The effects of the uneven bottom on horizontal longitudinal correlations of acoustic field in deep water

The spatial coherence of sound field in the reflection area of deep-water bottom is studied by analyzing the data of an acoustic propagation experiment conducted in the South China Sea. Some different oscillation patterns of the measured horizontal longitudinal correlations of acoustic field are observed for two different tracks in the flat bottom and the uneven bottom environments when the source distance ranges from 29 km to 35 km. Numerical analysis based on the ray theory is carried on to explain the causes of the differences. The experimental and numerical results show that the travel time differences between the one-bottom-reflection(1 BR) rays, which make a major contribution to the sound field of the 1 BR area in flat bottom environment, decrease gradually with increasing horizontal distance. Then the phase of the interference cyclically changes within 2π, leading to the correlations’ periodical oscillation in deep water with flat bottom. In the uneven bottom environment, however, due to the reflectionblockage effects of the sea hill on the 1 BR sound beams, the sound transmission losses(TLs) increase significantly in the reflection area of the sea hill. What’s more, the horizontal-longitudinal correlations decreasing slightly in general don’t show obvious cyclical oscillation any longer in the corresponding reflection area because of the complex interference of the relatively more eigenrays. This work is of significance to analyze the detection performance of the sonar array in deep water with the complicated bathymetry environment.