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Nov 1990

Volume 88, Issue S1, pp. S1-S200

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back to top Session 6UW: Underwater Acoustics: Scattering and Medium Variability
Contributed Papers
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The study of ocean water inhomogeneities by acoustic methods (A)

Victor A. Akulichev

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S130-S131 (1990); (2 pages)

Online Publication Date: 14 Aug 2005

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This paper considers the results of experimental investigation into sound propagation through extended ocean water areas containing large‐scale inhomogeneities in the form of frontal water mass zones or mesoscale inhomogeneities in the form of eddies. A comparison is made between the measured sound field and the corresponding numerical calculations. The changes in sound signal levels caused by large‐scale inhomogeneities of ocean water in crossing frontal zones and eddies in the North Pacific and South Indian Oceans are shown. Distinctive characteristics of sound signal transition from the deep‐water sound channel in subtropical waters to the near‐surface underwater sound channel in sub‐Arctic or sub‐Antarctic waters (and vice versa) are discussed. The possibility of sounding different large‐scale ocean water inhomogeneities by backscattering of powerful pulse signals is considered. Results of sound backscattering from frontal zones and from eddy perturbations are presented.
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A nondestructive bottom characterization with a model tank experiment (A)

Juan I. Arvelo, Jian Ren Yuan, Herbert Überall, and Louis A. Hargenrader

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S131-S131 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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The theory of normal modes has been applied to the inverse scattering problem. The model accounts for multiple elastic and absorptive bottom sediments. The characterization is performed at a number of frequencies according to the depth of the water column. Transmission loss measurements with range are supplied to a modified Levenberg‐Marquardt minimization model at each frequency. The highest frequency is resolved first to characterize the uppermost sediment of the ocean floor. After convergence to a set of acoustic properties for that sediment, the next lower frequency is used to evaluate the acoustic properties of the next deeper sediment. Some experimental measurements by Hundley and Glegg [J. Acoust. Soc. Am. Suppl. 1 87, S132 (1990)] are used to test this inverse scattering method in a model tank with an epoxy sediment over a concrete subbottom. [Work supported by ONR.]
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Projection of the far‐field target strengh of a continuously illuminated rigid sphere from the near field (A)

F. Dale Groutage, Robert D. Kollars, and Ernest Swenson

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S131-S131 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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Simulated near‐field measurements of composite scattered and continuous plane‐wave illumination pressure fields were used to accurately predict the far‐field target strengh of a rigid sphere. A theoretical diffraction model of a rigid sphere [R. V. Waterhouse, J. Acoust. Soc. Am. 75, 695–706 (1983)] was verified and used to simulate scattering in the geometric near field. Separation of the incident and scattered fields was based on wave‐vector filtering where a two‐dimensional spatial FFT was used to centralize the incident field. Following successful decompsition of incident and scattered fields, the complex near‐field pressures were appropriately windowed and then projected to the far field via the Helmholtz integral [J. Hald, B&K Tech. Rev. 1, 5–9 (1989)]. Several supporting simulations were completed that optimized the parameters to allow a small measurement array for cost effective implementation.
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Fresnel zone effects in the scattering of sound by intermediate length cylinders (A)

Daniel T. DiPerna and Timothy K. Stanton

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S131-S131 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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The backscattering behavior of straight cylinders is examined whose “intermediate” lengths are comparable to the diameter of the first Fresnel zone of the source/receiver pair. This “transitional region” is complex in nature as the cylinders occupy a finite number of Fresnel zones (≈ 1–5) and, in general, can only be described numerically. The scattering is described by first adapting the deformed cylinder formulation [T. K. Stanton, J. Acoust. Soc. Am. 86, 691–705 (1989)] to the point‐source/point‐receiver combination. Numerically evaluating this expression showed the scattering characteristics to be dominated by Fresnel zone effects—oscillations in the backscatter versus length curve caused by constructive and destructive wave interferences due to phase shifts from contributions along the cylinder axis. An experiment was performed that involved measurement of backscatter versus cylinder length, and there is reasonable agreement between the results and the trend as predicted by the approximate theory. [Work supported by ONR.]
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Acoustic scattering from free field and buried spherical shells (A)

Joseph L. Lopes and Douglas G. Todoroff

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S131-S131 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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A measurement was conducted to investigate the effect of sediment loading on the characteristic elastic response of an object. Backscattered form functions were obtained for free field and buried 2% and 11% spherical shells. The results have shown that the free‐field form function for the 2% shell is dominated by the nondispersive s0 mode while the 11% shell is dominated by the antisymmetric Lamb mode a0. Upon sediment loading, the a0 resonances shifted to higher frequencies. The results of this experiment are in good agreement with theoretical calculations.
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A variational model for bubbly liquids: Reflection from a liquid‐bubbly liquid interface (A)

