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- ItemRearrangement of stresses in fault zones - detecting major issues of coupled hydraulic–mechanical processes with relevance to geothermal applications(Göttingen : Copernicus Publ., 2014) Ziefle, G.The South German Molasse Basin provides favourable conditions for geothermal plants. Nevertheless, micro-seismic events occur in the vicinity of the geothermal Unterhaching Gt2 well and seem to be caused by the geothermal plant. The injection and production are located in an existing fault system. The majority of seismic events takes place at a horizontal distance of 500 m or less of the borehole. However, none of the seismic events are located in the injection reservoir but in fact at a significantly greater depth. A deeper process understanding of the interacting thermal–hydraulic–mechanical effects in the vicinity of the well is desired. This article presents a significantly simplified 2-D model, investigating interactions of the stress field in the vicinity of the geothermal well and movements in the fault system. This might be of special interest, as the operation of the geothermal plant might lead to changes in the material and fracture properties on the one hand and in the equilibrium state on the other. A detailed description of the model, as well as various parameter studies, is presented. It can be seen that boundary conditions such as direction of the stress field in relation to the fault system, geometry of the fault system and parameters of the fractures have a significant influence on stresses in the proximity of the geothermal well. A variation in the spatial stress field in some parts of the fault system is to be expected. For the chosen assumptions the dimension of this variation is about 25% of the assumed stresses. Future work on this model might focus on the characteristics of the fault system, as well as on the influence of the coupled thermal–hydraulic–mechanical effects.
- ItemEffectiveness of acidizing geothermal wells in the South German Molasse Basin(Göttingen : Copernicus Publ., 2013) Schumacher, S.; Schulz, R.In Germany, many hydro-geothermal plants have been constructed in recent years, primarily in the region of Munich. As the host formation here mainly consists of carbonates, nearly all recently drilled wells have been acidized in order to improve the well yield. In this study, the effectiveness of these acid treatments is analyzed with respect to the amount of acid used and the number of acid treatments carried out per well. The results show that the first acid treatment has the largest effect, while subsequent acidizing improves the well only marginally. Data also indicate that continued acidizing can lead to degradation of the well. These findings may not only be important for geothermal installations in Germany but also for projects, for example, in Austria, France or China where geothermal energy is produced from carbonate formations as well.
- ItemWeibull-distributed dyke thickness reflects probabilistic character of host-rock strength([London] : Nature Publishing Group UK, 2014) Krumbholz, Michael; Hieronymus, Christoph F.; Burchardt, Steffi; Troll, Valentin R.; Tanner, David C.; Friese, NadineMagmatic sheet intrusions (dykes) constitute the main form of magma transport in the Earth’s crust. The size distribution of dykes is a crucial parameter that controls volcanic surface deformation and eruption rates and is required to realistically model volcano deformation for eruption forecasting. Here we present statistical analyses of 3,676 dyke thickness measurements from different tectonic settings and show that dyke thickness consistently follows the Weibull distribution. Known from materials science, power law-distributed flaws in brittle materials lead to Weibull-distributed failure stress. We therefore propose a dynamic model in which dyke thickness is determined by variable magma pressure that exploits differently sized host-rock weaknesses. The observed dyke thickness distributions are thus site-specific because rock strength, rather than magma viscosity and composition, exerts the dominant control on dyke emplacement. Fundamentally, the strength of geomaterials is scale-dependent and should be approximated by a probability distribution.
- ItemTransboundary geophysical mapping of geological elements and salinity distribution critical for the assessment of future sea water intrusion in response to sea level rise(Munich : EGU, 2012) Jørgensen, F.; Scheer, W.; Thomsen, S.; Sonnenborg, T.O.; Hinsby, K.; Wiederhold, H.; Schamper, C.; Burschil, T.; Roth, B.; Kirsch, R.; Auken, E.Geophysical techniques are increasingly being used as tools for characterising the subsurface, and they are generally required to develop subsurface models that properly delineate the distribution of aquifers and aquitards, salt/freshwater interfaces, and geological structures that affect groundwater flow. In a study area covering 730 km2 across the border between Germany and Denmark, a combination of an airborne electromagnetic survey (performed with the SkyTEM system), a high-resolution seismic survey and borehole logging has been used in an integrated mapping of important geological, physical and chemical features of the subsurface. The spacing between flight lines is 200–250 m which gives a total of about 3200 line km. About 38 km of seismic lines have been collected. Faults bordering a graben structure, buried tunnel valleys, glaciotectonic thrust complexes, marine clay units, and sand aquifers are all examples of geological structures mapped by the geophysical data that control groundwater flow and to some extent hydrochemistry. Additionally, the data provide an excellent picture of the salinity distribution in the area and thus provide important information on the salt/freshwater boundary and the chemical status of groundwater. Although the westernmost part of the study area along the North Sea coast is saturated with saline water and the TEM data therefore are strongly influenced by the increased electrical conductivity there, buried valleys and other geological elements are still revealed. The mapped salinity distribution indicates preferential flow paths through and along specific geological structures within the area. The effects of a future sea level rise on the groundwater system and groundwater chemistry are discussed with special emphasis on the importance of knowing the existence, distribution and geometry of the mapped geological elements, and their control on the groundwater salinity distribution is assessed.
