3D VTEM inversion for delineating sub-vertical shear zones in the West African gold belt

Abstract

Accurately delineating sub-vertical graphitic thrust faults and shear systems is one of the major strategies followed in West African gold exploration. Due to the dense vegetation and limited outcrops, airborne electromagnetic (AEM) surveys have been routinely flown for geological mapping. We present a case study for practical 3D inversion of VTEM data acquired in Ghana. The geology is very complex, and consists of carbonaceous shear zones containing multiple thrust faults. Certain types of gold mineralization are directly associated with the shear zones which manifest themselves as sub-vertical conductors. We compare results from 3D AEM inversion to those obtained from traditional AEM interpretation methods, including conductivity depth imaging, decay constant analysis, profile anomaly picking, Fraser filtering, and plate modelling. We show that even with no a priori information, 3D AEM inversion accurately delineates the sub-vertical conductors, and recovers the most accurate representation of the geology. Interpretations based on CDIs are shown to be unreliable.