Magnetic method for multi-scale exploration of the Franceville Basin

The purpose of this thesis is to develop new tools in magnetic method to characterize geological units. First, we present theoretical and numerical developments of the potential field theory for magnetic method in space and frequency domains. We generalize the reduction-to-the-pole transformation so that symmetrical sources cause symmetrical magnetic functions. Thus, the superpotential functions are newly defined for magnetic methods. The magnetic anomaly tensor is defined as the superpotential second derivatives. These two new functions have interesting symmetry properties that help magnetic maps interpretation. Second, we apply our results to analyze aeromagnetic data from the Franceville Basin collected in Gabon by AREVA NC for mining exploration. The aim of this study is to analyze these data in order to establish a new structural map and identify potential uranium targets. The metallogenic model of the Francevillian uranium helps us to link the presence of potential uranium deposits with the existence of magnetic anomalies. A multi-scale study is carried out in Fourier domain to explore sub-surface faults and the deep basement. The magnetic tensor acts as directional filtering and enables us to locate 2D structures. To conclude, we gather the results in a structural map providing the structuralist new information regarding uranium deposits of economic interest.