Christopher Feuillade

Effects of water‐depth mismatch on matched‐field localization in shallow water

This article discusses the impact of incorrect estimates of the water column depth on matched‐field source localization in a shallow water environment. Computer calculations were performed for the case of a nominal 100‐m depth water column subject to water‐depth variations of up to ±3.5 m, which would be caused by long‐period ocean swell or by tidal changes. The environment was assumed to be range independent (by proper choice of the geometry); thus the question of rough surface scattering was not an issue. The calculations incorporated source depths of 25, 50, and 75 m, a propagation distance of 4 km, an acoustic frequency of 150 Hz, and a linear vertical receiving array. The array consisted of 21 hydrophones with an interelement spacing of 2.5 m, and it spanned the center one‐half of the water column (25‐ to 75‐m depth). The matched‐field algorithm utilized in this study is the high‐resolution maximum‐likelihood estimator. A primary result of the work is that, as the output of the matched‐field processo...

A low‐frequency acoustic scattering model for small schools of fish

A new low‐frequency scattering model for small to moderately sized fish schools has been developed. The model, which uses a mathematical formalism based upon the harmonic solution of sets of coupled differential equations, allows a verified swimbladder scattering ‘‘kernel’’ for the individual fish to be incorporated. It includes all orders of multiple scattering interactions between fish, and calculates the aggregate scattering field by coherent summation. Application to simulated ensembles of closely spaced fish indicates significant deviations from incoherent scattering returns. Peak target strength reductions, and shifts in the resonance frequency, appear due to multiple scattering. The target strength also varies strongly with frequency as a result of interference effects. When applied to widely dispersed ensembles, the model reproduces the results of incoherent scattering. For larger ensembles, at greater depth, the model predicts sharply reduced target strength values around the main resonance. The ...

Environmental mismatch in shallow‐water matched‐field processing: Geoacoustic parameter variability

The effects of variations in geoacoustic environmental parameters on the performance of a matched‐field localization processor in shallow water were investigated. The SUPERSNAP propagation model was used to generate a reference acoustic pressure field and to simulate an ‘‘experimentally detected’’ field due to an acoustic ‘‘source.’’ These were then correlated using a maximum‐likelihood estimator for selected degrees of mismatch of environmental parameters. It was found that small perturbations in a downward‐refracting summer water sound‐speed profile of ±1σ (i.e., ±1 standard deviation) from average measured values caused severe degradation in localization performance, with predictions of source range and depth becoming highly unstable. However, similar perturbations of an almost isospeed winter profile caused comparatively little degradation. Similarly, perturbations in the sediment sound‐speed profile of up to ±1σ from average measured values were possible while still giving stable and reliable estimat...

Acoustically coupled gas bubbles in fluids: Time-domain phenomena

In previous work [C. Feuillade, J. Acoust. Soc. Am. 98, 1178-1190 (1995)] a coupled oscillator formalism was introduced for describing collective resonances, scattering, and superresonances, of multiple gas bubbles in a fluid. Subsequently, time-domain investigations of the impulse response of coupled systems have disclosed the exact conditions which determine whether the ensemble scattering behavior should be described using: either (a), a multiple scattering; or (b), a self-consistent methodology. The determining factor is the Q of the individual scatterers, and their typical spatial separations in the medium. For highly damped or sparse systems, e.g., scattering from loose schools of swimbladder fish, or from a gassy seabed containing entrained bubbles, the multiple scatter counting approach should be applicable. For more strongly coupled systems, e.g., a dense cloud of resonating bubbles in the water column, energy exchange may be due primarily to radiative cycling rather than scattering, in which case a self-consistent approach is indicated. The result has implications for both volume and bottom scattering applications.

Shallow‐water matched‐field localization off Panama City, Florida

The results of a shallow‐water localization experiment, performed 19 miles south of Panama City, Florida in October 1985, are presented. The experiment involved a 450‐Hz source placed 2.2 km from a vertical array of 16 hydrophones in ∼33 m of water. The experimental site was essentially range independent with a flat, hard, sandy bottom. Successful passive localization of the source was obtained using a maximum‐likelihood matched‐field processor. Studies were undertaken to determine the robustness of the localization to variation of the following parameters: water depth, sediment sound‐speed profile, sediment density and attenuation, and array tilt. It was found that, in order to ensure localization accuracy and robustness, the environmental parameters important to know well are the water depth, sediment sound speed, and array tilt. However, the matched‐field processor is much more tolerant to inaccuracies in estimates of the sediment density and attenuation. This corresponds clearly with the results of tw...