- ItemMonitoring hillslope moisture dynamics with surface ERT for enhancing spatial significance of hydrometric point measurements(Munich : EGU, 2015) Hübner, R.; Heller, K.; Günther, T.; Kleber, A.Besides floodplains, hillslopes are basic units that mainly control water movement and flow pathways within catchments of subdued mountain ranges. The structure of their shallow subsurface affects water balance, e.g. infiltration, retention, and runoff. Nevertheless, there is still a gap in the knowledge of the hydrological dynamics on hillslopes, notably due to the lack of generalization and transferability. This study presents a robust multi-method framework of electrical resistivity tomography (ERT) in addition to hydrometric point measurements, transferring hydrometric data into higher spatial scales to obtain additional patterns of distribution and dynamics of soil moisture on a hillslope. A geoelectrical monitoring in a small catchment in the eastern Ore Mountains was carried out at weekly intervals from May to December 2008 to image seasonal moisture dynamics on the hillslope scale. To link water content and electrical resistivity, the parameters of Archie's law were determined using different core samples. To optimize inversion parameters and methods, the derived spatial and temporal water content distribution was compared to tensiometer data. The results from ERT measurements show a strong correlation with the hydrometric data. The response is congruent to the soil tension data. Water content calculated from the ERT profile shows similar variations as that of water content from soil moisture sensors. Consequently, soil moisture dynamics on the hillslope scale may be determined not only by expensive invasive punctual hydrometric measurements, but also by minimally invasive time-lapse ERT, provided that pedo-/petrophysical relationships are known. Since ERT integrates larger spatial scales, a combination with hydrometric point measurements improves the understanding of the ongoing hydrological processes and better suits identification of heterogeneities.
- ItemZero-Offset VSP Monitoring of CO2Storage: Impedance Inversion and Wedge Modelling at the Ketzin Pilot Site(New York, NY : Hindawi, 2014) Götz, Julia; Lüth, Stefan; Krawczyk, Charlotte M.; Cosma, CalinAt the CO2 storage pilot site near the town of Ketzin (35 km west of Berlin, Germany) the sandstone reservoir at 630 m–650 m depth is thin and heterogeneous. The time-lapse analysis of zero-offset VSP measurements shows that CO2-induced amplitude changes can be observed on near-well corridor stacks. Further, we investigate whether CO2-induced amplitude changes in the monitoring data can be used to derive geometrical and petrophysical parameters governing the migration of CO2 within a brine saturated sandstone aquifer. 2D seismic-elastic modelling is done to test the processing workflow and to perform a wedge modelling study for estimation of the vertical expansion of the CO2 plume. When using the NRMS error as a measure for the similarity between the modelled and recorded repeat traces, the best match is achieved for a plume thickness of 6-7 m within the reservoir sandstone of 8 m thickness. With band limited impedance inversion a velocity reduction at the top of the reservoir of 30%, influenced by casing reverberations as well as CO2 injection, is found. The relation of seismic amplitude to CO2 saturated layer thickness and CO2-induced changes in P-wave velocities are important parameters for the quantification of the injected CO2 volume.
- ItemMulti-stage emplacement of the Götemar Pluton, SE Sweden: new evidence inferred from field observations and microfabric analysis, including cathodoluminescence microscopy(Berlin ; Heidelberg : Springer, 2012) Friese, Nadine; Vollbrecht, Axel; Tanner, David C.; Fahlbusch, Wiebke; Weidemann, MiriamThe emplacement of the Mesoproterozoic Götemar Pluton into Paleoproterozoic granitoid host rocks of the Transscandinavian Igneous Belt is re-examined by microfabric analysis, including cathodoluminescence microscopy. Field data on the pluton-host rock system are used to strengthen the model. The Götemar Pluton, situated on the Baltic Shield of SE Sweden, is a horizontally zoned tabular structure that was constructed by the intrusion of successive pulses of magma with different crystal/melt ratios, at an estimated crustal depth of 4–8 km. Initial pluton formation involved magma ascent along a vertical dike, which was arrested at a mechanical discontinuity within the granitoid host rocks; this led to the formation of an initial sill. Subsequent sill stacking and their constant inflation resulted in deformation and reheating of existing magma bodies, which also raised the pluton roof. This multi-stage emplacement scenario is indicated by complex dike relationships and the occurrence of several generations of quartz (Si-metasomatism). The sills were charged by different domains of a heterogeneous magma chamber with varying crystal/melt ratios. Ascent or emplacement of magma with a high crystal/melt ratio is indicated by syn-magmatic deformation of phenocrysts. Complex crystallization fabrics (e.g. oscillatory growth zoning caused by high crystal defect density, overgrowth and replacement features, resorbed and corroded crystal cores, rapakivi structure) are mostly related to processes within the main chamber, that is repeated magma mixing or water influx.
