Optical detection of ensonified capillary-gravity waves using polarimetric imaging

The optical detection of surface capillary-gravity waves induced by underwater sound has many potential applications that range from the detection of sound-generating underwater objects to airborne bathymetric surveys. While multiple lab-based efforts have measured acoustically generated surface capillary-gravity waves, we report on a recent field-based measurement using polarimetric imaging. A controlled acoustic source was placed 10 m below a lake surface and emitted sound in the 500 Hz to 10000Hz frequency range. The lake surface was imaged using a polarimetric camera mounted 7 m above the lake surface. Measurable short-lived surface capillary-gravity waves (∼3 mm wavelength) were observed in the polarimetric camera images during ensonification of the lake surface. Changes were observed in both the omnidirectional and directional wave spectra. In the omni-directional wavenumber spectrum, enhanced capillary wave activity at high wavenumbers was observed for acoustic source frequencies in the 2–5 kHz range. Additionally, ensonification was observed to result in the amplitude and wavenumber modulation (enhancement/diminution) of existing wind-generated surface gravity-capillary waves. In the directional spectra, while ambient gravity-capillary waves showed a spreading function with stronger downwind versus upwind propagation, the acoustically generated gravity-capillary waves showed minimal impact on the directionality of the wave spectra.