Abstract
An experiment was conducted in the Gulf of Mexico to evaluate a technique for measuring modal attenuations. Use of short pulses allowed individual normal modes to be observed and then identified by comparing observed pressure distributions and arrival times with those predicted by a normal mode model. Variable depth sources for 400, 750, and 1500 Hz were mounted on a fixed platform. A vertical string of hydrophones was suspended from a ship, which anchored at several ranges. The measurements were made under downward refracting conditions in 30 m of water with a level sand bottom. For the first two modes at 400 Hz, vertical pressure distributions, relative arrival times and time spreading due to dispersion were in good agreement with theory. Higher modes and modes at 750 Hz and 1500 Hz were not identified. By comparing peak amplitudes at different ranges, a mode attenuation coefficient of 1.3 dB/km was determined for the first mode at 400 Hz.
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