TY - GEN A1 - Ziegler, Moritz O. A1 - Heidbach, Oliver A1 - Reinecker, John A1 - Przybycin, Anna M. A1 - Scheck-Wenderoth, Magdalena T1 - A multi-stage 3-D stress field modelling approach exemplified in the Bavarian Molasse Basin T2 - Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe N2 - The knowledge of the contemporary in situ stress state is a key issue for safe and sustainable subsurface engineering. However, information on the orientation and magnitudes of the stress state is limited and often not available for the areas of interest. Therefore 3-D geomechanical-numerical modelling is used to estimate the in situ stress state and the distance of faults from failure for application in subsurface engineering. The main challenge in this approach is to bridge the gap in scale between the widely scattered data used for calibration of the model and the high resolution in the target area required for the application. We present a multi-stage 3-D geomechanical-numerical approach which provides a state-of-the-art model of the stress field for a reservoir-scale area from widely scattered data records. Therefore, we first use a large-scale regional model which is calibrated by available stress data and provides the full 3-D stress tensor at discrete points in the entire model volume. The modelled stress state is used subsequently for the calibration of a smaller-scale model located within the large-scale model in an area without any observed stress data records. We exemplify this approach with two-stages for the area around Munich in the German Molasse Basin. As an example of application, we estimate the scalar values for slip tendency and fracture potential from the model results as measures for the criticality of fault reactivation in the reservoir-scale model. The modelling results show that variations due to uncertainties in the input data are mainly introduced by the uncertain material properties and missing S-Hmax magnitude estimates needed for a more reliable model calibration. This leads to the conclusion that at this stage the model's reliability depends only on the amount and quality of available stress information rather than on the modelling technique itself or on local details of the model geometry. Any improvements in modelling and increases in model reliability can only be achieved using more high-quality data for calibration. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 556 KW - in-situ stress KW - induced seismicity KW - geothermal-reservoirs KW - geomechanical model KW - fault reactivation KW - alpine foreland KW - map project KW - km depth KW - orientation KW - system Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-409806 SN - 1866-8372 IS - 556 ER - TY - THES A1 - Ziegler, Moritz O. T1 - The 3D in-situ stress field and its changes in geothermal reservoirs T1 - Das 3D in-situ Spannungsfeld und seine Änderungen in Geothermiereservoiren N2 - Information on the contemporary in-situ stress state of the earth’s crust is essential for geotechnical applications and physics-based seismic hazard assessment. Yet, stress data records for a data point are incomplete and their availability is usually not dense enough to allow conclusive statements. This demands a thorough examination of the in-situ stress field which is achieved by 3D geomechanicalnumerical models. However, the models spatial resolution is limited and the resulting local stress state is subject to large uncertainties that confine the significance of the findings. In addition, temporal variations of the in-situ stress field are naturally or anthropogenically induced. In my thesis I address these challenges in three manuscripts that investigate (1) the current crustal stress field orientation, (2) the 3D geomechanical-numerical modelling of the in-situ stress state, and (3) the phenomenon of injection induced temporal stress tensor rotations. In the first manuscript I present the first comprehensive stress data compilation of Iceland with 495 data records. Therefore, I analysed image logs from 57 boreholes in Iceland for indicators of the orientation of the maximum horizontal stress component. The study is the first stress survey from different kinds of stress indicators in a geologically very young and tectonically active area of an onshore spreading ridge. It reveals a distinct stress field with a depth independent stress orientation even very close to the spreading centre. In the second manuscript I present a calibrated 3D geomechanical-numerical modelling approach of the in-situ stress state of the Bavarian Molasse Basin that investigates the regional (70x70x10km³) and local (10x10x10km³) stress state. To link these two models I develop a multi-stage modelling approach that provides a reliable and efficient method to derive from the larger scale model initial and boundary conditions for the smaller scale model. Furthermore, I quantify the uncertainties in the models results which are inherent to geomechanical-numerical modelling in general and the multi-stage approach in particular. I show that the significance of the models results is mainly reduced due to the uncertainties in the material properties and the low number of available stress magnitude data records for calibration. In the third manuscript I investigate the phenomenon of injection induced temporal stress tensor rotation and its controlling factors. I conduct a sensitivity study with a 3D generic thermo-hydro-mechanical model. I show that the key control factors for the stress tensor rotation are the permeability as the decisive factor, the injection rate, and the initial differential stress. In particular for enhanced geothermal systems with a low permeability large rotations of the stress tensor are indicated. According to these findings the estimation of the initial differential stress in a reservoir is possible provided the permeability is known and the angle of stress rotation is observed. I propose that the stress tensor rotations can be a key factor in terms of the potential for induced seismicity on pre-existing faults due to the reorientation of the stress field that changes the optimal orientation of faults. N2 - Kenntnis des derzeitigen in-situ Spannungszustandes der Erdkruste ist essenziell für geotechnische Anwendungen und seismische Gefährdungsabschätzungen, welche auf physikalischen Beobachtungen basieren. Jedoch sind die Spannungsinformationen jedes Datenpunktes unvollständig und die Menge an vorhandenen Datenpunkten ist normalerweise nicht groß genug, um