Constraints on the source of unrest at the Aluto-Langano geothermal field, Ethiopia, inferred from 3-D interpretation of MT measurements

The global energy demand is ever rising and renewable energies are considered to be a major contributor to any future energy mix. A promising candidate is geothermal energy as it is carbon-neutral and readily available in regions that may have no access to conventional energy resources. Geothermal power generation is most attractive in volcanic regions with ready access to shallow high enthalpy systems. As for instance in Iceland and New Zealand, where a well established infrastructure allows profitable exploitation of geothermal resources accounting in a large part for the local energy production. One of the privileged regions possessing a remarkable, but so far largely untapped geothermal potential is the East African Rift system (EARS). The EARS is an active continental break-up zone hosting numerous young volcanic systems with most of them concentrated along its eastern branch between Mozambique and Ethiopia. Considerable progress in geothermal exploration along the EARS is so far limited to Kenya and Ethiopia, where first geothermal power plants have been installed during the 90s. Currently several geothermal projects are in progress in these regions and a considerable development of the renewable energy sector is expected in the near future. One plant is under construction at Corbetti volcano in Ethiopia, once completed it is estimated to generate over 1000 MW electric power and hereby meant to be Africa’s largest geothermal power plant (Reykjavik Geothermal, 2014). Recently the International Renewable Energy Agency (IRENA) presented a strategy to build a Clean Energy Corridor stretching from Ethiopia to South Africa to exploit the excellent renewable energy potential along the EARS focusing on hydro, geothermal, solar and wind power (IRENA Headquarters, 2013). The aim of this project is to meet the increasing energy demand of the rapidly growing economies in East Africa by massive investment in renewable energy. It is worth noting that the advantage of geothermal sources compared to other renewable sources like wind, solar and hydro power is their independence from weather conditions and their constant output with availability around the clock. The region of interest addressed in this study is the Main Ethiopian Rift System, which encompasses a number of volcanoes that have been identified as potential high enthalpy geothermal systems in the past (Endeshaw, 1988). Some of them are known to be actively deforming with reoccurring periods of uplift and setting as indicated by satellite observations (Biggs et al., 2011). One of the regions where temporal changes take place is the Aluto-Langano volcanic complex. It hosts Ethiopia’s currently only producing geothermal power plant, which taps a geothermal system with fluid temperatures exceeding 350 ◦C (Gianelli and Teklemariam, 1993). The observed periods of uplift at Aluto took place in 2004 and 2008, they affected a region of around 100 km2 and were followed by periods of subsidence. The power plant is located in the center of the deforming region where the maximum amplitudes of unrest occur. This state of play clearly raises the question of the unrest’s implication on the plant in terms of productivity and geohazard. The working hypothesis is that the causative source for the deformation is either in the hydrothermal reservoir, in a deeper magmatic system or in coupled magmatic-hydrothermal system. The aim of this thesis is to discriminate between the different scenarios and to delineate the nature of the deforming source. In order to do this we conducted magnetotelluric (MT) measurements. This geophysical induction method uses natural occurring time-varying electromagnetic fields to decipher subsurface electrical conductivities and is especially sensitive to high conducting zones, as hydrothermal and magmatic reservoirs usually are (Muñoz, 2014). Furthermore it easily covers the necessary exploration depth down to approximately 10 km. In the past years MT has been successfully implemented in geothermal research and has proved to be a reliable and cost-efficient method in identifying high enthalpy geothermal systems on the basis of subsurface conductivities. This is supported by recent and ongoing developments of efficient computational numerical methods, which make it capable to interpret and to invert for MT data in a fully 3-D manner. The study addressed in this thesis involved the whole process of organizing and planning a field campaign, including logistics and customs clearance. The field measurements in Ethiopia were conducted together with a team of scientists from Addis Ababa University, ETH Zurich, the Geological Survey of Ethiopia and local people from the survey region. In total we installed 46 MT sites covering the extent of the Aluto volcanic complex. The acquired data were processed, modeled and interpreted in context of interdisciplinary studies previously conducted at the Aluto volcanic complex and in the Main Ethiopian Rift System. Our recovered 3-D models reveal an electrical resistivity distribution, which is in accord with the conceptual reservoir model of a high enthalpy geothermal system, where a low resistive clay cap overlies the more resistive upflow zone (Johnston et al., 1992). Our models provide no evidence for an active magmatic system, this is why we conclude that the source of unrest is most likely situated within the shallower part of the hydrothermal system. In order to put constraints on possible mechanisms that might trigger the cyclic periods of uplift and setting we studied publications on the analysis of well data and fluids from Aluto that were mainly published in the 90s. These studies consistently report major changes over time in the hydrothermal regime of the geothermal field and reveal complex water-rock interaction processes taking place in at least the upper 2.6 km of the reservoir as known from well logs (e.g. Gizaw, 1993; Teklemariam et al., 1996). On the basis of these findings we argue in favor of two different kinematic mechanisms that might trigger the observed unrest: The first mechanism is related to the hydro-mechanical behavior of clay minerals and their tendency to swell and shrink when exposed to changes of water saturation and pore water chemistry (de Siqueira et al., 1999; Xu et al., 2006). The second mechanism we refer to is thermoelastic expansion of fractured rock consequent to forced advection of hot fluids (Bonafede, 1991; Troiano et al., 2011). All in all it is very likely that fluids act as causal agent driving kinematic mechanism that finally result in the observed ground