Subsurface imaging with low frequency SAR. Field validation in France and Egypt using a ground-penetrating radar

We study the capabilities of low frequency radar systems to sound the subsurface in arid countries. This approach is based on the coupling between two complementary radar techniques: the airborne Synthetic Aperture Radar (SAR) used in L-band (1.2 GHz) for imaging large scale subsurface structures, and the Ground-Penetrating Radar (GPR) used between 500 and 900 MHz for sounding soils at a local scale, from the surface down to several meters. In this paper, we first recall the results obtained on the Pyla dune (France). This site is a bare sandy area presenting large subsurface structures (paleosoils) at varying depths. A polarimetric analysis of airborne SAR data, as well as the GPR sounding experiment, shows that subsurface scattering occurs at several places. The SAR penetration depth is estimated by inverting a simple scattering model for which the subsurface structure, i.e. geometric and dielectric properties, is determined by the GPR data analysis. The recent results obtained on the well-known site of Bit Safsaf (southern Egypt) are then presented. The comparison between L-band SAR and GPR sections shows that penetration effects occur in many places, revealing rich subsurface structures. These results suggest that airborne radar systems in a lower frequency range (P-L band) should be able to detect soil structures down to several meters, leading to innovative Earth observation systems for geological and hydrogeological mapping in arid regions.