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In central Europe, the majority of the CO2 emissions in the energy sector are related to the provision of building and process heat. Due to seasonal fluctuations in demand, especially to provide heat for residential and industrial buildings, local storage of excess heat in summer for utilization in winter is becoming increasingly important. With the current state of technology and foreseeable developments, sufficient amounts of heat can only be stored underground, taking advantage of the large available storage volumes. In contrast to typical near-surface aquifer thermal energy storage (ATES), the utilization of deep reservoirs enables the storage of much larger thermal energies due to potentially significantly higher injection temperatures (e.g. > 80 °C).Previous studies demonstrated the high potential of deep reservoirs for high-temperature (HT) ATES, in particular for former hydrocarbon reservoirs in the Upper Rhine Graben. However, these studies focused on thermo-hydraulic processes, only rarely considering the impact of coupled mechanical processes. Using the case study of the DeepStor project, a demonstrator for HT-ATES under development in the north of Karlsruhe (Germany), the present study investigates the influence of coupled thermo-hydraulic-mechanical (THM) processes during the operation of HT-ATES systems.In particular, we investigate the impact of seasonal HT-ATES with biannual injection/production cycles on the stress distribution in the subsurface and subsequently caused displacements in the reservoir and the surface as well as the shear capacity at faults. This study further aims at improving the understanding of poro- and thermoelastic processes related to HT-ATES. Whereas the thermoelastic component dominates the vertical displacements at the top of the reservoir, the uplift at the surface is primarily controlled by the poroelastic component. Furthermore, an assessment of potential risks such as surface uplift or shear capacity at faults is performed. Our results show that surface uplift is primarily controlled by the reservoir depth, Young’s modulus, and the injection/production flow rate.

A number of six in situ hydraulic fracturing experiments were carried out at the Äspö Hard Rock Laboratory (Sweden) in 2017 in a depth of 410 m. Here we present electric self-potential monitoring during the conventional and the step-wise cyclic injection experiments HF2 and HF3. Electric self-potential data were acquired through a two-sensor array, each including nine measuring probes and one base probe, that were installed at the 410 m and 280 m levels. The experimental borehole F1 is drilled in the direction of Shmin, perpendicular to the expected fracture plane. The self-potential sensors are installed sub-parallel to Shmin at level 410 at a distance of 50-75 m to the borehole F1 and sub-perpendicular to Shmin at level 280 m at a distance of 150-200 m to F1. The self-potential data were measured with a sampling rate of 1 Hz. Here, we propose a 1-D modelling of the streaming potential that approximates the measured self-potential data. These streaming potential gradients ∆V are estimated from the simulated pressure signals and the coupling coefficient.

SUMMARY Current geophysical joint inversion methods can be roughly divided into structurally or property driven. While structural coupling plays a key role in fully heterogeneous environments, it is blind to the actual physical properties, which limit its application. On the other hand, approaches based on direct property-correlations promise strong coupled models using petrophysical linkages derived from rock physics or through the search for cross-property relationships in the objective function directly, for example, fuzzy-clusters or correspondence maps. We apply a correspondence maps (CM) methodology to the joint estimation of density and magnetization in 3-D environments using gravity and magnetic data. CM joint inversion has yet to be applied to the specific combination of gravity and magnetic data, nor compared to other approaches like cross-gradient coupling or fuzzy-clustering. The CM searches for cross-property functional relationships; it has the potential to allow a gradual property variation that is not restricted to a predetermined number of homogeneous zones. We tested the algorithm using two illustrative synthetic examples to analyse the compositional and geometrical improvements and thoroughly investigate its capacity to recover a study model based on potential features in a geothermal scenario using polynomials. We investigated the case of using lower and higher polynomials than required for recovering the second synthetic model. The nature of the inversion suggested instability when high-grade polynomials were used, which was successfully overcome by gradually increasing the number of coefficients. The strategy was satisfactorily implemented to the field data. We applied the algorithm over the Los Humeros geothermal field in Mexico and fit a quadratic relationship between density and magnetization that exhibits two main zones consistent with previous works. The first one contains relatively low density and high magnetization. The second zone contains low density and low magnetization values. In this last zone, we emphasize that an observed link between low density, low magnetization, and major fault zones with high relevance to geothermal exploration is resulting from joint inversion, only. We achieved an enhanced petrophysical and structural integrated subsurface imaging in synthetic and field cases at the expense of slightly increased RMS misfit value.

At the Äspö hard rock underground laboratory in Sweden, six in situ hydraulic fracturing experiments took place at 410 m depth. A multistage hydraulic fracturing approach is tested with a low environmental impact, e.g., induced seismicity. The idea is to mitigate induced seismicity and preserve the permeability enhancement process under safe conditions. The fractures are initiated by two different injection systems (conventional and progressive). An extensive sensor array is installed at level 410 m, including simultaneous measurements of acoustic emissions, electric self-potential, and electromagnetic radiation sensors. The monitoring catalog includes more than 4300 acoustic emission events with estimated magnitudes from the continuous monitoring setup (in-situ sensors between 1-100 kHz). The experiment borehole F1 is drilled in the direction of Shmin, perpendicular to the expected fracture plane. Two electromagnetic radiation sensors are installed and aligned to (i) Shmin and (ii) the expected fracture plane with a sampling rate of 1 Hz and a frequency range between 35-50 kHz. The self-potential sensors are installed at level 410 with a distance of 50-75 m from the borehole F1, including nine measuring probes and one base probe. A second self-potential setup is deployed at level 280 m in the far-field with a distance of 150-200 m from F1. The self-potential data were measured with a sampling rate of 1 Hz. For the first time (to our knowledge), the results of electric and electromagnetic monitoring of two hydraulic stimulation at meter-scale are presented.

The storage of low-temperature heat in the near surface underground is already widely used worldwide, whereas only little attempts of storing of higher temperatures have been made so far. However, this can become increasingly important in the heat transition strategy by contributing with this technology to medium and peak load supply. Previous works have demonstrated that a large part of the required thermal energy can be efficiently stored in former oil reservoirs in Tertiary sediments of the URG. The advantage of using depleted oil reservoirs as HT-ATES is that they have a lower overall project risk due to the knowledge of the subsurface from the exploration history and that the geological and geophysical data, which are mostly available, allow a more reliable forecast of efficiency and development costs.KIT in Karlsruhe has now planned the "Deepstor" HT-ATES research infrastructure for its Northern Campus. The site is adjacent to the former Leopoldshafen oil field and can draw on a large amount of data from boreholes for assessment prior to drilling. The targeted reservoirs is the several meters in thickness consisting of fine-grained calcareous sandstones from the Oligocene Froidfontaine Formation at a depth of approx. 1,300 meters.The baseline of this study is the interpretation of petrophysical data such as resistivity-, sonic-, gamma and SP-logs to derive hydrogeological and geothermal parameters in 15 deep wells in vicinity of the planned drilling site. An integrated data analysis is performed with simulations to gain a quantitative understanding of the fluid and heat flow of the HT-ATES site and to predict the storage and recuperation capacity. Poro-perm values from core material are used to calibrate the results. The aim is a statistically based assessment of the storage site for planning and cost estimation of the research infrastructure.This work should provide further insights for the future development of geothermal heat storage and enable the integration of a HT-ATES in the KIT campus.

