Investigation of the Geothermal Potential in the Eastern Side of Olkaria Domes Using Magnetotelluric Method

Abstract

High-temperature geothermal resources are commonly found along rift margins due to the intrusion of magma into the shallow crust, as is evident in the Kenyan Rift Valley. Among the volcanic centers in this region, Olkaria stands out as the most extensively explored and is currently in an advanced stage of geothermal development. Despite its success as a major energy source, further investigations are on-going to optimize the development of its geothermal fields. In particular, the area east of the Olkaria domes remains largely unexplored, with only one well, OW-922, drilled to date. Unfortunately, this well failed to sustain discharge during testing. This highlights the need for a comprehensive study to assess the geothermal potential and improve the understanding of the field in this region. To address this, both 2D and 3D magnetotelluric (MT) inversions were performed using data from 50 soundings. The obtained models were analysed to map the subsurface structures influencing the geothermal system on the eastern side of the domes. A spatial median filter was applied to correct for static shift effects on the MT data. Dimensionality analysis was then used to classify the type of the subsurface structures. Phase Tensor analysis, one of the dimensionality tools, indicated low skew values at short periods (10²–1s), suggesting predominantly 1D and 2D structures at shallow depths. In contrast, higher skew values observed at longer periods (10 s) pointed to the presence of complex 3D structures at greater depths. The regional structures are striking in a general trend of NE-SW as observed from the geo-electric strike estimation method. The resistivity models obtained from 2D and 3D inversion revealed three layers varying with depth. The upper near-surface resistive layer of > 100 Ωm, extending almost to 1 km above sea level. Beneath this layer is a conductive zone of about < 10 Ωm, which could be the possible reservoir seal. The third layer has a relatively high resistivity of > 80 Ωm, representing the possible reservoir zone ranging between 2.5 to 4 km.