Investigation and quantitative analysis on the acoustic energy tobogganing in the upslope waveguide of continental slope area

Abstract

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 Pad 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.