The Upper Rhine Graben (URG) has shown multiple geopotentials during the past century: hydrocarbons have been produced mainly from porous, but also from fractured Cenozoic-Mesozoic reservoirs. Fractured Mesozoic and Paleozoic rocks comprise important geothermal reservoirs that are currently exploited in the URG. Heat storage is a new geopotential under consideration for the KIT Campus North, approximately 10 km north of the city of Karlsruhe, south-west Germany. For spatio-temporal structural analysis, volume calculation, well-path planning and thermo-hydraulic modelling the 3D-structure of the subsurface down to a depth of about 4 km is crucial. We used available 3D-seismic and borehole data for structural analysis, quantification of formation thicknesses and the geometry of sedimentary successions. Significant fault shadows occur at depths greater than 1.5 km in the footwall of major faults and reduce the reliability of their location but still give a good image of faults and their respective displacements.Sandstone-bearing Cenozoic graben-filling sedimentary successions that previously were exploited for hydrocarbons are currently investigated for their heat storage potential of excess heat. Additionally, fractured Mesozoic and Paleozoic rocks are considered for deep geothermal heat supply. Cenozoic reservoir rocks dip approximately 5° to the East. The studied area is dominated by the two major (N)NE-(S)SW-striking Leopoldshafen and Stutensee growth faults showing displacements of several hundred meters. Major syndepositional normal faulting along the Leopoldshafen fault occurred during early Miocene (Hydrobia beds, Landau Formation). Increase of syn-tectonic sedimentary thicknesses from East to West indicate graben-interior migration of fault activity with time. In the hanging wall of the Stutensee fault shallow-rooted, ENE-dipping en-echelon normal faults, linked by relay ramps, and small NNW-striking graben structures displace Cenozoic strata by several tens of meters, but apparently do not cut the respective major fault, indicating that both minor and major faults were either concomitantly active or that major faults postdate minor fault activity. While the minor faults root in older Tertiary successions, both the Stutensee and the Leopoldshafen Fault are rooting in the crystalline basement of yet unknown petrographic composition. A slip and dilatation tendency analysis was performed to reduce the risk of induced seismicity for well-path planning on basis of published stress models, drilling induced tensile fracture analysis, and borehole breakouts of deep borehole data comprising Mesozoic strata and on borehole breakouts in the Cenozoic successions. The maximum horizontal stress direction (SH) trends N-S, resulting in a relatively high potential for both (oblique) normal fault slip and dilation and hence for relatively high geothermal potentials in the deep subsurface.

In regions with favourable subsurface condition, geothermal resources provide a significant contribution to the reduction of man-made CO2 emissions. Its economic utilization often requires reservoir engineering that bears the risk of a number of environmental challenges such as induced seismicity, groundwater pollution or radioactive scaling.In order to develop a socially feasible geothermal utilization concept in the Upper Rhine Graben close to the city of Karlsruhe we designed a research approach in which interdisciplinary (between natural and social sciences) knowledge production is combined with transdisciplinary knowledge production. This means that besides the collaboration of scientists of various disciplines, stakeholder and citizens s from surrounding communities get the possibility to take part in the project through workshops and interviews. The results of those transdisciplinary interactions will be integrated through translation into technical parameters in the technical design of a geothermal utilization concept. For the development of utilization scenarios, technical criteria were adopted into technical parameter ranges. Furthermore, socio-ecological criteria such as “no induced seismicity” are translated into technical parameters by an experience-based approach. The resulting scenarios are substantiated by numerical models that address the energy outcome. They will be reflected in a second stakeholder workshop. Finally, recommendations for a geothermal heat utilisation concept will be formulated.

Energy transition involves an increasing demand for renewable energies. Room heating and hot water account for the majority of the energy demand of private households. Thus, seasonal storage of excess heat produced during the summertime and extracted during the wintertime is of paramount importance. High temperature heat storage in the subsurface may be realized in abandoned hydrocarbon reservoirs worldwide as these reservoirs have already been extensively characterized concerning their geology, geo- and petrophysical properties as well as their depths and geometries. Although these hydrocarbon reservoirs are relatively well characterized, their potential use for geothermal heat storage has not yet been investigated. Here we focus in a first approach on abandoned oilfields of the Upper Rhine Graben (URG) in SW Germany with the aim to assess their potentials for geothermal heat storage. While geothermal production commonly targets fractured reservoirs to obtain economically viable flowrates, geothermal heat storage will aim at reservoirs with high porosities. As the productivity of hydrocarbon reservoirs is commonly controlled by their porosities, they appear as viable targets for high temperature heat storage.We have characterized 20 abandoned hydrocarbon reservoirs in the URG, which were productive for more than five years, in Cenozoic sandstones in depths of approximately 200 – 1800 m. Our characterization is based on published data of their production histories, reservoir geology, and petrophysical properties. Most reservoirs in the URG are stacked reservoirs with inflow of hydrocarbons into the borehole from multiple stratigraphic units, as for example in Landau and Leopoldshafen, biasing an assignment of respective reservoir productivity. For heat storage injection pressure needs to be well controlled to avoid undesired hydraulic fracturing. Therefore, (theoretically) infinite reservoirs with high transmissivities appear to be more attractive and less risky than confined reservoirs.The production histories of the various hydrocarbon reservoirs show typical patterns with a rapid increase of the annual production, followed by a slower decrease of production (tailing) before hydrocarbon production was shut down. Most reservoirs in Cenozoic sandstones show porosities of 10 to 20% with some extreme values of up to 30%. Associated permeabilities vary from 0.1 to 100 mD with some extreme values of up to 1000 mD. Data show a non-linear relationship between porosity and permeability. During hydrocarbon production water-oil ratios increase until unalluring water-oil were produced. We evaluate the potential of abandoned hydrocarbon reservoirs in the URG for heat storage by developing generic numerical models to constrain limiting conditions. The uncertainties of input parameters and their impact on heat storage potential will be addressed by a sensitivity analysis. Potential reservoirs for heat storage may be defined based on energy recovered over invested energy (EROI). Preliminary results of our numerical models show a strong dependency of the storage potential on increasing flowrates as well as on decreasing thermal conductivities of the reservoir and especially the confining layers below and above.

