High-frequency propagation in a bubbly medium

During a shallow-water, direct-path experiment off the Panama City, Florida coast a storm passed through the area that gave rise to both a change in sound speed of about 50 m/s and pulse-to-pulse variations that were significantly larger than those found during relatively benign periods before and after the storm. The depth of the water was about 10 m with the propagation path over a range of about 60 m at mid-depth. Very narrow beam pulsed signals from 20 to 135 kHz were transmitted at 1-s intervals. Measurements of the void fraction of bubbles showed a bubble concentration that can account for the change in the speed of sound. Fluctuations in the arrival times can be attributed to small changes in the average bubble concentration along the propagation path. Both scattering and dispersion due to the presence of small bubbles will be discussed. Comparison of measurements before, during, and after the storm period will be presented. [Work supported by NRL and ONR.]