COMPARISON OF DATA AND MODEL PREDICTIONS OF CURRENT, WAVE AND RADAR CROSS-SECTION MODULATION BY SEABED SAND WAVES

摘要

A comprehensive set of ERS SAR images with highresolution bathymetry and environmental data was analysed to estimate the relationship between seabed sand waves and the modulation of the normalised radar cross-section (NRCS) in C-band VV. Wind speeds near 4 m/s and fetch-limited wind-wave conditions appear favourable for imaging of sand waves. Empirical Modulation Transfer Functions (MTF's) were compared with MTF's based on a 2-scale wave-current interaction model. The model reproduces measured phase shifts well; magnitudes are slightly too low. The model predicts phase shifts near 90o while the two-scale mechanism is dominant. Therefore, the tilting long waves relax fast enough to remain near equilibrium with the local current gradient. Seabed reconstruction from SAR images with this model reproduces most sand wave crests. Loss of effective resolution, ascribed to insufficient averaging over the tilting long waves, is seen as the main limitation for charting of sand waves.