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Mar 2008

Volume 123, Issue 3, pp. 1211-EL38

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Range-dependent waveguide scattering model calibrated for bottom reverberation in a continental shelf environment

Ameya Galinde, Ninos Donabed, Mark Andrews, Sunwoong Lee, Nicholas C. Makris, and Purnima Ratilal

J. Acoust. Soc. Am. Volume 123, Issue 3, pp. 1270-1281 (2008); (12 pages) | Cited 3 times

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An analytic model is developed for scattering from random inhomogeneities in range-dependent ocean waveguides using the Rayleigh–Born approximation to Green’s theorem. The expected scattered intensity depends on statistical moments of fractional changes in compressibility and density, which scatter as monopoles and dipoles, respectively, and the coherence volume of the inhomogeneities. The model is calibrated for ocean bottom scattering using data acquired by instantaneous wide-area ocean acoustic waveguide remote sensing (OAWRS) and geophysical surveys of the ONR Geoclutter Program. The scattering strength of the seafloor on the New Jersey shelf, a typical continental shelf environment, is found to depend on wave number k, medium coherence volume Vc, and seabed depth penetration factor Fp following a 10 log10(FpVck4) dependence. A computationally efficient numerical approach is developed to rapidly compute bottom reverberation over wide areas using the parabolic equation by exploiting correlation between monopole and dipole scattering terms and introducing seafloor depth penetration factors. An approach is also developed for distinguishing moving clutter from statistically stationary background reverberation by tracking temporal and spatial fluctuations in OAWRS intensity images.
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43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.30.Ft Volume scattering

Bottom profiling by correlating beam-steered noise sequences

Chris H. Harrison and Martin Siderius

J. Acoust. Soc. Am. Volume 123, Issue 3, pp. 1282-1296 (2008); (15 pages) | Cited 7 times

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It has already been established that by cross-correlating ambient noise time series received on the upward and downward steered beams of a drifting vertical array one can obtain a subbottom layer profile. Strictly, the time differential of the cross correlation is the impulse response of the seabed. Here it is shown theoretically and by simulation that completely uncorrelated surface noise results in a layer profile with predictable amplitudes proportional to those of an equivalent echo sounder at the same depth as the array. The phenomenon is simulated by representing the sound sources as multiple random time sequences emitted from random locations in a horizontal plane above a vertical array and then accounting for the travel times of the direct and bottom reflected paths. A well-defined correlation spike is seen at the depth corresponding to the bottom reflection despite the fact that the sound sources contain no structure whatsoever. The effects of using simultaneously steered upward and downward conical beams with a tilted or faceted seabed and multiple layers are also investigated by simulation. Experimental profiles are obtained using two different vertical arrays in smooth and rough bottom sites in the Mediterranean. Correlation peak amplitudes follow the theory and simulations closely.
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43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
43.30.Nb Noise in water; generation mechanisms and characteristics of the field
43.30.Re Signal coherence or fluctuation due to sound propagation/scattering in the ocean

Passive fathometer processing

Peter Gerstoft, William S. Hodgkiss, Martin Siderius, Chen-Fen Huang, and Chris H. Harrison

J. Acoust. Soc. Am. Volume 123, Issue 3, pp. 1297-1305 (2008); (9 pages) | Cited 8 times

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Ocean acoustic noise can be processed efficiently to extract Green’s function information between two receivers. By using noise array-processing techniques, it has been demonstrated that a passive array can be used as a fathometer [ Siderius, et al., J. Acoust. Soc. Am. 120, 1315–1323 (2006) ]. Here, this approach is derived in both frequency and time domains and the output corresponds to the reflection sequence. From this reflection sequence, it is possible to extract seabed layering. In the ocean waveguide, most of the energy is horizontally propagating, whereas the bottom information is contained in the vertically propagating noise. Extracting the seabed information requires a dense array, since the resolution of the bottom layer is about half the array spacing. If velocity sensors are used instead of pressure sensors, the array spacing requirement can be relaxed and simulations show that just one vertical velocity sensor is sufficient.
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43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.60.Pt Signal processing techniques for acoustic inverse problems

Joint time/frequency-domain inversion of reflection data for seabed geoacoustic profiles and uncertainties

