Peter C. Mignerey

Observations of matched-field autocorrelation time in the South China Sea

Matched-field processors can suffer degradation in dynamic environments due to mismatch between data and replica vectors. One measure of such degradation is the autocorrelation time scale of the data vectors. Matched-field autocorrelation times for 300- and 500-Hz narrow-band signals have been extracted from data acquired during the 2001 Asian Seas International Acoustics Experiment (ASIAEX). The acoustic signals were received on a vertical hydrophone array located near the South China Sea (SCS) shelf break at a range of 18.9 km from the acoustic sources. The water depth along the propagation path ranged from 100 to 125 m. For acoustic signal propagation through a diurnal internal tide that takes the form of a uniform depression of the thermocline, the matched-field correlation time at 300 Hz is approximately /spl tau//sub 1/2/=15 min or, nondimensionally, n/sub 1/2/=200 000 cycles. For acoustic signal propagation through packets of nonlinear internal waves, the correlation time for the 300-Hz signal is approximately /spl tau//sub 1/2/=2 min or n/sub 1/2/=30 000 cycles. A nonlinear internal wave packet entering the acoustic propagation path at 11:40 UTC on May 7 was observed to be unambiguously associated with a drop in the signal correlation time. A two-dimensional advective, frozen-ocean acoustic propagation model produces matched-field correlation times that are qualitatively similar to these observations. It is concluded that, in this environment, nonlinear internal wave packets propagating in the acoustic path shorten the lifetime of replica vectors and cause matched-field processor degradation.

Multichannel deconvolution of an acoustic transient in an oceanic waveguide

The temporal signature and source location of an acoustic pulse propagating in an oceanic waveguide is estimated from passively sensed multichannel data on a vertical hydrophone array. A theoretical solution for this inverse source problem is presented that incorporates both a broadband generalization of matched‐field processing to estimate the source location, and a Bayesian based deconvolution algorithm operating on multichannel array data to extract the source signature. The algorithms are implemented on a massively parallel computer architecture. The theory was tested on experimental data using a chirp signal recorded on a vertical hydrophone array located approximately one convergence zone from the source. Results indicate that deconvolving the Green’s function using multichannel data is important for stabilizing the inversion in the presence of environmental mismatch. The additional information about the source time series, obtained over slightly different propagation channels, limits the solution s...

Broadband source signature extraction using a vertical array

Source signatures have been extracted from acoustic data received on a large vertical array. Multichannel deconvolution methods were used in conjunction with broadband coherent and incoherent matched‐field processors for source signature reconstruction. The signatures were generated by broadband (25–110 Hz) sources located near the first and second convergence zones, approximately 48 and 97 km from the array. Cross‐correlation coefficients between known source signatures and deconvolved estimates indicate that faithful reconstructions of both magnitude and phase can be obtained. Use of incoherent noise components from the main diagonals of cross‐spectral density matrices produced correlation coefficients ranging from 0.77 to 0.93. Degradation primarily appeared as increased noise in the reconstructed signature. Attempts to include the full spatial coherence of the ambient noise by using off‐diagonal matrix components, resulted in correlation coefficients approximately 10% lower. This effect is caused by n...

Acoustic observations of internal tides and tidal currents in shallow water

Significant acoustic travel-time variability and frequency shifts of acoustic intensity level curves in broadband signal spectrograms were measured in the East China Sea during the summer of 2008. The broadband pulses (270-330 Hz) were transmitted from a fixed source and received at a bottomed horizontal array, located at the 33 km range. The acoustic intensity level curves of the received signals indicate regular frequency shifts that are well correlated with the measured internal tides. Similarly, regular travel-time shifts of the acoustic mode arrivals correlate well with the barotropic tides and can be explained by tidal currents along the acoustic propagation track. These observations indicate the potential of monitoring internal tides and tidal currents using low-frequency acoustic signals propagating at long ranges.

