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Arctic Acoustic Measurements at 50 kHz

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An acoustic transmission experiment was conducted in conjunction with development operations of the Unmanned Arctic Research Submersible UARS system off Fletchers Ice Island T-3. Transmissions from a low directivity, 50-kHz projector on the submersible part of the UARS acoustic tracking system were received at transducers suspended beneath the ice and then recorded. The profile of the ice immediately above the UARS was measured throughout the run and the UARS acoustic tracking system provided complete knowledge of the changing measurement geometry. The data were analyzed to yield the amplitude reflection coefficient as a function of the nominal grazing angle with the ice undersurface and the shift in reflection area, the sea water attenuation coefficient, and signal fluctuation statistics. The amplitude reflection coefficient was found to be highly variable and independent of grazing angle for angles from 10 deg to 40 deg the reflected signal had short-term fluctuations with a standard deviation on the order of 5 dB. The mean coefficient, however, varied about unity by typically - 6 dB in a somewhat periodic manner which was related to a secondary 50 to 100 foot wavelength component present in the measured ice roughness spectra. The measured attenuation coefficient at a frequency of 50 kHz, a temperature of -1.62 deg C, a salinity of 31.9, and a pressure of 4.8 atmospheres was 11.0 dB per kiloyard. This value confirms Greenes arctic measurements but is some 5 dB less than that predicted by Schulkin and Marsh. The standard error of this measurement was 0.72 dB, which indicates that over the ranges used in the experiment 500-yd maximum the direct path signal fluctuations ascribable to the medium were small.

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  • Physical and Dynamic Oceanography
  • Acoustics

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