Joint Inversion of Gravity and Magnetic Data for Basement Geology in the Chad Basin

This study applies joint inversion of gravity and magnetic potential field data to improve characterisation of basement geology beneath the thick sedimentary cover of the Chad Basin in northeastern Nigeria. The Chad Basin contains important groundwater resources in the multi-layer Chad Formation and potential undiscovered hydrocarbon resources, but the deep basement geology is poorly constrained due to the absence of deep drilling. Jointly inverting gravity and magnetic data with cross-gradient coupling exploits the complementary sensitivity of the two methods to produce more geologically consistent basement models than separate inversion. This study processes Bouguer gravity data from a 70-station ground survey supplemented by satellite gravity (EGM2008) and aeromagnetic data from the NGSA archive for the Maiduguri-Monguno area of Borno State. Data are levelled, quality-filtered, and converted to equivalent formats. 2D joint inversion using the SIMPEG open-source platform applies cross-gradient constraints linking density and magnetic susceptibility structure. Initial models are constrained by available seismic reflection interpretations and borehole data from the Lake Chad Basin Authority. Model uncertainty analysis estimates parameter confidence. Findings reveal a Precambrian basement topography varying from 1.8 to 4.8 kilometres depth across the study area, with a prominent basement graben beneath the central Borno Basin coinciding with the regional gravity low. Two basement ridges interpreted as inliers of dense Archean crust create magnetic highs that could be confused with mineralisation targets without the density constraint. The study demonstrates that joint inversion significantly reduces the ambiguity inherent in single-method basement interpretation in thick sedimentary basins.

Keywords: joint inversion, gravity, magnetic data, Chad Basin, basement geology.