Conference contributions

Abstract

In highly sensitive and complex environments, such as closed repositories, it is crucial to enhance the information content of planned (geo)physical measurements while keeping the costs to a minimum. Previous studies have proposed methods to optimize both sensor positions and measurement configurations for Electrical Resistivity Tomography (ERT) surveys in subsurface environments with static or moving targets. This study extends Optimal Experimental Design (OED) strategies for geoelectrical measurements by incorporating information from active time-dependent transport processes in the subsurface. Three distinct approaches for process monitoring are presented and applied to a simulated diffusive-advective transport process across multiple time steps. The methods aim at focusing the survey only on the relevant part of the model, in this case the model region that is affected by the transport process. All methods consider uncertain model input parameters by introducing an uncertainty factor in the ranking function. The study introduces a purely model-driven and a purely data-driven time-dependent OED approach. The former relies solely on model predictions to focus the survey, while the latter utilizes previously acquired data to generate predictive focusing masks for the next dataset. Additionally, a hybrid approach combining simulated transport distance and already acquired datasets is outlined. Comparative analyses show that the adaptively designed, time-dependent OED approaches result in increased image quality compared to both standard surveys as well as time-independent OED methods. For slow transport processes or small monitoring intervals, the purely data-driven approach is deemed most suitable, as it does not involve model predictions and, therefore, avoids potential uncertainties in model parametrization. Conversely, for faster transport processes or monitoring strategies with larger intervals, the approaches that (partly) incorporate model predictions show the most promising results.

Cite as

Menzel, N. and Uhlemann, S. and Wagner, F. M. (2024): Strategies for geoelectrical monitoring of subsurface fluid transport processes using Optimized Experimental Design. 84. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, 10.-14. März, Jena.

Abstract

Die Vorführung von seismischen Experimenten in Innenräumen für die Öffentlichkeitsarbeit ist oftmals nicht direkt möglich. Idealisierungen oder Miniaturisierungen sind in solchen Fällen erforderlich. Daher haben wir ein Exponat zur Veranschaulichung von seismischen Wellen in Tischgröße konzipiert. Mit unterschiedlich schweren und großen Fallgewichten, die von einem Gestell aus verschiedenen Höhen fallen gelassen werden, können seismische Wellen erzeugt und mit einem RaspberryShake aufgezeichnet werden. Es wurden verschiedene Materialien (Sand, Schaumstoff und Styropor) verwendet, um deren Einfluss auf die Wellenform zu illustrieren. Für die Aufzeichnung und Visualisierung wurde eine Webapplikation entwickelt, welche die Daten des RaspberryShakes kontinuierlich anzeigte. Dazu wurde über einen STA-LTA-Trigger eine Aufzeichnungsmöglichkeit implementiert, so dass verschiedene Seismogramme verglichen werden konnten. Darüber hinaus wurden Gamification-Elemente eingebaut. So konnten Teilnehmer versuchen vorab aufgezeichnete Seismogramme zu reproduzieren. Außerdem konnten, ähnlich wie bei der Jahrmarktattraktion Hau den Lukas, Signale einer bestimmten Stärke erzeugt werden. Hier sollte dann aber nicht eine möglichst starke Amplitude erzeugt werden, sondern eine vorgegebene Amplitude möglichst genau getroffen werden. Ergänzend wurden noch didaktisch aufbereitete Materialien zur Erklärung von aktiver Seismik und der Untergrunderkundung geliefert. Das Exponat wurde bereits erfolgreich auf der RWTH-Wissenschaftsnacht 5 vor 12 im Herbst 2023 eingesetzt und wird stetig weiterentwickelt.

Cite as

Boxberg, M. S. and van Meulebrouck, J. and Balza Morales, A. and Menzel, N. and Wagner, F. M. (2024): Ein Exponat zur Veranschaulichung von seismischen Wellen für die Öffentlichkeitsarbeit. 84. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, 10.-14. März, Jena.