J. A. Hawkins, Jr. and A. Bedford

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S131-S131 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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Recently, K. W. Commander and A. Prosperetti [J. Acoust. Soc. Am. 85, 732–746 (1989)] presented a model for the acoustics of bubbly liquids containing a distribution of bubbles sizes. They also analyzed the reflection and transmission of waves normally incident on a layer of bubbly liquid. In this paper the reflection and transmission of oblique plane waves at the interface between a liquid and a bubbly liquid are analyzed. The analysis is based on a variational theory for the acoustics of a bubbly liquid with a distribution of bubble sizes [J. A. Hawkins, Jr. and A. Bedford, J. Acoust. Soc. Am. Suppl. 1 86, S42 (1989)]. It is shown that the reflection and transmission coefficients are relatively insensitive to the bubble size distribution.
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Performance limitations of hull‐mounted sonar arrays in the presence of air bubbles (A)

C. de Moustier and B. J. Sotirin

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S131-S132 (1990); (2 pages)

Online Publication Date: 14 Aug 2005

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Analyses of seafloor echoes received by a hull‐mounted multibeam echosounder in various sea states, wind speeds, and ship's operating conditions made it possible to quantify the performance limitations of the sonar due to bubble masking. This performance assessment included, for each weather and ship's steering conditions: the number of transmit/receive cycles lost per unit time, the relative increase in noise level during bubble masking events, and the corresponding signal‐to‐noise ratio (SNR). For sea states ranging from 1 to 5, the number of pings lost per unit time is directly related to the ship's speed and course. Depending on the ship, this number can reach 40% in sea state 5 for ship's speeds in excess of 9 kn while heading into the seas. Likewise, the presence of air bubbles during a receive window seems to account for an increase in the background noise level of 8 to 10 dB in sea state 3, 10 to 13 dB in sea state 4, and over 15 dB in sea state 5. In addition, variations of 20 to 30 dB in SNR for near‐specular returns have been observed, from one ping to the next, during bubble masking events. In sea states 4 and above, such variations can adversely affect seafloor acoustic backscatter measurements made with hull‐mounted arrays. [Research sponsored by ONR and IFREMER.]
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On differential phase measurements with the SeaMARC II bathymetric sidescan sonar system (A)

M. Masnadi‐Shirazi and C. de Moustier

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S132-S132 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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The SeaMARC II sonar system, operated by the University of Hawaii, determines bathymetry by measuring the phase difference produced by seafloor echoes received at two rows of transducers, roughly half a wavelength apart, and by converting these measurements to angles of arrival as a function of time. Such phase measurements are affected by noise and interferences from multiple reflections on the ocean surface and bottom. Furthermore, the noise tends to mask the distortions due to these interferences. Working in the complex domain with acoustic data recorded at sea in the Fall of 1989, a simple time‐averaging filter was found to reduce the noise while preserving distortions due to interferences and hence allowing their investigation. Complex ping stacking, over an area with little change in relief, also gave a clear view of the interference phenomena. As a result, in addition to the expected multiple reflection interferences, evidence of cross‐talk between the port and starboard pairs of transducer arrays was found, and mapping of differential phase angles into acoustic angles of arrival required an “effective distance” between the two transducer rows almost 50% larger than the physical distance. Implications of these findings for bathymetric measurements and applicability of the phase processing method presented here will be discussed.
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Improvement in the prediction of sonobuoy detection ranges for ASW aircraft using on‐site acoustic measurements (A)

Charles Wiseman

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S132-S132 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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Sonobuoy detection ranges predicted at shore stations for use by ASW (antisubmarine warfare) aircraft are based on historical data and on XBT (expendable bathythermograph) data that are typically at least 6 to 12 h old. The accuracy of such predictions is questionable as shown by measurements and analysis. Prediction accuracy needs to be improved to increase the effectiveness and lower the costs of antisubmarine search and barriers using sonobuoys dropped from ASW aircraft. These predictions can be improved by measuring the on‐site sound propagation loss between a specially modified air‐dropped electronic SUS charge as the known sound source and the acoustic receivers in the air‐dropped sonobuoys. The derived propagation loss, used in conjunction with the XBT‐based predictions, can be used to more effectively select sonobuoy depths and patterns. The SUS charge can be modified to be neutrally buoyant and provide 30 ping seconds of preprogrammed transmissions over a 6 h life. The transmissions can be shaped for low probability of intercept or mimicry. Similar applications to improve sonar effectiveness for surface ships and ASW helicopters are also addressed.
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The curtain effect in a multiple convergence zone environment: Implications for ambient noise and bottom and surface reverberation levels (A)

David G. Browning, Peter D. Herstein, and Raymond J. Christian

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S132-S132 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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Over multiple convergence zone propagation ranges the relative spreading loss per zone, although initially high, decreases with range. The other major component of propagation loss, attenuation, remains unchanged and eventually becomes greater than the rate of spreading loss. The range at which this crossover occurs—the curtain effect [Browning et al., J. Acoust. Soc. Am. Suppl. 1 80, S54 (1986)]—is highly frequency dependent. At low frequencies the curtain effect occurs at long ranges; this allows sources or scatterers in the second convergence zone or beyond to impact the received level since they suffer only a relatively small additional loss compared to the spreading loss to the first convergence zone. For a given distribution of noise source levels or scattering strengths it is estimated what would be the resulting background level and the relative importance of events, for example, a comparison between medium strength multiple events at medium ranges and a strong event at long range. This analysis is then extended to higher frequencies where the curtain effect occurs at a range of one convergence zone or less. [Work supported by NUSC.]
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