Combined geophysical investigations of the characteristics of a regional fault zone for groundwater development in a basement complex terrain of South-west Nigeria

This paper presents the results of an integrated electrical resistivity and magnetic survey of a strike slip fault in a basement complex terrain with poor well/borehole yield. It aims to characterize the fault zone and determine its geometry, width and depth extent and attitude that will enable an assessment of its prospectivity for groundwater development. The magnetic profiles imaged a fault block characterized by a low (negative) total field magnetic anomaly with a width extent ranging from 112 to 150 m, a depth extent in excess of 150 m and a near vertical disposition. The resistivity structures delineate low resistivity near vertical discontinuities within the high resistivity fresh basement host rock that correlate across the five traverses investigated. The north-western edge of the magnetic fault block and the resistivity anomalous zone correlates with the geo-referenced strike slip fault. The width of the low resistivity zone ranges from 75 to 125 m with depth extent in excess of 150 m. Four subsurface geologic/geoelectric layers were delineated. These include a relatively thin topsoil underlain by a clay/sandy clay weathered basement; a partly weathered/fractured basement column and a basal fresh basement with depth to rock head of 1.6-15.0 m. A zone with thick column (up to 117 m) of partly weathered/fractured basement correlates across the traverses and significantly with the anomalous magnetic and resistivity zones. The study concludes that the thicker region of partly weathered/fractured basement column within the fault zone which is likely to have enhanced permeability and storage capacities has potential for sustainable groundwater development. It also demonstrates the effectiveness of integrated 1D and 2D electrical resistivity imaging and magnetic profiling in structure (fault) identification, characterization, subsurface sequence delineation and groundwater potential assessment in a typical basement complex environment.