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Mar 2003

Volume 113, Issue 3, pp. 1181-1756

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Beam intensity striations and applications

T. C. Yang

J. Acoust. Soc. Am. Volume 113, Issue 3, pp. 1342-1352 (2003); (11 pages) | Cited 6 times

Online Publication Date: 28 Feb 2003

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The single-element spectrogram for a continuous broadband signal, plotted as a function of range, has been shown to exhibit striated bands of intensity maxima and minima. The slope of the striations is an invariant of the modal interference and is described by a waveguide invariant parameter “beta.” The striation pattern is analyzed and modeled in this paper for the beam outputs of a horizontal line array obtained by conventional beamforming. Array beamforming makes it possible to measure the waveguide invariant parameter for weak signals due to the enhancement of signal levels by the array gain over that of a single element. It is shown that the signal beam spectrogram as a function of range exhibits the same striation pattern as that (predicted) for a single element. Specifically, for a broadside signal, the beam striation is identical to that of a single-element plus a constant signal gain. For a nonbroadside target, the signal beam intensity will be modified by a frequency-bearing dependent signal gain due to the signal spread over multiple beams, nevertheless the beam spectrogram retains the same striation pattern (slope) as for a single element. The sidelobe beams (outside the canonical cones containing the signal arrivals) exhibit an entirely different striation pattern as a function of frequency and range. For array processing, it is shown that a fast range-rate, close range target and a distant, slow range-rate interference source will have a different striation pattern (slope) in the corresponding beam spectrograms as a function of time, assuming no prior knowledge of the source ranges. The difference in the striations between the beam spectrograms can be used in array processing to suppress the interference contribution. A 5–7 dB interference suppression is demonstrated using simulated data. © 2003 Acoustical Society of America.  
Show PACS
43.30.Bp Normal mode propagation of sound in water
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

Acoustic scattering from a thermally driven buoyant plume revisited

John Oeschger and Louis Goodman

J. Acoust. Soc. Am. Volume 113, Issue 3, pp. 1353-1367 (2003); (15 pages) | Cited 3 times

Online Publication Date: 28 Feb 2003

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Far-field weak scattering theory is applied to the case of high-frequency broad-bandwidth acoustic scattering from a thermally generated buoyant plume in a controlled laboratory environment. To first order, the dominant scattering mechanism is thermally driven sound-speed variations that are related to temperature deviations from ambient. As a result, the received complex acoustic scattering is a measure of the one-component three-dimensional Fourier transform of the temperature difference field measured at the Bragg wave number. The Bragg wave number vector is the difference between the scattered and incident wave vectors. Solving for its magnitude yields the Bragg scattering condition; this is the Fourier component of the plume variability that produces scattering. Results are presented for multistatic scattering from unstable and turbulent plumes using a parallel scattering geometry. The data justify application of the far-field weak scattering theory to the present case of a thermal plume. As a consequence, quantitative results on medium variability can be inferred using high-frequency broad-bandwidth acoustic scattering. Particular attention is given to the role of anisotropy of the variability of the scattering field in determining the validity of far-field Bragg scattering. © 2003 Acoustical Society of America.
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43.30.Ft Volume scattering
43.30.Vh Active sonar systems

Modeling signal loss in surficial marine sediments containing occluded gas

Trevor Gardner

J. Acoust. Soc. Am. Volume 113, Issue 3, pp. 1368-1378 (2003); (11 pages) | Cited 1 time

Online Publication Date: 28 Feb 2003

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The presence of occluded gas in inland lakes, harbor muds, and surficial marine sediments is well documented. Surficial gassy sediments cause underlying beds to be acoustically impenetrable to seismic surveys; therefore, the modeling of signal loss arising from mudline reflection and transmission absorption is of particular interest. The Anderson and Hampton [J. Acoust. Soc. Am. 67, 1890–1903 (1980)] model for attenuation in gassy sediments was evaluated against the physical and acoustical properties of eight laboratory silty clay soils containing different amounts of occluded gas in bubbles of 0.2- to 1.8-mm diameter. The model was shown to give good agreement with measured data over the lower frequencies of bubble resonance and above resonance. It did not agree with measured data at frequencies below resonance, for which the model did not simulate the bulk properties of the gassy soils. The Mackenzie [J. Acoust. Soc. Am. 32, 221–231 (1960)] model for reflection loss was also examined for the gassy soils. The maximum reflection losses of 6 dB, at a grazing angle of 40°, does not wholly support speculation by Levin [Geophysics 27, 35–47 (1962)] of highly reflective pressure-release boundaries arising from substantial reflection and absorption losses in gassy sediments. It was found that mudlines formed from sediments with significant occluded gas may be successfully penetrated, although the substantial absorption loss arising from signal transmission through the sediment prevents penetration of the surficial layers to much beyond a meter in depth. © 2003 Acoustical Society of America.
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43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.30.Vh Active sonar systems

