A comparison of the potential for using optical and SAR data for geological mapping in an arid region: the Atar site, Western Sahara, Mauritania

Abstract. Our research in Mauritania focused mainly on the study of phenomena that partially or entirely masked surficial formations overlying bedrock. The analysis of ERS radar images compared to that of optical SPOT data identified geomorphological and geological objects for which the SAR provides complementary information at a regional scale; interpretation difficulties caused by, for example, shadow and lay-over phenomena; and mapping potential using geocoded ERS 1 images, notably for regional geomorphology, the sedimentology of sand bodies, and the structure of bedrock, where possible below windblown cover. The non-geocoded image must be interpreted with care to avoid the phenomenon of dip-direction inversion. The 'unwrapping of slopes' by geocoding gives a picture of the global layout of units, particularly those of the substratum in areas with medium to high topographic relief. Main faults are clearly seen on radar images at small scale and show up under conditions similar to those of optical data, but fractures that are (sub)parallel to the direction of the incident signal can be invisible on the radar data. Occasionally, the 'transparency' phenomenon enables us to 'see' below thin sand cover.