Daniel Kiefhaber

Water wave measurement from stereo images of specular reflections

A new optical instrument for the study of ocean waves, the Reflective Stereo Slope Gauge, has been developed. Its purpose is to measure ocean wave field parameters that are crucial to the air–sea exchange of momentum, heat and gases. The instrument combines a statistical wave slope measurement method similar to Cox and Munks sun glitter technique with a dedicated stereo camera and associated illumination setup for direct wave height measurements. The instrument output was validated under controlled conditions in a wind-wave facility.

Development of a reflective stereo slope gauge for the measurement of ocean surface wave slope statistics

An optical instrument for the measurement of surface ocean wave statistics has been developed and is presented in this thesis. Based on the reflective stereo slope gauge (RSSG) principle, it can simultaneously measure wave slope and height statistics. The instrument comprises a stereo camera setup and two light sources built from infrared LEDs (940 nm). Slope statistics are derived from the statistical distribution of the positions of specular reflections in images of the water surface. The parallax of the reflections in the stereo images gives the distance of the camera to the water surface which can be used to infer wave height statistics. A laboratory version of the instrument has been built, calibrated and tested in experiments at the Aeolotron wind wave facility. Two-dimensional slope probability distributions for slopes in the range of -0.05 < s_x,s_y < 0.05 were obtained for a range of wind speeds and clean water as well as surface slick conditions. The mean square slope of the surface was derived and compared to data from simultaneous reference measurements. The two data sets are found to agree well for lower wind speeds, at higher wind speeds significant deviations occur. The causes for these deviations have been identified and taken into account in designing the instrument for field measurements. This ocean version of the instrument is ready to accompany heat exchange measurements aboard a research vessel in the Baltic Sea in June 2010.