Summary HT-ATES (high-temperature aquifer thermal energy storage) systems represent a future technology to shift large amounts of excess heat from summer to winter using reservoirs in the deep underground. Among others, depleted hydrocarbon-bearing reservoir formations may constitute favorable storage conditions for HT-ATES. This study characterizes these reservoirs in the Upper Rhine Graben (URG) and quantifies their heat storage potential numerically. Assuming a doublet system with semi-annual injection and production cycles, injection at 140 °C in a reservoir with an ambient temperature of 70 °C leads to an annual storage capacity of up to 12 GWh and significant recovery efficiencies increasing up to 82 % after ten years of operation. Our sensitivity analysis of operational conditions identifies reservoir conditions (e.g. permeability or thickness) and the drilling configuration (horizontal/vertical) as the most influencing parameters. With about 90 % of the investigated reservoirs in the URG potentially transferable into HT-ATES systems, our analyses reveal a large storage potential of these oil reservoirs.

Besides temperature, mineralogy, residence time and dilution are crucial for assessing water-rock interaction intensity. The geothermal system at the Villarrica-Quetrupillan-Lanin volcanic chain, Southern Chile, is located across a prominent lithological transition from plutonic rocks of the North Patagonian Batholith (NPB) to volcano-sedimentary units. With the goal to investigate the impact of lithology on medium-enthalpy geothermal fluids, 15 hot spring discharges were sampled and analyzed for anthropogenic tracers and isotopic composition and compared to the analyses of 31 reservoir rock analogues. Comparison of strontium isotope signatures between rock analogues and hot spring discharges allow an allocation of associated reservoir rocks. Chlorofluorocarbons quantify the dilution of the geothermal springs by shallow groundwater, ranging from almost CFC-free samples to dilution with modern meteoric water by up to 50 %. Fluids discharging from plutonic rocks have low proportions of dilution with modern waters, while hot springs discharging from the volcano-sedimentary rocks have a higher and variable dilution with modern waters. The fractionation of oxygen isotopes of the SO 4 -H 2 O system reveals reservoir temperature estimates of 80–100 °C in the plutonic sequence matching discharge temperatures. For the springs discharging from volcano-sedimentary units higher reservoir temperatures of 100–140 °C are calculated. On basis of the analysis, a conceptual reservoir model can be derived. Fast fluid ascent is indicated along Liquine-Ofqui fault system in the NPB by similar discharge and reservoir temperatures and low surficial dilution rates. Large differences in discharge and reservoir temperatures in the Cura-Mallin formation along with a high influx of surficial water may be attributed to a more branched pathway pattern. In conclusion lithology maybe an important factor when coming to the utilisation of geothermal resources.

International audience; The growing demand for energy, natural resources and urban expansion during the last two centuries increased human interference with the geosphere far beyond geothermal usage. The increasing number of large-scale projects intervening the area of life of communities raised public concerns related to their environmental and social impact. Integration of public concerns into such projects should therefore go beyond outreach and communication measures. It requires an open approach to inclusive governance structures with respect to designing research and development processes and to modify technological options. Geoethical concepts emphasize that geoscientific knowledge may assist society in decision making as well as in dealing with risks, user conflicts and environmental threats on local, regional and global scale in order to support more sustainable practices at the intersection of human beings and the geosphere.In the present article, we analyse the social response to recent geothermal development and identify the precondition for public acceptability of geothermal projects. On this basis, the potential contribution of a GeoLaB, a Geothermal Laboratory in the crystalline Basement, to a geoethic approach in geothermal research and technology development is discussed. The underground research laboratory is planned as an infrastructure to answer scientific challenges and to offer the necessary transparency to interact with the public. The GeoLaB approach aims on transparent, tangible science and can serve to enhance mutual understanding of stakeholder groups. It may increase public awareness on geothermal research and potentially enhance the opportunity for public approval of planned activities. As a generic site, GeoLaB can develop scientific-technological solutions for a responsible exploitation of geothermal energy accompanied by sociological studies. The underground research laboratory will serve as a platform for science communication, participation and dialogue of stakeholders from industry, politics, administration and society. This complies with the comprehension of responsible research in a geoethical sense.

International audience; An increasing interest in magnetotelluric monitoring of hydraulic stimulation experiments and of natural earthquakes currently requires a detailed investigation of the limits of this method. Our study contributes to this discussion with a unique quasi-continuous and long-term magnetotelluric monitoring of different injection and production experiments at the Rittershoffen geothermal site in Alsace (France). Here, we provide a first evaluation of the data and an outlook on more specific challenges. The geothermal doublet at Rittershoffen is operated through two wells GRT1 and GRT2. The magnetotelluric monitoring covers the end of drilling phase of GRT2, mostly, production from, but also injection into this well, injection into GRT1 and a circulation experiment. Magnetotelluric data were processed successfully using remote referencing. Transfer functions show particular variation pattern for different operations, i.e. an increase in uncertainty, conductivity and phase during test operation with a preferential direction sub-parallel to Shmin, i.e. perpendicular to the expected extension of the fractures controlling the reservoir. In particular fluid injection, either into GRT2 or GRT1 causes a strong decrease in resistivity by up to one order of magnitude in the YX component between about 8–25 s of period. However, the comparison between the temporal distribution of the occurring anomalies and the magnetic field intensity reveals an amplifying effect on the anomalies. In conclusion, we confirm earlier findings on the influence of the magnetic field intensity on magnetotelluric monitoring and at the same time show the lower limit of monitoring in particular monitoring of injection into deep geothermal reservoirs.

To date, magnetotelluric monitoring of processes during reservoir engineering of geothermal systems have been carried out only at three sites world-wide. Here, we add a new survey at the Reykjanes peninsular (Iceland). The MT data acquisition at RN-15/IDDP-2 covered the last third of the drilling period and thermo-hydraulic stimulation. Drilling was accompanied by temporal total fluid losses of up to 60 L/s as well as randomly distributed induced seismicity. Our experimental results of a two-months magnetotelluric monitoring during the deepening of the RN-15/IDDP-2 well on the Reykjanes peninsular (Iceland) to 4’659 m are in line with earlier observations on decreasing resistivities at periods of a few s to about 20 s (or up to about 40 s) in conjunction with fluid injection. Simple models indicate however that it is not the fluid volume itself that causes the anomaly. Moreover, here, temporal decreases in the electric resistivities occur at 0.2-20 s with minima at periods of about 0.4 s 1-2 days ahead of clusters of seismic events with magnitudes up to ML 1 order of magnitudes over a short period range and is thus considered noise induced in the subsurface. This study aims at adding an essential dataset to the general discussion on MT monitoring of reservoir processes. The results show a temporal relation between decreasing apparent resistivity and (i) the geomagnetic field activity, (ii) the fluid losses up to 60 L/s, as well as (iii) mechanic processes occurring in the reservoir before clusters of induced seismicity on the other hand.

