, or the emission frequency. It has been established that the approximate classical expression for the invariant correctly describes the slope of interference lines only in ranges of variation in the grazing angles of modes where the dependence of the cycle length of their corresponding Brillouin waves on the ray parameter is quite smooth and monotonic. The paper studies the formation of the spatial–frequency interference structure of an acoustic field propagating from a shallowwater isovelocity waveguide to a relatively deep-water waveguide with a near-surface sound channel.

Spatial Divergence of Multimode Acoustic Beams Formed by a Vertical Array in a Refracting Oceanic Waveguide

We consider the patterns manifested during the formation of the spatial intensity distribution of an acoustic field excited by a vertical array in a deep-water oceanic waveguide with an underwater sound channel open to the surface and an acoustically transparent bottom. The study focuses on the dependence of the spatial divergence of multimode beams on the angle of compensation of an array emitting a tone signal. It is established that if the size of the array aperture significantly exceeds the optimal size, then at a certain angle of compensation appreciably differing from zero, a multimode beam forms with minimal diffraction divergence even in the absence of bending points and smooth extrema of the dependence of the length of the ray cycle on the ray parameter.

A three-dimensional sono-elastic method of ships in finite depth water with experimental validation

In order to conduct the comprehensive analysis of fluid-structure interactions, acoustic radiation and acoustic propagation, a three-dimensional sono-elastic method in the frequency domain is proposed in this paper. In this method, the three-dimensional hydroelasticity theory of floating structures is incorporated with the hydro-acoustic propagation theory by introducing frequency domain Green's functions appropriate to the ocean hydro-acoustic waveguide environment. The simple source boundary integral approach is adopted to obtain the sono-elastic response in the frequency domain. In order to deal with the irregular frequency problem in this approach, a Closed Virtual Impedance Surface (CVIS) method is put forward. At last, the sono-elastic method is validated by calculating the acoustic radiation of a floating elastic spherical shell and an experiment of the underwater acoustic radiation of two ring-stiffened cylindrical shell models.

The effect of the small-scale variability of sound velocity on the zonal structure of acoustic fields in oceanic waveguides

The effect of the small-scale variability of sound velocity on the zonal structure of acoustic fields in oceanic waveguides

Stochastic matched-field localization of an acoustic source based on principles of Riemannian geometry.

Passive localization of acoustic sources is treated within a geometric framework where non-Euclidean distance measures are computed between a cross-spectral density estimate of received data on a vertical array and a set of stochastic replica steering matrices, rather than traditional replica steering vectors. A processing scheme involving matrix-matrix comparisons where steering matrices, as functions of the replica source coordinates, naturally incorporate environmental variability or uncertainty provides a general framework for considering the acoustic inverse source problem in an ocean waveguide. Within this context a subset of matched-field processors is examined, based on recent advances in the application of non-Euclidean geometry to statistical classification of data feature clusters. The matrices are interpreted abstractly as points in a Riemannian manifold, and an appropriately defined distance measure between pairs of matrices on this manifold defines a matched-field processor for estimating source location. Acoustic simulations are performed for a waveguide comprising both a depth-dependent sound-speed profile perturbed by linear internal gravity waves and a depth-correlated surface noise field, providing an example of the viability of this approach to passive source localization in the presence of sound-speed variability.

Geometrical-Acoustics Approximation in the Study of the Low-Frequency Pulse Propagation in a Near-Bottom Oceanic Waveguide

We present the results of experimental studies of propagation of the low-frequency pulsed signals in the deep-water region of the Indian Ocean under conditions of a waveguide open from the bottom side when the signal from a near-surface source, which is received by the near-bottom receiver, is a sequence of reflections from the bottom and the ocean surface. The spatiotemporal diagram showing the amplitudes of the received pulses as functions of the distance and the relative time of their arrival is used for the data processing. It is shown that using a simple model of the stratified waveguide with a constant bottom and geometrical-optics approximation for calculating the tone-signal field, one can achieve good agreement between the experimental and theoretical results. In particular, this allows one to determine the effective waveguide depth for the frequency range 5–40 Hz and estimate the reflecting-boundary characteristics. The obtained results can also be used for predicting the parameters of the low-frequency signal propagating in the waveguides of this kind.

