Friedemann Samrock Publications Content


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Dambly, M.L.T., F. Samrock, A. Grayver, H. Eysteinsson , and M.O. Saar, Geophysical imaging of the active magmatic intrusion and geothermal reservoir formation beneath the Corbetti prospect, Main Ethiopian Rift , Geophysical Journal International, 236, pp. 1764-1781, 2024. [Download] [View Abstract]Silicic volcanic complexes in the Main Ethiopian Rift (MER) system host long-lived shallow magma reservoirs that provide heat needed to drive geothermal systems. Some of these geothermal systems in Ethiopia appear to be suitable for green and sustainable electricity generation. One such prospect is located at the Corbetti volcanic complex near the city of Awassa. High-resolution imaging of the subsurface below Corbetti is of imminent importance, not only because of its geothermal potential, but also due to reported evidence for an ongoing magmatic intrusion. In this study, we present a new subsurface 3-D electrical conductivity model of Corbetti obtained through the inversion of 120 magnetotelluric stations. The model elucidates a magmatic system under Corbetti and reveals that it is linked to a magma ponding zone in the lower crust. Magma is transported through the crust and accumulates in a shallow reservoir in form of a magmatic mush at a depth of 4 kmb.s.l. below the caldera. The imaged extent and depth of the shallow magma reservoir is in agreement with previous geodetic and gravimetric studies that proposed an ongoing magmatic intrusion. Interpreting our model with laboratory-based conductivity models for basaltic and rhyolitic melt compositions suggests that Corbetti is seemingly in a non-eruptible state with∼6–16 vol. percent basaltic melt in the lower crust and∼20–35 vol. percent rhyolitic melt in the upper crust. With these observations, Corbetti’s magmatic system shares common characteristics with volcanic complexes found in the central MER. Specifically, these volcanic complexes are transcrustal two-stage magmatic systems with magma storage in the lower and upper crust that supply heat for volcano-hosted high-temperature geothermal systems above them. According to the presented subsurface model, a cross-rift volcano-tectonic lineament exerts first-order controls on the magma emplacement and hydrothermal convection at Corbetti. Our study depicts hydrothermal convection pathways in unprecedented detail for this system and helps identify prospective regions for future geothermal exploration. 3-D imaging of both the Corbetti’s magmatic and associated geothermal systems provides key information for the quantitative evaluation of Corbetti’s geothermal energy potential and for the assessment of potential volcanic risks.

Dambly, M.L.T., F. Samrock, A. Grayver, and M.O. Saar, Insights on the interplay of rifting, transcrustal magmatism and formation of geothermal resources in the central segment of the Ethiopian Rift revealed by 3-D magnetotelluric imaging, Journal of Geophysical Research: Solid Earth, 128, 2023. [Download] [View Abstract]The Main Ethiopian Rift is accompanied by extensive volcanism and the formation of geothermal systems, both having a direct impact on the lives of millions of inhabitants. Although previous studies in the region found evidence that asthenospheric upwelling and associated decompression melting provide melt to magmatic systems that feed the tectono-magmatic segments in the rift valley, there is a lack of geophysical models imaging these regional and local scale transcrustal structures. To address this challenge, we use the magnetotelluric method and image subsurface electrical conductivity to examine the magmatic roots of Aluto volcano, quantify and interpret the melt distribution in the crust considering established concepts of continental rifting processes and constrain the formed geotherma system. Specifically, we combined regional (maximum 30 × 120 km2) and local (15 × 15 km2) magnetotelluric data sets and obtained the first multi-scale 3-D electrical conductivity model of a segment of the central Main Ethiopian Rift. The model unravels a magma ponding zone with up to 7 vol. % melt at the base of the crust (30 − 35 km b.s.l.) in the western part of the rift and its connection to Aluto volcano via a fault-aligned transcrustal magma system. Melt accumulates at shallow crustal depths (≥ 4 km b.s.l.), thereby providing heat for Aluto’s geothermal system. Our model suggests that different volcano-tectonic lineaments in the rift valley share a common melt source. The presented model provides new constraints on the melt distribution below a segment of the rift which is important for future geothermal developments and volcanic hazard assessments in the region.

