Determination of the Aquifer System of the Northern Sector of Bida Basin, Nigeria Using Electrical Resistivity Method

The northern sector of the Bida Basin extends from Gulu in the south to Kontagora in the north, where it contacts the crystalline rock of the Basement Complex system. It lies between latitudes 5o80' and 10o50' N and longitudes 4o50' and 7o00' E. The Basin consists of Maastrichtian sedimentary deposits comprising of the Bida Sandstone, Sakpe Ironstone, Enagi Siltstone and Batati Ironstone Formations, with a total sedimentary pile of between 3500 to 4500 m. The aquifer is composed mainly of coarse to medium grained sandstone. Geoelectric survey was conducted using the Schlumberger configuration to a total AB/2 separation of 300 m. 140 Vertical Electrical Sounding (VES) points were established along nine profiles spread across the Basin. The resulting curves were interpreted both qualitatively and quantitatively using visual inspection of the field curve, curve fitting using standard master and subsidiary curves as well as by computer iteration (Interpex IX1D). The results indicate that the dominant curve types are the AA, QQH and KQ. Four groups of curve types were obtained in the Basin, Group 1 (H, K), Group 2 (A, AA, KHK), Group 3 (HK, HKQ, KHKQ, KHKHK, QQH) and Group 4 (QHK, KQQ, KQ, QHK, Q). Based on these four groups the Basin was divided into a northern, central and southern sectors as well as geological contact areas. Geoelectric sections, developed from geoelectric logs show that three types of aquifers exist at various depths; a perched aquifer with a depth of between 5 to 20 m, a semi confined / confined aquifer at between 30 to 60m and an unconfined aquifer with undetermined thickness. The topsoil has a resistivity range of 50-500 Ohm-meters, ferruginised sandstone, 800-2000 Ohm-meters, sandy clay, 150-500 Ohm-meters, sandstone 300-1500 Ohm-meters and clay/shale, 10 to 100 Ohm-meters. The aquifer is represented by the sandstone.