John S. Perkins

An approximation to the three‐dimensional parabolic‐equation method for acoustic propagation

We have reported previously [Ralph N. Baer, J. Acoust. Soc. Am. 69, 70–75 (1981)] on an algorithm, based on the parabolic approximation to the reduced wave equation, for the propagation of sound in three dimensions in the ocean. We present here a simpler algorithm: solve N two‐dimensional problems and combine the results to form an approximate three‐dimensional solution. Analytic and numerical results show that this N×2D approach is an excellent approximation to the original algorithm for realistic ocean environments, even those where fronts and eddies are present, provided redirection of energy in azimuth due to boundary interaction is not important. We compare the two computer models based on these algorithms for several test cases by considering how they distribute energy spatially, and by simulating the performance of a hypothetical horizontal array of hydrophones placed in the calculated complex‐valued acoustic fields. In an example of propagation through a large and strong ocean eddy, the N×2D metho...

Optimal time‐domain beamforming with simulated annealing including application of a priori information

A fast simulated annealing algorithm is developed for an optimal time‐domain beamformer. The optimal beamformer has greater resolution than the standard frequency‐domain beamformers, which discard information by averaging the data to form a correlation matrix and remove degrees of freedom by collapsing the number of unknown parameters. The optimal ambiguity function uses the data in raw form and depends on all of the unknown source parameters. This approach is practical with simulated annealing. A specialized simulated annealing algorithm is required to search for the source bearings and time series because the parameters are analogous to a mixture of substances with different freezing points. Examples are presented to demonstrate that the optimal beamformer can be enhanced significantly with a priori information and to illustrate the effects of source level and bandwidth, noise level, array size, and number of sources.

The matched-phase coherent multi-frequency matched-field processor

Coherent multi-frequency matched-field processing is investigated using a matched-phase coherent matched-field processor. Its main difference from previous coherent processors is that the relative phases of the Fourier components contained within the recorded signal are not assumed to be known a priori. Rather they are considered free parameters that can be determined using a global functional minimization algorithm. Additionally, this processor uses only the cross-frequency terms, making it less susceptible to the detrimental effects of ambient noise; in one example, this processor shows a five decibel improvement over a similar coherent processor. Along with its increased sensitivity with respect to the broadcast source levels, this coherent processor exhibits superior range resolution as compared with multi-frequency incoherent processors, due to the cross-frequency interference of the vertical eigenmodes. Within this work we explore the efficacy of the algorithms used to determine the relative phases along with the performance of the matched-phase coherent processor itself, performed within the context of data collected during an event from the SWellEx-96 experiment. Performance comparisons between this processor, an incoherent processor, and another coherent processor are demonstrated using this data set.

The Kirchhoff approximation and first‐order perturbation theory for rough surface scattering

The conventional Kirchhoff approximation for rough surface scattering does not reduce to perturbation results in the limit of small surface heights. It is shown here how to modify the conventional Kirchhoff approximation by using appropriate half‐space Green’s functions. The resulting expressions for the differential cross section for scattering from rough surfaces reduce to the conventional Kirchhoff results and the perturbation results in the appropriate limits.

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.

Overview of the reverberation modeling workshops

Two workshops on reverberation modeling are being conducted under joint sponsorship from the Program Executive Office C4I, PMW 180 (as funded by the Office of the Oceanographer of the Navy) and the Office of Naval Research. The overall goal of these workshops is to evaluate recent progress made in reverberation‐related research efforts and to make recommendations for further development. The first workshop was held in November 2006 and the second is scheduled for March 2008. The focus of the first workshop was on reverberation from the environment, while the second workshop will emphasize system characteristics. At the first workshop, fifteen different reverberation models were represented and extensive comparisons were carried out before, during, and after the workshop. We present the approach used to conduct the first workshop, discuss issues that have been identified, and outline tentative plans for the second workshop. [Work supported by ONR and PMW 180.]

Environmental signal processing: Three‐dimensional matched‐field processing with a vertical array

Matched‐field processing in a three‐dimensional (range and azimuthal dependent) environment provides the possibility of localizing a source in bearing, as well as in range and depth, with a purely vertical array since the environment itself breaks the azimuthal symmetry of the vertical array. This added dimension in aperture is a result of including environmental complexity in the matched‐field process. The processing is referred to here as environmental signal processing (ESP) and is demonstrated by simulation. A Gulf Stream environment where sound speed and bathymetry vary in three dimensions is selected for this simulation.

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.

Rayleigh method for scattering from random and deterministic interfaces

This paper describes Rayleigh methods for computing plane‐wave scattering amplitudes for rough interface scattering. For Dirichlet, fluid–fluid, and fluid–solid interfaces, the Rayleigh–Fourier (RF) method is shown to give good results for interfaces with slopes exceeding the limit of the Rayleigh hypothesis. In the case of sinusoidal Dirichlet surfaces, described by z=h cos(2πx/L), the method breaks down for 2πh/L∼2. For gentler surfaces the RF method gives high‐quality results more cheaply than boundary integral equation or least‐squares methods. In the case of fluid–solid interfaces, results of the RF method, which are apparently presented here for the first time, compare favorably with published results obtained by extinction theorem methods. It is concluded that the RF method is a suitable candidate for studying rough interface scattering by Monte Carlo simulations, even for fluid–solid interfaces.

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 ...