Resolving Power of the Interferometric Method of Source Localization

Results of a computer simulation of the resolution of three noise sources with different intensities against the background of isotropic noise in an oceanic waveguide are presented. A comparative analysis of the error in determining their coordinates (bearing, radial velocity, distance, depth) is performed. A computational experiment is implemented based on the interferometric method of source localization using vector-scalar receivers.

Underwater acoustic source localization using generalized regression neural network.

In this paper, machine learning is introduced to source localization in underwater ocean waveguides. Source localization is regarded as a supervised learning regression problem and is solved by generalized regression neural network (GRNN). As a feed-forward network, GRNN is built using training data with fixed structure and configuration. The normalized sample covariance matrix (SCM) formed over a number of snapshots, and the corresponding source position are used as the input and output for GRNN. The source position can be estimated directly from the normalized SCM with GRNN; the proposed approach is thus in theory data driven. In addition, there is only one parameter, the spread factor, to be learned for GRNN. The optimal spread factor is determined using cross-validation. The regression method of GRNN is compared with the classification method of feed-forward neural network (FNN), as well as the classical method of matched field processing (MFP) for vertical array data from the SWellEx-96 experiment. The results show that GRNN achieves a satisfactory localization performance that outperforms both FNN and MFP. The proposed approach provides an alternative way for underwater source localization, especially in the absence of a priori environmental information or an appropriate propagation model.

Time delay estimation via zero-crossing of envelope

Time-delay estimation of an acoustic signal in an ocean waveguide is a challenging due to the received signal corrupted by noise and phase distortion caused commonly by reflection of the transmitted signal traveling to a remote receiver via more than one path. In this study, envelope-based Hilbert transform method is applied to enhance the accuracy of time-delay estimation. Through the mathematical interpretation and numerical simulations, the proposed method is compared to the conventional time-delay estimation methods (e.g., cross correlation and zero-crossing of Hilbert transform). It is found that the envelope-based Hilbert transform method more efficiently estimates the time-delay than the conventional methods and resolve ambiguity of time delay estimate.

Study of the Coherence of Acoustic Fields of High-Frequency Noise Sources in a Randomly Inhomogeneous Ocean

A model is been developed for the coherence of high-frequency hydroacoustic fields of noise sources in a randomly inhomogeneous ocean. We analyze the dependences of the coherence of hydroacoustic fields of noise sources on the mutual position of the source and receiver system, the type of waveguide, and the parameters of random inhomogeneities of an oceanic waveguide. We discuss the possibilities of underwater acoustic observation.

Direct recovery of wave-penetrable scatterers in a stratified ocean waveguide

We shall derive a direct sampling method for the recovery of wave-penetrable inhomogeneous scatterers in a three-dimensional stratified ocean waveguide. The proposed method is simple, straightforward and computationally efficient since no matrix inversions, optimizations or solutions of ill-posed linear systems are involved. Moreover, it is highly tolerant to noise and is applicable with a few scattered fields corresponding to only two or four incidences. In addition, a priori knowledge of either the number of disconnected components or the physical features of the unknown obstacles is not required by the direct sampling method. A mathematical derivation is provided for its validation and several extensive numerical experiments are shown to evaluate the effectiveness and feasibility of the newly proposed method.