Samrock, F., A. Grayver, M.L.T. Dambly, M.R. Müller, and M.O. Saar, Geophysically guided well siting at the Aluto-Langano geothermal reservoir, Geophysics, 88, pp. 1-43, 2023. [Download] [View Abstract]Volcano-hosted high-temperature geothermal reservoirs are powerful resources for green electricity generation. In regions where such resources are available, geothermal energy often provides a large share of a country’s total power generation capacity. Sustainable geothermal energy utilization depends on the successful siting of geothermal wells, which in turn depends on prior geophysical subsurface imaging and reservoir characterization. Electromagnetic resistivity imaging methods have proven to be a key tool for characterizing magma-driven geothermal systems because resistivity is sensitive to the presence of melt and clays that form through hydrothermal alteration. Special emphasis is often given to the “clay cap,” which forms on top of hydrothermal reservoirs along the flow paths of convecting geothermal fluids. As an example, the Aluto-Langano volcanic geothermal field in Ethiopia was covered with 178 densely spaced magnetotelluric (MT) stations. The 3D electrical conductivity model derived from the MT data images the magma body that acts as a heat source of the geothermal system, controlling geothermal convection and formation of alteration zones (commonly referred to as clay cap) atop the geothermal reservoir. Detailed 3D imaging of the clay cap topography can provide direct insight into hydrothermal flow patterns and help identify potential “upflow” zones. At Aluto all productive geothermal wells were drilled into zones of clay cap thinning and updoming, which is indicative of underlying hydrothermal upflow zones. In contrast, nonproductive wells were drilled into zones of clay cap thickening and lowering, which is an indicator for underlying “outflow” zones and cooling. This observation is linked to fundamental characteristics of volcano-hosted systems and can likely be adapted to other geothermal fields where sufficiently detailed MT surveys are available. Therefore, high-resolution 3D electromagnetic imaging of hydrothermal alteration products (clay caps) can be used to infer the hydrothermal flow patterns in geothermal reservoirs and contribute to derisking geothermal drilling projects.

Samrock, F., A.V. Grayver, O. Bachmann, Ö. Karakas, and M.O. Saar, Integrated magnetotelluric and petrological analysis of felsic magma reservoirs: Insights from Ethiopian rift volcanoes , Earth and Planetary Science Letters, 559/116765, 2021. [Download] [View Abstract]Geophysical and petrological probes are key to understanding the structure and the thermochemical state of active magmatic systems. Recent advances in laboratory analyses, field investigations and numerical methods have allowed increasingly complex data-constraint models with new insights into magma plumbing systems and melt evolution. However, there is still a need for methods to quantitatively link geophysical and petrological observables for a more consistent description of magmatic processes at both micro- and macro-scales. Whilst modern geophysical studies provide detailed 3-D subsurface images that help to characterize magma reservoirs by relating state variables with physical material properties, constraints from on-site petrological analyses and thermodynamic modelling of melt evolution are at best incorporated qualitatively. Here, we combine modelling of phase equilibria in cooling magma and laboratory measurements of electrical properties of melt to derive the evolution of electrical conductivity in a crystallizing silicic magmatic system. We apply this framework to 3-D electrical conductivity images from magnetotelluric studies of two volcanoes in the Ethiopian Rift. The presented approach enables us to constrain key variables such as melt content, temperature and magmatic volatile abundance at depth. Our study shows that accounting for magmatic volatiles as an independent phase is crucial for understanding electrical conductivity structures in magma reservoirs at an advanced state of crystallization. Furthermore, our results deepen the understanding of the mechanisms behind volcanic unrest and help assess the long-term potential of hydrothermal reservoirs for geothermal energy production.

Morschhauser, A., A.V. Grayver, A. Kuvshinov, F. Samrock, and J. Matzka, Tippers at island geomagnetic observatories constrain electrical conductivity of oceanic lithosphere and upper mantle, Earth, Planets and Space, 71/17, pp. 1-9, 2019. [Download] [View Abstract]Geomagnetic field variations as recorded at geomagnetic observatories are important for global electromagnetic studies. However, this data set is rarely used for studying the local electrical conductivity at depths $<200$ km. The main reasoning being that given a single geomagnetic observatory, one can at most constrain the one-dimensional (1-D) conductivity structure beneath it. At the same time, tippers, magnetic transfer functions resolving these depths, are zero for any 1-D conductivity distribution. We show that the ocean induction effect alleviates these limitations for observatories on islands and develop a method to invert tippers for a 1-D conductivity profile in the presence of three-dimensional conductivity structure due to bathymetry. This allows to recover 1-D upper mantle conductivity profiles at remote oceanic locations where little or no knowledge is available and that would otherwise be difficult to access. We apply the method to Gan in the Indian Ocean and to Tristan da Cunha in the South Atlantic, and the obtained conductivity profiles indicate a normal oceanic mantle and elevated conductivities, respectively, which fits well with their geological settings.