- ItemCyclostratigraphic studies of sediments from Lake Van (Turkey) based on their uranium contents obtained from downhole logging and paleoclimatic implications(Berlin ; Heidelberg : Springer, 2015) Baumgarten, H.; Wonik, T.A deep drilling campaign was performed at Lake Van (Turkey) to enhance the understanding of the paleoenvironmental conditions of the Middle East. Cores were collected, and the sediments are mainly composed of clayey silts and tephra deposits. Spectral gamma ray data were acquired (0–210 m below lake floor), and the uranium data were used for cyclostratigraphic studies to estimate the sedimentation rates and the time of deposition. Detection and analysis of climate cycles require continuous sedimentation and cannot be applied to these sediments which include numerous tephra layers. Therefore, these layers were removed, and a synthetic log was created (cumulative thickness of the tephra ≈50 m; remaining lacustrine sediments ≈160 m). High amplitudes were detected and correlated to Milanković cycles. Their evolution was analyzed using the sliding window technique. The sedimentation rates varied from 22 to 33 cm/ka in the upper section. The sediments between the lake floor and a depth of 210 m were deposited over a period of 587 ka. Our results agree with core interpretations (e.g., correlation of total organic carbon with marine isotope stages) which suggest a time span of deposition of 600 ka. High-frequency cycles were detected and correlated with interstadials from the North Greenland δ18O record for the past 75 ka. The two datasets agreed closely. We conclude that climate signals, even on sub-Milanković scale, are imprinted in uranium of these sediments, and cyclostratigraphic methods are applicable if event layers are subtracted from the sediment record.
- ItemNumerical modelling of climate change impacts on freshwater lenses on the North Sea Island of Borkum using hydrological and geophysical methods(Munich : EGU, 2012) Sulzbacher, H.; Wiederhold, H.; Siemon, B.; Grinat, M.; Igel, J.; Burschil, T.; Günther, T.; Hinsby, K.A numerical, density dependent groundwater model is set up for the North Sea Island of Borkum to estimate climate change impacts on coastal aquifers and especially the situation of barrier islands in the Wadden Sea. The database includes information from boreholes, a seismic survey, a helicopter-borne electromagnetic (HEM) survey, monitoring of the freshwater-saltwater boundary by vertical electrode chains in two boreholes, measurements of groundwater table, pumping and slug tests, as well as water samples. Based on a statistical analysis of borehole columns, seismic sections and HEM, a hydrogeological model is set up. The groundwater model is developed using the finite-element programme FEFLOW. The density dependent groundwater model is calibrated on the basis of hydraulic, hydrological and geophysical data, in particular spatial HEM and local monitoring data. Verification runs with the calibrated model show good agreement between measured and computed hydraulic heads. A good agreement is also obtained between measured and computed density or total dissolved solids data for both the entire freshwater lens on a large scale and in the area of the well fields on a small scale. For simulating future changes in this coastal groundwater system until the end of the current century, we use the climate scenario A2, specified by the Intergovernmental Panel on Climate Change and, in particular, the data for the German North Sea coast. Simulation runs show proceeding salinisation with time beneath the well fields of the two waterworks Waterdelle and Ostland. The modelling study shows that the spreading of well fields is an appropriate protection measure against excessive salinisation of the water supply until the end of the current century.
- ItemSpectral induced polarization measurements for predicting the hydraulic conductivity in sandy aquifers(Munich : EGU, 2013) Attwa, M.; Günther, T.Field and laboratory spectral induced polarization (SIP) measurements are integrated to characterize the hydrogeological conditions at the Schillerslage test site in Germany. The phase images are capable of monitoring thin peat layers within the sandy aquifers. However, the field results show limitations of decreasing resolution with depth. In comparison with the field inversion results, the SIP laboratory measurements show a certain shift in SIP response due to different compaction and sorting of the samples. The SIP data are analyzed to derive an empirical relationship for predicting the hydraulic conductivity (K). In particular, two significant but weak correlations between individual real resistivities (ρ') and relaxation times (τ), based on a Debye decomposition (DD) model, with measured K are found for the upper groundwater aquifer. The maximum relaxation time (τmax) and logarithmically weighted average relaxation time (τlw) show a better relation with K values than the median value τ50. A combined power law relation between individual ρ' and τ with K is developed with an expression of A · (ρ')B · (τlw)C, where A, B and C are determined using a least-squares fit between the measured and predicted K. The suggested approach with the calculated coefficients of the first aquifer is applied for the second. Results show good correlation with the measured K indicating that the derived relationship is superior to single phase angle models as Börner or Slater models.