schlüssige Ergebnisse zu erzielen. Daher ist eine eingehende Untersuchung des in-situ Spannungsfeldes, welche durch 3D geomechanisch-numerische Modellierung geleistet wird, erforderlich. Jedoch ist die räumliche Auflösung der Modelle begrenzt und der resultierende Spannungszustand ist großen Unsicherheiten unterworfen, welche die Aussagekraft der Ergebnisse beschränken. Zusätzlich gibt es zeitliche Änderungen des Spannungsfeldes, welche durch natürliche Prozesse bedingt oder menschengemacht sind. In meiner Dissertation behandle ich diese Herausforderungen in drei Manuskripten, welche (1) die Orientierung des derzeitigen Spannungszustandes, (2) die 3D geomechanisch-numerische Modellierung des in-situ Spannungszustandes und (3) das Phänomen injektionsinduzierter zeitlicher Rotationen des Spannungstensors zum Thema haben. In dem ersten Manuskript präsentiere ich die erste umfassende Spannungsdatensammlung von Island mit insgesamt 495 Einträgen. Dafür habe ich Bilddatenlogs aus 57 Bohrlöchern in Island auf Indikatoren der maximalen horizontalen Spannungsorientierung hin untersucht. Diese Studie ist die erste ganzheitliche Spannungsuntersuchung, welche sich auf verschiedene Indikatoren der Spannungsorientierung stützt und in einem geologisch sehr jungen und tektonisch aktiven Gebiet auf einem Mittelozeanischen Rücken an Land liegt. Es zeigt sich, dass selbst sehr nahe an der Plattengrenze eine tiefenunabhängige, eindeutige Spannungsorientierung existiert. In dem zweiten Manuskript präsentiere ich einen kalibrierten 3D geomechanisch-numerischen Modellierungsansatz des in-situ Spannungszustandes des bayrischen Molassebeckens welches den regionalen (70x70x10km³) und den lokalen (10x10x10km³) Spannungszustand untersucht. Um diese zwei Modelle zu verbinden, habe ich ein mehrstufigen Modellansatz entworfen, welcher eine zuverlässige und effiziente Methode darstellt um Randbedingungen und Initialbedingungen für das kleinere Modell aus dem größeren Modell abzuleiten. Des Weiteren quantifiziere ich die Unsicherheiten in den Modellergebnissen, welche im Allgemeinen durch geomechanisch-numerische Modellierung und im Speziellen durch den Mehrstufenansatz entstehen. Ich zeige, dass die Signifikanz der Modellergebnisse hauptsächlich durch die Unsicherheiten in den Materialeigenschaften sowie der geringen Anzahl vorhandener Spannungsmagnitudendaten zur Kalibrierung reduziert wird. In dem dritten Manuskript untersuche ich das Phänomen injektionsinduzierter zeitlicher Rotationen des Spannungstensors und deren kontrollierende Parameter. Ich führe eine Sensitivitätsanalyse mit einem generischen 3D thermo-hydro-mechanischen Modell durch. Darin zeige ich, dass die Schlüsselparameter, welche die Rotationen des Spannungstensors kontrollieren, die Permeabilität des Reservoirgesteins als der entscheidende Faktor, die Injektionsrate und die initiale Differenzspannung sind. Insbesondere für geothermische Systeme nach dem Hot-Dry-Rock-Verfahren mit einer geringen Permeabilität weisen die Ergebnisse auf große Rotationen des Spannungstensors hin. Gemäß diesen Ergebnissen kann die initiale Differenzspannung in einem Reservoir abgeschätzt werden, sollte die Permeabilität bekannt sein und der Winkel der Spannungsrotation beobachtet werden. Ich schlage vor, dass Spannungsrotationen ein Schlüsselfaktor in Bezug auf das Potenzial für induzierte Seismizität sind, welche auf prä-existierenden Störungen entsteht, die durch die Reorientierung des Spannungsfeldes optimal orientiert werden. KW - stress KW - stress changes KW - induced seismicity KW - geothermal KW - geomechanical modelling KW - Spannung KW - Spannungsänderungen KW - induzierte Seismizität KW - Geothermie KW - geomechanische Modellierung Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-403838 ER - TY - JOUR A1 - Wang, Lei A1 - Kwiatek, Grzegorz A1 - Rybacki, Erik A1 - Bonnelye, Audrey A1 - Bohnhoff, Marco A1 - Dresen, Georg T1 - Laboratory study on fluid-induced fault slip behavior: the role of fluid pressurization rate JF - Geophysical research letters : GRL N2 - Understanding the physical mechanisms governing fluid-induced fault slip is important for improved mitigation of seismic risks associated with large-scale fluid injection. We conducted fluid-induced fault slip experiments in the laboratory on critically stressed saw-cut sandstone samples with high permeability using different fluid pressurization rates. Our experimental results demonstrate that fault slip behavior is governed by fluid pressurization rate rather than injection pressure. Slow stick-slip episodes (peak slip velocity < 4 mu m/s) are induced by fast fluid injection rate, whereas fault creep with slip velocity < 0.4 mu m/s mainly occurs in response to slow fluid injection rate. Fluid-induced fault slip may remain mechanically stable for loading stiffness larger than fault stiffness. Independent of fault slip mode, we observed dynamic frictional weakening of the artificial fault at elevated pore pressure. Our observations highlight that varying fluid injection rates may assist in reducing potential seismic hazards of field-scale fluid injection projects.
Plain Language Summary Human-induced earthquakes from field-scale fluid injection projects including enhanced geothermal system and deep wastewater injection have been documented worldwide. Although it is clear that fluid pressure plays a crucial role in triggering fault slip, the physical mechanism behind induced seismicity still remains poorly understood. We performed laboratory tests, and here we present two fluid-induced slip experiments conducted on permeable Bentheim sandstone samples crosscut by a fault that is critically stressed. Fault slip is then triggered by pumping the water from the bottom end of the sample at different fluid injection rates. Our results show that fault slip is controlled by fluid pressure increase rate rather than by the absolute magnitude of fluid pressure. In contrast to episodes of relatively rapid but stable sliding events caused by a fast fluid injection rate, fault creep is observed during slow fluid injection. Strong weakening of the dynamic friction coefficient of the experimental fault is observed at elevated pore pressure, independent of fault slip mode. These results may provide a better understanding of the complex behavior of fluid-induced fault slip on the field scale. KW - fault slip KW - fluid injection KW - induced seismicity KW - fluid pressurization KW - rate KW - stick-slip KW - fault creep Y1 - 2020 U6 - https://doi.org/10.1029/2019GL086627 SN - 0094-8276 SN - 1944-8007 VL - 47 IS - 6 PB - Wiley CY - Hoboken, NJ ER - TY - JOUR A1 - Wang, Lei A1 - Kwiatek, Grzegorz A1 - Rybacki, Erik A1 - Bohnhoff, Marco A1 - Dresen, Georg T1 - Injection-induced seismic moment release and laboratory fault slip BT - implications for fluid-induced seismicity JF - Geophysical research letters N2 - Understanding the relation between injection-induced seismic moment release and operational parameters is crucial for early identification of possible seismic hazards associated with fluid-injection projects. We conducted laboratory fluid-injection experiments on permeable sandstone samples containing a critically stressed fault at different fluid pressurization rates. The observed fluid-induced fault deformation is dominantly aseismic. Fluid-induced stick-slip and fault creep reveal that total seismic moment release of acoustic emission (AE) events is related to total injected volume, independent of respective fault slip behavior. Seismic moment release rate of AE scales with measured fault slip velocity. For injection-induced fault slip in a homogeneous pressurized region, released moment shows a linear scaling with injected volume for stable slip (steady slip and fault creep), while we find a cubic relation for dynamic slip. Our results highlight that monitoring evolution of seismic moment release with injected volume in some cases may assist in discriminating between stable slip and unstable runaway ruptures. KW - induced seismicity KW - seismic moment release KW - fluid injection KW - stick slip KW - fault creep KW - acoustic emission Y1 - 2020 U6 - https://doi.org/10.1029/2020GL089576 SN - 0094-8276 SN - 1944-8007 VL - 47 IS - 22 PB - American Geophysical Union CY - Washington ER - TY - THES A1 - Sen, Ali Tolga T1 - Inversion of seismic source parameters for weak mining-induced and natural earthquakes T1 - Inversion von seismischen Quellparametern für schwache Bergbau-induzierte und natürliche Erdbeben N2 - The purpose of this thesis is to develop an automated inversion scheme to derive point and finite source parameters for weak earthquakes, here intended with the unusual meaning of earthquakes with magnitudes at the limit or below the bottom magnitude threshold of standard source inversion routines. The adopted inversion approaches entirely rely on existing inversion software, the methodological work mostly targeting the development and tuning of optimized inversion flows. The resulting inversion scheme is tested for very different datasets, and thus allows the discussion on the source inversion problem at different scales. In the first application, dealing with mining induced seismicity, the source parameters determination is addressed at a local scale, with source-sensor distance of less than 3 km. In this context, weak seismicity corresponds to event below magnitude MW 2.0, which are rarely target of automated source inversion routines. The second application considers a regional dataset, namely the aftershock sequence of the 2010 Maule earthquake (Chile), using broadband stations at regional distances, below 300 km. In this case, the magnitude range of the target aftershocks range down to MW 4.0. This dataset is here considered as a weak seismicity case, since the analysis of such moderate seismicity is generally investigated only by moment tensor inversion routines, with no attempt to resolve source duration or finite source parameters. In this work, automated multi-step inversion schemes are applied to both datasets with the aim of resolving point source parameters, both using double couple (DC) and full moment tensor (MT) models, source duration and finite source parameters. A major result of the analysis of weaker events is the increased size of resulting moment tensor catalogues, which interpretation may become not trivial. For this reason, a novel focal mechanism clustering approach is used to automatically classify focal mechanisms, allowing the investigation of the most relevant and repetitive rupture features. The inversion of the mining induced seismicity dataset reveals the repetitive occurrence of similar rupture processes, where the source geometry is controlled by the shape of the mined panel. Moreover, moment tensor solutions indicate a significant contribution of tensile processes. Also the second application highlights some characteristic geometrical features of the fault planes, which show a general consistency with the orientation of the slab. The additional inversion for source duration allowed to verify the empirical correlation for moment normalized earthquakes in subduction zones among a decreasing rupture duration with increasing source depth, which was so far only observed for larger events. N2 - Ziel dieser Dissertation ist es ein automatisches Inversionsschema zur Ableitung von Parametern für Punkt- und finite Quellen von "schwachen" Erdbeben im Sinne von aussergewöhnlich kleinen Erdbeben an oder unterhalb der Magnitudenschwelle für Standard-Inversionsroutinen zu entwickeln. Der verwendete Inversionsansatz baut auf bestehender Inversionssoftware auf, die methodische Arbeit zielt hauptsächlich auf die Entwicklung und das Tuning des optimierten Inversionsablaufs. Das resultierende Inversionsverfahren ist für verschiedene Datensätze getestet und erlaubt somit eine Diskussion über das Herdinversionsproblem auf unterschiedlichen Skalierungen. In der ersten Anwendung, die sich mit induzierter Seismizität in Minen beschäftigt, werden die Herdparameter im lokalen Bereich mit einer Herd-Empfänger Entfernung von weniger als 3 km bestimmt. In diesem Zusammenhang bezieht sich die "schwache" Seismizität auf Ereignisse mit Magnituden unter Mw 2.0, welche sehr selten das Ziel von automatisierten Herdinversionsroutinen sind. Die zweite Anwendung berücksichtigt den regionalen Datensatz der Nachbebesequenz des Maule (Chile) Erdbebens von 2010, aufgezeichnet durch Breitband-Stationen in einer maximalen Entfernung von 300 km. In diesem Fall umfasst der Magnitudenbereich Ereignisse mit Magnituden kleiner als Mw 4.0. Auch dieser Datensatz wird hier als "schwache" Seismizität interpretiert, da die Analyse solch moderater Seismizität normalerweise nur durch Routinen für Momententensorinversion erfolgt, welche keinerlei Auflösung für die Herddauer oder finite Herdparameter beinhaltet. In dieser Arbeit wird ein automatisiertes, mehrstufiges Inversionsverfahren auf beide Datensätze angewendet mit dem Ziel sowohl Punktquellen-Herdparameter für