Groundwater Exploration using Geoelectric Technique in Oru-Ijebu, South-West Nigeria

Abstract

Forty Vertical Electrical Soundings (VES) were carried out using Allied Ohmega resistivity meter with schlumberger configuration in parts of Oru-Ijebu to determine the necessary geoelectrical parameters for delineating the aquiferous zones. Four to five distinct geoelectric layers were delineated from the survey area except for VESORU21 which exhibited six geoelectric layers namely the top soil, clayey sand/laterite, clay and granite gneiss. The first layer serves as the topsoil with high variable resistivity ranging from 11.2Ωm (VESORU7) to 153.8Ωm (VESORU25) with their corresponding thickness of 0.9m to 3.8m respectively. The thickness of the entire top soil of the investigated area ranges from 0.4m to 3.8m. The second layer is composed of sand, clay sand, laterite and partly sandy clay. The resistivity of clay/clayey sand overburden varying from 16.2Ωm with a thickness of 8.7m in VESORU17 and 484.3Ωm with a thickness of 3m VESORU6. The highest resistivity exhibited by the second layer is 974 Ωm in VESORU30 while its lowest resistivity is 21.9Ωm in VESORU34. The resistivity of the third layer which stands as a weathered lithology ranges from 17.2Ωm with a thickness of 10.3m at VESORU7 to 233.2Ωm at VESORU23 whose thickness is indeterminate due to current termination in the field to 196 Ωm; typically diagnostic of clay/clayey sand horizon except beneath VESORU1, VESORU2, VESORU11, VESORU30, VESORU32, VESORU34, and VESORU36 where the inferred lithology is sandstone, fresh basement and partly sandy-clay. The fourth and fifth layers are composed of fresh basement formation notably granite-gneiss rocks with resistivity values ranging from 88.5Ωm VESORU20 to 384Ωm in VESORU15. The maximum aquifer thickness is encountered beneath VESORU25. The weathered layer for the study area is thick enough for groundwater accumulation making it a very prolific one. VESORU1, VESORU2 and VESORU23 are marked by a low groundwater yield due to the dip and the thickness.