Jan Dettmer, Stan E. Dosso, and Charles W. Holland

J. Acoust. Soc. Am. Volume 123, Issue 3, pp. 1306-1317 (2008); (12 pages) | Cited 5 times

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This paper develops a joint time/frequency-domain inversion for high-resolution single-bounce reflection data, with the potential to resolve fine-scale profiles of sediment velocity, density, and attenuation over small seafloor footprints ( ∼ 100 m). The approach utilizes sequential Bayesian inversion of time- and frequency-domain reflection data, employing ray-tracing inversion for reflection travel times and a layer-packet stripping method for spherical-wave reflection-coefficient inversion. Posterior credibility intervals from the travel-time inversion are passed on as prior information to the reflection-coefficient inversion. Within the reflection-coefficient inversion, parameter information is passed from one layer packet inversion to the next in terms of marginal probability distributions rotated into principal components, providing an efficient approach to (partially) account for multi-dimensional parameter correlations with one-dimensional, numerical distributions. Quantitative geoacoustic parameter uncertainties are provided by a nonlinear Gibbs sampling approach employing full data error covariance estimation (including nonstationary effects) and accounting for possible biases in travel-time picks. Posterior examination of data residuals shows the importance of including data covariance estimates in the inversion. The joint inversion is applied to data collected on the Malta Plateau during the SCARAB98 experiment.
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43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.60.Pt Signal processing techniques for acoustic inverse problems

Controlled and in situ target strengths of the jumbo squid Dosidicus gigas and identification of potential acoustic scattering sources

Kelly J. Benoit-Bird, William F. Gilly, Whitlow W. L. Au, and Bruce Mate

J. Acoust. Soc. Am. Volume 123, Issue 3, pp. 1318-1328 (2008); (11 pages) | Cited 1 time

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This study presents the first target strength measurements of Dosidicus gigas, a large squid that is a key predator, a significant prey, and the target of an important fishery. Target strength of live, tethered squid was related to mantle length with values standardized to the length squared of −62.0, −67.4, −67.9, and −67.6 dB at 38, 70, 120, and 200 kHz, respectively. There were relatively small differences in target strength between dorsal and anterior aspects and none between live and freshly dead squid. Potential scattering mechanisms in squid have been long debated. Here, the reproductive organs had little effect on squid target strength. These data support the hypothesis that the pen may be an important source of squid acoustic scattering. The beak, eyes, and arms, probably via the sucker rings, also play a role in acoustic scattering though their effects were small and frequency specific. An unexpected source of scattering was the cranium of the squid which provided a target strength nearly as high as that of the entire squid though the mechanism remains unclear. Our in situ measurements of the target strength of free-swimming squid support the use of the values presented here in D. gigas assessment studies.
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43.30.Sf Acoustical detection of marine life; passive and active
43.30.Ft Volume scattering
43.20.Fn Scattering of acoustic waves

Demonstration of the invariance principle for active sonar

Jorge E. Quijano, Lisa M. Zurk, and Daniel Rouseff

J. Acoust. Soc. Am. Volume 123, Issue 3, pp. 1329-1337 (2008); (9 pages) | Cited 5 times

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Active sonar systems can provide good target detection potential but are limited in shallow water environments by the high level of reverberation produced by the interaction between the acoustic signal and the ocean bottom. The nature of the reverberation is highly variable and depends critically on the ocean and seabed properties, which are typically poorly known. This has motivated interest in techniques that are invariant to the environment. In passive sonar, a scalar parameter termed the waveguide invariant, has been introduced to describe the slope of striations observed in lofargrams. In this work, an invariant for active sonar is introduced. This active invariant is shown to be present in the time–frequency structure observed in sonar data from the Malta Plateau, and the structure agrees with results produced from normal mode simulations. The application of this feature in active tracking algorithms is discussed.
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43.30.Vh Active sonar systems
43.30.Bp Normal mode propagation of sound in water
43.30.Zk Experimental modeling
43.60.Ac Theory of acoustic signal processing

Waveguide invariant focusing for broadband beamforming in an oceanic waveguide

Hailiang Tao and Jeffrey L. Krolik

J. Acoust. Soc. Am. Volume 123, Issue 3, pp. 1338-1346 (2008); (9 pages) | Cited 4 times

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The performance of broadband sonar array processing can degrade significantly in shallow-water environments when interference becomes angularly spread due to multipath propagation. Particularly for towed line arrays near endfire, elevation angle spreading of multipath interference often results in masking of weaker sources of interest. While adaptive beamforming in a series of narrow frequency bands can suppress coherent multipath interference, this approach requires long observation times to estimate the required narrowband covariance matrices. To form wideband covariance matrices which can be estimated with less observation time, plane-wave focusing methods have been used to avoid interference covariance matrix rank inflation. This paper extends wideband focusing to the case of coherent multipath interference. The approach presented here, called waveguide invariant focusing (WIF), exploits a robust relationship for the frequency dependence of horizontal wave number differences. Unlike matched-field methods, WIF does not model multipath wave fronts but rather makes the interference appear to occupy the same rank-one subspace across frequency. This permits formation of wideband covariance matrices without interference rank inflation. Simulation experiments in a realistic ocean environment indicate that adaptive beamforming using WIF covariance matrices can provide a significant array gain improvement over conventional adaptive methods with limited observation time.
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43.30.Wi Passive sonar systems and algorithms, matched field processing in underwater acoustics
43.60.Fg Acoustic array systems and processing, beam-forming
43.60.Gk Space-time signal processing, other than matched field processing
43.30.Bp Normal mode propagation of sound in water
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