Effect of the internal tide on midfrequency transmission loss in the Shallow Water 2006 Experiment.

Abstract

Nonlinear internal waves depress the high‐gradient region of the thermocline. After the waves have passed, it may take several hours for the thermocline to rise to its prewave level. To examine consequent acoustic effects, two acoustic data sets collected 18–19 August in SW06 were analyzed with one at a fixed range of 550 m and the other along a tow track out to 8.1 km. Both data sets allow the effects of the rising thermocline on acoustic propagation to be examined over a range of temporal and spatial scales. Signals transmitted are in the midfrequency band 1.5–10.5 kHz. The gradual rising of the thermocline is shown to change acoustic intensity by 5 and 2 dB for the fixed and towed source data, respectively. Using nearby oceanographic moorings, a simple plane wave ocean model is developed to provide time‐dependent environmental input to broadband acoustic modeling assuming range‐independency. Modeling results are shown to produce mean transmission loss results consistent with experimental observations at both the short and the long ranges. The results suggest that the rising thermocline effect of nonlinear internal waves on mean transmission loss is observable, significant, and predictable. The result has implications on geoacoustic inversion. [Work supported by ONR.]