ALBERT

Description: In the context of the provision of clean renewable energy, the utilization of the subsurface of urban areas has been gaining more attention. Especially major urban centers like the city of Berlin, Germany, have moved into focus because of the major share in global CO2-Emissions. One of the most promising mitigation factors for at least part of these emissions, is to rely on geothermal energy resources. The subsurface of Berlin is characterized by a complex geological setting beneath which different, coupled heat transport processes interact and are also overprinted by anthropogenic activities to a certain degree. A major challenge herein is the specific present-day utilization scenario, featuring the production of groundwater from the shallow subsurface (drinking water supply) and extraction of shallow geothermal heat from the underground. Therefore, to predict and reconstruct observed temperature, pressure and mass distributions a precise knowledge of the geological configuration and physical parameters of the subsurface is key. In this thesis I attempt to provide answers to the aforementioned issues by relying on numerical modeling studies which are based on a detailed 3D reconstruction of the underground and on physical principles of heat-, fluid- and mass transport. I present models of increasing complexity and detail concerning all physical processes and boundary conditions at work, enabling detailed representations of the present state, and a reconstruction of the natural state of the system, that is, before any human intervention. Based on the understanding gained from these studies, I derive some conclusive asserts on sustainable utilization scenarios in the underground of Berlin, and based on similarities, of other large urban centers. An initial set of models investigated, for a first time, the effect of major surface water bodies (lakes and rivers) on the thermal and hydraulic configuration of the subsurface. The results of these models show that major surface water bodies significantly modify the shallow to intermediate geothermal and hydrogeological setting especially where connected to areas showing a certain degree of overprinting by human activities. Due to groundwater production and associated lower hydraulic heads near the surface water bodies, forced infiltration from the latter is predicted, which fits observations closely. The thermal modifications are on the order of interest for shallow geothermal installations and also connect to a shift in areas most promising for exploitation. The second set of models focused on reconstructing the natural state, that is, the state before pumping activities commenced. This step was deemed necessary in order to be able to quantify the modifications to the subsurface hydrothermal configuration as related to human activities. The reconstructed "natural" state shows a complete replenishment of the depression cones, resulting in a shift of recharge and discharge areas, where rivers and lakes display gaining conditions only. In contrast, models that integrate pumping activities, illustrate that the effects of subsurface production is larger, both in magnitudes and areal as well as depth extents, than previously captured by models that relied on fixed hydraulic surface boundary conditions only. The presence of active wells provides a more realistic representation of flow rates, the net results of which is a sensitive modification of predicted fluid pathways, in agreement with the monitored hydraulics in Berlin (as exemplary demonstrated for the site of Karolinenhöhe). In a latter stage, I carried out a quantitative study on the energetic potential of the underground, by conducting a systematic analysis of the geothermal potential stored at different levels beneath the city of Berlin. Two approaches were chosen to identify and quantify promising areas for geothermal utilization. The first approach makes direct use of modeled temperature distribution at different depth levels. These highly nonuniform distributions reflect a heterogeneous potential of heat in place in the underground. In the shallow subsurface, utilization for shallow geothermal heat production and storage could be opted for. In the deep subsurface heat and electricity production are deemed possible. Building on this, the second approach investigated two possible deep geothermal target horizons by simulating a hydrothermal power-plant utilizing the predicted temperatures as well as reservoir geometry and physical properties. The connected geothermal potential of the deeper subsurface then shows nominally promising results, depicting up to ~10 MWth for a single virtual power plant at the most promising locality, while locally 0 MWth are encountered as well. All of these model results show most importantly that the subsurface thermal, hydraulic and mass distribution is highly sensitive to the parameters under study, which highlights the amount of caution that should be given to any planned change in the utilization of this space. This relates to any planned geothermal utilization of the different groundwater compartments which should be studied in depth using the models of this thesis as starting conditions.

