The Chott El Djerid, Tunisia: Observation and Discussion of a SAR Phase Signature Over Evaporitic Soils

The presence of water in arid regions is correlated to large evaporitic deposits, with high concentration of salts. Polarimetric synthetic aperture radar (SAR) observations over such areas show high variations for both the amplitude and phase of the backscattered copolarized signal. This is due to a large dynamic range for both surface roughness and dielectric constant parameters, between the wet and dry seasons: crystallized salt is rough and presents a low dielectric constant, whereas saline water corresponds to smooth and conductive surfaces. We observed a complete seasonal cycle over the chott El Djerid playa, in southern Tunisia, using the 5.6-GHz C-band polarimetric SAR on-board RADARSAT-2. One SAR image acquisition was performed every 24 days, from February to September 2009. In addition to expected variations in the radar backscattered power, we observed significant changes in the phase difference between horizontally and vertically polarized channels. In order to explain such a phase effect, we first considered the arguments of the Fresnel reflectivity coefficient when approaching the Brewster angle, for materials presenting a high loss tangent. A more complete analytical modeling derived from the integral equation model approach confirmed this hypothesis.