Study of very-low-frequency sound propagation characteristics in the complex ocean space based on the spectral element method

The research on the acoustic field in the complex ocean space is carried out with the spectral element method (SEM). The study focuses on the characteristics of very-low-frequency (VLF) underwater acoustic propagation and the influence of seafloor relief on it. An axisymmetric model which takes the complex flow-solid coupling near the seafloor into account is constructed. Then the point source excitation and full-wave simulation in the SEM are realized. Through discussing in time and time-frequency domains, it is found that the energy structure of the Scholte wave is stable with no significant change in pulse width and frequency range after passing through several upstairs terrains. The propagation characteristics of VLF acoustic waves are affected by both the overall topography and local undulations. The acoustic transmission loss in almost real seafloor relief space, such as stepped terrain, cannot be effectively described by one single logarithmic expression only. In shallow sea areas with soft sediment and stepped-up topography, the interface waves excited by a sound source close to the seabed have a better ability to propagate long distances than the sound waves in the water. The overall acoustic transmission loss in the horizontal ocean space with soft sediment is approximated by the columnar expansion when the interface waves are effectively excited and received.