Abstract

Die Interpretation unabhängiger geophysikalischer Datensätze kann aufgrund des Mehrdeutigkeitsproblems eine Herausforderung darstellen. Daher wurden Inversionstechniken entwickelt, die verschiedene Datensätze zusammen invertieren, um weniger mehrdeutige multiphysikalische Bilder des Untergrunds zu erzeugen. Jüngst wurde ein neuer kooperativer Inversionsansatz vorgeschlagen, der minimale Entropiebeschränkungen verwendet. Das Hauptmerkmal dieses Ansatzes ist, dass er in der Lage ist, schärfere Grenzen innerhalb des Modells zu erzeugen. Wir haben diesen Ansatz in einem Open-Source-Software-Framework implementiert und systematisch seine Fähigkeiten und Anwendbarkeit auf Geoelektrik (ERT), Refraktionsseismik (SRT) und Magnetik untersucht. Zunächst führten wir eine Studie mit synthetischen 2D ERT- und SRT-Daten durch, um den Ansatz zu demonstrieren und den Einfluss der zu kalibrierenden Inversionsparameter zu untersuchen. Die Ergebnisse zeigen, dass die Verwendung des JME-Stabilisators (Joint Minimum Entropy) separaten, konventionellen glättungsbeschränkten Inversionen überlegen ist und verbesserte Bilder liefert. Als Nächstes haben wir die Methode mit 3D ERT- und Magnetfelddaten vom Rockeskyller Kopf, Westeifel, verwendet. Die unabhängige Inversion der Magnetfelddaten deutete bereits auf einen unterirdische vulkanische Diatrem hin, aber die gemeinsame Inversion mit JME bestätigte nicht nur die erwartete Struktur, sondern lieferte auch verbesserte Details im Abbild. Die multiphysikalischen Bilder beider Methoden sind in vielen Regionen des Modells konsistent, da sie ähnliche Grenzen erzeugen. Aufgrund der Empfindlichkeit der ERT-Messungen gegenüber den hydrogeologischen Bedingungen im Untergrund sind einige Strukturen nur in den ERT-Daten sichtbar. Diese Merkmale scheinen sich im Modell der magnetischen Suszeptibilität nicht durchzusetzen, was einen weiteren Vorteil und die Flexibilität des Ansatzes unterstreicht. Die Ergebnisse sowohl der synthetischen als auch der Felddaten lassen jedoch darauf schließen, dass vor der gemeinsamen Inversion eine sorgfältige Parameterprüfung erforderlich ist, um ein geeignetes Parameter- und Referenzmodell zu erhalten. Unsere Arbeit zeigt, dass die kooperative Inversion mit minimaler Entropie ein vielversprechendes Werkzeug für die geophysikalische Bildgebung ist, vorausgesetzt, dass die richtigen Einstellungen gewählt werden, und sie identifiziert auch einige Ziele für die zukünftige Forschung, um den Ansatz zu verbessern.

Cite as

Ziegon, A. H. and Boxberg, M. S. and Wagner, F. M. (2024): Kooperative Inversion mit minimaler Entropie und Anwendung auf Geoelektrik-, Seismik- und Magnetik-Daten. 84. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, 10.-14. März, Jena.

Abstract

Komplexe tektonische Verhältnisse der südlichen Erftscholle beeinträchtigen insbesondere auf kleinräumigen Skalen die natürlichen geologischen und hydrologischen Verhältnisse. Das präsentierte Gebiet nahe Euskirchen wird von mehreren NW-SE-gerichteten Verwerfungen durchzogen, deren Lage sowie Einfluss auf die vorherrschenden Bedingungen bereits in vergangenen Studien ermittelt wurde. Da sich diese Untersuchungen jedoch ausschließlich auf räumlich punktuelle Datenquellen stützen, enthalten die Ergebnisse grosse Unsicherheiten. Die in dieser Studie beschriebenen geophysikalischen Messungen sollen dabei helfen, die angenommenen Störungsverläufe im Arbeitsgebiet zu evaluieren und gegebenenfalls zu korrigieren. Seismische Refraktionstomografie (SRT) und elektrische Widerstandstomografie (ERT) wurden entlang von Messprofilen möglichst orthogonal zu den vermuteten Störungslagen durchgeführt. Ein Grossteil der Datenverarbeitung sowie die Inversionen wurden mittels der frei verfügbaren Software pyGIMLi (Rücker et al., 2017) durchgeführt. Zusätzlich zu den individuellen Inversionen der SRT- und ERT-Datensätze wurde der Structurally-Coupled Cooperative Inversion (SCCI) Algorithmus (Skibbe et al., 2018) verwendet, um die seismischen und geoelektrischen Daten gemeinsam zu invertieren. Diese Studie zeigt die Vorteile der individuellen und kombinierten Anwendung mehrerer geophysikalischer Methoden im Kontext oberflächennaher Untersuchungen, insbesondere hinsichtlich der Detektion kleinräumiger tektonischer Strukturen. Die Lage der Verwerfungen konnte im gesamten Arbeitsgebiet mittels geophysikalischer Tomografien identifiziert werden, sofern der vertikale Versatz an den Störungen gross genug ist, um von den Methoden dargestellt zu werden. Aufgrund der guten Auflösung der Einzelinversionen greift der SCCI-Algorithmus lediglich an den bereits erkennbaren lithologischen und hydrologischen Modellgrenzen und stellt diese verdeutlicht dar. Durch wiederholte Anpassung der Regularisierung nach jeder Iteration ermöglicht diese Methode den Austausch struktureller Informationen zwischen den individuellen geophysikalischen Datensätzen während der Inversion.

