The effects of source motion and bottom bathymetry on temporal coherence in shallow water propagation

Abstract

Previous studies on coherence have been focused on the effects of water column fluctuations on temporal coherence with a stationary source/receiver setup. However, with focus being turned to moving source setups there has been documented a significant drop in temporal coherence. With a moving platform, the propagation path will change based on relative source/ receiver position, and hence the bathymetry along the path will vary. Here, we will examine the effects that this bathymetric variation and related ship speed contribute to coherence loss. A range dependent parabolic equation model will be used to predict the temporal coherence for individual mode arrivals. A slowly varying random bottom will be introduced to the model and the coherence calculated for different ship speeds and for both radial and tangential tracks. Results will be compared with stationary source/receiver setups in order to determine at what ship speed/ bottom bathymetry does source motion become the driving factor in loss of coherence versus water column fluctuations from a stationary setup. Preliminary results indicate that at a speed of 2 knots, there is remarked loss of coherence at all modes except the first with even small variations in bottom bathymetry, which is in agreement with experimental results.