This study investigates the appropriateness of monodisperse carboxylated polystyrene spheres as clay colloid analogues. Colloid transport experiments using Na-illite are conducted within a parallel-plate fracture flow cell with an aperture of 0.75 mm at pH 5 under low ionic strength (1 mM NaCl) and laminar flow (7 mL/h) conditions. The effects of collector surface material (Grimsel granodiorite or acrylic glass), fracture orientation and residence time on colloid retention are examined. In order to avoid full surface coverage effects, ultra-trace (30 ppb) Na-illite colloid concentrations are used. Laser-induced breakdown detection is applied in order to detect and quantify the Na-illite colloids. The results are directly compared to the experiments of Stoll, et al. [1] who investigated the transport and surface interaction of carboxylated polystyrene spheres (25 and 1000 nm diameter) using the same experimental setup and conditions. It is concluded that polystyrene spheres are no ideal analogue to predict the mobility and transport behavior of a polydisperse clay colloid suspension. Despite the higher solid density, the Na-illite colloids show higher mobility compared to the polystyrene spheres without significant impact of surface roughness or gravity. The platelet shape, the non-uniform surface charge distribution and slower sedimentation may account for the observed differences.

The potential to provide base-load electricity as well as industrial and domestic heat places geothermal energy in a unique position in the future energy mix. As estimated by the International Energy Agency, geothermal power can contribute to 3.5% to the worldwide power and 3.9% to heat production by 2050. This includes the development of Enhanced Geothermal Systems (EGS) in low-enthalpy systems. The EGS technology is still in an early stage of development and the intended upscaling of power plant capacities presupposes the assurance of efficiency, profitability and safety of geothermal facilities. Pushing EGS technologies towards market maturity requires a long-term strategic approach and massive investments in research and development. Comprehensive multidisciplinary research programs combining fundamental and applied concepts tackling technological, economic and ecological and safety challenges along the EGS process chain are needed. The Karlsruhe Institute of Technology has defined a broad research program on EGS technology development following the necessity of a transdisciplinary approach. Research activities cover the whole process chain including system integration and span from fundamental to applied research across scales. The research concept is embedded in the national research program of the Helmholtz Association and is structured in four clusters: reservoir characterization and engineering; thermal water circuit; materials and geo-processes; and power plant operation. The proximity to industry closely interlinked with fundamental research forms the basis of a target-orientated concept. It combines the disciplines geosciences, geophysics, material sciences, engineering, geochemistry and numerics. The present paper aims at giving an overview of geothermal research at KIT and emphases the need for concerted research efforts at the international level to accelerate technological breakthrough of EGS as essential part of a future sustainable energy system.

In this paper, an underground experiment at the Aspo Hard Rock Laboratory (HRL) is described. Main goal is optimizing geothermal heat exchange in crystalline rock mass at depth by multistage hydraulic fracturing with minimal impact on the environment, that is, seismic events. For this, three arrays with acoustic emission, microseismicity and electromagnetic sensors are installed mapping hydraulic fracture initiation and growth. Fractures are driven by three different water injection schemes (continuous, progressive and pulse pressurization). After a brief review of hydraulic fracture operations in crystalline rock mass at mine scale, the site geology and the stress conditions at Aspo HRL are described. Then, the continuous, single-flow rate and alternative, multiple-flow rate fracture breakdown tests in a horizontal borehole at depth level 410 m are described together with the monitoring networks and sensitivity. Monitoring results include the primary catalogue of acoustic emission hypocentres obtained from four hydraulic fractures with the in situ trigger and localizing network. The continuous versus alternative water injection schemes are discussed in terms of the fracture breakdown pressure, the fracture pattern from impression packer result and the monitoring at the arrays. An example of multistage hydraulic fracturing with several phases of opening and closing of fracture walls is evaluated using data from acoustic emissions, seismic broad-band recordings and electromagnetic signal response. Based on our limited amount of in situ tests (six) and evaluation of three tests in Avro granodiorite, in the multiple-flow rate test with progressively increasing target pressure, the acoustic emission activity starts at a later stage in the fracturing process compared to the conventional fracturing case with continuous water injection. In tendency, also the total number and magnitude of acoustic events are found to be smaller in the progressive treatment with frequent phases of depressurization.

Mid-crustal conductors are a common phenomenon in magnetotelluric studies. In the Andean Cordillera of southern Chile, they appear to concentrate along major fault zones. A high-resolution, broad-band magnetotelluric survey including 31 stations has been

In extensional tectonic settings major structural elements such as graben boundary faults are typically oriented subparallel to the maximum horizontal stress component SHmax. They are often structurally accompanied by transfer zones that trend subparallel to the extension direction. In the Upper Rhine Graben, such transfer faults are typically characterized by strike-slip or oblique-slip kinematics. A major re-orientation of the regional stress field by up to 90° of the Upper Rhine Graben in the Early Miocene led to the present-day normal and strike-slip faulting regimes in the North and South of the Upper Rhine Graben, respectively, and a transition zone in-between. Consequently, conditions for fault frictional failure changed significantly. Moreover, it has been observed during tracer and stimulation experiments that such transfer faults may be of major importance for the hydraulic field of geothermal reservoirs under the present stress condition, especially, when located between production and injection well. In this context we have investigated slip and dilation tendencies (TS and TD) of major structural elements at reservoir scale for two representative geothermal sites, Bruchsal (Germany) and Riehen (Switzerland), located close to the Eastern Main Boundary Fault of the Upper Rhine Graben. We have evaluated the quality and uncertainty range of both tendencies with respect to potential variation in SHmax orientation. Despite significant differences in orientation of the structures and the stress regimes, the resulting variation of TS and TD reveal major similarities concerning the reactivation potential of both, the graben-parallel structures and the transfer faults. The conditions of criticality for tensile failure and non-criticality for shear failure suggest that transfer faults are most likely naturally permeable structures with low stimulation potential. This is in agreement with the absence of both immediate tracer recovery and seismicity in the studied geothermal sites.

Karlsruhe Institute of Technology (KIT) FONDAP-CONICYT 15090013 Andean Geothermal Center of Excellence (CEGA) FONDAP-CONICYT 15090013 BMBF-CONICYT International Scientific Collaborative Research Program FKZ 01DN14033/PCCI130025 National Agency for Research and Development (ANID)/Scholarship Program/DOCTORADO BECAS CHILE DAAD/2015-62150016

The impact of fracture geometry and aperture distribution on fluid movement and on non-reactive solute transport was investigated experimentally and numerically in single fractures. For this purpose a hydrothermally altered and an unaltered granite drill core with axial fractures were investigated. Using three injection and three extraction locations at top and bottom of the fractured cores, different dipole flow fields were examined. The conservative tracer (Amino-G) breakthrough curves were measured using fluorescence spectroscopy. Based on 3-D digital data obtained by micro-computed tomography 2.5-D numerical models were generated for both fractures by mapping the measured aperture distributions to the 2-D fracture geometries (x-y plane). Fluid flow and tracer transport were simulated using COMSOL Multiphysics®. By means of numerical simulations and tomographic imaging experimentally observed breakthrough curves can be understood and qualitatively reproduced. The experiments and simulations suggest that fluid flow in the altered fracture is governed by the 2-D fracture geometry in the x-y plane, while fluid flow in the unaltered fracture seems to be controlled by the aperture distribution. Moreover, we demonstrate that in our case simplified parallel-plate models fail to describe the experimental findings and that pronounced tailings can be attributed to complex internal heterogeneities. The results presented, implicate the necessity to incorporate complex domain geometries governing fluid flow and mass transport into transport modeling.

