Abstract
Internal solitary waves in the South China Sea have attracted attention because of their large amplitude and high rate of occurrence. Internal solitary waves have a substantial influence on underwater sound propagation and ambient noise. However, there have seldom been reports on the noise they cause. In this paper, we conducted an internal solitary waves cooperative observation experiment in the South China Sea in 2016. We analyzed the temperature, flow velocity and noise changes induced by internal solitary waves. The power spectra of noise generated by internal solitary waves at frequencies below 100 Hz was almost 20 dB higher than ambient noise. The observed low-frequency noise had uniform harmonics. Combined with the changes of flow velocity, we interpreted the low frequency noise as flow noise induced by vortex-induced vibration of internal solitary waves flowing past the cable mooring system. The noise spectra were related to the position of the cable where the hydrophone was mounted. The closer they were to the middle of the cable, the greater the vibration amplitude, and the stronger the noise. This study provided a passive acoustic monitoring and warning method for high marine currents.
Highlights
Internal solitary waves (ISWs), characterized by their large amplitudes, high rate of occurrence and strong nonlinearity, are widely distributed in global oceans with crest lengths of up to 200 km [1]
We found that the low-frequency flow noise was produced by the cable vibrating under the impact of ISWs, named vortex-induced vibration (VIV)
We analyzed the noise during three large ISWs and found that ISWs produced strong noise at low frequencies
Summary
Internal solitary waves (ISWs), characterized by their large amplitudes, high rate of occurrence and strong nonlinearity, are widely distributed in global oceans with crest lengths of up to 200 km [1]. In addition to sound propagation characteristics, ISWs have been observed to contribute substantially to ambient noise by breaking waves on the sea surface, increasing the internal velocity of currents, and stirring marine sediments [12]. The intensity of flow noise decreases with increasing frequency It is usually observable on a mooring-mounted hydrophone array in the cable, but not detected on a fixed hydrophone on the seabed [23]. We observed the water temperature and flow velocity changes induced by ISWs as part of the 2016 Internal Solitary Wave Cooperative Observation Experiment in the South China Sea. we analyzed the effects of ISWs on noise using acoustic data from a mooring-mounted hydrophone array.
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