ASA 124th Meeting New Orleans 1992 October

3aAO3. Comparative acoustic measurements of the ocean bubble layer.

Peter H. Dahl

Appl. Phys. Lab., College of Ocean and Fishery Sci., Univ. of Washington, Seattle, WA 98105

Mark Trevorrow

Inst. of Ocean Sci., Sidney, BC V8L 4B2, Canada

Andrew T. Jessup

College of Ocean and Fishery Sci., Univ. of Washington, Seattle, WA 98105

During January 1992, the University of Washington, Applied Physics Laboratory (APL-UW) made acoustic surface scattering (20--50 kHz) and environmental measurements from the research platform FLIP, while the Institute of Ocean Sciences (IOS) measured the ocean surface bubble field using its SEASCAN sonar drifter, deployed from the USNS De Steiguer operating nearby FLIP. The experiment took place 400 n.mi. off the southern California coastline. Its goal was to obtain coordinated acoustic surface scattering, bubble field, and environmental sea state measurements to contribute to the understanding of the ocean bubble field, e.g., its spatial distribution and depth scale, and the role of bubbles in near-surface acoustic scattering and propagation. Measurements of low-grazing angle surface forward scattering were made using an omnidirectional source (suspended from a spar buoy tethered to FLIP), and receivers mounted on FLIP's hull. The portion of received signal scattered from the sea surface experienced variable pulse elongation and energy loss. The latter is attributed to the extinguishing effects of the ocean bubble field through which surface bounce paths must traverse, and can be related to the average bubble field density along the propagation path. The SEASCAN collected multifrequency (28--400 kHz) vertical incidence backscatter and horizontal-looking sidescan sonar measurements from the same bubble field, from which microbubble density versus radius distributions were derived. Results from these two complementary measurements of the bubble field, APL-UW attenuation based and IOS scattering based, will be discussed. [Work supported by ONT (APL-UW), and DREA (IOS).]