Samrock, F., A.V. Grayver, H. Eysteinsson, and M.O. Saar, Magnetotelluric image of transcrustal magmatic system beneath the Tulu Moye geothermal prospect in the Ethiopian Rift, Geophysical Research Letters, 2018. [Download] [View Abstract]Continental rifting is initiated by a dynamic interplay between tectonic stretching and mantle upwelling. Decompression melting assists continental break-up through lithospheric weakening and enforces upflow of melt to the Earth’s surface. However, the details about melt transport through the brittle crust and storage under narrow rift-aligned magmatic segments remain largely unclear. Here we present a crustal scale electrical conductivity model for a magmatic segment in the Ethiopian Rift, derived from 3-D phase tensor inversion of magnetotelluric data. Our subsurface model shows that melt migrates along pre-existing weak structures and is stored in different concentrations on two major interconnected levels, facilitating the formation of a convective hydrothermal system. The obtained model of a transcrustal magmatic system offers new insights into rifting mechanisms, evolution of magma ascent, and prospective geothermal reservoirs.

Kuvshinov, A., J. Matzka, B. Poedjono, F. Samrock, N. Olsen, and S. Pai, Probing Earth’s conductivity structure beneath oceans by scalar geomagnetic data: autonomous surface vehicle solution, Earth, Planets and Space, 68 (1)/189, 2016. [Download]

Samrock, F., A. Kuvshinov, J. Bakker, A. Jackson, and F. Shimeles, 3-D analysis and interpretation of magnetotelluric data from the Aluto-Langano geothermal field, Ethiopia, Geophysical Journal International, 202/3, pp. 1923-1948, 2015. [Download] [View 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.

Bakker, J., A. Kuvshinov, F. Samrock, A. Geraskin, and O. Pankratov, Introducing inter-site phase tensors to suppress galvanic distortion in the telluric method, Earth, Planets and Space: EPS, 67/1, pp. 160, 2015. [Download] [View Abstract]A common problem when interpreting magnetotelluric (MT) data is that they often are distorted by shallow unresolvable local structures, an effect known as galvanic distortion. We present two transfer functions that are (almost) resistant to galvanic distortion. First, we introduce the electric phase tensor, which is derived from the electric tensor, where the electric tensor relates the horizontal electric fields at a field and base site. The electric phase tensor is only affected by galvanic distortion, if present, at the base site. Second, we introduce the quasi-electric phase tensor, which is derived from the quasi-electric tensor, where the quasi-electric tensor relates the electric field at a field site with the magnetic field at a base site. The quasi-electric tensor is not affected by galvanic distortion. Using a synthetic data-set, we show that the sensitivity of the MT phase tensor, the quasi-electric phase tensor, and the electric phase tensor is comparable for our model under consideration. Furthermore, we demonstrate that stable (quasi-) electric phase tensors can be recovered from a real data-set with the use of existing processing software. In addition, we provide a formalism to propagate the uncertainties from the estimated (quasi-) electric and impedance tensors to their respective phase tensors. The uncertainties of the (quasi-) electric phase tensors are of the same order of magnitude as the uncertainties of the MT phase tensor. From our study, we conclude that the (quasi-) electric phase tensors are an attractive complement to the standard MT responses.

