The effects of a dynamic shallow water front on acoustic propagation

This paper investigates the effects of a dynamic shallow water front on narrowband (1 kHz) and broadband (50 Hz bandwidth centered at 1 kHz) acoustic propagation. A generic shallow water front is simulated with the Princeton Ocean Model. The sound velocity field is calculated as a function of range, depth and time. A parabolic equation acoustic model is used to simulate acoustic propagation from the warm to the cold side of the front. When the front is in geostrophic balance, acoustic mode coupling is from the higher order, bottom-interacting modes to lower order modes that become trapped in the upper 50 meters of the water column. In the narrowband case, acoustic propagation across the front is enhanced by 10 to 15 dB in the upper 50 meters due to mode coupling. In the broadband case, the coupling from higher order to lower order modes mitigates the multipath, causing the pulse to emerge from the cold side of the front almost undistorted in the upper 50 meters. However when the front becomes unstable and begins to move, acoustic propagation becomes highly variable and the above effects may be observed only intermittently. The study shows that acoustic propagation across a shallow water front can change significantly over a period of hours and hence the need to consider the dynamics of fronts when trying to draw general conclusions about their effects on acoustic propagation