Geophysical Contribution to Ecological Mapping of the Chad Basin in Borno and Yobe States, North East Nigeria

The shrinking of Lake Chad represents one of the most dramatic environmental changes in sub-Saharan Africa, and understanding the subsurface geology and hydrogeology of the Nigerian sector of the Chad Basin is essential for informed natural resource and ecological management in Borno and Yobe states. This professional study integrates airborne electromagnetic, gravity, and aeromagnetic data to map subsurface geological formations controlling groundwater occurrence and sediment thickness in the Nigerian Chad Basin. Helicopter-borne frequency-domain electromagnetic data acquired over a 40,000-square-kilometre area were inverted to produce three-dimensional resistivity models of the upper 200 metres of the basin fill. These models reveal the geometry of palaeochannels buried within the Quaternary Chad Formation, which represent the most productive aquifer horizons in the region. Bouguer gravity data interpretation constrains the total sediment thickness, ranging from less than 100 metres at the basement outcrops on the southern basin margin to over 2,000 metres near the international boundary. The integration of electromagnetic basement depth estimates with gravity models provides a reconciled basement topography map. Palaeochannel geometry delineated from the resistivity models is correlated with wetland distribution data to identify ecologically significant groundwater-dependent ecosystems. The study provides the Nigerian Hydrological Services Agency with an evidence-based groundwater resource atlas for the Chad Basin. Keywords: Chad Basin, airborne electromagnetics, groundwater, palaeochannel, ecological mapping