Aeromagnetic and Resistivity Surveying for Delineating Optimal Sites for Groundwater development in part of Minna, Sheet 164 SW, North-Central Nigeria

This study was conceptualised out of a need to help stem the continuous rising incidences of failed and abandoned wells in the study area. This situation is attributable to inadequate understanding of the hydrogeological and structural disposition of the area. The study area is underlain by crystalline rocks belonging to the Basement complex of Nigeria. These rocks comprise of Schist, Gneiss, Quartzite and Granite with minor occurrences of alluvial deposits. Analysis and interpretation of the Aeromagnetic data was used to map previously known and unknown fractures which are deemed to aid appreciable flow of water to drilled wells. These fractures, especially the near surface fractures were validated using interpreted VES data. The interpretation of the Vertical Electrical Sounding (VES) data was done following a calibration of various geoelectric layers from existing wells. Principal joint directions on the surface yielded primarily NE – SW trend while aeromagnetic data revealed lineaments trending principally in the ENE – WSW direction. These orientations correlate well with the general orientation of structures in the Basement Complex. VES data revealed three to four geoelectric layers comprising of the top soil at depths between 1 to 7 m and resistivity values in the range of 100 Ωm to 300 Ωm. This is followed by the weathered layer (for 3-layer system) or fractured layer (for 4-layer system). The last layer is the fresh basement rock of infinite resistivity. The weathered layers are generally shallow and have thicknesses in the range of 3 and 30 m while the weathered/fractured zone are the conductive layers which most likely are aquiferous. Optimal sites are delineated where the Aeromagnetic lineaments and surface structures intersect and where an appreciable thickness of the weathered layer is delineated. This study has been able to establish another zone of the aquiferous unit beyond the 200 m depth. The outcome of this research has been able to establish the need for both geological and hydrogeological investigations and the need to integrate at least two geophysical methods in groundwater exploration efforts in the Basement Complex.