NATIVE I VLF Signal Analysis. NATIVE I VLF Experiment Data Research and Analysis.
Final rept. 15 Apr 91-31 Oct 92,
TEXAS UNIV AT AUSTIN APPLIED RESEARCH LABS
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During the NATIVE I experiment conducted in the Blake Plateau area, very low frequency VLF sound waves transmitted at 7 and 10 Hz from a source towed at a depth of 122 m were sampled over the bottom half of the water column by an 18-element vertical hydrophone array anchored on the bottom 3170 m deep. Analysis included the comparison of measured beam levels with corresponding eigenray and adiabatic normal mode models at short 0.7 km - 20 km and long 35-60 km ranges. At short ranges, the VLF measurements and both models were consistent in indicating easily distinguishable direct, bottom, bottom surface, and bottom-surface-bottom interacting arrivals. At longel range, the arrivals typically consisted of three from the surface and three from the bottom, corresponding to two, three, and four bottom interactions. An anomalous bottom loss observed at 30-34 deg at 7 Hz appears to be the result of a slow hydrate layer in the sediment. Except for these angles, VLF bottom loss was low compared to typical values at higher frequency, and consistent with conventional modeled loss calculations for the pre-assessed geoacoustic parameters of the sediment.