Sound propagation in deep water with a sloping bottom

Abstract

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 dB 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 of 320 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 dB 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.