Analysis of sound field distribution of angle dimension in deep ocean bottom bounce area and its application to active sonar vertical beam pitch

Abstract

In the deep ocean environment, the sound energy from the direct path is very hard to illuminate a target located in the shadow zone. To solve the problem, the bottom bouncing technique, in which the vertical transmitting beamforming and the bottom bouncing are used to illuminate the target in the shadow zone, provides a potential way. However, due to the sound field interference in the bottom bouncing area in the deep ocean environment, the sound energy in the bottom bouncing area is fluctuant, which produces several discrete detectable areas. In order to detect underwater targets in these areas when using an active sonar with a transmitting vertical linear array, the selecting of reasonable vertical transmitting beam angles is necessary. Therefore, the relationship between the discrete detectable area and the transmitting vertical angle in deep ocean is very important for the active sonar using a transmitting vertical linear array. In this paper, it is shown that the sound field fluctuation in the bottom bouncing area is due to the energy fluctuation of the rays having different grazing angles. And the active sonar can achieve high noise gain in a detectable area when the vertical transmitting angle is equal to the grazing angle of the sound ray with the peak energy. To obtain a high noise gain of the active sonar using the vertical transmitting array, an efficient method to estimate the grazing angle of sound rays with peak energy according to the sound field distribution of angle dimension is proposed, which is helpful for selecting a group of best vertical transmitting angles. Meanwhile, a transmitting signal which contains a group of subpulses is designed. Furthermore, each subpulse is applied to the whole transmitting array by using the vertical steering to illuminate a certain detectable area in the bottom bouncing area. By doing so, a subpulse steered to the previously selected vertical angle will ensure a high transmitting array gain in a detectable area, and all of pulses will illuminate the whole detectable areas with high array gains almost simultaneously. Numerical simulations show that the proposed method is stable and efficient, and has good noise gain in the shadow zone (where almost no direct path exists) in the deep ocean environment.