STEREO IMAGING TECHNIQUES FOR THREE-DIMENSIONAL SURFACE WAVE MEASUREMENT: APPLICATIONS AND IMPROVEMENTS
Campbell, Alexander J
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Stereo imaging is used to measure surface waves in two different experimental setups. First, stereo imaging is used remotely measure waves interacting with three distinct ice types: brash, frazil, and pancake. Along-shore spatial frequency distributions show that pancake and frazil ices attenuate higher frequency waves faster than lower frequency waves, causing a decrease in mean frequency. In contrast, wave propagation through brash ice causes a rapid increase in dominant wave frequency, believed to be the result a non-linear energy transfer to higher frequencies caused by collisions between the brash ice particles. Total wave energy flux was observed to decrease exponentially in both pancake and frazil ice, whereas total energy flux remained constant in the brash ice due to thin layer thickness. Spatial energy flux distributions also reveal that wave reflection occurs at the boundary of each ice layer, with reflection strongest at the pancake/ice-free and frazil/brash interfaces and weakest at the brash/ice-free interface. These high resolution observations demonstrate the spatially variable nature of waves propagating through ice. Second, efficient flexible-baseline stereo imaging is developed to study boat wakes and Langmuir circulation over range of scale (up to 1000 m2). Given the difficulty associated with obtaining ground control points over a water surface, determining the exterior camera orientation is generally the most difficult part of field calibration. The innovative calibration technique accomplishes this using a two step process where exterior orientation is first estimated using a total station and then refined using a series of geometric and epipolar constraints. Processing times are also reduced using phase-only correlation and parallel processing. The first experiment explores the wake patterns generated by boats moving at different speeds. Analysis includes maximum wave height, crest length, wake angle, and interaction with ambient waves. Results processed using the new technique measured a water surface displacement time series to within 1.2 cm mean absolute error when compared with a wire wave gauge which is on the order of the camera resolution. The second experiment investigates waves in the presence of Langmuir circulation, including orientation of Langmuir cells to wave direction and influence on wave energy. Overall EFSI increases the applicability of stereo imaging to measure surface waves.