High-Frequency Acoustic Backscatter from the Sea Surface
High-Frequency Acoustic Backscatter from the Sea Surface

Author: W. I. Roderick

Publisher:

Published: 1984

Total Pages: 39

ISBN-13:

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This document contains the slide presentation entitled 'High-Frequency Acoustic Backscatter From the Sea Surface, ' given at the 107th meeting of the Acoustic Society of America on 8 May 1984 in Norfolk, Virginia. A high resolution scattering experiment was conducted in the shallow waters of the North Atlantic. A narrow beam parametric array, which was rotatable in both azimuth and elevation, was utilized as a broadband high-frequency acoustic projector. Acoustic surface scattering data were obtained at normal incidence and low grazing angles (less than 10 deg) as a function of acoustic transmit frequency and sea state conditions. Meteorologic and oceanographic data were obtained in concert with the acoustic measurements and included wind speed and direction, ocean surface wave spectra and currents, and ocean sound speed. Surface backscattering strength, Doppler spectra (shift and spread), and envelope statistics were some of the measured parameters. It will be shown that the Doppler spectra are approximately Gaussian and the spectral shift could be predicted from Bragg diffraction theory modified by the induced Doppler due to surface currents. The measured Doppler spreads, however, were greater than those predicted by composite roughness theory.

The Variability of High-Frequency Acoustic Backscatter from the Region Near the Sea Surface
The Variability of High-Frequency Acoustic Backscatter from the Region Near the Sea Surface

Author:

Publisher:

Published: 1997

Total Pages: 20

ISBN-13:

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Simultaneous and coincident measurements of acoustic and microwave backscatter from the air/sea interface were obtained during Phase 2 of the SAXON-FPN experiment in December 1992 and again in March 1993. The acoustic and microwave grazing angles were both set to 17 degrees, and the wavelengths were matched, being set to 2.14, 3.00, and 5.66 cm, corresponding to, respectively, acoustic frequencies of 26.5, 50, and 70 kHz and microwave frequencies of 5.3, 10, and 14 GHz. The results of our experiments show that the two scattering strengths are comparable at wind speeds below about 3 m/s but that the acoustic scattering strength increases much faster than the microwave scattering strength with increasing wind speed until reaching saturation. The temporal variability of acoustic backscattering from the region near the sea surface is examined for frequencies in the 30- to 70-kHz range. A variance spectrum of the scattering strength exhibits effects associated with three different processes described in the paper.

High Frequency Ocean Acoustics
High Frequency Ocean Acoustics

Author: Michael B. Porter

Publisher: American Institute of Physics

Published: 2004-12-07

Total Pages: 570

ISBN-13:

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All papers were peer-reviewed. Sound in the ocean is as fundamental as light in theatmosphere. The high-frequency acoustic band has been little studied in the past; however, new applications such as mine hunting, marine mammal tracking, and communications (the undersea internet) have generated tremendous interest. These peer-reviewed proceedings include 8 invited papers by leading experts in particular areas and collectively survey all aspects of current research in high-frequency acoustics.

High-Frequency Seafloor Acoustics
High-Frequency Seafloor Acoustics

Author: Darrell Jackson

Publisher: Springer Science & Business Media

Published: 2007-08-02

Total Pages: 634

ISBN-13: 0387369457

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This book is a research monograph on high-Frequency Seafloor Acoustics. It is the first book in a new series sponsored by the Office of Naval Research on the latest research in underwater acoustics. It provides a critical evaluation of the data and models pertaining to high-frequency acoustic interaction with the seafloor, which will be of interest to researchers in underwater acoustics and to developers of sonars. Models and data are presented so as to be readily usable, backed up by extensive explanation. Much of the data is new, and the discussion in on two levels: concise descriptions in the main text backed up by extensive technical appendices.

Doppler Spectra of Sea-Surface Backscatter at High Acoustic Frequencies
Doppler Spectra of Sea-Surface Backscatter at High Acoustic Frequencies

Author: W. L. Konrad

Publisher:

Published: 1982

Total Pages: 14

ISBN-13:

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The initial transfer of energy from atmospheric wind to the ocean surface, which would be a significant factor in ambient noise, may occur via the so-called Cat's Paw phenomena. To study this effect, an experiment was conducted at Seneca Lake for a moderately disturbed (no whitecaps) air-water interface. Doppler spectrum measurements of high-frequency acoustic waves backscattered from the wind-driven water surface indicate the presence of small-scale roughness that is convected downward by surface drift. For a slightly rippled surface, there is a sharp resonant peak corresponding to a convection velocity of approximately 0.4 m/s which is nearly independent of wind speed. The spectrum is broadened by phase modulation due to the orbital motion of large-scale gravity waves but remains skewed in the downwind direction. (Author).

Shallow Water Acoustic Backscatter and Reverberation Measurements Using a 68-kHz Cylindrical Array
Shallow Water Acoustic Backscatter and Reverberation Measurements Using a 68-kHz Cylindrical Array

Author: Timothy Cole Gallaudet

Publisher:

Published: 2001

Total Pages: 670

ISBN-13:

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The characterization of high frequency, shallow water acoustic backscatter and reverberation is important because acoustic systems are used in many scientific, commercial, and military applications. The approach taken is to use data collected by the Toroidal Volume Search Sonar (TVSS), a 68 kHz multibeam sonar capable of 360 deg imaging in a vertical plane perpendicular to its direction of travel. With this unique capability, acoustic backscatter imagery of the seafloor, sea surface, and horizontal and vertical planes in the volume is constructed from data obtained in 200 m deep waters in the Northeastern Gulf of Mexico when the TVSS was towed 78 m below the surface, 735 m astern of a towship. The processed imagery provides a quasi-synoptic characterization of the spatial and temporal structure of boundary and volume acoustic backscatter and reverberation. Diffraction, element patterns, and high sidelobe levels are shown to be the most serious problems affecting cylindrical arrays such as the TVSS, and an amplitude shading method is presented for reducing the peak sidelobe levels of irregular-line and non-coplanar arrays. Errors in the towfish's attitude and motion sensor, and irregularities in the TVSS's transmitted beampattern produce artifacts in the TVSS-derived bathymetry and seafloor acoustic backscatter imagery. Correction strategies for these problems are described, which are unique in that they use environmental information extracted from both ocean boundaries. Sea surface and volume acoustic backscatter imagery is used to explore and characterize the structure of near-surface bubble clouds, schooling fish, and zooplankton. The simultaneous horizontal and vertical coverage provided by the TVSS is shown to be a primary advantage, motivating further use of multibeam sonars in these applications.

Acoustic Backscattering from Sea Ice at 10-100kHz
Acoustic Backscattering from Sea Ice at 10-100kHz

Author:

Publisher:

Published: 1991

Total Pages: 119

ISBN-13:

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This report considers high frequency (10-100 kHz) acoustic backscattering from the underside of sea ice from a fundamental physical point of view. The underice acoustic backscatter problem involves several issues in wave propagation and scattering not typically encountered in backscattering from other geophysical media (e.g., the ocean surface and floor). It is thus of intrinsic scientific interest, as well as being of vital interest in underwater acoustic applications.