ASA 125th Meeting Ottawa 1993 May

5pUW11. VLF cw signal analysis.

Scott J. Levinson

Nancy R. Bedford

Stephen K. Mitchell

Joan C. Lange

Appl. Res. Labs., Univ. of Texas at Austin, Austin, TX 78713

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--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 longer range, the arrivals typically consisted of three from the surface and three from the bottom, corresponding to two, three, and four bottom interactions. The continuous range over which a particular arrival order dominated was 5 km or less. An anomalous bottom loss observed at 30(degrees)--34(degrees) 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 it is consistent with conventional modeled loss calculations for the pre-assessed geoacoustic parameters of the sediment. [Work supported by the ASW Environmental Acoustic Support (AEAS) Program of the Office of Naval Research.]