double-couple (DC) und volle Momententensor (MT) Modelle als auch für Herddauer und finite Herdparameter zu bestimmen. Ein Hauptergebnis dieser Analyse von schwachen Ereignissen ist die Vergrößerung von Momententensor-Katalogen, deren Interpretation wichtig werden könnte. Aus diesem Grund wurde ein neuartiger Cluster-Ansatz für Herdmechanismen angewendet um diese automatisch zu klassifizieren, der somit die Untersuchung der wichtigsten und sich wiederholenden Bruchprozessen erlaubt. Die Inversion des Datensatzes von induzierter Seismizität in Minen enthüllt das sich wiederholende Auftreten von ähnlichen Bruchprozessen, wobei die Herdgeometrie von der Form der Minentäfelung kontrolliert wird. Darüber hinaus deuten die Momententensorlösungen einen signifikanten Beitrag von tensilen Prozessen an. Auch die zweite Anwendung zeigt einige charackteristische, geometrische Besonderheiten der Bruchflächen auf, welche prinzipiell konsistent zur Orientierung der subduzierten Platte sind. Die zusätzliche Inversion nach der Herdzeit erlaubt die Verifizierung von empirischen Korrelationen von momentennormalisierten Erdbeben in Subduktionszonen zwischen einer abnehmenden Bruchdauer mit zunehmender Herdtiefe, welche bisher nur für größere Ereignisse beobachtet wurde. KW - Seismologie KW - induzierten Seismizität KW - Quellenumkehr KW - Momententensor KW - Aufenthaltsdauer KW - seismology KW - induced seismicity KW - source inversion KW - moment tensor KW - source duration Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-71914 ER - TY - JOUR A1 - Richter, Gudrun A1 - Hainzl, Sebastian A1 - Dahm, Torsten A1 - Zöller, Gert T1 - Stress-based, statistical modeling of the induced seismicity at the Groningen gas field BT - the Netherlands JF - Environmental earth sciences N2 - Groningen is the largest onshore gas field under production in Europe. The pressure depletion of the gas field started in 1963. In 1991, the first induced micro-earthquakes have been located at reservoir level with increasing rates in the following decades. Most of these events are of magnitude less than 2.0 and cannot be felt. However, maximum observed magnitudes continuously increased over the years until the largest, significant event with ML=3.6 was recorded in 2014, which finally led to the decision to reduce the production. This causal sequence displays the crucial role of understanding and modeling the relation between production and induced seismicity for economic planing and hazard assessment. Here we test whether the induced seismicity related to gas exploration can be modeled by the statistical response of fault networks with rate-and-state-dependent frictional behavior. We use the long and complete local seismic catalog and additionally detailed information on production-induced changes at the reservoir level to test different seismicity models. Both the changes of the fluid pressure and of the reservoir compaction are tested as input to approximate the Coulomb stress changes. We find that the rate-and-state model with a constant tectonic background seismicity rate can reproduce the observed long delay of the seismicity onset. In contrast, so-called Coulomb failure models with instantaneous earthquake nucleation need to assume that all faults are initially far from a critical state of stress to explain the delay. Our rate-and-state model based on the fluid pore pressure fits the spatiotemporal pattern of the seismicity best, where the fit further improves by taking the fault density and orientation into account. Despite its simplicity with only three free parameters, the rate-and-state model can reproduce the main statistical features of the observed activity. KW - induced seismicity KW - modeling KW - statistical seismology KW - forecast Y1 - 2020 U6 - https://doi.org/10.1007/s12665-020-08941-4 SN - 1866-6280 SN - 1866-6299 VL - 79 IS - 11 PB - Springer CY - New York ER - TY - THES A1 - Niemz, Peter T1 - Imaging and modeling of hydraulic fractures in crystalline rock via induced seismic activity T1 - Charakterisierung und Modellierung hydraulischer Brüche in Kristallingestein mit Hilfe induzierter Seismizität N2 - Enhanced geothermal systems (EGS) are considered a cornerstone of future sustainable energy production. In such systems, high-pressure fluid injections break the rock to provide pathways for water to circulate in and heat up. This approach inherently induces small seismic events that, in rare cases, are felt or can even cause damage. Controlling and reducing the seismic impact of EGS is crucial for a broader public acceptance. To evaluate the applicability of hydraulic fracturing (HF) in EGS and to improve the understanding of fracturing processes and the hydromechanical relation to induced seismicity, six in-situ, meter-scale HF experiments with different injection schemes were performed under controlled conditions in crystalline rock in a depth of 410 m at the Äspö Hard Rock Laboratory (Sweden). I developed a semi-automated, full-waveform-based detection, classification, and location workflow to extract and characterize the acoustic emission (AE) activity from the continuous recordings of 11 piezoelectric AE sensors. Based on the resulting catalog of 20,000 AEs, with rupture sizes of cm to dm, I mapped and characterized the fracture growth in great detail. The injection using a novel cyclic injection scheme (HF3) had a lower seismic impact than the conventional injections. HF3 induced fewer AEs with a reduced maximum magnitude and significantly larger b-values, implying a decreased number of large events relative to the number of small ones. Furthermore, HF3 showed an increased fracture complexity with multiple fractures or a fracture network. In contrast, the conventional injections developed single, planar fracture zones (Publication 1). An independent, complementary approach based on a comparison of modeled and observed tilt exploits transient long-period signals recorded at the horizontal components of two broad-band seismometers a few tens of meters apart from the injections. It validated the efficient creation of hydraulic fractures and verified the AE-based fracture geometries. The innovative joint analysis of AEs and tilt signals revealed different phases of the fracturing process, including the (re-)opening, growth, and aftergrowth of fractures, and provided evidence for the reactivation of a preexisting fault in one of the experiments (Publication 2). A newly developed network-based waveform-similarity analysis applied to the