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Aug 1999

Volume 106, Issue 2, pp. 523-L29

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General perturbative solution to wave scattering from a soft random cylindrical surface

Nikolaos C. Skaropoulos and Dimitrios P. Chrissoulidis

J. Acoust. Soc. Am. Volume 106, Issue 2, pp. 596-604 (1999); (9 pages) | Cited 1 time

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A perturbative solution to the problem of plane-wave scattering from a soft, randomly rough, cylindrical surface is obtained by use of the Rayleigh hypothesis and the assumption that coupled radiation modes can be present in the scattered wave. The wave amplitudes of the cylindrical-wave expansion of the scattered field are expressed as an asymptotic series in the root-mean-square of surface irregularities. The end result of the analysis is a concise recursive formula for the coefficients of that series. The solution is energy consistent up to fourth-order in the smallness parameter. Specific information about the validity and applicability of the solution is given by use of the mean boundary condition error, which is defined in this paper with a view to assessing the conformity of near-field results with the boundary condition. The numerical results show that the fourth-order approximation is more accurate than the second-order one, but it may, under certain conditions, exhibit higher mean boundary condition error. © 1999 Acoustical Society of America.  
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43.30.Hw Rough interface scattering
43.20.Fn Scattering of acoustic waves
43.40.Kd Impact and impact reduction, mechanical transients

Multiaspect identification of submerged elastic targets via wave-based matching pursuits and hidden Markov models

Paul Runkle, Lawrence Carin, Luise Couchman, Joseph A. Bucaro, and Timothy J. Yoder

J. Acoust. Soc. Am. Volume 106, Issue 2, pp. 605-616 (1999); (12 pages) | Cited 5 times

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This paper investigates classification of submerged elastic targets using a sequence of backscattered acoustic signals corresponding to measurements at multiple target-sensor orientations. Wavefront and resonant features are extracted from each of the multiaspect signals using the method of matching pursuits, with a wave-based dictionary. A discrete hidden Markov model (HMM) is designed for each of the target classes under consideration, with identification of an unknown target effected by considering which model has the maximum likelihood of producing the observed sequence of feature vectors. HMMs are stochastic models which are well suited to describing piecewise-stationary processes, and are appropriate for multiaspect classification due to the strong aspect dependence of the scattered fields for most realistic targets. After establishing the physical and geometric correspondence between multiaspect sensing and the HMM parameters, performance is assessed through consideration of measured acoustic data from five similar submerged elastic targets. Results are presented with and without additive noise. © 1999 Acoustical Society of America.
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43.30.Vh Active sonar systems
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries

Acoustic attenuation in very shallow water due to the presence of bubbles in rip currents

J. W. Caruthers, S. J. Stanic, P. A. Elmore, and R. R. Goodman

J. Acoust. Soc. Am. Volume 106, Issue 2, pp. 617-625 (1999); (9 pages) | Cited 3 times

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An experiment was performed just off the research pier at the Scripps Institute of Oceanography to determine the acoustic effects of small bubbles in very shallow water (∼6 m depth). The distance offshore was ∼300 m. The propagation lengths were 2–10 m, and the frequency range was from 39 to 244 kHz. During the experiment, rip currents passed through the field of measurement instruments. These rip currents were laden with bubbles created in the surf between the instruments and the shore. The effects of these rip currents on the spatial distributions of the resulting acoustic attenuation are discussed. From the attenuation data, the bubble distributions are calculated using a new iterative approach [Caruthers et al., in press, J. Acoust. Soc. Am.] that is based on the well-known resonant bubble approximation. Calculated bubble distributions varied from an essentially uniform lack of bubbles during quiescent periods to highly inhomogeneous and dense bubbly regions within rip events. Such observed distributions were consistent with measurements made by other investigators during the experiment. © 1999 Acoustical Society of America.
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43.30.Es Velocity, attenuation, refraction, and diffraction in water, Doppler effect
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.35.Bf Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in liquids, liquid crystals, suspensions, and emulsions

Waves in rotating conducting piezoelectric media

J. Wauer

J. Acoust. Soc. Am. Volume 106, Issue 2, pp. 626-636 (1999); (11 pages) | Cited 2 times

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The propagation of waves in a conducting piezoelectric solid is studied for the case when the entire medium rotates with a uniform angular velocity. For comparison, both the conventional electrically quasistatic theory and the fully dynamic Maxwell equations are taken into consideration. In completion, a generalized thermoelastic theory of piezoelectric bodies is incorporated. The governing dispersion relations are obtained to determine the effects of moderate rotation, thermal, and constant electrical conductivity on the finite phase velocity of the waves. Analysis is carried out for plane waves in an infinite medium but also for surface waves of a half-space. Finally, the radial vibrations of a hollow cylinder are addressed. The evaluations are specified for hexagonal crystals of (6 mm) class and a simple arrangement of the direction of wave propagation and the crystal and rotational axes. © 1999 Acoustical Society of America.
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43.40.At Experimental and theoretical studies of vibrating systems

Yellow Shark Spring 1995: Inversion results from sparse broadband acoustic measurements over a highly range-dependent soft clay layer

Martin Siderius and Jean-Pierre Hermand

J. Acoust. Soc. Am. Volume 106, Issue 2, pp. 637-651 (1999); (15 pages) | Cited 11 times

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In May 1995, SACLANTCEN performed broadband (200–800 Hz) acoustic measurements in the Giglio basin off the coast of Italy as part of the Yellow Shark inversion experiments. In this paper, inversion of sparse, broadband transmission loss (TL) measurements is investigated to determine bottom properties in strongly range-dependent (RD) situations commonly encountered in shallow water. The data are from the Elba–Formiche transect where water depth varies from 65 m at the acoustic projector location, to approximately 120 m at the 4-element vertical arrays deployed at ranges of 8, 16, 24, 32, and 40 km along the transect. The experimental site has a soft clay-layer bottom which varies in thickness from 3 to 10 m with a sound speed less than the water column. A modal analysis including coupling effect is given to explain the frequency bands for which high TL was observed as a function of range and depth. The TL measurements were inverted by matching the RD fields with model results. Using parabolic equation modeling, bottom geoacoustic parameters were varied in a marching search to fit the TL measured at the five ranges. The experimental results demonstrate that RD bottom properties such as sound speed and thickness of the slow clay layer can be obtained from broadband TL measurements sparsely distributed in range and depth. © 1999 Acoustical Society of America.
Show PACS
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.30.Zk Experimental modeling
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
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