A combined interpretation of electrical resistivity and airborne magnetic datasets for assessment and evaluation of a geothermal reservoir in South West Nigeria

Abstract

The surface occurrence and manifestation of warm and hot springs in a location are of utmost concern and require preliminary investigation to ascertain the reasons for this natural event. This event has also been observed in some other parts of Nigeria. So, the Ikogosi Warm Spring (IKGWS) area and environs were chosen as a study site located in the southwestern part of Nigeria because of their proximity. Thus, the causes and sources of the flow of hot and cold springs manifested on the surface in the area were explored. In this work, a combination of two geophysical techniques was used to describe and understand the configuration and processes of this typical event. The method involved the acquisition of resistivity data using a Terrameter (ABEM SAS 300C) from nine (9) vertical electrical soundings (VES) stations and supported by airborne magnetic data from the warm and cold sections of the study area. The VES analysis shows that the subsurface is made of four to six layers with resistivity ranging from 1.0 to 1334.7 Ωm. It also revealed that VES 1, 2 and 3 has resistivity ranging from 62.5-1082.8 Ωm with depths between 0.7-58.8 m and thickness 0.7-45.7 m while VES 4, 5 and 6 has resistivity ranging from 83.7-1334.7 Ωm with depth between 0.6-30.6 m and thickness 0.6-18.3 m. The general distribution of resistivity shows a very low value near the Ikogosi spring, revealing that resistivity ranged from 1 to 2540.3 Ωm, depth: 0.6 to 112.3 m, and thickness: 0.7 to 66.8 m. The results from heat flow computation from aeromagnetic investigations confirmed that there are zones in the IKGWS with anomalous heat flow, which have great prospects for geothermal energy exploration. The heat flow estimated for the IKGWS and environs ranged between 75 and 127 mW/m^2, and the average value of 100 mW/m^2, while the temperature gradient varied between 28 and 48 °C/Km and the mean value of 38 °C/Km.  Likewise, the Curie point depth (CPD) estimated ranged between 12 and 21 km and mean value of 16 km.