Directional Response of a Horizontal Linear Array to an Acoustic Source at Close Range in Deep Water

During an experiment conducted in the deep South China Sea, a bottom-mounted horizontal linear array (HLA) collected the radiated noise from a moving cooperative ship at a distance less than 13 km. Bearing-time records of the HLA through plane-wave beamforming showed significant bearing estimation errors and two or three split bearing tracks of the ship that leads to misjudgment of target numbers. To reveal the physics underlying the experimental phenomena, the directional response of the HLA to an acoustic source at close range was developed by normal mode theory. Numerical simulations were conducted to analyze the acoustic intensity distribution characteristics and the arrival structures of the acoustic field. Comparative results of beamforming obtained with normal mode and ray theory show that the ray model provides an easy way to predict the split bearings for HLA beamforming. When sources are near the endfire of the array, significant bearing estimation errors and the beam splitting effect for a large aperture array were the main issues. For sources near the broadside of the array, the beam broadening effect was the most significant concern. The effect of the array length on the beamforming anomalies and the array gain loss was also investigated. With the present theories, the HLA beamforming anomalies during the experiment were well explained.