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Apr 2005

Volume 117, Issue 4, pp. 1675-2625

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Phase speed and attenuation in bubbly liquids inferred from impedance measurements near the individual bubble resonance frequency

Preston S. Wilson, Ronald A. Roy, and William M. Carey

J. Acoust. Soc. Am. Volume 117, Issue 4, pp. 1895-1910 (2005); (16 pages) | Cited 2 times

Online Publication Date: 08 Apr 2005

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In the ocean, natural and artificial processes generate clouds of bubbles that scatter and attenuate sound. Measurements have shown that at the individual bubble resonance frequency, sound propagation in this medium is highly attenuated and dispersive. The existing theory to explain this behavior is deemed adequate away from resonance. However, due to excessive attenuation near resonance, little experimental data exists for a comparison with model predictions. An impedance tube was developed specifically for exploring this regime. The effective medium phase speed and attenuation were inferred from measurements of the surface impedance of a layer of bubbly liquid composed of air bubbles and distilled water, for void fractions from 6.2×10−5 to 5.4×10−4 and bubble sizes centered around 0.62 mm in radius. Improved measurement speed, accuracy, and precision is possible with the new instrument, and both instantaneous and time-averaged measurements were obtained. The phase speed and attenuation at resonance was observed to be sensitive to the bubble population statistics and agreed with an existing model [J. Acoust. Soc. Am. 85, 732–746 (1989)], within the uncertainty of the bubble population parameters. Agreement between the model and the data reported here is better than for the data that was available when the model was originally published. © 2005 Acoustical Society of America.
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43.30.Es Velocity, attenuation, refraction, and diffraction in water, Doppler effect

A phase-space Gaussian beam summation representation of rough surface scattering

Goren Gordon, Ehud Heyman, and Reuven Mazar

J. Acoust. Soc. Am. Volume 117, Issue 4, pp. 1911-1921 (2005); (11 pages) | Cited 1 time

Online Publication Date: 08 Apr 2005

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A Gaussian beam (GB) summation representation for rough surface scattering is introduced. In this scheme, the coherent and incoherent scattered fields are described by a phase-space summation of GBs that emanate from the rough surface at discrete set of points and directions. It thus involves stochastic GB2GB scattering matrices for the coherent and incoherent fields, and deterministic GB propagators. It benefits from the simplicity and accuracy of the latter, and can be used in applications involving propagation in complex scenarios comprising inhomogeneous media with rough surface boundaries. The GB2GB matrices are calculated from the statistical moments of the scattering amplitude, which are given either analytically or empirically. An analytical and numerical example for weakly rough surface is presented and discussed. Applications to the more complicated propagation scenario of doubly rough surface waveguide with multiple reflection phenomena will be presented in a follow-up publication. © 2005 Acoustical Society of America.
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43.30.Hw Rough interface scattering

Phase-space beam summation analysis of rough surface waveguide

Goren Gordon, Ehud Heyman, and Reuven Mazar

J. Acoust. Soc. Am. Volume 117, Issue 4, pp. 1922-1932 (2005); (11 pages) | Cited 1 time

Online Publication Date: 08 Apr 2005

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A Gaussian beam summation (GBS) formulation is introduced for a doubly rough boundary waveguide, wherein the coherent and incoherent scattered fields are decomposed into a discrete phase-space summation of Gaussian beams (GB) that emanate from the rough surfaces in all directions. The scheme involves deterministic GB propagators and stochastic GB-to-GB (GB2GB) scattering matrices for the coherent and incoherent fields, where each scattered beam is propagated inside the waveguide and is scattered again from the rough boundaries. The GB2GB matrices are calculated from the statistical moments of the scattering amplitude, which are given either analytically or empirically. An analytical and numerical example for a waveguide with weak boundary roughness is presented and discussed. The formulation reveals explicitly the phase-space footprint of the stochastic multiple scattering process at the rough boundaries, thus providing a cogent physical interpretation and an effective mathematical representation to the field. The formulation also accommodates the receiver’s pattern in the same phase-space format. Bistatic reverberations inside a rough surface waveguide as a function of the range and of the source and the receiver directions are thus examined as an implementation example. © 2005 Acoustical Society of America.
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43.30.Hw Rough interface scattering

Geoacoustic inversion in time domain using ship of opportunity noise recorded on a horizontal towed array

Cheolsoo Park, Woojae Seong, and Peter Gerstoft

J. Acoust. Soc. Am. Volume 117, Issue 4, pp. 1933-1941 (2005); (9 pages) | Cited 2 times

Online Publication Date: 08 Apr 2005

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A time domain geoacoustic inversion method using ship noise received on a towed horizontal array is presented. The received signal, containing ship noise as a source of opportunity, is time-reversed and then back-propagated to the vicinity of the ship. The back-propagated signal is correlated with the modeled signal which is expected to peak at the ship’s location in case of a good match for the environment. This match is utilized for the geoacoustic parameter inversion. The objective function for this optimization problem is thus defined as the normalized power focused in an area around the source position, using a matched impulse response filter. A hybrid use of global and local search algorithms, i.e., GA and Powell’s method is applied to the optimization problem. Applications of the proposed inversion method to MAPEX 2000 noise experiment conducted north of the island of Elba show promising results, and it is shown that the time domain inversion takes advantage of dominant frequencies of the source signature automatically. © 2005 Acoustical Society of America.
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43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography

