GEOELECTRIC EVALUATION OF THE GROUNDWATER POTENTIAL OF A SCHIST BELT- IKORO-EKITI LOCALITY, SOUTH WESTERN NIGERIA

Abstract

Geophysical investigation involving the Vertical Electrical Sounding (VES) and 2- D Dipole-Dipole electrical resistivity profiling was carried out at Ikoro Ekiti with a view to evaluating the groundwater potential of the schist underlain locality. Six traverses were established within the study area with each 500 m long. Dipole- Dipole profiling was carried out along three of the traverses with station separation of 25 m. Sixteen VES dataset were acquired along all the traverses using the Schlumberger array. The half current electrode spacing (AB/2) ranged from 1 – 225 m with maximum spread length of 450 m. The VES curves were preliminarily quantitatively interpreted by partial curve matching. The interpretation results (layer resistivities and thicknesses) were used as starting model parameters in 1-D computer assisted forward modeling with the WinRESIST software. The final interpretation results were used to generate geoelectric sections and maps. Dipole- Dipole data were used to prepare pseudosections and inverted to 2-D electrical images using the DIPPRO software. The geoelectric sections delineate five geoelectric layers which include the topsoil, laterite, the weathered layer, the partly weathered basement and the fresh basement rock. The topsoil varies in thickness from 0.5-5.3 m with layer resistivity values of between 38 and 336 ohm-m. The composition of the layer ranges from clay to sandy clay and clayey sand. The lateritic clay layer varies in thickness from 1.5-11.7 m with layer resistivity values of between 137 and 789 ohm-m. The weathered layer varies in thickness from 12.2-104.2 m with layer resistivity values of between 20 and 137 ohm-m. It is composed of clay and sandy clay. The partly weathered basement has resistivity and thickness values of 244 ohm-m and 22.6 m respectively. The fresh basement bedrock has resistivity values varying from 369-9997 ohm-m. The depth to the basement bedrock varies from 15.7-116.6 m. The isoresistivity map of the weathered layer displays resistivity values that are generally less than 100 ohm-m. The isopach maps of the weathered layer and the overburden delineate a suspected NW-SE trending basement depression on the northern flank. The 2-D resistivity structure along traverses TR2, TR4 and TR6 delineate four geologic layers. These are the topsoil, the lateritic layer, the weathered layer and the basement bedrock. The groundwater potential evaluation of the VES Stations based on thresholds of weathered layer resistivities and thicknesses and the nature of the basement bedrock shows that eleven VES Stations (68.75% of the total VES Station) display low groundwater potential rating while two (2) (12.5%) has low/medium groundwater potential rating and three (3) (18.75% of the total VES stations) have medium groundwater potential rating. It can therefore be concluded that the groundwater potential of Ikoro Ekiti Schist Belt is generally low.