Normal Mode Properties and Signal Coherence in Shallow Water Propagation

Abstract

Abstract : For many years, the 6.1 basic research communities have focused on the effects of internal waves on temporal coherence; whereas, Navy applied programs are more concerned with the randomizing effect of the combination of bottom bathymetry variations and platform motion on array (spatial) coherence. We find that it is not possible to isolate these two causes in the shallow ocean. In the deep ocean and for propagation by refracted paths, one need only consider the effects of internal waves to understand coherence. But in shallow oceans, propagation is generally by reflected paths and bottom variability can and does affect coherence and often is more randomizing that internal waves. We have found that for very low frequencies the bottom bathymetry variation are a small fraction of the acoustic wavelength and the bottom appears flat and internal waves alone determine coherence. At very high frequencies the bottom variations are a large fraction of the wavelength and so even the slightest sound speed variations randomize and de-correlate the signal even without internal waves. But for the practical mid-frequency range, (400Hz to 3kHz), the effects of each cause are interwoven and generally inseparable. For many years, the 6.1 basic research communities have focused on the effects of internal waves on temporal coherence; whereas, Navy applied programs are more concerned with the randomizing effect of the combination of bottom bathymetry variations and platform motion on array (spatial) coherence. We find that it is not possible to isolate these two causes in the shallow ocean. In the deep ocean and for propagation by refracted paths, one need only consider the effects of internal waves to understand coherence. But in shallow oceans, propagation is generally by reflected paths and bottom variability can and does affect coherence and often is more randomizing that internal waves.