Detection of extreme waves using radar-image sequences

A method is presented to localize wave groups spatially and spatio-temporally utilizing synthetic aperture radar (SAR) images and nautical radar-image sequences of the ocean surface. Extreme waves can grow in space and time as a result of wave group evolution. These wave groups have to be taken into account for instance for the design of offshore platforms, breakwaters or ships, because they can cause severe damage on those structures. To detect extreme waves, dominant wave groups are selected from SAR images and radar-image sequences by considering the wave envelope. A radar-image sequence is transformed into the wave-number frequency domain using a 3D Fourier transform where the signal of the ocean gravity waves is filtered using a band pass filter based on the dispersion relation for linear surface gravity waves. Thereafter, a 3D Hilbert transform is applied to the filtered complex Fourier coefficients, which are then transformed back into the spatio-temporal domain applying an inverse 3D Fourier transform. The resulting spatio-temporal complex envelope of the wave field is investigated for the dominant wave groups, by considering the amplitude of the complex envelope. With slight changes the algorithm can also be applied to single radar images. To test and verify the algorithm, several radar image sequences were acquired with the wave monitoring system WaMoS-II, which is based on a nautical radar operating in the X-band (9.5 GHz) near grazing incidence. The instrument was operated on towers in the North Sea. All these data sets are exploited with respect to the localization of extreme waves and wave groups.