Poster Session 1, Monday, October 3, 15:40–18:00
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 using Laser Doppler Vibrometry, we report on a recent field-based measurement using polarimetric imaging. A recently conducted experiment ensonified a lake surface with underwater sound with the goal of inducing optically-detectable surface capillary-gravity waves. A controlled acoustic source was placed 10 m below the lake surface, and emitted sound in the 500 Hz to 10,000 Hz frequency range with source levels that varied between 165 dB re 1 uPa and 171 dB re 1 uPa. The sea surface was imaged using a polarimetric camera mounted 7 m above the sea surface, mounted to a stable barge. Measurable short-lived surface capillary-gravity waves (~3 mm wavelength) were observed in the polarimetric camera images during ensonification of the sea 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.
Christopher Zappa, Lamont Doherty Earth Observatory of Columbia University, [email protected]
Kaus Ragukumar, Integral Consulting Inc., [email protected]
Grace Chang, Integral Consulting Inc., [email protected]
Frank Spada, Integral Consulting Inc., [email protected]