Remote sensing and geophysical investigations in French Guiana
Résumé
The combination of remote sensing and geophysics is of great importance for the mapping of substrate areas with interesting mineral potential. Remote sensing investigations carried out in French Guiana mainly with radar data is allowing to differentiate morphologies and lithologies, and to map large-scale structures, particularly in the volcanosedimentary units of the Northern Synclinorium. For the Palaeoproterozoic series, airborne total magnetic field provides better arguments to highlight structural features, while airborne radiometry delineates more accurate contrasts for plutonic varieties. On the one hand, uranium (U) and thorium (Th) are abundant in the plutonic areas, with variations revealing two main generations of granite. On the other hand, potassium (K) is highly concentrated in more pelitic terms of the sedimentary units. Airborne total magnetic field and SAR data provide information about the basaltic dykes, which cut through the Proterozoic terranes. Most of them were emplaced during the Early Jurassic onset of the Atlantic Ocean. However older dyke swarms, Proterozoic in age, reveal other textural or structural behaviour. Other results concerning the geomorphology and hydrogeology of the coastal plain have been obtained (fan-shaped arrangement, capture, migration of the estuaries, etc.). In the particular case of the preserved rain forest of French Guiana, the SAR backscattering is mainly due to the shape of the homogeneous canopy modulated by regional morphology. The geophysical data correspond to characteristics of the rocks themselves, on the one hand in-depth for magnetic data and from the other hand at the surface for the radiometric data. These airborne and space-borne data sets are highly complementary for drawing reconnaissance map at large-scale.