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May 1990

Volume 87, Issue S1, pp. S1-S164

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back to top Session II. Underwater Acoustics VI: Arctic Acoustics II
Contributed Papers
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Boundary element simulation of long‐range facet reverberation (A)

Henrik Schmidt and Peter Gerstoft

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S83-S83 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Simulation of seismoacoustic facet reverberation in an ocean waveguide requires solution of the full two‐way wave equation. Such simulations have been performed by means of finite difference and finite element methods. However, the computational requirements of these discrete methods have prohibited simulation for ranges larger than a few water depths. Here a hybrid boundary element‐wavenumber integration approach is presented to simulate long‐range facet reverberation. For a facet inhomogeneity in an otherwise horizontally stratified environment, the total field is expressed in terms of Green's theorem. By choosing a Green's function satisfying the boundary conditions at all horizontal interfaces, only the interface between the irregularity and the layered medium contributes to the surface integral in Green's theorem. Thus, only the boundary of the inhomogeneity needs to be discretized, with a spatial sampling independent of range. Here a modified version of SAFARI [H. Schmidt and F. B. Jensen, J. Acoust. Soc. Am. 77, 813–825 (1985)] is applied to compute all Green's functions involved in the boundary integral, allowing analytical integration of the influence functions over each boundary element. After solving the boundary element equations, the total reverberant field is again computed by means of SAFARI. The approach is both efficient and general, allowing the exterior as well as the interior region to be a stratified elastic medium. Simulations will be presented for reverberation from a salt dome buried in the seabed as well as from Arctic ice facets. [Work supported in part by ONR Arctic Program Office.]
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High‐frequency under‐ice reflection losses and spatial coherence (A)

Marcia A. Wilson and Roger W. Meredith

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S83-S83 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Propagation of high‐frequency acoustic signals under a multiyear pack ice results in a scattered acoustic field of fluctuating amplitude and coherence. High‐frequency scattering from under‐ice surfaces is complicated by out‐of‐plane and multiple scattering because acoustic wavelengths may be many orders of magnitude smaller than ice relief. In addition, the transition layer between water and ice, ice inhomogeneities, and ice anisotropy become important scattering sources. Measurements are presented of horizontal spatial coherence at 24 and 42 kHz from linear FM slide pulses of 800‐Hz bandwidth and 100‐ms duration. Measurements were made with a linear array of 16 hydrophones distributed over a 16‐m aperture and deployed 61 m below the ice surface. The data presented here were taken at a nominal range of 1000 m and results from array bearings at endfire, broadside, and 45° are compared. Results are presented for a fixed source at depths of 30, 61, and 91 m. Deconvolution of the source waveform is used to separate direct and reflected arrivals. Relative amplitudes of these arrivals are compared with what would be expected for a perfect reflector. The amplitude differences may be as high as 30 dB and the ice reflected coherence may increase slightly with range. In addition to hydrophone spacing, frequency, and source depth, the reflected arrival coherence is found to be sensitive to the physical location of the hydrophone in the aperture. This sensitivity is due to multipath interference from the scattering region. [Work supported by the Office of Naval Technology.]
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Random‐boundary structural and topographical effects on acoustic transmission in an ice‐covered channel (A)

Charles E. Ashley

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S83-S83 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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The relative effects of the upper and lower boundaries of an ocean channel on the intensity of an acoustic signal in the channel were investigated. The upper surface of this channel was assumed to be covered by an ice layer whose density and sound speed were taken to be random variables. In addition, the ice‐water interface was taken to consist of large‐scale two‐dimensional facets. Each facet was assumed to possess small random depth deviation, slope, and curvature. The lower water‐sediment interface of this channel is modeled in a similar manner. Ray theory was then used to study the effects of these boundaries on the intensity at a point receiver for an acoustic signal. Expressions were derived for the mean and variance of the acoustic intensity in terms of the statistics of the two boundaries. Relative effects of structure and topography were compared along with the relative effects of the ice and sediment boundaries. The distinctive acoustic consequences of boundaries whose properties have different means, variances, and horizontal correlations were examined.
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Scattering and reverberation measurements from the CEAREX 89 Arctic experiment (A)

Thomas J. Hayward

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S83-S83 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Reverberation data collected during the CEAREX 89 experiment in the Greenland Sea are analyzed to describe the spatial variability of surface and bottom reverberation and estimate short‐range backscattering strengths. Features of the time‐azimuth decay of reverberation are identified by comparison with model propagation calculations, and backscattering strengths are estimated from the data using the measured reverberation and calculated propagation losses. Statistical measures of variability are applied in an attempt to characterize the spatial variability of surface (under‐ice) and bottom reverberation. [This work was supported by the Office of Naval Research (ONR) Arctic Sciences Program, Code 1125AR.]
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Properties of iceberg underwater echoes (A)

