John A. Fawcett

A boundary integral equation method for acoustic scattering in a waveguide with nonplanar surfaces

A boundary integral equation method (BIEM) is formulated to compute the scattering of underwater sound from compact deformations of an oceanic waveguide’s surfaces. The method involves only integrations over the finite area of the waveguide surface deformations. Numerical computations are given for specific deformations of the ocean floor.

A derivation of the differential equations of coupled‐mode propagation

In this paper the differential equations of the coupled‐mode theory of acoustic propagation are derived. The derivation properly accounts for the boundary conditions imposed at an interface and gives rise to terms not normally found in other coupled‐mode formulations. The solutions of this couple‐mode system agree very well in numerical experiments with those obtained from a boundary integral equation method.

On the use of higher-order azimuthal schemes in 3-D PE modeling.

In this paper, the issue of using higher-order finite difference schemes to handle the azimuthal derivative term in a three-dimensional parabolic equation based model is addressed. The three-dimensional penetrable wedge benchmark problem is chosen to illustrate the accuracy and efficiency of the proposed schemes. Both point source and modal initializations of the pressure field are considered. For each higher-order finite difference scheme used in azimuth, the convergence of the numerical solution with respect to the azimuth is investigated and the CPU times are given. Some comparisons with solutions obtained from another 3-D model [J. A. Fawcett, J. Acoust. Soc. Am. 93, 2627-2632 (1993)] are presented. The numerical simulations show that the use of a higher-order scheme in azimuth allows one to reduce the required number of points in the azimuthal direction while still obtaining accurate solutions. The higher-order schemes have approximately the same efficiency as a FFT-based approach (in fact, may outperform it slightly); however, the finite difference approach has the advantage that it may be more flexible than the FFT approach for various PE approximations.

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.

Synthetic aperture processing for a towed array and a moving source

In this paper the use of synthetic aperture processing is examined for simultaneously estimating a source’s unknown rest frequency, relative radial speed, and bearing from towed array hydrophone data. By examining this three‐dimensional estimation problem, it is concluded that for the situation where the velocity and/or rest frequency of the source are unknown there is no advantage in using coherent synthetic aperture techniques instead of incoherent methods to estimate the source’s bearing. The case of multiple sources is also examined and it is concluded that although long‐time coherent processing can improve bearing resolution for two closely spaced targets, the same results can be obtained by taking a long‐time Fourier transform and then using conventional beamforming methods.

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...

Fourier synthesis of three‐dimensional scattering in a two‐dimensional oceanic waveguide using boundary integral equation methods

A numerical method is presented for computing the three‐dimensional pressure field generated by compact harmonic sources in a two‐dimensional stratified acoustic waveguide. This method can serve to model acoustic scattering from ridgelike bathymetry, or surface features such as ice ridges, in a realistic oceanic waveguide model. The basic technique entails synthesizing the three‐dimensional solution by using Fourier transform methods based on a sequence of two‐dimensional problems. Numerical results from computations based on a simple cosine‐bell ridge model and a more complex ice model are presented, and a comparison with the analogous strictly two‐dimensional model is also considered.

Scattering from small three‐dimensional irregularities in the ocean floor

A perturbation expansion is used in this article to investigate the scattering of acoustic energy from small three‐dimensional irregularities of the ocean floor. The concept of modal conversion is stressed. The ocean floor is taken to be the interface between the ocean fluid and a fast fluid bottom. Simple formulas for the modal conversion coefficients are given in terms of the two‐dimensional Fourier transform of the irregularity height function.

Using convolutional neural networks on perceptual and spectral features: Classification experiments on the TREX’13 dataset

Distinguishing objects of interest on the seafloor from clutter remains a key problem facing the automatic target recognition (ATR) community. Because scattering at high frequencies relates more to the geometry of the scatterer, traditional ATR on high frequency imaging sonars is known to suffer in areas of high clutter. Recently, there has been interest in low frequency (1–50 kHz) wideband imaging sonars, because the complex combination of external (geometric) and internal (elastic) scattering that occurs in this frequency range is thought to improve classification in areas of high clutter. This work investigates the classification problem of distinguishing unexploded ordnance (UXO) from clutter using image-based Convolutional Neural Networks on the TREX’13 dataset. This dataset consists of experimental and modelled acoustic backscatter for objects interrogated acoustically at 3–30 kHz across a range of aspects. The model data are used exclusively for training, and the experimental data are used exclusively for testing purposes. The generated feature-set is a combination of acoustic colour and perceptual features, which are derived from modelling how humans perceive timbre. Further in an effort to address the differing amplitude modulations between sets of model and experimental data and improve classified performance, several moving window normalizations will be investigated. Distinguishing objects of interest on the seafloor from clutter remains a key problem facing the automatic target recognition (ATR) community. Because scattering at high frequencies relates more to the geometry of the scatterer, traditional ATR on high frequency imaging sonars is known to suffer in areas of high clutter. Recently, there has been interest in low frequency (1–50 kHz) wideband imaging sonars, because the complex combination of external (geometric) and internal (elastic) scattering that occurs in this frequency range is thought to improve classification in areas of high clutter. This work investigates the classification problem of distinguishing unexploded ordnance (UXO) from clutter using image-based Convolutional Neural Networks on the TREX’13 dataset. This dataset consists of experimental and modelled acoustic backscatter for objects interrogated acoustically at 3–30 kHz across a range of aspects. The model data are used exclusively for training, and the experimental data are used exclusiv...

Directional modal scattering by a ridge

The three‐dimensional scattering of modes from a ridge is considered. A simple plane‐wave interpretation is demonstrated for the conversion of an incident mode into transmitted/reflected modes by interaction with a compact ridge. There are, however, particular angles of incidence for which the transmission/reflection coefficients of modal conversion exhibit singular‐type behavior. This results in a banded structure in the horizontal two‐dimensional plots of scattered modal power. Both ‘‘small’’ and ‘‘large’’ ridges are considered and a boundary integral equation method is used to compute the modal scattering from these ridges numerically.