南海深海典型不平整地形对声场垂直相关性的影响

海底地形变化对深海环境下的声传播和空间相关性有重要影响。基于2014年南海中南部海域不完全深海声道条件下大跨度垂直阵接收的声信号,选取平坦海底和典型不平整(存在小海底山和海底斜坡)海底两个不同传播方向上传播损失差异较大的距离,对声场垂直相关性进行了对比分析,并应用射线理论对相关性差异予以定性分析和机理解释。在不平整海底方向,在第一影区内小海底山遮挡区附近:在部分一次海底反射声线被小海底山遮挡的距离处,接收声信号主脉冲多途干涉结构相对平坦海底方向更加简单,垂直相关性增强;而在全部一次海底反射声线被小海底山遮挡的距离处,对声场起主要作用的变为二次海底反射声线,接收声信号主脉冲呈现多途干涉结构,垂直相关性降低。在第一会聚区附近:平坦海底环境下,由于两组水体中反转声线在空间垂直方向上的干涉效应,使得声场的垂直相关随接收深度增加呈现出周期性振荡的现象;而不平整海底方向,在50 km处,海底斜坡阻挡了一组水体反转声线,在大深度上只出现单会聚结构,观测不到垂直相关的周期性振荡现象;在57 km附近对声场起主要贡献的成分从水体中反转声线变成了多次海底反射声线,主脉冲多途展宽变宽,垂直相关性显著降低。研究结果对深海复杂海底地形环境下声场垂直相关性的深入分析及声呐阵列增益估计等具有重要意义。 

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.