Long-term, large-scale acoustic fluctuations in the Ulleung Basin

Abstract

An analysis of long-term, large-scale oceanographic conditions in the Southern Japan/East Sea has identified several interesting phenomena, including a previously undiscovered stable eddy. That analysis was based on temperature profile data observed daily with 23 PIES (pressure inverted echo sounder) instruments spaced at about 40 km throughout the Ulleung Basin during a 2-year period. In the present work, a gridded version of the PIES data was used to study the relation between oceanographic structure and acoustic transmission loss structure from 50 to 500 Hz throughout the area in terms of long-term fluctuations on the order of days to weeks. Significant correlations between oceanographic and acoustic variability were observed with different characteristics during summer and winter. The results indicate predictably stable acoustic paths that correlate with benign ocean conditions and predictably unstable acoustic paths that correlate with dynamic ocean features. The implication is that there are frequencies and sensor geometries that favor long-term acoustic monitoring with controlled uncertainty and that certain stable scenarios preclude the need for daily temperature sampling. [Work sponsored by ONR.]