Effects of uneven bottom on vertical correlations of sound field in the deep water area of the South China Sea

Variation of bathymetry has significant impact on the sound propagation and spatial correlations in deep water.Based on the deepwater experiment data with an incomplete sound channel in the South China Sea,the vertical correlation characteristics along two different propagation tracks with flat bottom and a typical uneven bottom (with the presence of the sea hill and the sloping bottom) are compared by analyzing the sound signals received by a largescale vertical line array.The comparison is focused on the ranges that endure significant transmission loss differences.Numerical analysis based on the ray theory is carried out to provide corresponding theoretical explanations for the variation of vertical correlations.In the uneven bottom environment,due to the blocking effect the sea hill exerts on the one-bottom-reflection rays,the vertical correlations vary with range.At certain distance,when part of the one-bottomreflection rays are blocked by the sea hill,the multipath structure of the main pulse received is relatively simpler and the vertical correlation coefficients are greater compared with the results in the flat bottom environment.At another distance,when all one-bottom-reflection rays are blocked,the existence of the sea hill leads to a more complex multipath structure of the main pulse and therefore the vertical correlation coefficients are less than those in the flat bottom environment.Near the first convergence zone,due to the reflection-blockage effects of bottom on one group of the refracted rays from water volume,the vertical correlation pattern in the uneven bottom environment is significantly different from that in the flat bottom environment.The vertical correlation coefficients at the range of 50 km do not oscillate periodically with the increase of the receiving depth anymore.The vertical correlations at the range of 57 km decrease significantly.It is because what mainly contribute to the sound field are the multi-bottom-reflection rays instead of the refracted rays from water volume.The work is of great significance to analyze the vertical correlation characteristics and estimate the sonar array gain in deep water with an uneven bathymetry.