Description: Knowledge of spatial and temporal distribution of fluids in the subsurface is crucialin a wide range of applications. During the production of crude oil typically high sa-line produced formation water is injected into the reservoir layer, aiming to push theoil towards production wells. While oil is commonly seen as an electrical insulator,the injected saline brines are characterised by low electrical resistivity. Thus, electro-magnetic (EM) methods and especially Controlled Source Electromagnetics (CSEM)attracted an increasing interest to monitor these resistivity changes inside the reser-voir over time.This thesis mainly reports on numerical aspects of modelling and inversion of landbased CSEM with particular focus towards hydrocarbon monitoring applications.Most of the presented developments were inspired by a superordinate research pro-ject including CSEM field surveys across an actively producing onshore oil field inNorthern Germany.In producing oil fields there exists a large number of steel-cased wells. Such existingoil field infrastructure and especially the presence of metal casings significantly altersthe propagation of EM fields in the subsurface. Their spatially unfavourable dimen-sions effectively prohibits a straightforward implementation into the modelling grid.Thus I developed a new modelling approach allowing consideration of such thin butvertically extended highly conductive structures including their mutual interaction.The developed methodology had been implemented into existing modelling and in-version codes. Using the new approach to investigate the influence of metal casingson CSEM data shows that they act as additional inductively coupled vertical electricdipole sources at depth and thereby increase resolution capabilities at depth. Thepresence of metal casings can thus be exploited by optimising the source receiverlayout in such a way that the strength of these additional vertical dipole sources ismaximised.An additional working package of the superordinate project was the measurement ofvertical electric fields in a shallow observation well. However, measurements of ver-tical electric fields requires long measurement dipoles to achieve satisfactory signal-to-noise ratios. Such extended dipoles span several modelling cells and are thereforein conflict with assumptions usually made for modelling, that receivers can be rep-resented as point dipoles. I therefore expanded the modelling and inversion codesto consider the physical receiver dimensions. The new algorithm implicitly considersimperfect alignment of the receiver with the corresponding field component. Withoutthe consideration of this effect inversion of vertical electric field measurements islikely to cause erroneous results.Finally I discuss different aspects of time-lapse inversion required to track changes influid saturation over time. The cascaded inversion scheme is applied to synthetic time-lapse data for a simplified oilfield undergoing brine flushing. The influence of variousinversion parameters in particular different regularisation techniques are examined.Surface based sources and receivers typically provide low sensitivity towards deeptargets in highly conductive backgrounds. Despite that using additional constraints,in particular a model weighting scheme together with energised steel casings allowedto track resistivity changes inside the reservoir based on synthetic time-lapse data.

