Characterization of Average Geoacoustic Bottom Properties from Expected Propagation behavior at Very Low Frequencies (VLF) Using a Towed Array Simulation

摘要

This study seeks to determine the feasibility of broad georeconnaissance data acquisition for average bottom properties collected via a variable-depth towed array, detecting low frequency self-noise created from the towship. It concentrates on shallow water regimes in which both near-and farfield array response features can be exploited from a single array. Bottom properties have been input to the SAFARI fast fullwave FFP model to create synthetic VLF towship data. The data are then processed at the array via synthetic beamforming techniques, allowing discrimination of normal acoustic, as well as evanescent and seismic interface modes. This latter information in particular can be shown to relate directly to crucial bottom properties such as average compressional sound speed in the bottom (for fluid geoacoustic bottoms) or, in some cases, the net shear speed (for ideal infinite half-space bottoms of sufficiently high rigidity). Multilayered bottoms are examined to determine the thickness for which a higher net bottom attenuation factor dominates the seismo-acoustic process. It is expected that for thick (fluid)layers one can effectively ignore the deeper basement, while for thin (fluid) sediment layers, basement (solid) properties dominate in the low frequency/near cutoff regime. (Reprints).