Geoelectrical Prospecting for Shallow Aquifers in Otukpo, North-Central Nigeria

Abstract

A two-dimensional (2D) geoelectrical resistivity field data, consisting of six profiles were collected in parts of Otukpo metropolis using the conventional Wenner Alpha array. The obtained 2D apparent resistivity data for each profile were processed individually using a 2D inversion code via RES2DWIN interpretation software. The 2D model resistivity images obtained from the inversion are presented as horizontal depth slices. The objective of this study is to ascertain the suitability of the investigated profiles for the development of shallow wells using resistivity imaging. The investigation was focused on the measurement and interpretation of the shallow subsurface characteristics that favors groundwater occurrence. The 2D Resistivity Imaging (RI) was utilized in fulfilling this objective employing the Wenner Alpha array. The near surface resistivity of the profiles tends to be high, in the range of 102 Ωm to 1254 Ωm. This could be attributed to the occurrence of laterite which is a product of intense chemical weathering and leaching of soluble iron-rich materials. It extends to a range of 1m to 12m depth on the profiles. The middle layer is indicative of moderate resistivity in the range of (< 50 Ωm) attributed to existence of clayey materials. The resistivity range of 1.01 Ωm to 5.29 Ωm (blue area) characterize the profiles and the resistivity of the profiles decreases with depth, indicating the existence of impermeable shale within a thickness of less than 20 meters depth on the profiles. From the investigation it was inferred that there are heterogeneous lithological formations in the shallow near subsurface of Otukpo area which suggest that wells constructed in the area should be confined to the silty-sand lithology which are overlapped above and underlain below by laterite and silty-clay. These wells may yield water sufficient for domestic purposes. The depth of the wells should be limited to the depth range of 18m to 20m where it terminates at the topmost shale layer of the profiles.