Description: Wissen über räumliche und zeitliche Verteilung von Fluiden im Untergrund is unerlässlich für eine Reihe von Anwendungen. Typischerweise wird während der Förderung von Rohöl salinares produziertes Formationswasser in die ölführende Formation injiziert um das Öl-Wasser-Gemisch in Richtung der Förderbohrungen zu spülen. Während Öl als elektrischer Isolator gilt, zeichnen sich die injizierten salinaren Fluide durch eine hohe elektrische Leitfähigkeit aus. Daher erfahren elektromagnetische Methoden und insbesondere Controlled Source Electromagentics (CSEM) zunehmendes Interesse diese Änderung des elektrischen Widerstands mit der Zeit zu überwachen. Diese Arbeit beschäftigt sich im wesentlichen mit numerischen Aspekten der Modellierung und Inversion von CSEM an Land mit speziellem Fokus auf der Überwachung der Kohlenwasserstoff Produktion. Die meisten der gezeigten Entwicklungen sind entwickelt im Zuge eines übergeordneten Forschungsprojektes inklusive CSEM Feldmessungen in einem produzierenden Ölfeld in Norddeutschland. Produzierende Ölfelder sind gekennzeichnet durch eine große Anzahl von Stahl verrohrten Bohrungen. Die Anwesenheit von Stahlinfrastruktur insbesondere von Stahlschutzrohren beeinflusst die Ausbreitung von elektromagnetischen Feldern im Untergrund. Deren unvorteilhafte Geometrie erlaubt keine direkte Berücksichtigung in dem Modellierungsgitter. Daher habe ich einen neuen Modellierungsanzatz entwickelt der es erlaubt solch dünne aber vertikal ausgedehnte hochgradig leitfähige Strukturen inklusive deren gegenseitige Wechselwirkung zu berücksichtigen. Die entwickelte Methode wurde in bestehende Modellierungs- und Inversionssoftware implementiert. Mithilfe dieses neuen Ansatzes konnte der Einfluss von Stahlverrohrungen auf CSEM Daten untersucht werden. Stahlverrohrungen wirken wie zusätzliche induktiv angeregte vertikale elektrische Dipolquellen im Untergrund und helfen daher die Auflösung in der Tiefe zu erhöhen. Die Anwesenheit von Stahlverrohrungen kann daher ausgenutzt werden in dem man die Quell-Empfänger-Geometrie in einer Art und Weise optimiert, die die Stärke dieser zusätzlichen vertikalen Dipolquellen maximiert. Ein weiteres Arbeitspaket des übergeordneten Forschungsprojektes bestand in der Messung von vertikalen elektrischen Feldern in flachen Beobachtungsbohrungen. Messungen des vertikalen elektrischen Feldes erfordert lange Messdipole um ein ausreichendes Signal-Rausch-Verhältnisses zu gewährleisten. Solch ausgedehnte Dipole überspannen mehrere Zellen des Modellierungsgitters und verletzen die übliche Annahme, wonach die Länge der Empfänger vernachlässigbar ist. Daher habe ich die bestehenden Modellierungs- und Inversionsprogramme erweitert um die physischen Dimensionen von elektrischen Feld Empfängern zu berücksichtigen. Der implementierte Algorithmus berücksichtigt implizit Abweichungen der Orientierung des Messdipols von der Richtung der zu messenden Feldkomponente. Ohne dieser Berücksichtigung führt eine Inversion von vertikalen elektrischen Feld Daten zu fehlerhaften Ergebnissen. Schließlich werden unterschiedliche Aspekte von time-lapse Inversion diskutiert, welche notwendig ist um Änderungen der Fluidzusammensetzung abzubilden und zu verfolgen. Eine kaskadiertes Inversionsschema wurde auf synthetische time-lapse Daten eines vereinfachten Ölfeldes angewendet. Untersucht wurde der Einfluss verschiedener Parameter insbesondere verschiedener Regularisierungstechniken. Sender und Empfänger an der Erdoberfläche sind typischerweise wenig sensitiv zu tiefen Strukturen in leitfähiger Umgebung. Anhand von synthetische Daten konnte gezeigt werden, dass das benutzen zusätzlicher Nebenbedingungen wie einer Modellgewichtung und dem ausnutzen von vorhandenen Stahlverrohrungen es dennoch erlaubt Änderungen innerhalb des Ölreservoirs zu lokalisieren.