massive AE activity supports the latter finding. To validate whether the reduction of the seismic impact as observed for the cyclic injection schemes during the Äspö mine-scale experiments is transferable to other scales, I additionally calculated energy budgets for injection experiments from previously conducted laboratory tests and from a field application. Across all three scales, the cyclic injections reduce the seismic impact, as depicted by smaller maximum magnitudes, larger b-values, and decreased injection efficiencies (Publication 3). N2 - Hydraulisch-stimulierte tiefengeothermale Systeme (Enhanced Geothermal systems, EGS) gelten als einer der Eckpfeiler für die nachhaltige Energieerzeugung der Zukunft. In diesen geothermalen Systemen wird heißes Tiefengestein durch Fluidinjektionen unter hohem Druck aufgebrochen, um Wegsamkeiten zur Erwärmung von Wasser oder anderen Fluiden zu schaffen. Beim Aufbrechen werden zwangsläufig kleine seismische Ereignisse ausgelöst (induzierte Seismizität), die in sehr seltenen Fällen an der Oberfläche spürbar sind, jedoch in extremen Fällen auch Schäden verursachen können. Die Kontrolle bzw. die Reduzierung der seismischen Aktivität in EGS ist daher ein entscheidender Punkt, damit diese Art der Energiegewinnung eine breite gesellschaftliche Akzeptanz findet. Grundlage dieser Dissertation ist eine Serie von kontrollierten, hydraulischen Bruchexperimenten mit Bruchdimensionen von einigen Metern. Die Experimente wurden in einer Tiefe von 410 m in kristallinem Gestein eines Versuchsbergwerks (Äspö Hard Rock Laboratory, Schweden) mit unterschiedlichen Injektionsstrategien durchgeführt. Die detaillierte Auswertung der Bruch-Experimente in dieser Dissertation zielt darauf ab, die Nutzbarkeit von hydraulischen Stimulationen (hydraulic fracturing, HF) in EGS zu untersuchen und das Verständnis von Bruchprozessen sowie der hydromechanischen Beziehung zur induzierten Seismizität zu verbessern. Um die Schallemissionsaktivität (acoustic emissions, AE), die durch 11 piezoelektrische AE-Sensoren kontinuierlich aufgezeichnet wurde, zu extrahieren und zu charakterisieren, wurde ein halbautomatischer, wellenformbasierter Detektions-, Klassifizierungs- und Lokalisierungsworkflow entwickelt. Mit Hilfe des resultierenden Katalogs von 20000 AEs wurde das Bruchwachstum detailliert kartiert und charakterisiert. Das Experiment mit der neuartigen, zyklischen Injektionsstrategie (HF3) weist einen geringeren seismischen Fußabdruck auf als die Standard-Injektionsstrategie. HF3 induzierte weniger AEs und eine kleinere Maximalmagnitude. Außerdem hatte das Experiment einen signifikant höheren b-Wert, was einer verringerten Anzahl von großen AEs relativ zur Anzahl der kleineren AEs entspricht. Darüber hinaus zeigte HF3 eine erhöhte Komplexität im Bruchmuster mit mehreren Brüchen bzw. einem Netzwerk von Brüchen. Im Gegensatz dazu entwickelten die Standard-Injektionen einzelne, ebene Bruchzonen (Publikation 1). Zusätzlich zu den induzierten AEs wurden transiente, langperiodische Signale auf den horizontalen Komponenten von zwei Breitband-Seismometern, die wenige Meter von den Brüchen installiert waren, ausgewertet. Diese Signale wurden als Neigungssignale interpretiert und mit modellierten Neigungssignalen verglichen. Der Vergleich zeigt unabhängig, dass hydraulische Brüche geöffnet wurden und bestätigt, dass die AE-basierte Analyse die Bruchgeometrie verlässlich kartieren kann. Die gemeinsame Betrachtung von AEs und Neigungssignalen offenbart verschiedene Phasen des Bruchprozesses: das (wiederholte) Öffnen des Bruches, das Bruchwachstum und das weitere Wachsen des Bruches nach dem Ende der Injektion. Außerdem liefert die Analyse Hinweise auf die Reaktivierung einer natürlichen Bruchzone in einem der Experimente (Publikation 2). Eine neuentwickelte und hier präsentierte Wellenform-Ähnlichkeitsanalyse, die Informationen des gesamten Sensornetzwerkes nutzt und zum ersten Mal auf einen umfangreichen AE-Katalog angewendet wurde, unterstützt diese Interpretation. Um zu validieren, ob die verringerte Seismizität während der zyklischen Injektion auf der Meter-Skala (Bergwerk) auf andere Maßstäbe übertragbar ist, wurden Energie-Budgets für Injektionsexperimente aus zuvor durchgeführten Laborversuchen und aus einem Tiefengeothermie-Projekt berechnet. Über alle drei Skalen hinweg zeigen die zyklischen Injektionen einen verringerten seismischen Fußabdruck mit kleineren Maximalmagnituden, größeren b-Werte und einem kleineren Verhältnis von seismisch-abgestrahlter zu injizierter Energie (Publikation 3). KW - induced seismicity KW - hydraulic fracturing KW - enhanced geothermal systems (EGS) KW - injection KW - deformation KW - acoustic emissions KW - fracture growth KW - injection scheme KW - basement rock KW - Schallemissionen KW - Grundgestein KW - Deformation KW - verbesserte geothermische Systeme KW - Bruchausbreitung KW - hydraulisches Aufbrechen KW - Induzierte Seismizität KW - Injektion KW - Injektionsschema Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-556593 ER - TY - JOUR A1 - Martínez-Garzón, Patricia A1 - Kwiatek, Grzegorz A1 - Bohnhoff, Marco A1 - Dresen, Georg T1 - Volumetric components in the earthquake source related to fluid injection and stress state JF - Geophysical research letters N2 - We investigate source processes of fluid-induced seismicity from The Geysers geothermal reservoir in California to determine their relation with hydraulic operations and improve the corresponding seismic hazard estimates. Analysis of 869 well-constrained full moment tensors (M-w 0.8-3.5) reveals significant non-double-couple components (>25%) for about 65% of the events. Volumetric deformation is governed by cumulative injection rates with larger non-double-couple components observed near the wells and during high injection periods. Source mechanisms are magnitude dependent and vary significantly between faulting regimes. Normal faulting events (M-w<2) reveal substantial volumetric components indicating dilatancy in contrast to strike-slip events that have a dominant double-couple source. Volumetric components indicating closure of cracks in the source region are mostly found for reverse faulting events with M-w>2.5. Our results imply that source processes and magnitudes of fluid-induced seismic events are strongly affected by the hydraulic operations, the reservoir stress state, and the faulting regime. KW - non-double-couple components KW - induced seismicity KW - geothermal KW - stress state KW - tensile opening KW - pore pressure Y1 - 2017 U6 - https://doi.org/10.1002/2016GL071963 SN - 0094-8276 SN - 1944-8007 VL - 44 IS - 2 SP - 800 EP - 809 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - López-Comino, José Ángel A1 - Cesca, Simone A1 - Niemz, Peter A1 - Dahm, Torsten A1 - Zang, Arno T1 - Rupture directivity in 3D inferred from acoustic emissions events in a mine-scale hydraulic fracturing experiment JF - Frontiers in Earth Science N2 - Rupture directivity, implying a predominant earthquake rupture propagation direction, is typically inferred upon the identification of 2D azimuthal patterns of seismic observations for weak to large earthquakes using surface-monitoring networks. However, the recent increase of 3D monitoring networks deployed in the shallow subsurface and underground laboratories toward the monitoring of microseismicity allows to extend the directivity analysis to 3D modeling, beyond the usual range of magnitudes. The high-quality full waveforms recorded for the largest, decimeter-scale acoustic emission (AE) events during a meter-scale hydraulic fracturing experiment in granites at similar to 410 m depth allow us to resolve the apparent durations observed at each AE sensor to analyze 3D-directivity effects. Unilateral and (asymmetric) bilateral ruptures are then characterized by the introduction of a parameter kappa, representing the angle between the directivity vector and the station vector. While the cloud of AE activity indicates the planes of the hydrofractures, the resolved directivity vectors show off-plane orientations, indicating that rupture planes of microfractures on a scale of centimeters have different geometries. Our results reveal a general alignment of the rupture directivity with the orientation of the minimum horizontal stress, implying that not only the slip direction but also the fracture growth produced by the fluid injections is controlled by the local stress conditions. KW - directivity KW - earthquake source KW - induced seismicity KW - hydraulic KW - fracturing KW - acoustic emissions Y1 - 2021 U6 - https://doi.org/10.3389/feart.2021.670757 SN - 2296-6463 VL - 9 PB - Frontiers Media CY - Lausanne ER - TY - JOUR A1 - Kwiatek, Grzegorz A1 - Martinez-Garzon, Patricia A1 - Plenkers, K. A1 - Leonhardt, Maria A1 - Zang, Arno A1 - von Specht, Sebastian A1 - Dresen, Georg A1 - Bohnhoff, Marco T1 - Insights into complex subdecimeter fracturing processes occurring during a water injection experiment at depth in Aspo Hard Rock Laboratory, Sweden JF - Journal of geophysical research : Solid earth N2 - We investigate the source characteristics of picoseismicity (M-w < -2) recorded during a hydraulic fracturing in situ experiment performed in the underground Aspo Hard Rock Laboratory, Sweden. The experiment consisted of six stimulations driven by three different water injection schemes and was performed inside a 28-m-long, horizontal borehole located at 410-m depth. The fracturing processes were monitored with a variety of seismic networks including broadband seismometers, geophones, high-frequency accelerometers, and acoustic emission sensors thereby covering a wide frequency band between 0.01 and 100,000Hz. Here we study the high-frequency signals with dominant frequencies exceeding 1000 Hz. The combined seismic network allowed for detection and detailed analysis of 196 small-scale seismic events with moment magnitudes M-W < -3.5 (source sizes of decimeter scale) that occurred solely during the stimulations and shortly after. The double-difference relocated hypocenter catalog as well as source parameters were used to study the physical characteristics of the induced seismicity and then compared to the stimulation parameters. We observe a spatiotemporal migration of the picoseismic events away and toward the injection intervals in direct correlation with changes in the hydraulic energy (product of fluid injection pressure and injection rate). We find that the total radiated seismic energy is extremely low with respect to the product of injected fluid volume and pressure (hydraulic energy). The radiated seismic energy correlates well with the hydraulic energy rate. The obtained fault plane solutions for particularly well-characterized events signify the reactivation of preexisting rock defects under influence of increased pore fluid pressure on fault plane orientations in good correspondence with the local stress field orientation. KW - induced seismicity KW - fracking KW - picoseismicity KW - seismomechanics KW - source parameters KW - maximum magnitude Y1 - 2018 U6 - https://doi.org/10.1029/2017JB014715 SN - 2169-9313 SN - 2169-9356 VL - 123 IS - 8 SP - 6616 EP - 6635 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Hofmann, Hannes A1 - Zimmermann, Günter A1 - Farkas, Márton Pál A1 - Huenges, Ernst A1 - Zang, Arno A1 - Leonhardt, Maria A1 - Kwiatek, Grzegorz A1 - Martinez-Garzon, Patricia A1 - Bohnhoff, Marco A1 - Min, Ki-Bok A1 - Fokker, Peter A1 - Westaway, Rob A1 - Bethmann, Falko A1 - Meier, Peter A1 - Yoon, Kern Shin A1 - Choi, JaiWon A1 - Lee, Tae Jong A1 - Kim, Kwang Yeom T1 - First field application of cyclic soft stimulation at the Pohang Enhanced Geothermal System site in Korea JF - Geophysical journal international N2 - Large-magnitude fluid-injection induced seismic events are a potential risk for geothermal energy developments worldwide. One potential risk mitigation measure is the application of cyclic injection schemes. After validation at small (laboratory) and meso (mine) scale, the concept has now been applied for the first time at field scale at the Pohang Enhanced Geothermal System (EGS) site in Korea. From 7 August until 14 August 2017 a total of 1756 m(3) of surface water was injected into Pohang well PX-1 at flow rates between 1 and 10 l s(-1), with a maximum wellhead pressure (WHP) of 22.8 MPa, according to a site-specific cyclic soft stimulation schedule and traffic light system. A total of 52 induced microearthquakes were detected in real-time during and shortly after the injection, the largest of M-w 1.9. After that event a total of 1771 m(3) of water was produced back from the well over roughly 1 month, during which time no larger-magnitude seismic event was observed. The hydraulic data set exhibits pressure-dependent injectivity increase with fracture opening between 15 and 17 MPa WHP, but no significant permanent transmissivity increase was observed. The maximum magnitude of the induced seismicity during the stimulation period was below the target threshold