风成噪声背景下垂直阵阵列信噪比随声源深度的变化规律

风成噪声是海洋中最广泛存在的环境噪声、是被动声纳处理性能的重要影响因素.它具有一个典型的物理性质是很大程度上只对应中高阶模态.阵列信噪比计入了阵列采样辐射声强、背景噪声功率和阵处理增益,是决定声纳阵列处理性能的关键物理量.本文研究了受风成噪声模态结构影响,典型夏季浅海环境中垂直阵阵列信噪比随声源深度的变化关系.在简正波模深函数采样完整的假设条件下,理论证明了阵列信噪比随声源深度的变化可近似为低阶模态幅度强度(模深函数模值的平方)随深度变化的线性叠加,且模态阶数越低,贡献越大;并且,在强风成噪声背景、显著负梯度环境下该变化规律可由1阶模态幅度强度随深度的变化近似独立表征.以上结果表明,在同一声源距离条件下,声源置于水体下半部分时的阵列信噪比比置于海面附近更大,并且在位于1阶模态峰值点所在深度附近时达到最大.典型负声速梯度浅海环境中的仿真实验结果对理论分析进行了验证,并表明在一定条件下阵列信噪比随声源深度的变化与声源距离近似无关.

Variation of signal-to-noise ratio of vertical array with sound source depth under wind-generated noise background

Wind-generated noise is ubiquitous in ocean environments and highly influences the passive sonar performance.Since it originates from sources near the ocean surface,one of its physical features is that it largely represents only the intermediate-and high-order modes.The array-level signal-to-noise ratio(SNR),which includes the array-sampled sound intensity,background noise power,and array gain,is an essential quantity determining the sonar array performance.What is investigated in this work is how the array-level SNR of the vertical line array(VLA)varies with the source depth in downward-refracting shallow water,contributed by the modal structure of the surface noise.On the assumption that the modes are well sampled,it is theoretically demonstrated that the SNR varying with the source depth can be approximated as a linear combination of the lower-order mode-amplitude intensities varying with the water depth.Particularly,when the surface noise especially dominates and the water channel is highly downward refractive,this variation can be represented nearly only by the 1st-order mode-amplitude intensity varying with depth.The structure is meaningful in practice.It suggests the SNR will be inherently larger when the source is submerged than it is near the ocean surface,and will be maximized at a source depth slightly below the 1st-order mode peak across different source ranges.The above assertions are demonstrated in a typical downward-refracting shallow-water channel;the effects from the dominant degree of the surface noise,sound speed gradient in water column,and array aperture are investigated numerically.The obtained results are shown below.(1)Under certain circumstances,the variation of SNR with source depth is nearly irrelevant to the source range.(2)When the surface noise is more significant,the largest SNR in a certain source range will be more significantly larger than the SNR for the source near the surface,the corresponding source depth will be closer to that presenting the 1st-order mode’s peak,and the variation of SNR with source depth is increasingly irrelevant to the source range.(3)A stronger downward-refracting sound speed also enhances this SNR superiority and irrelevance to the source range,but causes the source depth presenting the largest SNR to be more deviated from the 1st-order mode’s peak.(4)Although the structure is unraveled on the assumption that the VLA spans the full water column,it can be seen when the VLA does not but covers the low-order modes'main part;when the array aperture is insufficiently large it will become approximately periodic in the source range,with the source depth presenting the largest SNR fluctuating lightly and nearly periodically around the 1st-order mode peak.