Fracture porosity is a crucial parameter in hydrocarbon and geothermal reservoir exploration and a major challenge in the absence of nearby exploration wells. The Sankt Gallen geothermal project targets a fault zone that affects Mesozoic sediments at a depth of about 4500 m. Spatial extension of these sediments, a major fault zone and indication for graben structures in the crystalline basement are observed in 3D seismic. Both the graben and the fault zone coincide with negative gravity anomalies acquired and analyzed during this study. Forward modeling of gravity anomalies based on a 3D seismic survey is used to estimate possible fracture porosity. After stripping gravity effects of geothermally irrelevant geological units from the residual anomaly, most likely only local structures related to the fault zone account for remaining anomalies. Synthetic case study on the effect of density variation and considerable gas content in the well support possible fracture porosity between about 4% and 8%.

International audience; An indirect electromagnetic geothermometer is used for deep temperature estimations in the Soultzsous-Forêts geothermal area (France) using magnetotelluric sounding data. Validation of temperatureassessment carried out by comparison of the forecast temperature profile with temperature log fromthe deepest borehole has resulted in the relative extrapolation accuracy of less than 2%. It is foundthat the resistivity’s uncertainty caused by magnetotelluric inversion errors and by possible effects ofexternal factors very weakly affects the resulting temperature, with the latter being influencedmainly by the ratio between the borehole length and the extrapolation depth. The temperature crosssectionconstructed up to the depth 5000 m manifests local temperature maxima at large depthsbeneath the wells GPK2 and RT1/RT3. The analysis of the temperature profile in GPK2 locationbeneath 5000 m indicates that its behaviour continues to be of the conductive type (as in the depthrange of 3700 m–5000 m) up to the depth 6000 m, while manifesting convective type below thisdepth. Finally, application of the indirect electromagnetic geothermometer for the deep temperatureforecasting in the Rittershoffen site enabled us to constrain the location for future drilling.

The technical feasibility of Enhanced Geothermal Systems (EGS) has been demonstrated for the first time in fractured crystalline basement rocks at the Soultz-sous Forets project (France), thus creating a unique and vast data base. At this EGS reference site, different hydraulic and chemical stimulation procedures and experiments were performed in four wells at three different reservoir levels located between 2 and 5 km depth. These measures enhanced significantly the hydraulic yield of the three reservoirs, in some instances by about two orders of magnitude. In this compilation of hydraulic data, we summarize the achievements at Soultz during the development of three reservoirs by more than 15 major stimulations over a 20-year period between 1988 and 2007. We evaluate the efficiency of the different injection schemes used and provide details on the performance history and testing conditions. In addition to the 52 experiments described for the testing phase, this compilation includes nine tests under operational conditions conducted over the 2008–2013 period. The evolution of hydraulic yield resulting from various injection, production, and circulation experiments is a major achievement of the Soultz reservoir development. This experience points to two important results: 1) the amount of total volume circulated between wells has a very significant effect on reservoir performance and 2) given the large flow rate variation a common linear trend of pressure increase at higher fluid flow rates develops that manifests over all three reservoirs. A strong focus is on the well tests in the intermediate reservoir allowing for a characterization of productivity and injectivity indices. Our analysis showed that initial hydraulic conditions from single-well injection tests are comparable to each other in the three reservoirs, but individual fault zones may determine the stimulation behaviour. We identify progressive cyclic injection in combination with circulation between wells reaching high hydraulic yields at comparatively low pressure. The Soultz data suggest how to maximize injection and minimize induced seismicity. This unique data base illustrates the learning curve achieved in Soultz and provides a strong basis for further conceptual model developments.

Some of the major geothermal anomalies in central Europe are linked to tectonic structures within the top of crystalline basement, which modify strongly the top of this basement. Their assessment is a major challenge in exploration geophysics. Gravity has been proven to be suitable for the detection of mainly large scale lithological and structural inhomogeneities. Indeed, it is well known and proven by different wells that, for example, in northern Switzerland extended negative anomalies are linked to such structures. Due to depth limitation of wells, there vertical extension is often unknown. In this study, we have investigated the potential of gravity for the geometrical characterization of such basement structures. Our approach consists in the combination of the series of Butterworth filters, geological modelling and best-fitting between observed and computed residual anomalies. In this respect, filters of variable wavelength are applied to observed and computed gravity data. The geological model is discretized into a finite element mesh. Near-surface anomalies and the effect of the sedimentary cover were eliminated using cut-off wavelength of 10km and geological and seismic information.We analysed the potential of preferential Butterworth filtering in a sensitivity study and applied the above mentioned approach to part of the Swiss molasses basin. Sensitivity analyses reveal that such sets of residual anomalies represents a pseudo-tomography revealing the distribution of different structures with depth. This finding allows for interpreting negative anomalies in terms of 3-D volumes. Best-fitting then permits determination of the most likely 3-D geometries of such basement structures. Our model fits both, geological observations and gravity: among 10 deep boreholes in the studied area, six reach the respective units and confirm our distribution of the negative (and positive) anomalies.

Geothermal energy production from the deep subsurface requires a detailed knowledge of the relevant static and transient parameter distribution in the reservoir and host rock. In reservoir exploration, engineering and operation phases, both temporal and spatial subsurface parameters are acquired, evaluated and monitored in order to improve the reservoir performance. To support temporal and spatial data access to geothermal data sources, an efficient 3D/4D GIS is proposed in this study. We discuss theoretical and first practical approaches for the management of such temporal and spatial geothermal data. A first practical example is provided using the data acquired in the Soultz-sous-Forets geothermal project (France). A distributed software architecture, database design, and the concept for advanced query component with embedded simulations are presented. Finally, we give an outlook on the planned future research in 3D data management of subsurface, near-surface, and above-surface installations in other projects.

The interaction of monodisperse fluorescent carboxylated polystyrene colloids (25 nm and 1000 nm diameter) with a cut granodiorite surface (Grimsel granodiorite; Switzerland) and with acrylic glass is investigated both experimentally and numerically. Colloid transport experiments are conducted in a parallel plate type fracture flow cell with an aperture of 0.75 mm at pH 5 under low ionic strength (1 mM NaCl) and under laminar flow (7 mL/h) conditions. The study focuses on the effect of residence time, colloid size, collector material and fracture orientation on colloid retention. Long colloid residence times are achieved by stop-flow experiments. Using atomic force microscopy and, more specifically, the colloid probe technique surface roughness and force distance information of the collector material (granodiorite or acrylic glass) as a function of probe size (cantilever) are obtained. The experiments are modeled using COMSOL Multiphysics® (2-D numerical simulations). The experimental and the modeled results lead to the conclusion that large colloids (1000 nm diameter) undergo sedimentation and deposition on the surface during stop-flow. Collector interaction is not affected by the surface roughness variation. Contrariwise, for the investigated 25 nm colloids sedimentation does not play a role under the experimental conditions and collector interaction is triggered by surface inhomogeneities such as surface roughness.

