Impact of continental-slope bottom on ambient noise in the South China Sea

The impact of continental-slope bottom on the ocean ambient noise is analyzed by using experimental noise data received from the same depth at two locations on the continental slope and in the deep water, respectively. The noise spectral level and vertical directivity under different bottom conditions are calculated using a three-dimensional ray model, and then the influence of the bottom environment on ambient noise field is studied. The numerically predicted noise spectral level agrees well with the experimental data in terms of absolute amplitude and frequency trend. The experimental data and simulation results show that, in the shallow area of the continental shelf, the waveguide is not conducive to long-distance propagation for ship noise, therefore the low frequency (below 300 Hz) noise level is lower than that in the deep area at the same depth. Meanwhile, due to the bottom with hard characteristics in the shallow area, the high frequency (above 1 kHz) noise from the sea surface can also reach the receiver after being reflected by the seabed in addition to directly reaching the receiver. Therefore, the high frequency noise level in the shallow area on the continental shelf is higher than that in the deep area, and decreases with frequency slower. As a result, there is a crossover phenomenon of the ambient noise spectral levels on the continental slope and at the deep water with the increase of frequency.