Laurie T. Fialkowski

Modeling ambient noise in three‐dimensional ocean environments

A model is developed for the calculation of the spatial properties of the surface‐generated noise in a three‐dimensional ocean. This is an extension of the work of Kuperman and Ingenito [J. Acoust. Soc. Am. 67, 1988–1996 (1980)], which used a normal‐mode representation of the noise field in a stratified ocean. Noise fields are simulated for both point receivers and vertical line receivers. These examples show how the spatial and directional characteristics of the noise field are affected by the ocean environment. For example, as is apparent in ambient noise data, surface noise propagating at high angles over a sloping ocean bottom is deflected into shallower angles. Also, matched‐field processing simulations in three‐dimensional ocean environments can be done in a consistent manner: signals and surface‐generated noise are modeled by propagating through the same environment with the same theory.

Matched-field processing using measured replica fields

An approach for avoiding the problem of environmental uncertainty is tested using data from the TESPEX experiments. Acoustic data basing is an alternative to the difficult task of characterizing the environment by performing direct measurements and solving inverse problems. A source is towed throughout the region of interest to obtain a database of the acoustic field on an array of receivers. With this approach, there is no need to determine environmental parameters or solve the wave equation. Replica fields from an acoustic database are used to perform environmental source tracking [J. Acoust. Soc. Am. 94, 3335-3341 (1993)], which exploits environmental complexity and source motion.

The bi-azimuthal scattering distribution of an abyssal hill

High-resolution bistatic images of a typical abyssal hill on the western flank of the Mid-Atlantic Ridge are made with a low-frequency towed-array system operating remotely at 1/2 convergence zone (∼33.3 km) stand-off. Comparison with modeled images, generated from high-resolution supporting bathymetry sampled at 5-m intervals, roughly the wavelength scale, reveals that steep scarps return the strongest echoes because they project the largest area along the acoustic path from the source to receiver. Prominent returns deterministically image scarp morphology when the cross-range axis of the system’s resolution footprint runs along the scarp axis. Statistical fluctuations inherent in the scattered field prevent the system from distinguishing smaller-scale anomalies on the scarps, such as canyons and gullies (∼100–200 m scale), that would otherwise be resolvable in range, in certain bistatic geometries. The mean bi-azimuthal scattering distributions of the two major scarps on the abyssal hill are identical a...

Source localization in noisy and uncertain ocean environments

Interference from noise and uncertainties in the environmental parameters are arguably the two most serious limitations in matched-field processing (MFP). Among the techniques that have been developed for handling these difficulties are the noise-canceling processor [M. D. Collins, N. C. Makris, and L. T. Fialkowski, “Noise cancellation and source localization,” J. Acoust. Soc. Am. 96, 1773–1776 (1994)] and focalization [M. D. Collins and W. A. Kuperman, “Focalization: Environmental focusing and source localization,” J. Acoust. Soc. Am. 90, 1410–1422 (1991)]. The noise-canceling processor is a generalization of the Bartlett processor that is based on matching the covariance matrix of the data with replica covariance matrices of the signal and the noise. Simulations are presented to illustrate the performance of the noise-canceling processor when there are errors in the noise replica. Focalization is a generalization of MFP in which environmental parameters are included along with source parameters in the ...

Matched-field source tracking by ambiguity surface averaging

The method of matched-field source tracking by ambiguity surface averaging is demonstrated using low-frequency shallow-water acoustic data recorded on a vertical array. In this method, source motion is taken into account by averaging values from individual, short time-average matched-field ambiguity functions; each resulting average value corresponds to a trial source track. The result is a new ambiguity function which depends on six parameters: the three-dimensional coordinates of the initial and final source positions. A simplification to four parameters is made by assuming the source remains at a constant depth. Only mild restrictions are made on trial tracks considered, and the resulting ambiguity surfaces can be plotted as a function of final coordinates by holding the initial coordinates fixed at the optimal position. When applied to experimental data, the number of high sidelobes present in individual matched-field ambiguity surfaces is greatly reduced. Also, the technique appears robust to uncerta...

Environmental source tracking

Several matched‐field processing (MFP) techniques have been developed for localizing a fixed acoustic source. Environmental source tracking (EST) is an approach for determining the track of a moving source. In addition to being attractive from a practical point of view, EST can determine source track in situations in which MFP cannot determine source position. The performance of MFP is limited by the number of propagating modes at the array location. EST can overcome this limitation by exploiting source motion and environmental complexity. For example, EST may be possible when the receiver is placed in a region of the ocean that supports only one propagating mode, a situation in which single‐frequency MFP is impossible. MFP requires the use of either an array of receivers or multiple‐frequency data. EST requires only a single frequency and a single receiver. Problems such as array deployment and array tilt, which are important with MFP, are therefore irrelevant with EST.

Noise cancellation and source localization

A noise‐canceling processor is developed for matched‐field processing problems involving a signal buried in noise. This processor is based on modeling both signal and noise and searching the space of unknown parameters to achieve the best agreement between covariances. The noise‐canceling processor reduces to the Bartlett processor in the limit of high signal‐to‐noise ratio. The examples illustrate the localization of a source obscured by interference from ambient noise or a second source. The noise‐canceling processor is also applied to localize a silent object using scattered ambient noise.

Environmental inversion and matched-field tracking with a surface ship and an L-shaped receiver array

Acoustic data from the natural broadband signature of a quiet surface ship, recorded on the vertical leg of an L-shaped array, is used to invert for the local geo-acoustic parameters and the resulting effective environment is used for subsequent tracking of the surface ship using a matched-field tracking technique applied to the full array. The matched-field analysis includes a comparison of the incoherent product of the processed data from the horizontal and vertical subapertures with coherent processing of the data from the full L-shaped array. Subaperture processing is of interest since there is a (loose) requirement that the number of data snapshots be greater than or equal to the number of array elements. This presents averaging difficulties for large arrays when the source being observed is moving. Analyzing each array leg separately allows the use of a smaller number of snapshots from which averaged quantities are constructed. Taken separately, the vertical leg of the array provides range-depth inf...

Consistency and reliability of geoacoustic inversions with a horizontal line array

Geoacoustic inversions with a towed horizontal array are of interest for rapidly characterizing sediment properties over changing regions. To be of practical value, inversions must yield consistent and reliable results for consecutive or neighboring data. A method of determining inversion reliability a priori is delineated using an empirical approach and confirmed with inversion results in terms of consistency. Geoacoustic parameter hierarchy and resolvability are empirically analyzed using two different methods: one requires knowledge of the source function and the other does not. Inversion results using the two methods are compared using both synthetic data and experimental data from MAPEX2000. The inversions employ a global optimization technique which navigates the parameter space in directions aligned with valleys of the cost function, increasing inversion algorithm efficiency and disclosing parameter correlations and hierarchy.

Measured depth-dependence of waveguide invariant in shallow water with a summer profile

Acoustic-intensity striation patterns were measured in the time-frequency domain using an L-shaped array and two simultaneously towed broadband (350-650 Hz) sources at depths above and below the thermocline under summer profile conditions. Distributions of the waveguide invariant parameter β, extracted from the acoustic striation patterns, peak at different values when receivers are above or below the thermocline for a source that is below the thermocline. However, the distributions show similar characteristics when the source is above the thermocline. Experimental results are verified by a numerical analysis of phase slowness, group slowness, and relative amplitudes of acoustic modes.