Lithological Identification of the Subsurface Using Electrical Resistivity Method in Ikot Ebidang Town, Onna Local Government Area of Akwa Ibom State, Nigeria.

Abstract

This study aimed at identifying lithologies of the subsurface using Electrical Resistivity method in Ikot Ebidang Town, Onna Local Government of Akwa Ibom State. A total of eleven (11) vertical electrical soundings (VES) were carried out in some crucial locations within the town using both the Schlumberger and Wenner configuration for earth probing. The probing was done using the ABEM Tarrameter SAS 300B, with maximum electrode spacing of 400m (for Schlumberger) and 140m (for Wenner array). The field data were analyzed using IP 12 WIN software to obtain the subsurface resistivities, thicknesses and depth of different VES points within the study area. In this study, the lithologies of different layers were obtained based on layer’s resistivity and thickness. The geoelectric section of profile A and B were constructed and there was a gradual variation in the lithologies of the area showing the topsoil, granite, sand, quartz and bedrock in profile A. The resistivity of profile A ranges from 115Ωm – 830,000 Ωm. The geoelectric section of profile B showed lithologies including the topsoil, consolidated shale, granite, sand stone and bedrock. The resistivity of profile B ranges from 25.2 Ωm – 390,000 Ωm. The difference in lithologies of the above profiles is as the result of variation in resistivity values within the region. The overall resistivity and thickness values within the area ranges from 25.2 Ωm - 1,100,00 Ωm and 0.176m – 37.81m respectively. The isoresistivity maps of layer 1, 2 and 3 as well as isopach maps of layer 1 and 2 were plotted using surfer 29 software. The iso-resistivity maps of layer 1, 2 and 3 showed variation in resistivity with different colouration, indicating the apparent resistivity (100Ωm – 1,800Ωm), (-50,000Ωm – 450,000Ωm) and (-40,000Ωm – 4,000,000Ωm) respectively. The isopach maps of layer 1 and 2 showed variation in thicknesses with different colouration, indicating thicknesses (0.25m – 1.3m) and (0 – 2.5m) respectively. The maps showed areas with similar resistivity values and thicknesses based on map’s colouration.