深海海底山环境下声传播水平折射效应研究

声波在深海海底山环境中传播时，海底山会对声传播产生重要影响.2016年在南海深海进行了一次海底山环境下的声传播实验，观测到了由海底山引起的三维声传播效应，本文利用BELLHOP射线理论解释了海底山环境下的三维声传播机理.结果表明：声波在传播过程中与海底山作用后破坏了深海会聚区结构，导致传播损失增大，在海底山后形成具有明显边界的声水平折射区，利用二维声传播模型无法解释实验现象，海底山后声水平折射区实验测量的声场结构与N×2D模型计算结果存在明显差异，实验的传播损失比N×2D模型计算结果大10 dB.通过三维射线模型分析N×2D模型计算结果与实验结果存在明显差异产生的原因，发现由于声波水平折射作用，部分声线无法到达接收器，使得三维声传播效应对海底山后一定角度范围内声场影响较为明显.因此，深海海底山会引起明显的三维水平折射效应，应在水下目标探测和定位等应用中给予重视.

Horizontal refraction effects of seamounts on sound propagation in deep water

The seamounts usually have important effects on sound propagation in deep water. A sound propagation experiment was conducted in the South China Sea in 2016. One of the experimental goals is to investigate the three-dimensional(3D) effects of seamounts on sound propagation. Phenomena about horizontal refraction of acoustic waves are observed in different propagation tracks which go through the seamount along different directions when the source depth is 200 m. Ray methods (BELLHOP N×2D and 3D models) which can calculate sound field efficiently and show clear physical images, are used to analyze and explain the causes of the phenomena. The experimental and numerical results show that the convergent zone structures are destroyed by the direct blockage of seamount due to the multiple reflection of acoustic waves, which leads to the increase of transmission loss (TL), and horizontal-refraction zone with obvious boundaries appears behind the seamount. Some experiment phenomena cannot be explained by BELLHOP N×2D model in which the horizontal refraction effects are not taken into consideration. The experimental sound field structure behind the seamount is obviously different from N×2D model numerical result, i.e.the width of shadow zone based on the experimental data is wider than that calculated by N×2D model, and the width of strong horizontal-refraction zone from the experiment is narrower than the N×2D model result. Moreover, the TLs calculated by N×2D model is about 10 dB less than the experimental result in horizontal refraction zone. After analyzing the difference between experimental data and N×2D model numerical results by BELLHOP 3D model which contains the azimuth-coupling capability, it can be concluded that sound waves reach the receiver through horizontal refraction after the interaction with seamount when the source is located behind the seamount. The eigenrays obtained from 3D model are less than N×2D model numerical result because some of sound beams cannot reach the receiver as a result of the horizontal refraction effects, which leads to the experimental TLs larger than the numerical results calculated by N×2D model. Therefore, 3D effect of seamount has an obvious influence on sound field within a certain angle range behind the seamount, and the investigation of 3D effects of seamounts is meaningful for the sound propagation and target detection in deep water.