Matched-field processing gain degradation caused by tidal flow over continental shelf bathymetry

Temporally variable, range dependent sound-speed profiles measured during ebb flow and estimated for slack flow are used to quantify the variability of matched-field signal-processing gain degradation in shallow water propagation channels controlled by tidally driven stratified flow over variable bathymetry. Calculations along a 9.3 km range establish phase changes in the acoustic signal as the primary cause of a 3-9 dB degradation in the coherent matched-field processing output of a full water column vertical array. The work indicates that over a tidal cycle acoustic signal properties and matched-field processing gain can be expected to change continuously in a shallow water stratified channel that has bathymetry variability. Acoustic signals propagating in such tidal flow-controlled environments may be expected to display repeatable (over successive tidal cycles) and predictable changes in their phase coherent properties. These results suggest that matched-field processor replica fields used in the shelf/slope propagation environment will have to be updated regularly during a tidal cycle to maintain maximum processor gain.

Environmentally adaptive wedge modes

The notion of environmentally adaptive, curvilinear, uncoupled normal modes is developed by writing the Helmholtz equation in curvilinear coordinates. The depth-dependent vertical scaling function provides some coordinate freedom missing from the Cartesian formulation. This freedom permits adaptation of the normal mode approach to variable depth and variable sound-speed problems for environments that change slowly in range. A W.K.B. approximation to the eigenfunctions shows modal decoupling can be realized when the vertical scaling function is chosen equal to the reciprocal of the depth-dependent vertical wave number of the lowest mode. Numerical simulations demonstrate that a requirement for constant inter-modal ratios of the vertical wave number is valid throughout several wedges with different bottom boundary conditions. The horizontal refraction of such modes agrees closely with analytic solutions. For a wide-angle wedge, the acoustic field agrees qualitatively well with the benchmark solution of Dean and Buckingham [J. Acoust. Soc. Am. 93, 1319-1328 (1993)].

Horizontal refraction of 3‐D curvilinear wedge modes

For shallow‐water acoustic propagation, the wavelength is commensurate with the water depth but short compared to the horizontal extent of the problem. Under these conditions a sloping bottom causes the development of normal modes having wavefronts that are curved in the vertical direction. For simple slopes, such wedge modes have been shown to propagate with cylindrical wavefronts along characteristics in the horizontal plane. This work extends adiabatic wedge mode theory to regions of arbitrary bathymetry by constructing a three‐dimensional curvilinear coordinate system that follows the contours of the ocean bottom. The requirement for separation of the depth coordinate from the coupled horizontal coordinates produces a nonlinear differential equation for a potential field. The gradient of this field then gives the depth scale factors and curved shape of the wedge modes. A standard normal mode problem is then solved to obtain the curvilinear wedge modes and a ray‐trace method is used to study the horizontal motion of those modes. An overview of the model formulation and some examples will be presented.

Flow‐controlled acoustic propagation channels on continental shelves

Continental shelves have steep changes in bathymetric gradients. These gradients are caused by a wide variety of geological processes and are often associated with the shelf/slope break, canyons, and banks created during pauses in transgressive/regressive sea‐level cycles. Tidal flow over these bathymetric features causes temporal variability in the local sound‐speed field. The temporal variability is repeatable over successive semidiurnal tidal cycles. The flow ranges from subcritical to supercritical. The variability in the flow over gradients in the bathymetry can result in the formation of a stratified sound channel, perturbation of the channel by linear and noninternal waves, flow stagnation with separation, and hydraulic jumps that can have associated mixing events. Each of these flow types will cause unique sound‐speed fields that can impact the properties of an acoustic signal propagating through the region. Calculations that show the relative importance of some flow types on the temporal variabil...

Parallel simulation of broadband ambiguity surfaces

A connection machine was used to implement parallel algorithms for the solution of the normal mode equation and for matched‐field processing over a large frequency band. The all‐acoustic layered model consists of piecewise linear sound‐speed and density profiles with a free surface and an acoustic half‐space below the bottom. Individual processors are dedicated to one particular mode with the set of processors spanning the frequency band for numerous sound‐speed profiles. Ambiguity surfaces as a function of range, depth, and frequency were generated in parallel using a linear estimator. These images show considerable variation in the sidelobes as a function of frequency while the main peak remains at the source location. [Work supported by the Office of Naval Research.]

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.