Improved vertical array performance in shallow water with a directional noise field

The detection and localization performance of a vertical array against a submerged target in shallow water can be significantly improved if the noise field is directional and the signal arrives in the null (notch) of the noise field. This often happens in a summer environment with a downward refractive sound speed profile in which the surface generated noise field exhibits a notch in the noise vertical directionality distribution at mid (e.g., 500 Hz) frequencies. Using conventional beamforming, by steering the beams to the shallow arrival angle of the target signal, the (undesired) noise which arrives at steep angles can be suppressed. However, conventional beamforming often suffers from significant signal gain degradation due to multipaths in the shallow water environment. Matched-field processing yields a theoretical signal gain by coherently processing the signal, but its ability to reject the directional noise is limited. Matched-beam processing, which is matched-field processing in the beam domain, incorporates the advantages of matched-field processing for coherent signal integration, and conventional beamforming for noise rejection. For the summer environment studied, matched-beam processing yields the highest array gain, almost a twice longer detection range, and a superior capability in detecting a deep target compared with the other two processors. This is demonstrated using simulated signal and noise fields with a full water column vertical array.