3-D analysis and interpretation of magnetotelluric data from the Aluto-Langano geothermal field, Ethiopia

Abstract

The Main Ethiopian Rift Valley encompasses a number of volcanoes, which are known to be actively deforming with reoccurring periods of uplift and setting. One of the regions where temporal changes take place is the Aluto volcanic complex. It hosts a productive geothermal field and the only currently operating geothermal power plant of Ethiopia. We carried out magnetotelluric (MT) measurements in early 2012 in order to identify the source of unrest. Broad-band MT data (0.001-1000 s) have been acquired at 46 sites covering the expanse of the Aluto volcanic complex with an average site spacing of 1 km. Based on this MT data it is possible to map the bulk electrical resistivity of the subsurface down to depths of several kilometres. Resistivity is a crucial geophysical parameter in geothermal exploration as hydrothermal and magmatic reservoirs are typically related to low resistive zones, which can be easily sensed by MT. Thus by mapping the electrical conductivity one can identify and analyse geothermal systems with respect to their temperature, extent and potential for production of energy. 3-D inversions of the observed MT data from Aluto reveal the typical electrical conductivity distribution of a high-enthalpy geothermal system, which is mainly governed by the hydrothermal alteration mineralogy. The recovered 3-D conductivity models provide no evidence for an active deep magmatic system under Aluto. Forward modelling of the tippers rather suggest that occurrence of melt is predominantly at lower crustal depths along an off-axis fault zone a few tens of kilometres west of the central rift axis. The absence of an active magmatic system implies that the deforming source is most likely situated within the shallow hydrothermal system of the Aluto-Langano geothermal field.

The Electrical Structure of the Central Main Ethiopian Rift as Imaged by Magnetotellurics: Implications for Magma Storage and Pathways

“…Additionally, there is a systematic mismatch in the phase values for both the stations directly above and further to the east of the conductor, which are better reproduced in the preferred inversion model. This confirms that the presence of a deeper conductor is required to explain data from the western‐central section of the profile and is in accordance with the forward modeling of tipper data on Aluto by Samrock et al (). We also tested the sensitivity of the MT data to a conductive feature representing a shallow magma chamber under Aluto and found that the presence of such a feature would easily be detected by the data collected above it (see supporting information Figure S3).…”

“…Below, a resistive layer extends to about 15 km depth. Under Aluto, the subsurface is resistive at depths < 3 km, which confirms the results from the much higher‐resolution 3‐D model from Samrock et al (), but which was based on a smaller array aperture ‐ and hence limited depth resolution ‐ of MT stations around the volcano edifice. With the frequency content of our data and the profile length in this study, we would have identified a deeper conductive anomaly under or around Aluto that might be expected from significant amounts of magma or partial melt, known to be present from geodesy, seismology, and degassing studies, had it existed.…”

“…Samrock et al () performed a MT survey to investigate the electrical conductivity structure under Aluto. They collected high‐quality MT data in an array on and around the volcanic edifice and identified a shallow zone of higher conductivity as the clay cap over the hydrothermal system.…”

“…Further south in the Main Ethiopian Rift, Whaler and Hautot () found that the subsurface was significantly less conductive than in Afar but that there were still conductive features located below the rift axis as well as near the rift flanks. Samrock et al () investigated the central rift volcano Aluto that surprisingly did not exhibit a mid‐crustal conductive feature, although their study indicated a conductor beneath one of the rift flanks. Selway () investigated the electrical conductivity of the East African Rift and the Tanzanian Craton and in their model also found a conductive crustal body beneath Mount Kilimajaro.…”

Geothermal potential and origin of natural thermal fluids in the northern Lake Abaya area, Main Ethiopian Rift, East Africa

“…1). One example is the Aluto-Langano geothermal system in the central part of the Main Ethiopian Rift (MER), where InSar (Interferometric Synthetic Aperture Radar) analysis has identified ongoing surface deformation with cycles of uplift and subsidence (Biggs et al, 2011), and which hosts a welldeveloped hydrothermal reservoir and a productive geothermal field (Gizaw, 1993;Teklemarian et al, 1996;Samrock et al, 2015). Besides Aluto, knowledge of the geothermal potential of other areas in Ethiopia has increased in recent years; examples include i) Tendaho in the Afar depression (Didana et al, 2015), ii) Gedemsa-Sodere and iii) Corbetti (Teklemarian and Kebede, 2010;Kebede, 2014); these latter two are located within the MER.…”