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 highresolutionrnbathymetry and environmental data wasrnanalysed to estimate the relationship between seabedrnsand waves and the modulation of the normalised radarrncross-section (NRCS) in C-band VV. Wind speeds nearrn4 m/s and fetch-limited wind-wave conditions appearrnfavourable for imaging of sand waves. EmpiricalrnModulation Transfer Functions (MTF's) were comparedrnwith MTF's based on a 2-scale wave-current interactionrnmodel. The model reproduces measured phase shiftsrnwell; magnitudes are slightly too low. The modelrnpredicts phase shifts near 90o while the two-scalernmechanism is dominant. Therefore, the tilting longrnwaves relax fast enough to remain near equilibrium withrnthe local current gradient. Seabed reconstruction fromrnSAR images with this model reproduces most sandrnwave crests. Loss of effective resolution, ascribed torninsufficient averaging over the tilting long waves, isrnseen as the main limitation for charting of sand waves.