Radar imaging mechanism of marine sand waves at very low grazing angle illumination caused by unique hydrodynamic interactions

Abstract

The investigations carried out between 2002 and 2004 during six field experiments within the Operational Radar and Optical Mapping in monitoring hydrodynamic, morphodynamic and environmental parameters for coastal management (OROMA) project aimed to improve the effectiveness of new remote sensing monitoring technologies such as shipborne imaging radars in coastal waters. The coastal monitoring radar of the GKSS Research Center, Geesthacht, Germany, is based on a Kelvin Hughes RSR 1000 X band (9.42 GHz) vertical (VV) polarized river radar and was mounted on board the research vessel Ludwig Prandtl during the experiments in the Lister Tief, a tidal inlet of the German Bight in the North Sea. The important progress realized in this investigation is the availability of calibrated X band radar data. Another central point of the study is to demonstrate the applicability of the quasi‐specular scattering theory in combination with the weak hydrodynamic interaction theory for the radar imaging mechanism of the seabed. Radar data have been taken at very low grazing angles ≤2.6° of flood and ebb tide–oriented sand wave signatures at the sea surface during ebb tidal current phases. Current speeds perpendicular to the sand wave crest ≤0.6 m s−1 have been measured at wind speeds ≤4.5 m s−1 and water depths ≤25 m. The difference between the maximum measured and simulated normalized radar cross section (NRCS) modulation of the ebb tide–oriented sand wave is 27%. For the flood tide–oriented sand wave, a difference of 21% has been calculated. The difference between the minimum measured and simulated NRCS modulation of the ebb tide–oriented sand wave is 10%, and for the flood tide–oriented sand wave, a value of 43% has been derived. Phases of measured and simulated NRCS modulations correspond to asymmetric sand wave slopes. The results of the simulated NRCS modulation show the qualitative trend but do not always quantitatively match the measured NRCS modulation profiles because the quasi‐specular scattering theory at very low grazing angle is a first‐order theory.