Geophysical investigation involving the use of geoelectrical technique has been carried out to delineate buried geological structures around the proposed Ganawuri Dam Site North Central Nigeria. The objective of this study is to establish the depth to fresh basement in the area using the VES technique with a view to delineating those near surface structures that could be deleterious to any construction work embarked upon within the area and to proffer a befitting recommendation that would necessitate the construction of a dam to impound the flowing stream and develop a reservoir which could be used for irrigation. The area is underlain by the Crystalline Basement rocks composed of granite gneiss, with late diorite, basalts and pegmatite intrusions. The older units have been intruded by the biotite granite of the Younger Granite province, occurring as a ridge bordering the north eastern margin of the area. ABEM Terrameter SAS 1000C model was used and the symmetrical Schlumberger configuration adopted. A total of twenty (20) Vertical Electrical Sounding (VES) stations were established and fully occupied along three (3) different profiles (AA/, BB/ and CC/) in the study area. Preliminary input data from the field were fed into Zohdy software to generate real resistivities and depths to geoelectric layers. Three (3) geoelectric layers were interpreted. The first geoelectric layer is a thin lateritic top surface with resistivity range of 270-4100 ohm-m with average thickness of 1.94m. The second geoelectric layer is interpreted as the weathered basement with resistivity range of 16-3510 ohm-m and the thickness ranges from 1.06-29.2m. This layer was observed to be very clayey at VES points 5, 6, 7, 10, 12, 16 and 17. The third interpreted geoelectric layer represents the fresh basement with infinite resistivity trend and thickness. The VES section delineated presence of fault/shear zones at 13m, 22m, 13-41m and 13-21m at VES 2, 3, 18 and 19 respectively along profile AA/. Along profile BB/, fault/shear zones were observed at 25-36m below 27m and 28m at VES 13, 14 and 15 respectively. Fault/shear zones were equally delineated between 11-17m at VES 10 along profile CC/. These shear zones are known for their structural weaknesses. Any construction work to be sited within the area should take into account these zones and geotechnical method such as grouting should be applied to seal off these shear zones.
Keywords: Basement, Ganawuri, Geoelectric Layer, Schlumberger array, Seepage, Shear zone
