A COMPARISON OF THE POTENTIAL FOR USING OPTICAL AND SAR DATA FOR GEOLOGICAL MAPPING IN AN ARID REGION - THE ATAR SITE, WESTERN SAHARA, MAURITANIA

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