深海海底斜坡环境下的声传播

Sound propagation in deep water with a sloping bottom

海底地形变化对声传播具有很大影响,在南海深海区域海底斜坡环境下进行了一次声传播实验,实验显示倾斜海底环境下声传播损失出现了一些不同于平坦海底环境下的现象,分析并解释了海底地形变化对产生声传播差异的原因.结果表明,海底斜坡对声波的反射增强作用可使斜坡上方的声传播损失减少约5 d B.当声波第一次入射到达的海底位置有较小幅度的山丘(凸起高度小于1/10海深)时,海底小山丘即可对声波有反射遮挡作用,导致在其反射区特定传播距离和深度上出现倒三角声影区,比平坦海底环境下相同影区位置处的传播损失增大约8 d B,影响深度可达海面以下1500 m.而海底斜坡对声波的反射阻挡作用使得从海面反射及水体向下折射的会聚区结构消失,只剩下从水体向上折射的会聚结构.因此,海底地形对深海声传播影响较大,在水下目标探测和性能评估等应用中应予以重视.

Variation of bathymetry has a large effect on the sound propagation in deep water. An acoustic propagation experiment is carried out in the South China Sea. Some different propagation phenomena are observed for two different tracks in the flat bottom and the sloping bottom environments. Numerical analysis based on the parabolic equation model RAM（range-dependent acoustic model） is performed to explain the causes of the differences. The experimental and numerical results show that the transmission losses（TLs） decrease down to about 5 d B above the slope due to the reflection of the bottom, with a high-intensity region appearing below the sea surface. When a sea hill with a height of320 m, which is less than 1/10 of water depth, exists in the incident range of sound beams on bottom first time, the sound beams are blocked due to the reflection of the sea hill. Then their propagating directions are changed, which makes an inverted-triangle shadow zone appearing in the reflection area of the sea hill. Compared with the TL results in the flat bottom environment, TLs increase up to about 8 d B in the corresponding area of the first shadow zone, and the abnormal TL effects can reach a maximal depth of 1500 m. Consequently, the shadow amplification effect caused by a small variation of bathymetry in deep water for long-range/large-depth sound propagation should receive enough attention. Furthermore, the convergence-zone structure in the sloping environment is different from that in deep water with flat bottom. The first convergence zone caused by refractions from the water above the axis of sound channel disappears. There are only the sound beams refracted back from water below the axis of sound channel. The numerical simulations show that the reflection-blockage of sound beams caused by the sloping bottom is significant. When the source is located somewhere above the slope, sound beams with large grazing angles can be reflected by the sloping bottom, and only some sound beams with small grazing angles can be refracted in the water without touching the slope and then come into the depth range of the vertical line array（VLA）, forming the first part of the convergence zone refracted back from water. As the source moves farther from the VLA, the reflection-blockage of the sloping bottom becomes stronger. Sound beams are all reflected by the slope at a depth of about 3000 m, and they go through below the VLA, which leads to the absence of the first convergence zone caused by refractions from the water above the axis of sound channel. Therefore, the accuracy of bathymetry is meaningful for the sound propagation and target detection in deep water.