Philippe de Heering

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S83-S84 (1990); (2 pages)

Online Publication Date: 13 Aug 2005

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Eleven free‐floating and ice‐locked icebergs of estimated masses ranging between 300 and 2 million tons were observed by means of a 36‐kHz sonar mounted in the hull of the Canadian icebreaking freighter M.V. ARCTIC for the purpose of characterizing their acoustic target properties. The echo data showed that their average target strength could be modeled, with reasonable accuracy, as that of a smooth ice sphere with no internal reflections, of a mass equal to the iceberg estimated mass. The average echo duration was observed to be related to the radius of the same sphere. The maximum detection ranges in open water, which were generally limited by the downward refracting conditions, were consistent with the berg's estimated draft, whereas ice‐locked bergs exhibited detection ranges approximately twice as long. [Work supported by the Canarctic Shipping Company, Ottawa, Canada and Transport Canada.]
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Methods of computer‐aided analysis of non‐Gaussian noise and application to robust adaptive detection (A)

Ivars P. Kirsteins and Donald W. Tufts

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S84-S84 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A methodology is presented for the modeling of certain nonstationary and non‐Gaussian Arctic acoustic noise data. Some components of the noise, which give it its non‐Gaussian characteristics, can be individually modeled, synthesized, and subtracted to provide a Gaussian residual. Further, it is shown that this process can also be carried out when signals are present. Our motivation comes from the problem of the detection of weak signals in such noise. The methodology is a combination of adaptive differential quantization and adaptive signal estimation algorithms based on singular‐value decomposition of a data matrix that has been developed. The combination of adaptive differential quantization with low‐rank approximation to data matrices or estimated covariance matrices is believed to be a new and effective method for multivariable, robust, adaptive detection.
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Directivity of an acoustic monopole in a floating ice plate and implications to Arctic ambient noise (A)

Peter J. Stein, Steven E. Euerle, and James K. Lewis

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S84-S84 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Predictions were made of the directivity of higher‐frequency (1‐kHz) acoustic radiation into the water from a monopole source located in a floating ice plate (2.4 m thick). Theoretical calculations were conducted using the SAFARI propagation routine [H. Schmidt and J. Glattetre, J. Acoust. Soc. Am. 78, 2105–2114 (1985)] and by the method of steepest descents (P. J. Stein, “Monopole Source in a Floating Ice Plate,” MIT/WHOI Ph.D. thesis, 1986). The models show close agreement with each other and predict a strong downward directivity with most of the energy radiated at grazing angles greater than 70°. This implies that (1) higher‐frequency impulsive ice noises detected at an omnidirectional hydrophone are likely radiated from a relatively small area of ice directly above the receiver (the area increasing with receiver depth) and (2) the nonimpulsive (background) ambient noise at higher frequencies is dominated by radiation from ice events located within a horizontal range radius equal to roughly one water depth. Modeling also shows that nulls can occur for some radiation angles for particular source depths within the ice. Also, only small effects of a more realistic ice strength vertical profile over a uniform profile are seen. [Work supported by ONR.]
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Extraction of the seabed critical angle using ice cracking noise as a signal source (A)

Pierre Zakarauskas, Ronald I. Verrall, and Michael V. Greening

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S84-S84 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A technique is described for measuring the reflection coefficient of the Arctic seabed with a single vertical array of hydrophones. Naturally occurring ice cracks were used as the acoustic sources. This method circumvents the difficulties and expense of introducing artificial sound sources through the thick Arctic pack ice. The measurements were made in April with a 22‐element array suspended from the ice in 420 m of water. The range of the source is first determined using the direct arrival and multiple reflections from the seabed and under‐ice surface. Then the directivity index is determined using the direct arrival path only. A plot of the reflection coefficient versus grazing angle clearly indicates the value of the critical angle. The sound speed of the sea bottom corresponding to this critical angle agrees well with that measured from a bottom grab sample taken during the field trip. Finally, an interesting phenomenon was an anomalous increase of power at a grazing angle of 60°. This is associated with a leaky plate wave radiating at the ice‐water interface.
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Determination of compressional and shear wave speed profiles and attenuation in sea ice (A)

Suhramaniam D. Rajah, James A. Doutt, and George V. Frisk

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S84-S84 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Sea ice is a very heterogeneous material with brine channels, layers of varying density and crystal size, and a porous skeletal layer at the ice‐water interface. Acoustic propagation under ice in the Arctic has been studied by many investigators, and efforts to predict the measured field by theory has met with limited success. One of the likely causes is the lack of adequate information on the anelastic properties of sea ice. A cross‐hole tomography experiment designed to measure the spatial and temporal structure of the anelastic properties of sea ice will be described. Results of a lake experiment and an experiment conducted in the Canadian Arctic during 1989 will be presented.
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