Résumé
In Lake Chad basin, the Quaternary phreatic Aquifer (named hereafter QPA) presents large piezometric anomalies referred as domes and depressions. Depths of these piezometric anomalies are approximately 15 m and approximately 60 m, respectively. Leblanc et al., (2003) discovered that brightness temperatures from METEOSAT infrared images of the Lake Chad basin show a correlation with the QPA piezometry. Indeed, during wet seasons, domes are associated with warm brightness temperatures, at the contrary of the depressions, which appear cold in METEOSAT images. Through this observation, these authors proposed that this thermal behaviour results from an excess of evapotranspiration that can also explain the formation of the piezometric anomalies. However, data provided by temperature logs in oil wells lead us to propose another hypothesis. Temperature logs obtained in oil wells illustrate that in the ancient Lake Chad and in the Bornou depression, at the SW of the Lake, heat transport is made by convection in the approximately 3 km deep confined aquifers. Moreover, we have estimated that the heat fluxes at the surface of the ancient Lake Chad can reach 138 mW m-2 and in the Bornou depression, approximately 63 mW m-2. Others oil wells show that, at the exterior of the depression, the heat transport is conductive and the heat fluxes at the surface are approximately 50 mW m-2. These new observations permit us to propose that beneath the depressions, a cold and salty descending convective current suck the QPA. Beneath the dome, a warm and less salty ascending current creates an overpressure in the QPA. Now, to explain the link observed with the thermal behaviour, we propose that over the domes, as the QPA is warm, water can rises by capillarity. The piezometric depressions regions are associated with a presence of clay-rich soils at the surface, which makes difficult the exchange between the QPA and the atmosphere. However, we have discovered some giant dessication cracks that can facilitate the exchanges by increasing the vertical permeability of the clay cap horizon. These facts eventually explain the correlation between the observed brightness temperatures and the piezometric anomalies.