Island connected sea bed signatures observed by multi-frequency synthetic aperture radar

Multifrequency X-, C-, and L-band synthetic aperture radar (SAR) images of the northern sea area off the isle of Heligoland in the German Bight of the North Sea have been analysed. The data were collected during the SAR and X-band Ocean Nonlinearities Research Platform North Sea Experiment (SAXON-FPN) which was carried out in November, 1990. Different oceanographic phenomena are visible on simultaneously obtained SAR images. Wind streaks and a vortex street can be identified only on the X- and C-band SAR images. Elongated streaks of predominantly low radar return are related to near-shore reefs and are imaged on all available radar scenes. The imaging mechanism of these submarine ridges is investigated and discussed using with some modifications the simple Bragg relaxation model proposed by Alpers and Hennings. The improved model differs from the original version in the representation of the unperturbed wave-energy spectral density. Also the advection term and the phase modulation (velocity bunching) have been included in the model. Due to the improvements it is now possible to simulate the radar cross-section modulation with the same order of magnitude at 0.4 GHz (X-band) and 5.3 GHz (C-band) as well as at 1.3 GHz (L-band). However, the simulated radar cross-section modulation is still underestimated compared with the SAR data, but the phase of the calculated radar cross-section modulation agrees quite well with the SAR measurements.