Effects of environmental uncertainties on sonar detection performance prediction

Liewei Sha and Loren W. Nolte

J. Acoust. Soc. Am. Volume 117, Issue 4, pp. 1942-1953 (2005); (12 pages) | Cited 7 times

Online Publication Date: 08 Apr 2005

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The development of effective passive sonar systems depends upon the ability to accurately predict the performance of sonar detection algorithms in realistic ocean environments. Such environments are typically characterized by a high degree of uncertainty, thus limiting the usefulness of performance prediction approaches that assume a deterministic environment. Here we derive closed-form receiver operating characteristic (ROC) expressions for an optimal Bayesian detector and for several typical suboptimal detectors, based on a statistical model of environmental uncertainty. Various scenarios extended from an NRL benchmark shallow-water model were used to check the analytical ROC expressions and to illustrate the effect of environmental uncertainty on detection performance. The results showed that (1) optimal detection performance in an uncertain environment in diffuse noise depends primarily on the signal-to-noise ratio at the receivers and the rank of the signal matrix, where the rank is an effective representation of the scale of environmental uncertainty; (2) the ROC expression for the optimal Bayesian detector provides a more realistic performance upper bound than that obtained from conventional sonar equations that do not incorporate environmental uncertainty; and (3) detection performance predictions can be performed much faster than with commonly used numerical methods such as Monte Carlo performance evaluations. © 2005 Acoustical Society of America.
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43.30.Wi Passive sonar systems and algorithms, matched field processing in underwater acoustics
43.60.Gk Space-time signal processing, other than matched field processing

Bayesian sonar detection performance prediction in the presence of interference in uncertain environments

Liewei Sha and Loren W. Nolte

J. Acoust. Soc. Am. Volume 117, Issue 4, pp. 1954-1964 (2005); (11 pages) | Cited 3 times

Online Publication Date: 08 Apr 2005

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The detection performance of sonar systems can be greatly limited by the presence of interference and environmental uncertainty. The classic sonar equation does not take into account these two limiting factors and is inaccurate in predicting sonar detection performance. Here we have developed closed-form receiver operating characteristic (ROC) performance expressions for the Bayesian detector in the presence of interference in uncertain environments. Various scenarios extended from a NRL benchmark shallow-water model were used to test the analytical ROC expressions and to analyze the effects of interference and environmental uncertainty on detection performance. The results show that (1) the degradation on detection performance due to interference is greatly magnified by the presence of environmental uncertainty; (2) Bayesian sonar detection performance depends on the following fundamental parameters: the signal-to-noise ratio, the rank of the signal matrix, and the signal-to-interference coefficient; (3) the proposed analytical ROC performance predictions can be computed much faster than performance evaluations with commonly used Monte Carlo techniques. © 2005 Acoustical Society of America.
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43.30.Wi Passive sonar systems and algorithms, matched field processing in underwater acoustics
43.60.Gk Space-time signal processing, other than matched field processing

Concurrent inversion of geo- and bio-acoustic parameters from transmission loss measurements in the Yellow Sea

Orest Diachok and Stephen Wales

J. Acoust. Soc. Am. Volume 117, Issue 4, pp. 1965-1976 (2005); (12 pages)

Online Publication Date: 08 Apr 2005

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This paper describes results of a simultaneous inversion of bio-acoustic parameters of fish (anchovies) and geo-acoustic parameters of the bottom from transmission loss (TL) measurements in the Yellow Sea, which were reported by Qiu et al. [J. Sound Vib. 220, 331–342 (1999)]. This data set was selected because the bio-absorptivity at their site was extremely large, 40 dB at 1.3 kHz at 5 km, and measurements were made between multiple source and receiver depths and ranges. Measurements were made at night when anchovies are generally dispersed. Replica fields were calculated with a normal mode model, which incorporates bio-absorption layers. The inversion was based on minimizing the rms difference, Δ, between measured and calculated values of TL at all ranges and source and receiver depths, and involved a simultaneous search for bio-layer depth, bio-layer thickness, bio-alpha, geo-sound speed, and geo-alpha. The resultant small value of Δ, ±1.7 dB, confirmed that the model, which was assumed in replica field calculations, was realistic, and that inverted parameters were meaningful. In particular, the inverted depth of the bio-absorption layer, 6.9±0.3 m, was consistent with theoretical calculations of the depth, 5.8±1 m, of 10-cm-long anchovies Engraulis japonicus, the dominant species in the Yellow Sea. © 2005 Acoustical Society of America.
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43.30.Sf Acoustical detection of marine life; passive and active

