Omnidirectional and directional sea surface backscattering strength measurements at low frequencies and low grazing angles

Limited experimental data exists on surface backscattering strengths at low acoustic frequencies (below 300 Hz) and low grazing angles (below 20°) for deep ocean environments. We report here preliminary results of such measurements taken during four days of a sea test in the North Atlantic in Septem...

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Bibliographic Details
Published in:The Journal of the Acoustical Society of America Vol. 73; no. S1; p. S97
Main Authors: Erskine, F. T., Dundore, D. M., Schifter, D. E., Shaffer, J. D.
Format: Journal Article
Language:English
Published: 01-05-1983
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Summary:Limited experimental data exists on surface backscattering strengths at low acoustic frequencies (below 300 Hz) and low grazing angles (below 20°) for deep ocean environments. We report here preliminary results of such measurements taken during four days of a sea test in the North Atlantic in September 1980 (sea states varied from 2 to 3). Four sets of approximately 20 small explosive charges (5-lb TNT-equivalent; bubble frequency of 77 Hz) were dropped beneath a towed acoustic array. We measured the direct path surface reverberation by processing the first 3 s of data, for mean grazing angles down to 8°, prior to receipt of the first bottom bounce at 6 s past detonation. The data were averaged over six frequency bands of 20-Hz width from 150 to 270 Hz and were reduced to scattering strengths versus mean grazing angle and frequency by using the NRL reverberation model to remove propagation effects and to compute the grazing angles versus time. Typical omnidirectional surface scattering strengths decrease from −55 to −70 dB for mean grazing angles from 20° to 8° with little (<5 dB) frequency dependence over the measured frequency range. These results are consistent with previously published measurements. Further, the data have been beamformed to examine the azimuthal directionality of the surface backscatter (sea direction varied from 27° to 110° relative to array heading), and preliminary results will be reported. [Data acquisition funded by Defense Mapping Agency; analyses supported by Naval Electronic Systems Command, Code 612.]
ISSN:0001-4966
1520-8524
DOI:10.1121/1.2020661