Samrock, F., and A. Kuvshinov, Tippers at island observatories: Can we use them to probe electrical conductivity of the Earth's crust and upper mantle?, Geophysical Research Letters - AGU Journal, 40/5, pp. 824-828, 2013. [Download] [View Abstract][1] For decades, time series of hourly-mean values of the geomagnetic field measured on a global network of observatories have been routinely used to recover the electrical conductivity distribution in midmantle depths. Nowadays, most observatories provide data in the form of minute-means. This allows for analysis of short-period geomagnetic variations, which, in principle, contain information about geoelectric structures in the crust and upper mantle. However, so far these data have been ignored for induction studies of the Earth due to a theoretical preconception. In this paper, we demonstrate that short-period responses (tippers) at island observatories, being large owing to the ocean effect, are also sensitive to 1-D structures and thus can be used for probing the Earth. This means that a huge amount of data that was not exploited hitherto for induction studies should be reconsidered as a useful source of information about geoelectric structures in oceanic regions where our knowledge is still very limited.

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Dorj, P., F. Samrock, and B. Erdenechimeg, Update of Geothermal Development of Mongolia, Proceedings World Geothermal Congress, 2020. [View Abstract]A first large scale detailed geophysical exploration work in Arkhangai province (a largest geothermal active zone) is done between 2019 and 2020. Based on the result of this geophysical exploration work a combined geothermal district heating and power production plant will be built in Arkhangai province in the coming few years. Ground source heat pump application is broadly introduced in the country using ground water and soil heating system.

Samrock, F., A.V. Grayver, B. Cherkose, A. Kuvshinov, and M.O. Saar, Aluto-Langano Geothermal Field, Ethiopia: Complete Image Of Underlying Magmatic-Hydrothermal System Revealed By Revised Interpretation Of Magnetotelluric Data, Proceedings World Geothermal Congress 2020, 2020. [Download] [View Abstract]Aluto-Langano in the Main Ethiopian Rift Valley is currently the only producing geothermal field in Ethiopia and probably the best studied prospect in the Ethiopian Rift. Geoscientific exploration began in 1973 and led to the siting of an exploration well LA3 on top of the volcanic complex. The well was drilled in 1983 to a depth of 2144m and encountered temperatures of 320°C. Since 1990 Aluto has produced electricity, albeit with interruptions. Currently it is undergoing a major expansion phase with the plan to generate about 70MWe from eight new wells, until now two of them have been drilled successfully. Geophysical exploration at Aluto involved magnetotelluric (MT) soundings, which helped delineate the clay cap atop of the hydrothermal reservoir. However, until now geophysical studies did not succeed in imaging the proposed magmatic heat source that would drive the observed hydrothermal convection. For this study, we inverted 165 of a total of 208 MT stations that were measured over the entire volcanic complex in three independent surveys by the Geological Survey of Ethiopia and ETH Zurich, Switzerland. For the inversion, we used a novel 3-D inverse solver that employs adaptive finite element techniques, which allowed us to accurately model topography and account for varying lateral and vertical resolution. We inverted MT phase tensors. This transfer function is free of galvanic distortions that have long been recognized as an obstacle in MT inversion. Our recovered model shows, for the first time, the entire magmatic-hydrothermal system under the geothermal field. The up-flow of melt is structurally controlled by extensional rift faults and sourced by a lower crustal basaltic mush reservoir. Productive wells were all drilled into a weak fault zone below the clay cap. The productive reservoir is underlain by an electrically conductive upper-crustal feature, which we interpret as a highly crystalline rhyolitic mush zone, acting as the main heat source. Our results demonstrate the importance of a dense MT site distribution and state-of-the-art inversion tools in order to obtain reliable and complete subsurface models of high enthalpy systems below volcanic geothermal prospects.

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Cherkose, B., H. Mizunaga, and F. Samrock, Imaging Resistivity Structures of High-Enthalpy Geothermal Systems Using Magnetotelluric Method: A case study of Aluto-Langano geothermal field in Ethiopia, Proceedings, 7th African Rift Geothermal Conference, pp. 1-13, 2018. [View Abstract]Resistivity imaging using magnetotelluric (MT) method in high-enthalpy geothermal systems is an effective tool to identify conductive clay layers that cover the geothermal systems and to detect a potential reservoir. In this paper, we use MT data from Aluto-Langano geothermal field, a high-enthalpy geothermal field in Ethiopia with the objective of constructing a 3-D resistivity model of the subsurface to better understand the hydrothermal system of the area by correlating it with geology, alteration information from wells and a conceptual model of a high-enthalpy field. In particular, identifying the low resistivity zone associated with clay alteration zones (the cap rock) and delineating potential reservoir are the main tasks in this study. The result of the 3-D inversion identifies three main resistivity structures: a high-resistivity layer associate with unaltered volcanic rocks at shallow depths, a low-resistivity layer related to argillic alteration clay products, predominantly smectite and a gradually increasing high resistivity region at greater depths related to the formation of high-temperature alteration minerals such as chlorite and epidote. The recovered resistivity model corresponds very well with the conceptual model for high-enthalpy volcanic geothermal systems where an increase in resistivity below a highly conductive surficial layer (clay cap), reflect an increase in temperature with depth. This is a common signature of high-temperature geothermal systems.