Long range acoustic imaging of the continental shelf environment: The Acoustic Clutter Reconnaissance Experiment 2001

Purnima Ratilal, Yisan Lai, Deanelle T. Symonds, Lilimar A. Ruhlmann, John R. Preston, Edward K. Scheer, Michael T. Garr, Charles W. Holland, John A. Goff, and Nicholas C. Makris

J. Acoust. Soc. Am. Volume 117, Issue 4, pp. 1977-1998 (2005); (22 pages) | Cited 8 times

Online Publication Date: 08 Apr 2005

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An active sonar system is used to image wide areas of the continental shelf environment by long-range echo sounding at low frequency. The bistatic system, deployed in the STRATAFORM area south of Long Island in April–May of 2001, imaged a large number of prominent clutter events over ranges spanning tens of kilometers in near real time. Roughly 3000 waveforms were transmitted into the water column. Wide-area acoustic images of the ocean environment were generated in near real time for each transmission. Between roughly 10 to more than 100 discrete and localized scatterers were registered for each image. This amounts to a total of at least 30 000 scattering events that could be confused with those from submerged vehicles over the period of the experiment. Bathymetric relief in the STRATAFORM area is extremely benign, with slopes typically less than 0.5° according to high resolution (30 m sampled) bathymetric data. Most of the clutter occurs in regions where the bathymetry is locally level and does not coregister with seafloor features. No statistically significant difference is found in the frequency of occurrence per unit area of repeatable clutter inside versus outside of areas occupied by subsurface river channels. © 2005 Acoustical Society of America.
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43.30.Vh Active sonar systems

Time-reversal imaging for classification of submerged elastic targets via Gibbs sampling and the Relevance Vector Machine

Nilanjan Dasgupta and Lawrence Carin

J. Acoust. Soc. Am. Volume 117, Issue 4, pp. 1999-2011 (2005); (13 pages) | Cited 1 time

Online Publication Date: 08 Apr 2005

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Time-reversal imaging (TRI) is analogous to matched-field processing, although TRI is typically very wideband and is appropriate for subsequent target classification (in addition to localization). Time-reversal techniques, as applied to acoustic target classification, are highly sensitive to channel mismatch. Hence, it is crucial to estimate the channel parameters before time-reversal imaging is performed. The channel-parameter statistics are estimated here by applying a geoacoustic inversion technique based on Gibbs sampling. The maximum a posteriori (MAP) estimate of the channel parameters are then used to perform time-reversal imaging. Time-reversal implementation requires a fast forward model, implemented here by a normal-mode framework. In addition to imaging, extraction of features from the time-reversed images is explored, with these applied to subsequent target classification. The classification of time-reversed signatures is performed by the relevance vector machine (RVM). The efficacy of the technique is analyzed on simulated in-channel data generated by a free-field finite element method (FEM) code, in conjunction with a channel propagation model, wherein the final classification performance is demonstrated to be relatively insensitive to the associated channel parameters. The underlying theory of Gibbs sampling and TRI are presented along with the feature extraction and target classification via the RVM. © 2005 Acoustical Society of America.
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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
43.60.Lq Acoustic imaging, displays, pattern recognition, feature extraction

Protocols for calibrating multibeam sonar

Kenneth G. Foote, Dezhang Chu, Terence R. Hammar, Kenneth C. Baldwin, Larry A. Mayer, Lawrence C. Hufnagle, Jr. , and J. Michael Jech

J. Acoust. Soc. Am. Volume 117, Issue 4, pp. 2013-2027 (2005); (15 pages) | Cited 19 times

Online Publication Date: 08 Apr 2005

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Development of protocols for calibrating multibeam sonar by means of the standard-target method is documented. Particular systems used in the development work included three that provide the water-column signals, namely the SIMRAD SM2000/90- and 200-kHz sonars and RESON SeaBat 8101 sonar, with operating frequency of 240 kHz. Two facilities were instrumented specifically for the work: a sea well at the Woods Hole Oceanographic Institution and a large, indoor freshwater tank at the University of New Hampshire. Methods for measuring the transfer characteristics of each sonar, with transducers attached, are described and illustrated with measurement results. The principal results, however, are the protocols themselves. These are elaborated for positioning the target, choosing the receiver gain function, quantifying the system stability, mapping the directionality in the plane of the receiving array and in the plane normal to the central axis, measuring the directionality of individual beams, and measuring the nearfield response. General preparations for calibrating multibeam sonars and a method for measuring the receiver response electronically are outlined. Advantages of multibeam sonar calibration and outstanding problems, such as that of validation of the performance of multibeam sonars as configured for use, are mentioned. © 2005 Acoustical Society of America.
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43.30.Xm Underwater measurement and calibration instrumentation and procedures
43.58.Vb Calibration of acoustical devices and systems
43.30.Yj Transducers and transducer arrays for underwater sound; transducer calibration
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