High flow rate injection and related hydromechanical interaction are the most important factors in reservoir development of Enhanced Geothermal Systems (EGS). GeoLaB, a new generic geothermal underground research laboratory (URL), is proposed for controlled high flow rate experiments (CHFE) to address limited comprehension of coupled processes connected to EGS reservoir flow conditions. As analogue for typical EGS development, CHFE require specific hydromechanical conditions including a connected fracture network in crystalline basement rock, sufficient hydraulic fracture transmissivities, a strike-slip to normal faulting tectonic regime, controllable hydraulic boundary conditions, and hydrothermal alteration fracture fillings that improve conditions for hydromechanical interaction. With the aim to identify most appropriate areas for future site selection, four criteria have been established based on the EGS reference site of Soultz. Two URLs in crystalline basement worldwide approximate the requirements of a new generic GeoLaB and may be used for accompanying experimentation. Besides favourable geological, hydraulic, and stress conditions, the vicinity to long-term EGS production favours the southern Black Forest as potential region for GeoLaB. Therefore, an exemplary site assessment has been carried out at “Wilhelminenstollen” in the southern Black Forest (Germany). New remote sensing, hydrochemical, and geophysical analyses as well as reactivation potential, and stress modelling were added to complement existing geological and hydrogeological information. At this site, reactivation potential analysis reveals two local maxima prone for shear reactivation as strike-slip faults. The highest lineament density is observed for the N110°E strike direction that is associated with both slip and dilation tendency maxima. Clay minerals occur in fractures and the matrix. Local, partly water-bearing fractures, when partly filled with ore minerals, were connected to veins in the tunnel using shallow geophysical methods. Hydrochemical data reveal infiltration of the tunnel water from at least 500 m above the tunnel. The results suggest a crystalline basement with a fracture network that is regionally connected and water-conducting. Hydraulic conductivity in the southern Black Forest granite is estimated to amount to about 4.5·10−8 m s−1 at 500 m depth. The hydraulic boundary conditions exclude unknown drainage. Analyses of the influence of topography on orientation and magnitude of the maximum stress indicate a minimum overburden of about 500 m for regional reactivation to be valid. In conclusion, the southern Black Forest and in particular “Wilhelminenstollen” offers favourable condition for CHFE. Final decision on the GeoLaB site is to be drawn from forthcoming exploration wells.

In the Himalayan chain the collision of India into Eurasia has produced some of the most complex crustal interactions along the Himalayan–Alpine Orogen. In NW Bhutan, middle to late Miocene deformation has been partitioned between conjugate strike-slip faulting, E–W extension along the Yadong-Gulu graben and kilometre-scale folding. To better understand the late deformation stages and their implications for the evolution of the eastern Himalayas, the palaeomagnetism in the erosional remnant of the Tethyan Himalayan rocks outcropping in NW Bhutan has been studied. Their position to the south of the trace of the inner South Tibetan Detachment, to the south of the Tibetan Plateau offers a unique possibility to study the Tertiary rotation of the Himalayas. Pyrrhotite is the carrier of the characteristic magnetisation based on 270–325 °C unblocking temperatures. The age of the remanence is ca. 13 Ma indicated by illite 40 K/ 40 Ar cooling ages and a negative fold test. Small circle intersection method applied to the pyrrhotite components shows a ca. 32° clockwise rotation with respect to stable India since 13 Ma. We suggest that this clockwise rotation is related to strain partitioning between NE-directed shortening, sinistral-slip along the Lingshi fault, and east–west extension. This represents a field-based explanation and a minimum onset age for present-day eastward motion of the upper-crust of SE-Tibet and NE-Himalayas.

With the aim of investigating the possibilities of magnetotelluric methods for the exploration of potential Enhanced Geothermal System (EGS) sites in the Upper Rhine valley, a 2-D magnetotelluric (MT) survey has been carried out on a 13 km long profile across the thermal anomaly in the area of the geothermal power plant of Soultz-sous-Forets in the winter 2007/08. Despite strong artificial noise, processing using remote referencing and Sutarno phase consistent smoothing revealed significant results from 10 out of 16 sites. Indication for 1-D structures was found in the shortest periods, 2-D effects in the periods up to 40 s, and 3-D effects in the long period range. Since 3-D effects were found in the longer periods, 2-D inversion was carried out for periods smaller than 40 s. The results of the inversion are consistent with the geology of the geothermal site and distinguish well the sediments from the granitic basement including the structures given by the faults. A conductive anomaly with a resistivity of about 3 Ωm has been found at a depth down to 2000 m in the area of the Soultz and Kutzenhausen faults, which is attributed to geothermal processes.

All geological models are subject to several kinds of uncertainty. These can be classified into three different types: data imprecision and quality, inherent randomness and incomplete knowledge. With our approach, we address uncertainties introduced by input data quality in complex three-dimensional (3D) models of subsurface structures (geological models). As input data, we consider parameters of geological structures, i.e. formation and fault boundary points and orientation measurements. Our method consists of five steps: construction of an initial geological model with an implicit potential-field method, assignment of probability distributions to data positions and orientation measurements, simulation of several input data sets, construction of several model realisations based on these simulated data sets and finally the visualisation and analysis of the uncertainties. We test our approach in two generic models, a simple graben setting and a complex dome structure. The first model shows that our approach can evaluate uncertainties from different structures and their interaction. Furthermore, it indicates that the final uncertainty of the model is not simply the sum of all input data uncertainties but complex interactions exist. The second example demonstrates that our approach can handle full three-dimensional settings with overturned surfaces. Results of the uncertainty simulation can be visualised and analysed in several ways, ranging from borehole histograms to uncertainty maps. For complex 3D visualisation and analysis, we use indicator functions. When we apply these, we can treat the visualisation of uncertainties in complex settings in a self-consistent manner. With our simulation method, it is possible to analyse and visualise the uncertainties directly introduced by imprecision in the input data. Our approach is intuitive and straight-forward and suitable in both simple and complex geological settings. It enables detailed insights into the model quality, even for the non-expert. In cases where a geological model is the basis for geophysical simulation, it opens-up the way to geological data-driven ensemble modelling and inversion.