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Samrock, F., Constraints on the source of unrest at the Aluto-Langano geothermal field, Ethiopia, inferred from 3-D interpretation of MT measurements, Dissertation, ETH Zurich, 155 pp., 2015. [Download] [View Abstract]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 con- ventional 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 construc- tion 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 mas- sive 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 in- dependence 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 temper- atures exceeding 350 ◦ C ( Gianel li and Teklemariam , 1993). The observed periods of uplift at Aluto took place in 2004 and 2008, they affected a region of around 100 km 2 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 delin- eate the nature of the deforming source. In order to do this we conducted magnetotel- luric (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 ( Mu ̃noz , 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 iden- tifying 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 plan- ning a field campaign, including logistics and customs clearance. The field measurements in Ethiopia were conducted together with a team of scientists from Addis Ababa Uni- versity, 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 in- terdisciplinary 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 sys- tem, 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 pub- lications 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 hydrother- mal 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 ten- dency 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 level oscillations. v Based on geomagnetic transfer functions, which provide information on lateral resis- tivity contrasts we conclude that the dominating occurrence of melt is most likely at lower crustal depths along a N-S elongated off-axis zone of volcanism west of the Main Ethiopian Rift System rather than under the Aluto volcanic complex. This interesting finding is well constraint by previous magnetotelluric and seismic studies ( Whaler and Hautot , 2006; Bastow et al. , 2011; Kim et al. , 2012) and it clearly shows the impor- tance of making a regional MT survey in order to fully understand the thermal regime in the rifting zone. Understanding the plumbing system associated with the volcanoes in this region could also have a major impact on geothermal exploration and on the future deployment of geothermal power plants in Ethiopia. Widespread development of geothermal energy in the rift could meet a major part of the local energy demand resulting in a vast benefit for the Ethiopian nation.

