Time-varying broadband interference patterns in the acoustic field during a shallow water, fixed-fixed experiment

Abstract

During a joint experiment between the Marine Physical Laboratory and the Applied Research Lab, Penn State University in Fall, 2000, continuous-broadband signals in the 100–400 Hz band were transmitted from a moored source to a large-aperture horizontal hydrophone array on the ocean bottom in 175-m water. Spectrograms over a 24-hour period were created from the data from several array elements. These long-duration spectrograms show sinuous interference patterns formed by the regions of modal constructive interference that can be easily traced throughout the entire 24-hour period. The time dependence of the amplitude and center frequency of these individual serpentine structures are correlated with variations in environmental conditions. In particular, their frequency-meandering nature is associated both with tidally driven changes in water depth and the watch circle of the moored source. Temporal oscillations in their amplitude occur with the same periods as those of internal waves. Abrupt jumps in their frequency content occur at the same time as rapid thickening of the upper mixed layer. A simple analytical model is used to predict many of these characteristics. [Work supported by ONR, Code 321US.]