The current knowledge on thermal water resources in carbonate rock aquifers is presented in this review, which also discusses geochemical processes that create reservoir porosity and different types of utilisations of these resources such as thermal baths, geothermal energy and carbon dioxide (CO2) sequestration. Carbonate aquifers probably constitute the most important thermal water resources outside of volcanic areas. Several processes contribute to the creation of porosity, summarised under the term hypogenic (or hypogene) speleogenesis, including retrograde calcite solubility, mixing corrosion induced by cross-formational flow, and dissolution by geogenic acids from deep sources. Thermal and mineral waters from karst aquifers supply spas all over the world such as the famous bath in Budapest, Hungary. Geothermal installations use these resources for electricity production, district heating or other purposes, with low CO2 emissions and land consumption, e.g. Germany’s largest geothermal power plant at Unterhaching near Munich. Regional fault and fracture zones are often the most productive zones, but are sometimes difficult to locate, resulting in a relatively high exploration uncertainty. Geothermal installations in deep carbonate rocks could also be used for CO2 sequestration (carbonate dissolution would partly neutralise this gas and increase reservoir porosity). The use of geothermal installations to this end should be further investigated.

Background: The possible signature of deep fluids originating from processes occurring during infiltration or circulation in a remote valley in the Black Forest (Germany), a typical infiltration area in the crystalline basement which represents a possible recharge zone of one of the major heat flux density anomalies in central Europe, has been hydrochemically characterised. Chemical and isotopic compositions of two warm springs and several cold springs in Bürchau as well as water from the Badenweiler Spa were sampled three times during June and October 2013. Methods: A number of 70 water samples were taken at natural outlet conditions and analysed for major and trace elements, water H/O-isotope ratios and sulphur isotope ratios of dissolved sulfate. A chlorofluorocarbon (CFC) analysis was conducted to determine the underground residence time of the thermal water. To assess water-rock interactions seven rock samples representing the occurring lithological units of the study area were prepared to thin sections for polarization microscopic analysis. Results: The main spring in Bürchau discharging water with a temperature up to T = 18.1°C at flow rates of approximately 0.2 L s$^{-1}$ is Na-Ca-HCO$_{3}$-dominated and generally low mineralised (total dissolved solids (TDS) of about 150 mg L$^{-1}$). Even lower mineralisation and temperature of a nearby spring indicates further dilution with shallow groundwater. With respect to cold springs in the vicinity, the thermal water in Bürchau is slightly enriched in Cl, B, Li, Rb and Cs. In nearby granites, sericitisation of plagioclase and oxidation of pyrite to goethite have been identified. The stable isotope composition of H and O suggests meteoric origin of the water. Chlorofluorocarbon (CFC) analyses indicate that 30% to 40% of the thermal water in Bürchau is younger than 60 to 70 years. Reservoir temperatures have been estimated to 40°C to 80°C using sulphate and quartz geothermometers. Conclusions: Thus, the circulation time of the other 60% to 70% of the thermal water is longer than 70 years. Estimated reservoir temperatures in a range of 40°C to 80°C correspond to an infiltration depth of about 1,600 to 1,800 m.

A palaeomagnetic study has been carried out in the Tethyan Himalaya (TH; the northern margin of Greater India). Twenty-six palaeomagnetic sites have been sampled in Triassic low-grade metasediments of western Dolpo. Two remanent components have been identified. A pyrrhotite component, characterized by unblocking temperatures of 270–335 °C, yields an in situ mean direction of D=191.7°, I=−30.9° (k=29.5, α95=5.7°, N=23 sites). The component fails the fold test at the 99% confidence level (kin situ/kbed=6.9) and is therefore of postfolding origin. For reason of the low metamorphic grade, this pyrrhotite magnetization is believed to be of thermo-chemical origin. Geochronological data and inclination matching indicate an acquisition age around 35 Ma. The second remanence component has higher unblocking temperatures (>400 °C and up to 500–580 °C range) and resides in magnetite. A positive fold test and comparison with expected Triassic palaeomagnetic directions suggest a primary origin. The postfolding character of the pyrrhotite component, and its interpreted age of remanence acquisition, implies that the main Himalayan folding is older than 35 Ma in the western Dolpo area. This study also suggests that the second metamorphic event (Neo-Himalayan) was more significant in the Dolpo area than the first (Eo-Himalayan) one. A clockwise rotation of 10–15° is inferred from the pyrrhotite component, which is compatible with oroclinal bending and/or rotational underthrusting models. This rotation is also supported by the magnetite component, indicating that no rotation of the Tethyan Himalaya relative to India took place before 35 Ma.

Secondary magnetic remanences residing in pyrrhotite and anisotropy of magnetic susceptibility (AMS) were studied in low-grade metamorphic carbonates of the Tethyan Himalaya in Nar/Phu valley (central Nepal) and used for interpretation of tectonic deformations. The characteristic remanence (ChRM) is likely of thermomagnetic origin related to post-peak metamorphic cooling occurring after the Eohimalayan phase (35–32 Ma). The ChRM postdates small-scale folding (main Himalayan folding F1 and F2) as shown by a negative fold test of site mean directions at 99% confidence level, and has been probably acquired between 32 and 25 Ma. Late-orogenic long-wavelength folding associated with the Chako antiform (CA) is recorded by the spatial dispersion of ChRM directions and the distribution of the main axes of the AMS tensor. The mean tilting of the ChRM direction since remanence acquisition (c20–30j) approximately coincides with the tilting of the CA (31j) at the study area indicating that the pyrrhotite remanence predates the CA (CA formed at

In metacarbonates of the Lesser (LH) and Tethyan (TH) Himalayas of Kumaon/Garhwal (N-India) characteristic remanent magnetisations carried by pyrrhotite (unblocking temperatures: 250^330‡C) and magnetite (demagnetising spectra: 15^50 mT) have been identified. Negative fold tests indicate remanence acquisition after the main folding phase, which is of short-wavelength character and occurs during the early orogenese of the Himalayas. A thermal or thermochemical origin of magnetisation is likely and the age of remanence acquisition is indicated to be about 40 Ma by 40 K/ 39 Ar cooling and 40 Ar/ 39 Ar crystallisation ages. In the Kumaon LH a long-wavelength tilting is indicated by a distribution of the remanence directions along a small-circle in N^S direction. Steepening of the remanence directions in the TH related to ramping on the Main Central Thrust (MCT) was not observed, in contrast to other related studies. In the Alaknanda valley of LH a 38 > 8 Ma age of remanence acquisition is supported by comparison of observed inclinations to the apparent polar wander path of India. Clockwise rotation of 20.3 > 11.7‡ (LH/Alaknanda valley) and 11.3 > 8.5‡ (TH) with respect to the Indian plate is observed, indicating that there is no significant evidence for rotational shortening along the MCT since about 40 Ma. Our results suggest that most of rotational underthrusting and oroclinal bending has not been accommodated by the MCT, but by the main thrusts south of it. The latest Miocene/Pliocene age of the Main Boundary Thrust indicates that oroclinal bending is a late-orogenic process.