Sound scattering by an air bubble near a plane sea surface

This paper presents a study of low-frequency sound scattering by an air bubble near a flat pressure-release surface. Using a self-consistent approach to describe multiple-scattering interactions, and a monopole approximation for the individual scatterer, the complete scattering amplitude for a bubble near a surface is derived. Radiation, thermal, and viscous damping effects are incorporated. The method leads to simple theoretical expressions which show that the presence of the surface modifies the resonance frequency and damping of the bubble, and modulates the scattering amplitude. The analytic formula for the modified resonance frequency compares favorably with earlier theoretical results, and with experimental data reported by M. Strasberg [J. Acoust. Soc. Am. 25, 536–537 (1953)]. At off-resonance frequencies, when the bubble is close to the surface and thermal and viscous dampings are not included, the results obtained agree with those of the modal series solution presented by Gaunaurd and Huang [IEEE...

A stable data‐adaptive method for matched‐field array processing in acoustic waveguides

The presence of a “modal noise” component leads to estimator instability when Capons maximum likelihood (ML) method is applied to the processing of data from a vertical array in an acoustic waveguide. The physics of the waveguide forces signal vectors and noise vectors alike to be projected onto the span of the “mode” vectors, when the number of sensors (N) exceeds the number of propagating modes (M). The instability occurs whenever the (single snapshot) N × 1 data vectors have the form x = Us + Uγ + white noise, where the matrix U is N × M (sampling the normal modes at the hydrophone locations and independent of the actual acoustic disturbances present), and s and γ correspond to signal and ambient noise sources, respectively. This condition arises in normal‐mode and local normal‐mode propagation. The dominant eigenvectors of R−1 (where R is the cross‐spectral matrix) are sensitive to slight inaccuracies in the calculation of R−1 in ways that affect the performance of the ML estimator. Following transfo...

Sector‐focused stability methods for robust source localization in matched‐field processing

Capon’s maximum likelihood method (MLM) is being used increasingly for improved localization and high resolution of sources in matched‐field processing (MFP) in a waveguide environment. When the noise component is dominated by modal noise (that is, noise due to excitation of the modal structure of the waveguide by distant sources of acoustic energy, e.g., wave action, distant shipping), the MLM can become unstable; when the number of modes (M) supported by the waveguide is considerably less than the number of sensors (N), a ‘‘reduced’’ MLM is available to stabilize the processing [Byrne et al., J. Acoust. Soc. Am. 87, 2493–2502 (1990)]. In this paper the sector‐focused stability (SFS) methods of Byrne and Steele (Proc. IEEE., ICASSP 1987) are employed to treat the case of M nearly equal to N. Simulations, using a shallow‐water range‐independent environment, are presented to illustrate the increase in stability of SFS over MLM in the presence of phase errors on the sensors or more general mismatch.

A nonlinear matched‐field processor for detection and localization of a quiet source in a noisy shallow‐water environment

Matched‐field processing is a technique that has been developed for the detection and localization of an acoustic source in an underwater environment. Conventional matched‐field processing utilizes cross correlations between the complex acoustic pressures measured from the elements of a submerged array of hydrophones and theoretically predicted complex acoustic pressures obtained from an appropriate model of the environment with an assumed source location. Modal matched‐field processing has been recently developed for the detection and localization of an acoustic source in a shallow‐water waveguide environment, where modal propagation of acoustic energy dominates. Modal matched‐field processing utilizes the cross correlations between predicted complex modal amplitudes and complex modal amplitudes calculated from the measured complex pressures. The ambient noise field in a shallow‐water environment has a correlated component that propagates to the hydrophone array through the discrete, trapped modes of the...

Matched‐field processing enhancement in a shallow‐water environment by incoherent broadband averaging

Incoherent broadband averaging is a method that uses the broadband nature of an acoustic source to acquire additional gain against ambient noise and to decrease background variance common to matched‐field ambiguity surfaces. In this paper, the relationship between the performance of incoherent broadband averaging and averaging parameters (i.e., bandwidth and the number of ambiguity surfaces averaged) is explored, and general guidelines for choosing these parameters are described. Expressions relating environmental parameters to the width and spacing of the sidelobes are obtained. These expressions predict the minimum bandwidth needed for incoherent broadband averaging for the case where one sidelobe dominates. Computer simulations in a Pekeris waveguide using a simulated but realistic noise field are used to verify the theoretical results. The simulations confirm the existence of the minimum bandwidth below which no effective averaging occurs. If the bandwidth equals or exceeds the minimum bandwidth, meas...