Improvement of Detection and Localization Performance Using the Receiving Array Response Difference Between Ocean Noise and Signal in Shallow Water

It was observed that when the short vertical line array (SVLA) is located in the deeper part of the water column, where sound velocity is lower, a groove always exists in the receiving array response in the horizontal direction for distant sound sources in the shallower part of the water column, where the sound velocity is higher. Normal mode modeling is used to explain this result. According to the normal mode theory, the receiving array response of the SVLA to a distant sound source can be expressed in terms of modal beams weighted in accordance with the normal mode amplitude. This modal representation offers a physical interpretation of the receiving array response to a distant sound source in terms of normal modes. The environmental effects of the shape of the sound velocity profile and geo-acoustic properties of the seabed on the receiving array response are analyzed. Based on the results, three conditions for the existence of the groove in the receiving array response are obtained: 1) a gradient in the sound velocity profile, 2) an SVLA in a water column in which the sound velocity is lower and low-order normal modes are trapped, and 3) a distant sound source in a shallow water column in which the sound velocity is higher, and acoustic source couples weakly with low-order normal modes and strongly with high-order normal modes. Finally, the receiving array response of the SVLA to ocean noise and distant sound source are analyzed and discussed using the Mediterranean Sea data. It is shown that the receiving array response to ocean noise differs from that to a distant sound source. Utilizing this difference, the array can be steered carefully to improve the output signal-to-noise ratio and increase the passive detection range against a submerged target in shallow water.