陆架斜坡海域上坡波导环境中声能量急剧下降现象及其定量分析

摘要

The toboggan in acoustic energy will appear at the top of the slope when the sound wave radiated by a shallow water source propagates in an upslope waveguide of the continental slope area. The grazing angles of the sound rays reflected by the ocean bottom will increase in the upslope waveguide, which leads to the acoustic energy tobogganing in the shallow water at the top of the slope. In this paper, the range of acoustic energy tobogganing (RAET) at a specified depth is defined to study this phenomenon. The transmission loss (TL) is calculated by the parabolic-equation acoustic model that ie applied to the range-dependent waveguide. The RAET is defined by an average transmission loss in the abyssal water and in the shallow water corresponding to the depth. The acoustic energy toboggan is explained using the ray-based model, and the effects of source location change on it are demonstrated, including the source depth and the range away from the bottom of the slope. The sound rays from a shallow water source which transmit in the upslope waveguide can be divided into two types: one is incident to the interface vertically and will return to the water along the original path; the other is that the rays will transmit towards the sound source (the deep sea direction). However, all of them will no longer spread forward after they have transmitted to a certain distance, leading to the acoustic energy tobogganing in shallow water. The analysis results show that the RAET becomes larger with source depth increasing, and the energy toboggan phenomenon will disappear when the source is deep enough. However, the range of source away from the slope bottom has less effect on RAET. Numerical simulations are conducted in a continental upslope environment by the RAM program based on the split-step Padé algorithm for the parabolic equation. The simulation results show as follows. 1) The TL will increase rapidly after the waves have transmitted to a certain range away from the bottom of the slope when the source depth is 110 m, and the TLs is 140–160 dB propagating to the shallow water at the top of the slope. 2) The RAET will enlarge orderly when the source depths are 110 m, 550 m and 800 m respectively, and the energy toboggan phenomenon will disappear when the source depth is more than 800 m. 3) Fix the source depth at 110 m and move it along the deep sea, then the RAET will greatly varies when the distance between the source and the slope bottom changes ina range of 1–15 km. However, the RAET remain almost constant at 69.8 km when the distance between the source and the slope bottom changes in a range of 16–50 km.%深度较浅的声源其辐射声波在陆架斜坡海域上坡传播时,在斜坡顶端会出现声能量急剧下降现象.利用射线声学模型分析了造成这一现象的原因,并根据抛物方程声场模型计算的深海和浅海平均传播损失定义了"声能量急剧下降距离",定量分析了声源位置对该现象的影响.结果表明:声源深度对"声能量急剧下降距离"影响较大,而声源与斜坡底端水平距离对其影响较小;当声源深度变大时,部分掠射角较小的声线最终能够达到斜坡顶端,致使"声能量急剧下降距离"增大,继续增加声源深度,将导致上坡声能量急剧下降现象消失.利用抛物方程声场模型对陆架斜坡海域上坡声传播进行数值仿真,结合"声能量急剧下降距离"的定义,计算并比较了声源位置不同时该距离的变化,数值计算结果验证了理论分析.