Water Surface Capillary Wave Simulation and Detection Using Optical Method

The long-range transmission of sound waves in water has been widely discussed, where the surface of water is often considered as smooth and rigid. In theory, tiny capillary waves can be induced when mechanic waves hit the interface of two substances with different wave speeds. In this paper, the shape of water-air interface is analyzed when a far-field plane acoustic wave from underwater reaches water surface. In conclusion, the amplitude of water surface wave is at submicron level, which decreases in the exponential form, and the attenuation coefficient increases greatly with the increase of frequency. Based on theories discussed, a fusion sensing system is proposed for the detection of underwater sound source from air. To build up this system, a laser vibrometer device connected to a computer with picometer resolution, is needed. At 2nm detection threshold, this detection method is able to find a 10Hz,140dB underwater sound source at 2.45m away. It is hard for underwater structures to eliminate noise in low frequency, thus monitoring water surface capillary wave is a promising new way for underwater noise detection.