Hydrographic Parameter Maps Deduced From CCD Image Sequences of the Water Surface Supplemented by In-Situ Wave Gauges


A new technique, which utilizes a combination of image sequences acquired by CCD cameras and in-situ data sets, is used to observe and analyze inhomogeneous spatiotemporal wave fields in hydraulic wave tanks. The analysis technique allows the determination of hydrographic parameters, like the near-surface current velocity vector, the water depth, and full directional calibrated wave spectra. After a geometric transformation of the image sequence from image coordinates to world coordinates the sequence is transformed to the three-dimensional wavenumber-frequency spectrum. Spectral filtering techniques allow a directional-frequency separation for each single wave of the sea-state signal. By an inverse transformation to the spatial domain noise-reduced phase and power maps of single waves are retrieved. The wavenumbers of these waves are determined by a fitting algorithm on a high-resoluted spatial scale. The composition of waves from all directions and frequencies results in a local wavenumber image spectrum. In-situ wave gauges at single points in the imaged area are used to calibrate the image spectra. This method is useful for impact studies on marine structures or beach studies in wave facilities. The analysis methods have been tested for optical sensors and as well for nautical X-band radar image sequences. This gives oceanographers, offshore, and coastal engineers a tool to study the spatio-temporal behaviour of complex wave fields in hydraulic wave tanks and in nature.
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