Improvement in matched field processing using the CLEAN algorithm

H. C. Song, J. de Rosny, and W. A. Kuperman

J. Acoust. Soc. Am. Volume 113, Issue 3, pp. 1379-1386 (2003); (8 pages) | Cited 4 times

Online Publication Date: 28 Feb 2003

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Adaptive matched field processing such as the minimum variance distortionless processor (MV) provides excellent sidelobe (or ambiguity) suppression capability in source localization given a perfect knowledge of the ocean environment. Unfortunately, this processing is very sensitive to sources of mismatch and robust adaptive algorithms are then employed such as a white noise constraint (WNC) often at the expense of insufficient sidelobe control. The CLEAN algorithm was introduced in radio astronomy [Astron. Astrophys. Suppl. Ser. 15, 417–426 (1974)] to produce a high quality image of the sky by reducing sidelobe-induced artifacts. In this paper, the CLEAN concept is extended to matched field processing. Numerical simulations and experimental data demonstrate that matched field processing combined with the CLEAN algorithm can improve performance, especially when a weak source is masked by sidelobes from a much stronger source. © 2003 Acoustical Society of America.
Show PACS
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

Absolute measurements of total target strength from reverberation in a cavity

David A. Demer, Stephane G. Conti, Julien De Rosny, and Philippe Roux

J. Acoust. Soc. Am. Volume 113, Issue 3, pp. 1387-1394 (2003); (8 pages) | Cited 6 times

Online Publication Date: 28 Feb 2003

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A new method was developed to acoustically measure the density and total scattering cross-section (σt) or total target strength [TTS = 10 log10(σt/4π)] of objects in motion in a highly reflective cavity [J. De Rosny and P. Roux, J. Acoust. Soc. Am. 109, 2587–2597 (2001)]. From an ensemble of pulse-echo recordings, the average contribution of the scatterer(s) to the reverberation within the cavity provides a measurement of the scattering mean free path. The latter was shown through theory and experiment to be proportional to the volume of the cavity and inversely proportional the product of the mean σt and number of scatterers. Here, the TTS measurement uncertainty is characterized using standard metal spheres as references. Theoretical TTS was calculated for multiple copper and tungsten carbide standard spheres (Cu: 60.0 30.05 and 23 mm and WC: 38.1 and 33.4 mm diameters, respectively), using well-described theory for scattering from elastic spheres and the optical theorem. Measurements of TTS were made over a wide bandwidth (30–120 kHz) and compared to their theoretical values. Measurements were made in a corrugated, cylindrical, galvanized-steel tank with 25 or 50 l of fresh water at a temperature of 21±1 °C. The results indicate the method can provide TTS measurements that are accurate to at least 0.4 dB with an average precision of ±0.7 dB (95% confidence interval). Discussed are the requisite cavity volumes and signal-to-noise ratios for quality measurements of TTS, tank volume, and/or numerical abundance of mobile targets. Also discussed are multiple potential applications of this technique in bioacoustical oceanography. © 2003 Acoustical Society of America.  
Show PACS
43.30.Xm Underwater measurement and calibration instrumentation and procedures
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.30.Sf Acoustical detection of marine life; passive and active
43.80.Ev Acoustical measurement methods in biological systems and media

Analysis of a compliantly suspended acoustic velocity sensor

James A. McConnell

J. Acoust. Soc. Am. Volume 113, Issue 3, pp. 1395-1405 (2003); (11 pages) | Cited 10 times

Online Publication Date: 28 Feb 2003

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The dynamics of a compliantly suspended acoustic velocity sensor having a spherical geometry are analyzed using theory and experiment. The analysis starts with a review of the motion associated with an unconstrained solid sphere when subjected to an acoustic plane wave in an unbounded inviscid fluid medium. The theory is then modified to account for the inclusion of an inertial sensor and an external suspension system. Accordingly, the open-circuit receiving response of a geophone-based and accelerometer-based device is derived. Density variations associated with the sphere and the surrounding fluid medium are assessed along with the effects fluid viscosity. Wave effects in the sphere and the suspension system are also analyzed. © 2003 Acoustical Society of America.
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43.30.Yj Transducers and transducer arrays for underwater sound; transducer calibration
43.58.Fm Sound level meters, level recorders, sound pressure, particle velocity, and sound intensity measurements, meters, and controllers
43.30.Xm Underwater measurement and calibration instrumentation and procedures
43.38.Ar Transducing principles, materials, and structures: general

Green’s function estimation using secondary sources in a shallow water environment

Philippe Roux and Mathias Fink

J. Acoust. Soc. Am. Volume 113, Issue 3, pp. 1406-1416 (2003); (11 pages) | Cited 33 times

Online Publication Date: 28 Feb 2003

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This work provides a new way to measure the Green’s function between two points in an acoustic channel without emitting a pulse by any of the two points. The Green’s function between A and B is obtained from a set of secondary sources in the guide by averaging either the correlation or the convolution of the signals received in A and B. A theoretical approach based on mode propagation in a monochromatic regime is presented. Results are then extended to the time domain. Estimation of the Green’s function is performed numerically in a range-independent and a range-dependent environment. Application to discreet acoustic communications is discussed. © 2003 Acoustical Society of America.
Show PACS
43.30.Zk Experimental modeling
43.60.Gk Space-time signal processing, other than matched field processing
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