Instantaneous 3D Continental-Shelf Scale Imaging of Oceanic Fish by Multi-Spectral Resonance Sensing Reveals Group Behavior during Spawning Migration

The migration of extensive social groups towards specific spawning grounds in vast and diverse ocean environments is an integral part of the regular spawning process of many oceanic fish species. Oceanic fish in such migrations typically seek locations with environmental parameters that maximize the probability of successful spawning and egg/larval survival. The 3D spatio-temporal dynamics of these behavioral processes are largely unknown due to technical difficulties in sensing the ocean environment over wide areas. Here, we use ocean acoustic waveguide remote sensing (OAWRS) to instantaneously image immense herring groups over continental-shelf-scale areas at the Georges Bank spawning ground. Via multi-spectral OAWRS measurements, we capture a shift in swimbladder resonance peak correlated with the herring groups’ up-slope spawning migration, enabling 3D spatial behavioral dynamics to be instantaneously inferred over thousands of square kilometers. We show that herring groups maintain near-bottom vertical distributions with negative buoyancy throughout the migration. We find a spatial correlation greater than 0.9 between the average herring group depth and corresponding seafloor depth for migratory paths along the bathymetric gradient. This is consistent with herring groups maintaining near-seafloor paths to both search for optimal spawning conditions and reduce the risk of predator attacks during the migration to shallower waters where near-surface predators are more dangerous. This analysis shows that multi-spectral resonance sensing with OAWRS can be used as an effective tool to instantaneously image and continuously monitor the behavioral dynamics of swimbladder-bearing fish group behavior in three spatial dimensions over continental-shelf scales.

Interferometric Direction Finding by a Vector-Scalar Receiver

A method for interferometric direction finding of a broadband sound source in an oceanic waveguide by a single vector-scalar receiver is presented. The method is based on the double Fourier transform of the interference pattern formed during motion. The efficiencies of the proposed direction finding method and the method based on measuring the delay times of signals arriving at spaced scalar receivers are compared based on the natural experiment results. The noise immunity of the interferometric direction finding method is considered.

Performance Evaluation of Acoustic Model-Based Blind Channel Estimation in Ocean Waveguides

Blind channel estimation (BCE) which does not require the knowledge of the source signals becomes increasingly important in various underwater acoustic applications. Unlike the conventional methods that utilize the statistical properties of the received signals, this paper proposes an acoustic model-based BCE method that exploits the properties of the acoustic propagation and the acoustic environmental information. This method is implemented by localizing the source with a matched-field processing algorithm first, then, uses the physical model of the ocean waveguides to determine the channel impulse response. Its performance is investigated by making comparisons with the mode-based artificial time reversal and the ray-based synthetic time reversal methods, using the array data measured from the SWellEx-96 experiment at four source-array ranges. Cross-correlation coefficient and normalized projection misalignment are adopted to evaluate the performance of the proposed method, and the simulation results for different multipath channels confirm the effectiveness of the proposed method.

Angular Resolution Enhancement Provided by Nonuniformly-Spaced Linear Hydrophone Arrays in Ocean Acoustic Waveguide Remote Sensing

Uniformly-spaced apertures or subapertures of large, densely-sampled, discrete linear receiver arrays are often used in remote sensing to increase the signal-to-noise ratio (SNR) by coherent beamforming that reduces noise coming from directions outside the signal beam. To avoid spatial aliasing or the presence of grating lobes in real spatial directions, the uniformly-spaced array inter-element spacing d sets a limit on the maximum frequency f max < c / 2 d of signals suitable for beamforming with the array, where c is the medium’s wave propagation speed. Here, we show that a nonuniformly-spaced array, for instance, formed by combining multiple uniformly-spaced subapertures of a nested linear array, can significantly enhance the array angular resolution while simultaneously avoiding dominant grating lobes in real angular space, even for signals with frequencies beyond the maximum that the array is designed for. The array gain, beam width, and maximum grating lobe height are quantified for the Office of Naval Research Five Octave Research Array (ONR-FORA) for various combinations of its uniformly-spaced subapertures, leading to nonuniformly-spaced subarrays. Illustrative examples show angular resolution enhancement provided by the nonuniformly-spaced ONR-FORA subarrays over that of its uniformly-spaced individual subaperture counterparts in both active and passive ocean acoustic waveguide remote sensing, drawn from measurements in the Gulf of Maine 2006 Experiment.