Samrock, F., Elektrisch hochleitfähige makroskopische Strukturen - ein alternatives Modell zur Erklärung scheinbarer Mantelanisotropie unter der känozoischen Vulkanprovinz Deutschlands, MSc Thesis, Georg-August-Universität zu Göttingen, 163 pp., 2010. [Download PDF] [View Abstract]Die känozoische Vulkanprovinz Deutschlands ist eine Region, die während des Känozoi- kums im Tertiär bis hinein ins Quartär Schauplatz aktiven Vulkanismuses war. Die hierbei entstandenen Vulkane erstrecken sich über ca. 300 km entlang einer Reihe von der Eifel im Westen Deutschlands über den Vogelsberg, die Rhön bis zur Heldburger Gangschar in Teilen Thüringens und Bayerns (Wedepohl und Baumann, 1999). Der Vogelsberg in Hessen stellt mit rund 2500 km2 das größte zusammenhängende Vulkangebiet Mitteleuropas dar (Walter, 1995). Entsprechend ihrer interessanten geologischen Vergangenheit ist die känozoische Vulkan- provinz, die neben ihrer vulkanischen Aktivität von einer sich über gesamt Europa er- streckenden Riftstruktur durchkreuzt wird (Ziegler, 1992), langwährender Untersuchungs- gegenstand geophysikalischer Forschung mit verschiedensten Explorationsmethoden. Ein Schwerpunkt liegt hierbei in den Methoden der geophysikalischen Tiefenforschung, die es erlauben Aussagen über die Struktur und die Dynamik des Mantels zu treffen. Seismo- logische Messungen konzentrieren sich auf die Region des Rheinischen Schildes. Mit der Durchführung des großangelegten Eifel Plume Projekts in den Jahren 1997 - 1998 erhoffte man sich anhand seismologischer Messungen klärende Antworten auf die kontrovers dis- kutierte Plumehypothese zu finden. Zwar konnten unter der Eifel seismische low-velocity Anomalien nachgewiesen werden (Ritter u. a., 2001; Keyser, Ritter und Jordan, 2002), eine Klärung der Plumehypothese steht jedoch weiter aus. Die Hypothese an sich stößt vieler- orts auf Ablehnung (Meyer und Foulger, 2007). Die Analyse der Aufspaltung von Scherwellen (SKS-Scherwellen-Splitting ) ergab Hinweise auf eine seismische Anisotropie unter dem Rheinischen Schild. Eine Tiefenauflösung, mit der die Quellregion der Anisotropie bestimmt werden könnte, ist mit dieser Methode nicht möglich (Savage, 1999). Mit hoher Wahrscheinlichkeit liegt sie jedoch im oberen Mantel, da die Kruste aufgrund ihrer geringen Mächtigkeit zu keiner signifikanten Aufspaltung von Scherwellen führt (Walker u. a., 2005). Als Ursache für die seismische Anisotropie gel- ten Olivinkristalle, die aufgrund von durch den Mantelfluss induzierten Spannungsfeldern ausgerichtet werden (Zhang und Karato, 1995). Die Olivinkristalle sind bezüglich der Lauf- zeiten seismischer Wellen entlang ihrer kristallographischen Achsen anisotrop (Kumazawa und Anderson, 1969). Olivin stellt mit ca. 70% den mineralogischen Hauptbestandteil des Mantels dar. Neben seismologischen Untersuchungen war und ist die känozoische Vulkanprovinz Unter- suchungsgegenstand der elektromagnetischen Tiefenforschung. Hinweise auf die Existenz eines Eifelplumes konnte aber auch diese bisher nicht erbringen (z.B. Kuras, 1998). Jedoch konnte mit Hilfe der Magnetotellurik im gesamten Gebiet der känozoischen Vulkanprovinz eine teils tiefenabhängige Anisotropie der elektrischen Leitfähigkeit σ festgestellt werden (Hönig, 1998; Bahr u. a., 2000; Leibecker u. a., 2002; Gatzemeier und Moorkamp, 2005, u.a.). Eine Eigenschaft, die die Magnetotellurik auszeichnet, ist ihre genauere Tiefenauf- lösung, die auf den periodenabhängigen Eindringtiefen der magnetischen und elektrischen Feldvariationen beruht. Die tiefenabhängige Anisotropie untergliedert sich in zwei Berei- che - die Kruste und den Mantel. Generell ist die elektromagnetische Streichrichtung, d.h. die Richtung der hohen Leitfähigkeit, in Kruste und Mantel nicht identisch. Die elek- tromagnetische Streichrichtung in der Kruste orientiert sich vornehmlich an geologischen Großstrukturen, wie den Terrangrenzen. Als verantwortlicher Leitfähigkeitsmechanismus kommen hier in erster Linie vernetzte leitfähige Phasen, wie salinare Fluide oder Graphit, in Frage. Sie konzentrieren sich in krustalen Kluft- und Risssystemen, die sich entlang einer durch die tektonische Spannung vorgegebenen Vorzugsrichtung ausbilden. Das Hauptaugenmerk dieser Arbeit liegt auf der Struktur und der Dynamik des oberen