Palaeomagnetic measurements were carried out on low-grade metamorphic carbonates, of Mesozoic age from the Shiar area (85.1°E, 28.6°N) of the Tethyan Himalaya (TH) in north central Nepal. Two characteristic remanence components carried by pyrrhotite (ChRM 1 ) and magnetite (ChRM 2 ) could be identified by their unblocking temperature spectra of 270–340 and 430–580°C, respectively. Fold tests are not significant, due to the uniform bedding of all sites. However, according to results from other areas of the TH, the pyrrhotite component has been probably acquired as a secondary (p)TRM during exhumation and cooling; thus the age of remanence acquisition can be related to the last cooling event (25–17 Ma in the surrounding areas). The inclination of the magnetite component matches the value expected from the Indian APWP. This may the primary origin of the ChRM 2 . Pyrrhotite site-mean directions show a small-circle distribution, with a best fit parallel to the N–S direction. Backtilting to the expected inclination ( I exp ) by intersection of the remanence small-circle with the small-circle of constant I exp yields a clockwise block rotation of 30–35° with respect to the Indian Plate. Characteristics of the pyrrhotite component (small-circle distribution of site-means, secondary origin, (p)TRM with unblocking temperatures below about 300°C), allow the interpretation of the chronologic order of the thermo-tectonic history: (i) an earlier main folding phase at elevated temperatures; (ii) a later event of cooling through about 300°C coinciding with the acquisition of ChRM 1 ; (iii) clockwise block rotations with respect to the Indian Plate and (iv) long-wave folding as the youngest tectonic event.

In Mesozoic metacarbonates of the Tethyan Himalayas (Shiar Khola area, Central Nepal) two characteristic remanent magnetisations (ChRM1 and ChRM2) were identified by their unblocking temperature spectra. The ChRM1 is carried by pyrrhotite (unblocking temperature: 270–360°C) and the ChRM2 by magnetite (unblocking temperature spectra: 430–580°C). The temperature-related formation of pyrrhotite at the expense of primary magnetite during low-grade metamorphism in marly carbonates allows the determination of thermal gradients by the pyrrhotite/magnetite ratio. This new method can be used as a geothermometer for T≤300°C in low-grade metamorphic carbonates, where other methods are not available. This method is applied for the first time in the Tethyan Himalayas of Central Nepal. In the Shiar Khola valley, systematic variations in the ferrimagnetic content of the metacarbonates along an E–W profile were detected by the ratio of remanence intensity of pyrrhotite to magnetite, derived from natural remanent magnetisation (RPYR/MAG) and saturation magnetisation (SPYR/MAG). Over a stretch of 10 km the RPYR/MAG and SPYR/MAG increase from W to E from ~0.42 to ~0.91 and ~0.48 to ~1.0, respectively. Based on temperature estimates, the eastern part experienced upper anchizone–epizone (~250–300°C) conditions, while the western part underwent only diagenesis (~200°C). The temperature gradient and the temperature ranges suggested are consistent with the findings of the calcite twin lamellae geothermometry which is a non-magnetic method.

Background This study aims on investigating the regional flow field of the Soultz and adjacent geothermal fields located on the western side of the central Upper Rhine Graben and thus to provide insight into the origin of the 70% of the geothermal fluid coming from the regional inflow in the deep reservoir of the Soultz site. In an integrative approach, we consolidate conceptual models on fluid flow in the central Upper Rhine Graben. Methods Based on a 3D geological model and a new 3D temperature interpolation, we tackle the relation between tectonic structures and the occurrence of advection/convection along favourably oriented fault zones. Using sequential Butterworth filters, we study the distribution of negative residual anomalies in a pseudo-tomography down to a depth of about 6 to 8 km. Results We derived N-S-striking V-shaped negative anomalies that are consistent with the orientation of fault zones revealing major temperature anomalies to their east. Conclusions Following the concept of negative anomalies revealing zones of increased fracture porosity, and in agreement with fluid-chemistry, our findings suggest infiltration of meteoric water through the graben boundary fault and along preferential flow pathways that merge at intermediate depth. Up-flow of thermal water mixed most likely with brine from the deeper eastern part of the graben occurs along W-dipping typically rather steep structures.

The total amount of crustal shortening within the Indian plate includes straight forward and rotational shortening. The complex tectonic history of the western Himalaya impedes a direct observation of rotational shortening between India and its northern margin. A combination of palaeomagnetic results and tectonic structures along a ∼300 km NW–SE section, from NW-Zanskar (33.9°N, 76.5°E) to Malari (30.0°N, 79.6°E) in northern India, enable a separation of rotations on different scales, correction of the observed rotations, and quantification of uniform rotational shortening. New palaeomagnetic results from metacarbonates of the Tethyan Himalaya in Sarchu, Losar, and Spiti and earlier studies from NW-Zanskar and Malari reveal a characteristic remanent magnetisation carried by pyrrhotite (unblocking spectrum: 250–330°C). Negative fold tests (99% level) indicated remanence acquisition after the main folding of the Himalaya. The formation of pyrrhotite during thermal events (T>200°C) suggests a remanence acquisition related to exhumation and cooling. Temperatures of about 200–300°C indicate a thermochemical remanence acquisition with an Eohimalayan age, dated in two areas by 40K/39Ar techniques on illites. The resulting rotation angles in respect to stable India show an ‘anomalous’ counterclockwise rotation in NW-Zanskar and a trend of decreasing clockwise rotations to the SE. These variations may be attributed to meso-scale (∼100 km) tectonic features and oroclinal bending. For the first time block rotations with respect to stable India are separated into local, meso-scale and regional significant ones. After elimination of local and meso-scale effects, using structural analysis of late orogenic features, modelled rotations due to oroclinal bending were subtracted. Regionally significant rotations of 0–20° clockwise are interpreted to be caused only by uniform rotational underthrusting.

Thermal sources are common manifestations of geothermal energy resources in Alpine regions. The up-flow of the fluid is well-known to be often linked to cross-cutting fault zones providing a significant volume of fractures. Since conventional exploration methods are challenging in such areas of high topography and complicated logistics, 3D geological modeling based on structural investigation becomes a useful tool for assessing the overall geology of the investigated sites. Geological modeling alone is, however, less effective if not integrated with deep subsurface investigations that could provide a first order information on geological boundaries and an imaging of geological structures. With this aim, in the present paper the combined use of 3D geological modeling and gravity surveys for geothermal prospection of a hydrothermal area in the western Alps was carried out on two sites located in the Argentera Massif (NW Italy). The geothermal activity of the area is revealed by thermal anomalies with surface evidences, such as hot springs, at temperatures up to 70 °C. Integration of gravity measurements and 3D modeling investigates the potential of this approach in the context of geothermal exploration in Alpine regions where a very complex geological and structural setting is expected. The approach used in the present work is based on the comparison between the observed gravity and the gravity effect of the 3D geological models, in order to enhance local effects related to the geothermal system. It is shown that a correct integration of 3D modeling and detailed geophysical survey could allow a better characterization of geological structures involved in geothermal fluids circulation. Particularly, gravity inversions have successfully delineated the continuity in depth of low density structures, such as faults and fractured bands observed at the surface, and have been of great help in improving the overall geological model.