Radar Imaging Mechanism of the Seabed: Results of the C-STAR Experiment in 1996 with Special Emphasis on the Relaxation Rate of Short Waves due to Current Variations

Abstract

During the field experiment of the Coastal Sediment Transport Assessment using SAR imagery project of the Marine Science and Technology program of the European Commission an Air?Sea Interaction Drift Buoy (ASIB) system was equipped with special sensors and instruments to measure the position, the water depth, the surface current velocity and direction, the modulation characteristics of short-wave energies, and relevant air?sea interaction parameters due to undulations in the seabed. The ASIB system was operated from on board a research vessel and the data were measured while the buoy drifted in the tidal currents across sand waves of the study area. All buoy measurements were analyzed by computing frequency spectra of low and high frequency waves (scalar spectra between 0.1 and 50 Hz). The whole range of short water waves was recorded by these in situ measurements on board the buoy, which is responsible for the backscattering of commonly used air- and spaceborne imaging radars. A comprehensive dataset of wave energy density spectrum modulations above sand waves was produced. Normalized Radar Cross Section (NRCS) modulations of a selected P-band airborne Experimental-Synthetic Aperture Radar (E-SAR) image were compared with wave energy density spectrum variations at the appropriate short surface gravity Bragg-wave frequency measured along the drift path of the ASIB system. The NRCS and wave energy density modulation depths agreed within a factor of 2. Using the obtained in situ measurements from the ASIB system the relaxation rate ? of short water waves due to current variations above submarine sand waves was calculated by applying a first-order weak hydrodynamic interaction theory. The relaxation rate ? dependence on several responsible hydrodynamic air?sea interaction parameters was calculated as a function of wavenumber k in the range of P-, L-, C-, and X-band radar Bragg waves for three different mean wind speed regimes of Uw = 0.8 m s?1, Uw = 3.8 m s?1, and Uw = 7.4 m s?1. Several published parameterizations of ? showed that this parameter increases with wavenumber and wind speed. Results show that ? increases also with wind speed Uw but decreases with wavenumber k. This can possibly imply that the wind growth relaxation rate ?w is not equivalent with the relaxation rate ? of short waves due to current variations above submarine sand waves as a function of k. The analysis can also imply that the Bragg scattering mechanism seems to be insufficient to explain completely alone the NRCS modulation due to the seabed via surface current gradients especially at higher radar frequencies.