Description: The Antarctic continent is almost entirely ~99% covered by a thick ice layer impeding classical in-situ measurements. It hence remains one of the least geophysically known areas on Earth. Little is known about the structure and the thermal and rheological properties of its lithosphere. Since the state of the lithosphere is strongly linked to near-surface processes such as ice dynamics or glacial isostatic adjustment (GIA) as well as the deeper, convecting mantle, knowledge of those properties is crucially important when modeling the coupled systems. This cumulative thesis consists of three published scientific papers that together characterize the lithosphere of Antarctica in terms of strength, temperature, density, composition and upper crustal properties. As a measure of strength, the effective elastic thickness Te was derived by cross-spectral analysis of the gravity field with the adjusted topography. The fan wavelet technique was employed to, for the first time, calculate variations of Te over the entire continent by means of admittance and coherence analysis. The same gravity and topography data was then combined with tomography models constrained by mineral physics equations in an iterative inversion scheme to develop a 3D density, thermal and compositional model of the Antarctic lithosphere and upper mantle. Seismic data on crustal structures was further employed to create a new Moho and crustal density model. In order to investigate upper crustal structures and properties, corrections of the gravity effect of isostatic compensation of geological loads were further applied to the isostatic gravity anomalies. The resulting so-called decompensative gravity anomalies were translated into sediment distributions previously hidden below the ice sheet. A general division of the Antarctic lithosphere is confirmed by all parameters under study. A transition is visible along the Transantarctic Mountains. Whether the mountain chain belongs to West Antarctica (WANT) or East Antarctica (EANT) has been under question, but especially the estimates of Te indicate a closer connection to WANT. Apart from this general division, lithospheric fragmentation was discovered within EANT. Cratonic fragments of Precambrian origin exhibiting high depletion, low temperatures and high Te were detected in Dronning Maud Land, in Wilkes Land and close to the South Pole. The latter two are likely part of the Mawson craton. Lithospheric weakening combined with an almost primitive upper mantle exists in the Lambert Graben and was probably the result of rifting in the East Antarctic Rift System. The obtained decompensative gravity anomalies correspond well to known sedimentary basins such as the Lambert Graben and the Filchner-Ronne Ice shelf. They also suggest the presence of large sedimentary deposits that were not ore only sparsely mapped previously. Therefore, this thesis provides a comprehensive model of the lithosphere of Antarctica and a basis for further analysis of its coupling with the deep mantle and surface processes. As such, the resulting model facilitates surface heat flux modeling and estimates of upper mantle viscosity crucial for GIA modeling.

Description: Exploring the interior of the Earth, particularly at seismogenic depths is a long sought goal for seismologists in order to better characterize (e.g. nucleation, occurence) devastating earthquakes, perform monitoring and better estimate future seismic risks. The success of traditional imaging methods is highly related to how densely the target area is sampled with ray paths between the source and receivers. In case of low seismicity and scarce earthquake-receiver distribution, employing seismic ambient noise, a ubiquitous source, is proved to be an alternative approach. The cross-correlations of seismic ambient noise can be used to estimate the Green’ s function of the medium and thus to determine the Earth’ s structure. The main goal of this study is to provide a comprehension on spatial and temporal variation of seismic velocity field at different phases of the seismic cycle. To achieve this, a structural investigation was performed applying seismic ambient noise analysis in the eastern Sea of Marmara region. Another application is in order to monitor potential velocity changes associated with a major earthquake occurred in Van region, eastern Turkey. In the first part of this thesis, once the necessary pre-processing steps applied to the raw data, ambient noise cross correlation technique is used in order to investigate the crustal structure surrounding the Çınarcık Basin (offshore Istanbul), hosting Princes’ Islands segment of the North Anatolian Fault Zone which is in the final phase of the seismic cycle. Low velocity zones of Çınarcık and Thrace Basins in the study area are successfully observed in group velocity dispersion curves, proving ambient noise cross correlation approach to be a feasible and powerful tool for imaging. As a latter step, a 1D S-wave velocity structure is obtained from an average dispersion curve and employed in the inversion process to present the improvement in hypocenter determination. In the second part, potential variations in the seismic velocity associated with the 2011 M7.1 Van earthquake is investigated by exploting the repeatibility of seismic ambient noise. Six-month data framing the mainshock is investigated. A coseismic velocity decrease directly related to the Van mainshock is observed in the near vicinity of the earthquake hypocenter. This change in velocity is explained by a coseismic stress change at depth taking into account the frequency band of interest. Moreover, a correlation between the coseismic velocity decrease with minimum distance of the respective ray path to the mainshock hypocenter and the amount of coseismic slip is presented.