Electromagnetic Characterisation of Basement Fractures for Geothermal Exploration in the Benue Trough, North East Nigeria

The Benue Trough is associated with volcanic and sub-volcanic features that suggest elevated crustal heat flux, and deep basement fracture systems may act as conduits for geothermal fluid circulation if adequately characterised. This study applies audiomagnetotelluric and magnetotelluric sounding methods to characterise deep basement fractures in the vicinity of known thermal spring occurrences in the Middle Benue Trough, Taraba and Benue states. MT/AMT stations were occupied at 48 sites using broadband recording over the frequency range of 0.001 to 10,000 hertz, sampling resistivity structures from the near surface to approximately 20 kilometres depth. Phase tensor analysis was applied to identify dimensionality effects prior to inversion, confirming a predominantly 2D electrical structure along the survey profiles. Two-dimensional inversion using the OCCAM algorithm produced resistivity-depth models revealing a major low-resistivity zone of less than 5 ohm-metres extending to depths of 6 to 12 kilometres beneath the Wikki Warm Springs thermal feature in Bauchi State. This conductor is interpreted as a fluid-saturated fracture network in basement rock that facilitates deep groundwater circulation. The depth of the source zone is inconsistent with purely near-surface heating, supporting a geothermal gradient of approximately 45 degrees Celsius per kilometre in this area. The study provides the first deep electromagnetic characterisation of a Nigerian geothermal system and delineates a drill target for geothermal exploration. Keywords: magnetotelluric, geothermal, Benue Trough, basement fractures, thermal spring