of M-w 2.0 and additional knowledge about the stimulated reservoir was gained. Additionally, the technical feasibility of cyclic injection at field scale was evaluated. The major factors that limited the maximum earthquake magnitude are believed to be: limiting the injected net fluid volume, flowback after the occurrence of the largest induced seismic event, using a cyclic injection scheme, the application of a traffic light system, and including a priori information from previous investigations and operations in the treatment design. KW - Cyclic soft stimulation (CSS) KW - induced seismicity KW - risk mitigation KW - enhanced geothermal systems (EGS) KW - granite KW - Pohang (Korea) Y1 - 2019 U6 - https://doi.org/10.1093/gji/ggz058 SN - 0956-540X SN - 1365-246X VL - 217 IS - 2 SP - 926 EP - 949 PB - Oxford Univ. Press CY - Oxford ER - TY - THES A1 - Grigoli, Francesco T1 - Automated seismic event location by waveform coherence analysis T1 - Die automatische Lokalisierung von Mikroerdbeben durch die Wellenform Kohärenzanalyse N2 - Automated location of seismic events is a very important task in microseismic monitoring operations as well for local and regional seismic monitoring. Since microseismic records are generally characterised by low signal-to-noise ratio, such methods are requested to be noise robust and sufficiently accurate. Most of the standard automated location routines are based on the automated picking, identification and association of the first arrivals of P and S waves and on the minimization of the residuals between theoretical and observed arrival times of the considered seismic phases. Although current methods can accurately pick P onsets, the automatic picking of the S onset is still problematic, especially when the P coda overlaps the S wave onset. In this thesis I developed a picking free automated method based on the Short-Term-Average/Long-Term-Average (STA/LTA) traces at different stations as observed data. I used the STA/LTA of several characteristic functions in order to increase the sensitiveness to the P wave and the S waves. For the P phases we use the STA/LTA traces of the vertical energy function, while for the S phases, we use the STA/LTA traces of the horizontal energy trace and then a more optimized characteristic function which is obtained using the principal component analysis technique. The orientation of the horizontal components can be retrieved by robust and linear approach of waveform comparison between stations within a network using seismic sources outside the network (chapter 2). To locate the seismic event, we scan the space of possible hypocentral locations and origin times, and stack the STA/LTA traces along the theoretical arrival time surface for both P and S phases. Iterating this procedure on a three-dimensional grid we retrieve a multidimensional matrix whose absolute maximum corresponds to the spatial and temporal coordinates of the seismic event. Location uncertainties are then estimated by perturbing the STA/LTA parameters (i.e the length of both long and short time windows) and relocating each event several times. In order to test the location method I firstly applied it to a set of 200 synthetic events. Then we applied it to two different real datasets. A first one related to mining induced microseismicity in a coal mine in the northern Germany (chapter 3). In this case we successfully located 391 microseismic event with magnitude range between 0.5 and 2.0 Ml. To further validate the location method I compared the retrieved locations with those obtained by manual picking procedure. The second dataset consist in a pilot application performed in the Campania-Lucania region (southern Italy) using a 33 stations seismic network (Irpinia Seismic Network) with an aperture of about 150 km (chapter 4). We located 196 crustal earthquakes (depth < 20 km) with magnitude range 1.1 < Ml < 2.7. A subset of these locations were compared with accurate locations retrieved by a manual location procedure based on the use of a double difference technique. In both cases results indicate good agreement with manual locations. Moreover, the waveform stacking location method results noise robust and performs better than classical location methods based on the automatic picking of the P and S waves first arrivals. N2 - Die automatische Lokalisierung seismischer Ereignisse ist eine wichtige Aufgabe, sowohl im Bereich des Mikroseismischen Monitorings im Bergbau und von Untegrund Aktivitäten,  wie auch für die lokale und regionale Überwachung von natürlichen Erdbeben. Da mikroseismische Datensätze häufig ein schlechtes Signal-Rausch-Verhältnis haben müssen die Lokalisierungsmethoden robust gegen Rauschsignale und trotzdem hinreichend genau sein. Aufgrund der in der Regel sehr hochfrequent aufgezeichneten Messreihen und der dadurch sehr umfangreichen Datensätze sind automatische Auswertungen erstrebenswert. Solche Methoden benutzen in der Regel automatisch gepickte und  den P und S Phasen zugeordnete Ersteinsätze und Minimieren die Summe der quadratischen Zeitdifferenz zwischen den beobachteten und theoretischen Einsatzzeiten. Obgleich das automatische Picken der P Phase in der Regel sehr genau möglich ist, hat man beim Picken der S Phasen häufig Probleme, z.B. wenn die Coda der P Phase sehr lang ist und in den Bereich der S Phase hineinreicht. In dieser Doktorarbeit wird eine Methode vollautomatische, Wellenform-basierte Lokalisierungsmethode entwickelt, die Funktionen des Verhältnisses "Short Term Average / Long Term Average"  (STA/LTA) verwendet und keine Pickzeiten invertiert. Die STA/LTA charakteristische Funktion wurde für unterschiedliche Wellenform Attribute getestet, um die Empfindlichkeit für P und S Phasen zu erhöhen. Für die P Phase wird die STA/LTA Funktion für die Energie der Vertikalkomponente der Bodenbewegung benutzt, wohingegen für die S Phase entweder die Energie der horizontalen Partikelbewegung oder eine optimierte Funktion auf Basis der Eigenwertzerlegung benutzt wird. Um die Ereignisse zu lokalisieren wird eine Gittersuche über alle möglichen Untergrundlokalisierungen durchgeführt. Für jeden räumlichen und zeitlichen Gitterpunkt werden die charakteristischen Funktionen entlang der theoretischen Einsatzkurve aufsummiert. Als Ergebnis erhält man eine 4-dimensionale Matrix über Ort und Zeit des Ereignisses, deren Maxima die wahrscheinlichsten Lokalisierungen darstellen. Um die Unsicherheiten der Lokalisierung abzuschätzen wurden die Parameter der STA/LTA Funktionen