Vertical Correlation of the Acoustic Field in Deep Water Measured with Explosive Sources

In this paper we report the measurement and analysis on vertical correlations of acoustic signals received by a vertical line array deployed near the sea bottom in deep water. The source data were from one calibration experiment of sound exposure level of explosive charges. The source–receiver range is about 17 km. The sound transmission duct is the well-known reliable acoustic path (RAP). The vertical correlation coefficients are estimated at center frequencies of 300, 600, 1000 and 1500 Hz with 1/3 octave bands. Two types of explosive charges are used with nominal source depths of 50 and 300 m, respectively. The variation of vertical correlation coefficients strongly relies on source depth and center frequency. For each scene, ten groups of experiment data exhibit uniform variation of vertical correlation coefficients with fractional differences due to the fluctuation of ocean waveguide. Besides, the noise-free numerical modeling results reach good agreements with the measured ones, which reflect the stability of RAP as a sound transmission duct in the space/frequency domain. Due to the obvious arrival structures of rays, the theoretical solution of vertical correlation coefficient is derived based on the ray theory. At last, the effects of source–receiver geometry on the variability of vertical correlation are discussed.

Detection, localization, and classification of multiple ocean vehicles over continental-shelf regions with passive ocean acoustic waveguide remote sensing

Multiple ocean vehicles, including both surface ships and unknown submerged vehicles, can be simultaneously monitored over instantaneous continental-shelf scale regions via passive ocean acoustic waveguide remote sensing (POAWRS) by employing a large-aperture densely sampled coherent hydrophone array system. Here, the approach is demonstrated for multiple merchant ships present in the Norwegian Sea in 2014. The sounds radiated underwater by ocean vehicles, dominated by narrowband tonals and cyclostationary signals, are detected in the beamformed spectrograms. Coherent beamforming of the receiver array data significantly enhances the signal-to-noise ratio of the ship-radiated signals enabling detection of ocean vehicles roughly two orders of magnitude more distant in range than a single hydrophone. The estimated bearing-time trajectory of a sequence of detections are employed to determine the horizontal location of each vehicle using the moving array triangulation technique. The estimated locations are verified by comparison with historical database of merchant ships present in the region. Several time-frequency characteristics are extracted from the detected signals and used to classify and track the ships.

Detection in an uncertain underwater waveguide

Detection of signals in oceanic waveguides is complex because of uncertainties in the underwater channel and potentially suboptimal location of receiving phones. Accurate detection and estimation are a direct result of optimization with respect to frequency, environmental parameters, and receiver location. We here study the impact of these parameters on detection and propose methods for improvement, including detection processor calibration for optimal performance. We complement our work by investigating detection performance with real data collected at different sensors in a pool. In addition to conventional hydrophones, we also use vector sensors. The rationale behind using a vector sensor is that particle velocity components may show different noise and channel characteristics such as a different number of modes and different dispersive behavior, which are tightly related to detection. We study several detectors and evaluate their performance. [Work supported by ONR and NSF.]

Blind deconvolution of sources of opportunity in ocean waveguides using bilinear channel models

Blind deconvolution of sources of opportunity in ocean waveguides using bilinear channel models

Finite impulse response of an ocean acoustic waveguide by compressed sensing

The usual approach to calculating the time domain acoustic response of an ocean waveguide is to Fourier synthesize it from a regularly sampled collection of frequency domain solutions. Depending on the details of the waveguide and the excitation signal, the resulting time series may be very sparse with significant time intervals of relative quiet between energetic multipath arrivals. This observed sparsity in the time domain suggests a computational advantage to synthesizing the time series by solving a convex optimization problem involving a much smaller set of solutions computed at random frequencies. The efficiency of the compressed sensing approach will depend on the sparsity of the time domain solution; the greater the sparsity in the time domain signal the greater the expected advantage of the compressed sensing approach. In this talk we examine the compressed sensing method of time series synthesis for a sequence of waveguides that range from extremely simple and ideal waveguides where the sparsity is expected to be great to the more realistic and complicated where the sparsity is expected to be reduced. [This work was supported by the U.S. Office of Naval Research.]