Fred Fisher and research with acoustic vector sensors; Marine Physical Laboratory’s vertical array of directional acoustic receivers and ocean noise

Abstract

Fred Fisher had boundless enthusiasm for all topics acoustic. A chance encounter with him in the hallway usually led to a half-hour discussion of the latest research efforts at the lab and recent results he found exciting. In the 1980s, Fred became interested in the problem of identifying the physical phenomena forming the pedestal about the horizontal in vertical directionality measurements of the deep ocean’s low-frequency noise field. Two competing mechanisms had been proposed: downslope conversion of coastal shipping and noise from high latitude winds coupling into the deep sound channel due to the shoaling of the sound channel axis. The relative contributions of these two mechanisms possibly could be separated if the azimuthal ambiguity of a vertical line array of hydrophones somehow could be broken. Therefore, Fred proposed to build a vertical array of ‘‘DIFAR’’ sensors, which led to the design and construction of the Marine Physical Lab’s Vertical ‘‘DIFAR’’ Array. This talk will reminisce a bit about Fred as well as present some results from an ambient noise experiment conducted in 1992 on the continental shelf using the Vertical DIFAR Array co-deployed with MPL’s freely drifting vector sensors, the Swallow floats. [Work supported by ONR and ONT.]