willkürlich verändert und das Ereignis relokalisiert. Die Punktwolke aller möglichen Lokalisierungen gibt ein Maß für die Unsicherheit des Ergebnisses. Die neu entwickelte Methode wurde an einem synthetischen Datensatz von 200 Ereignissen getestet und für zwei beobachtete Datensätze demonstriert. Der erste davon betrifft induzierte Seismizität in einem Kohlebergbau in Norddeutschland. Es wurden 391 Mikrobeben mit Magnituden zwischen Ml 0.5 und 2.0 erfolgreich lokalisiert und durch Vergleich mit manuell ausgewerteten Lokalisierungen verifziert.Der zweite Datensatz stammt von einem Anwednung auf des Regionale Überwachungsnetz in der Region Campania-Lucania (Süditalien)  mit 33 seismischen Stationen und einer Apertur von etwa 150 km. Wir konnten 196 Erdbeben mit Tiefen < 20 km und Magnituden zwischen Ml 1.1 und 2.7 lokalisieren. Eine Untergruppe der eigenen Lokalisierungen wurde mit den Lokalisierungen einer Standard Lokalisierung sowie einer  hochgenauen Relativlokalisierung verglichen. In beiden Fällen ist die Übereinstimmung mit den manuellen Lokalisierungen groß. Außerdem finden wir, dass die Wellenform Summations Lokalisierung ronbust gegen Rauschen ist und bessere Ergebnisse liefert als die Standard Lokalisierung, die auf dem automatischen Picken von Ersteinsatzzeiten alleine basiert. KW - earthquake location KW - induced seismicity KW - microseismicity Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-70329 ER - TY - JOUR A1 - Dahm, Torsten A1 - Cesca, Simone A1 - Hainzl, Sebastian A1 - Braun, Thomas A1 - Krüger, Frank T1 - Discrimination between induced, triggered, and natural earthquakes close to hydrocarbon reservoirs: A probabilistic approach based on the modeling of depletion-induced stress changes and seismological source parameters JF - Journal of geophysical research : Solid earth N2 - Earthquakes occurring close to hydrocarbon fields under production are often under critical view of being induced or triggered. However, clear and testable rules to discriminate the different events have rarely been developed and tested. The unresolved scientific problem may lead to lengthy public disputes with unpredictable impact on the local acceptance of the exploitation and field operations. We propose a quantitative approach to discriminate induced, triggered, and natural earthquakes, which is based on testable input parameters. Maxima of occurrence probabilities are compared for the cases under question, and a single probability of being triggered or induced is reported. The uncertainties of earthquake location and other input parameters are considered in terms of the integration over probability density functions. The probability that events have been human triggered/induced is derived from the modeling of Coulomb stress changes and a rate and state-dependent seismicity model. In our case a 3-D boundary element method has been adapted for the nuclei of strain approach to estimate the stress changes outside the reservoir, which are related to pore pressure changes in the field formation. The predicted rate of natural earthquakes is either derived from the background seismicity or, in case of rare events, from an estimate of the tectonic stress rate. Instrumentally derived seismological information on the event location, source mechanism, and the size of the rupture plane is of advantage for the method. If the rupture plane has been estimated, the discrimination between induced or only triggered events is theoretically possible if probability functions are convolved with a rupture fault filter. We apply the approach to three recent main shock events: (1) the M-w 4.3 Ekofisk 2001, North Sea, earthquake close to the Ekofisk oil field; (2) the M-w 4.4 Rotenburg 2004, Northern Germany, earthquake in the vicinity of the Sohlingen gas field; and (3) the M-w 6.1 Emilia 2012, Northern Italy, earthquake in the vicinity of a hydrocarbon reservoir. The three test cases cover the complete range of possible causes: clearly human induced, not even human triggered, and a third case in between both extremes. KW - induced seismicity KW - probabilistic discrimination KW - hydrocarbon field KW - triggered earthquake KW - seismic hazard KW - earthquake Y1 - 2015 U6 - https://doi.org/10.1002/2014JB011778 SN - 2169-9313 SN - 2169-9356 VL - 120 IS - 4 SP - 2491 EP - 2509 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Bentz, Stephan A1 - Martinez-Garzon, Patricia A1 - Kwiatek, Grzegorz A1 - Dresen, Georg A1 - Bohnhoff, Marco T1 - Analysis of Microseismicity Framing M-L > 2.5 Earthquakes at The Geysers Geothermal Field, California JF - Journal of geophysical research : Solid earth N2 - Preparatory mechanisms accompanying or leading to nucleation of larger earthquakes have been observed at both laboratory and field scales, but conditions favoring the occurrence of observable preparatory processes are still largely unknown. In particular, it remains a matter of debate why some earthquakes occur spontaneously without noticeable precursors as opposed to events that are preceded by an extended failure process. In this study, we have generated new high-resolution seismicity catalogs framing the occurrence of 20 M-L > 2.5 earthquakes at The Geysers geothermal field in California. To this end, a seismicity catalog of the 11 days framing each large event was created. We selected 20 sequences sampling different hypocentral depths and hydraulic conditions within the field. Seismic activity and magnitude frequency distributions displayed by the different earthquake sequences are correlated with their location within the reservoir. Sequences located in the northwestern part of the reservoir show overall increased seismic activity and low b values, while the southeastern part is dominated by decreased seismic activity and higher b values. Periods of high injection coincide with high b values and vice versa. These observations potentially reflect varying differential and mean stresses and damage of the reservoir rocks across the field. About 50% of analyzed sequences exhibit no change in seismicity rate in response to the large main event. However, we find complex waveforms at the onset of the main earthquake, suggesting that small ruptures spontaneously grow into or trigger larger events. KW - induced seismicity KW - earthquake nucleation KW - The Geysers KW - earthquake sequences Y1 - 2019 U6 - https://doi.org/10.1029/2019JB017716 SN - 2169-9313 SN - 2169-9356 VL - 124 IS - 8 SP - 8823 EP - 8843 PB - American Geophysical Union CY - Washington ER -