Deep-sea infrasonic noise during typhoon processes in the northern South China Sea

Ocean infrasonic sound information has expansive application prospects in underwater acoustic target detection, ocean acoustic tomography, and other fields. The data recorded by an acoustic mooring from 17:00 on August 6th to 1:00 on August 27th, 2012, Beijing time, on the deep-sea basin in the northern South China Sea, have been studied to analyze the characteristics of deep-sea infrasonic noise during typhoon processes. It is shown that the infrasonic noise level of 10-20 Hz is mainly distributed between 82-100 dB, which is affected by ship noise and is not correlated with sea surface wind speed. When there is no typhoon in the northern South China Sea, the infrasonic noise level will periodically increase and decrease, with a fluctuation period of about M₄ tidal cycle. The fluctuation pattern of infrasonic noise disappears during typhoons, and the noise level remains at low level. After conducting studies on spatial filtering and sound propagation simulation, it is believed that the infrasonic noise radiated by ships is affected by the changes of sea water sound speed at the propagation path, resulting in fluctuations in noise levels. When there is a typhoon in the northern South China Sea, a large number of ships will choose to avoid the typhoon, and the ship noise sources will be significantly reduced, resulting in the low level of noise levels.