Abstract:

As part of an experiment to determine the distribution and acoustical effects of bubbles near the surf zone, resonators, cameras, a coherent Doppler, and horizontally oriented Doppler sonars were deployed in the vicinity of Scripps Pier. With these instruments, evolution of the waves as they traveled toward the shore were detected and the bubble size distributions, rip currents, turbulence dissipation, and patterns of bubble cloud motion were measured. Propagation tests at 100 kHz along a 230-m path between the end of the pier and the surf were conducted. Bubble size measurements in the surf and on a bottom-mounted frame allowed identification of the spatial evolution of bubble characteristics. The sonar measurements show bubble clouds ejected offshore at discrete intervals and at speeds of ~0.3 m/s. Pulse propagation into the surf was occasionally completely attenuated by bubble clouds. However, it is interesting that prior to total blocking of the signal the pulse arrival time is delayed, implying an apparent decrease in sound speed. This is unexpected since our frequency (100 kHz) was above the resonance of the dominant bubbles. An explanation is proposed in terms of selective blocking of direct path propagation by bubble clouds. [Work supported by ONR Contracts Nos. N00014-96-C-6030 and N00014-97-1-0288.]