Mantels. Dessen elektromagnetische Streichrichtung liegt unter der känozoischen Vulkan- provinz mit großer Konsistenz in Ost-West-Richtung entlang der Aufreihung der vulka- nischen Gebiete (Gatzemeier, 2001). Nach Norden hin ist eine Änderung der elektroma- gnetischen Streichrichtung auf Nord-Süd zu beobachten, während der Anisotropiefaktor im Süden Deutschlands deutlich schwächer wird (Moorkamp, 2003). Der Anisotropiefaktor ist das Verhältnis der elektrischen Leitfähigkeit σI in Streichrichtung und der elektrischen Leitfähigkeit σ⊥ senkrecht zur Streichrichtung. Die elektromagnetische Anisotropie im obe- ren Mantel wurde bisher hauptsächlich mit der Diffusion von Wasserstoffionen H+ in Olivin erklärt (Bahr u. a., 2000; Gatzemeier, 2001; Gatzemeier und Moorkamp, 2005, u.a.). Ähn- lich wie die seismische Anisotropie in Olivin ist dessen auf der Diffusion von H+-Ionen beruhende elektrische Leitfähigkeit bezüglich seiner kristallographischen Achsen anisotrop (Karato, 1990). Ferner stimmt die Richtung der hohen Leitfähigkeit mit der Richtung der hohen seismischen Geschwindigkeit überein. So ist die weltweit vielfach beobachtete Über- einstimmung von seismischer und elektrischer Anisotropie (Simpson, 2001; Gatzemeier und Moorkamp, 2005; Walker u. a., 2005) anhand einer gemeinsamen Grundlage, nämlich der Ausrichtung von Olivin, erklärbar. Jedoch gibt es hierfür auch Gegenbeispiele: Hamilton, Jones, Evans u. a. (2006) beobachteten keine Übereinstimmung von seismischer und elektri- scher Anisotropie in Südafrika. Sie schlossen daraus, dass die für die seismische Anisotropie verantwortliche Region entweder in größeren Tiefen liegt oder dass die hierfür verantwortli- chen Mechanismen keine signifikanten elektrischen Eigenschaften aufweisen. Die Forschung auf diesem Gebiet ist also längst nicht abgeschlossen. Jüngste, erste direkte Labormessun- gen der Leitfähigkeit von Olivin brachten sogar völlig gegensätzliche Ergebnisse zutage (Wang u. a., 2006; Yoshino u. a., 2006). Ungeachtet dessen liegt die elektrische Anisotropie im oberen Mantel unter der känozo- ischen Vulkanprovinz mit einem Anisotropiefaktor von A = σI/σ⊥ > 100 in einem Be- reich, der mit der H+-Diffusion in Olivin nicht erklärbar ist. Vernetzte partielle silikatische Schmelzen entfallen als alternative Erklärung. Sie besitzen zwar eine höhere Leitfähigkeit, ihr notwendiger Anteil von 10% im oberen Mantel kommt aus Gründen der Stabilität je- doch nicht in Frage. Jüngste Forschungen an karbonatischen Schmelzen ergaben, dass deren Leitfähigkeit drei Größenordnungen über der silikatischer Schmelzen liegt (Gaillard u. a., 2008a). Damit genü- gen bereits geringe Mengen, um hohe Leitfähigkeiten zu erzeugen. Aufgrund der extremen Seltenheit ihrer Erstarrungsgesteine wurden karbonatische Schmelzen zur Erklärung von Leitfähigkeitsanomalien im Mantel bisher meist nicht berücksichtigt. Die Anisotropie der Leitfähigkeit muss prinzipiell nicht von einem intrinsisch anisotropen homogenen Mantel herrühren, sondern kann auch durch makroskopische laterale Leitfähig- keitskontraste verursacht sein. Es läge dann ein heterogener Mantel vor. Die Unterschei- dung zwischen einer „echten“ Anisotropie (homogener Mantel) und einer „scheinbaren“ Anisotropie (heterogener Mantel) geschieht mittels der Methode der geomagnetischen Tie- fensondierung (Schmucker, 1970), mit der laterale Leitfähigkeitskontraste aufgedeckt wer- den können. In dieser Arbeit wird ein 3D-Modell vorgestellt, das eine sehr gute Datenanpassung auf- weist und völlig auf das Eingliedern anisotroper Schichten verzichtet. Stattdessen wird ein heterogener Mantel postuliert. Es wird gezeigt, dass die elektrische Anisotropie in einer hochleitfähigen makroskopischen Struktur im Mantel begründet sein kann, die mit der Methode der geomagnetischen Tiefensondierung nicht aufgelöst wird. Die hochleitfähige Struktur wird durch das Vorhandensein karbonatischer Schmelzen unter den Vulkanen der känozoischen Vulkanprovinz erklärt. Die Existenz der karbonatischen Schmelzen wird ab- schließend auf der Grundlage geochemischer Analysen von Magmen diskutiert. Ferner wird gezeigt, dass die bisher oft vernachlässigten hochleitfähigen Sedimente im Norden Deutsch- lands einen erheblichen Effekt haben und eine wichtigen Beitrag zur Erklärung der Daten leisten.