Hydrographic parameter maps retrieved from nautical radar image sequences of inhomogeneous water surfaces

Abstract

The radar backscatter from the ocean surface, called sea clutter, is modulated by the surface wave field, and in addition in coastal areas by hydrodynamic interactions of the tidal current with the variable bottom topography. Using a nautical X-band radar image sequences of the water surface are acquired. The bottom signatures are static during a single measurement, whereas the wave signature, whose spatio-temporal evolution is given by the dispersion relation of surface gravity waves, is dynamic. These signatures are separated by frequency filtering. The analysis of the static radar signatures is performed in order to detect long term changes of the bathymetry. A nautical X-band radar was mounted on the island of Sylt in the German Bight in the period from February to June 1997, and from December 1998 to April 1999. The observed area is of high interest because of morphological changes. The static radar signatures have altered in a noticeable way between the two observation periods. A method was developed to estimate the near-surface current, the water depth and calibrated surface wave spectra from the dynamic radar signature, with is inhomogeneous in coastal areas due to the effects of wave refraction and diffraction. Along with the presented methods the nautical radar is a versatile instrument to detect morphodynamic changes together with the causal forces.