Cite as

Menzel, N. and Klitzsch, N. and Altenbockum, M. and Müller, L. and Wagner, F. M. (2023): Prospektion von Verwerfungen auf der südlichen Erftscholle mittels ERT und SRT. 83. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, 5.-9. März, Bremen.

Abstract

As part of the Lower Rhein Embayment (LRE), the Southern Erft block is characterized by a complex tectonic setting that may influence hydrological and geological conditions on a local as well as regional level. The area presented in this study is located near Euskirchen in the south of North Rhine-Westphalia and traversed by several NW-SE-oriented fault structures. Past studies based on the lithological description of borehole cores and hydrological measurements stated that the present faults affect the local groundwater conditions throughout the targeted area. However, since the tectonic structures were located based on a sparse foundation of geological borehole data, the results include considerable uncertainties. Therefore, it was decided to re-evaluate and refine the assumed fault locations by conducting geophysical measurements. Seismic Refraction Tomography (SRT) as well as Electrical Resistivity Tomography (ERT) was performed along seven measurement profiles with a length of up to 1.1 km. To allow a sufficient degree of model resolution, the electrode spacing was set to 5 m and halved for areas proximate to assumed fault locations. The geophone spacing was set to 2.5 m for all conducted seismic surveys. A large portion of data processing and inversion was performed with the open-source software package pyGIMLi (Rücker et al., 2017). In addition to compiling individual resistivity and velocity models for all deduced measurements, both ERT and SRT datasets were jointly inverted using the Structurally Coupled Cooperative Inversion (SCCI). This algorithm strengthens structural similarities between velocity and resistivity by adapting the individual regularizations after each model iteration. This study emphasizes the benefit of multi-method geophysics to detect small-scale tectonic features. The surveys allowed to identify the fault locations throughout the area of interest, provided that the vertical displacements are large enough to be detected by the measurements. Previously assumed locations of the tectonic structures diverge from the new evidence based on ERT and SRT surveys. Especially in the western and eastern parts of the research area, differences between the survey results and formerly assumed locations are in the order of 100 m. Seismic and geoelectric measurements further indicate a fault structure in the southern part of the area, which remained undetected by past studies. The joint inversion provides minor improvements of the geophysical models, as most of the individually inverted datasets already provide results of good quality and resolution. Therefore, the effect of the SCCI algorithm is limited to underlining lithological and hydrological boundaries that are already present in the individually inverted ERT- and SRT-models.

Cite as

Menzel, N. and Klitzsch, N. and Altenbockum, M. and Müller, L. and Wagner, F. M. (2023): Prospection of faults in the Southern Erftscholle with Refraction Seismics and Electrical Resistivity Tomography. EGU General Assembly, Vienna, 23–28 April 2023.

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Kowalski, J. and Boxberg, M. S. and Grundmann, J. T. and de Vera, J. P. P. and Heinen, D. and Funke, O. (2023): The TRIPLE project - Towards technology solutions for life detection missions. EGU General Assembly, 23–28 April 2023.

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Boxberg, M. S. and Chen, Q. and Plesa, A.-C. and Kowalski, J. (2023): Ice Transit and Performance Analysis for Cryorobotic Subglacial Access Missions on Earth and Europa. EGU General Assembly, 23–28 April 2023.

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Wagner, F. M. and Klahold, J. and Hauck, C. (2023): An open framework for time-lapse petrophysical joint inversion of geophysical permafrost monitoring data. European Conference on Permafrost (EUCOP23), Puigcerdà, Spain, 18-22 June 2023.

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Hauck, C. and Buckel, J. and Dafflon, B. and Delaloye, R. and Etzelmüller, B. and Farzamian, M. and Flores Orozco, A. and Herring, T. and Hilbich, C. and Isaken, K. and Keuschnig, M. and Kneisel, C. and Kunz, J. and Lambiel, C. and Lewbowicz, A. and Magnin, F. and Maierhofer, T. and Mollaret, C. and Morard, S. and Scandroglio, R. and Tomaskovicova, S. and Uhlemann, S. and Vieria, G. and Wagner, F. M. and Wee, J. (2023): Repeated electrical resistivity tomography surveys for analysis of ground ice loss from various permafrost areas of the world. European Conference on Permafrost (EUCOP23), Puigcerdà, Spain, 18-22 June 2023.

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Mordard, S. and Hilbich, C. and Mollaret, C. and Pellet, C. and Wagner, F. M. and Westermann, S. and Hauck, C. (2023): Assessment of permafrost degradation in the Alps by applying the thermal model CryoGrid Community Model (version 1.0) validated by Petrophysical Joint Inversion of geophysical data. European Conference on Permafrost (EUCOP23), Puigcerdà, Spain, 18-22 June 2023.

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Kowalski, J. and Boledi, L. and Boxberg, M. S. and Chen, Q. and Simson, A.L. (2023): Cryotwin − Digital infrastructure for virtually-assisted preparation and analysis of cryo-robotic exploration missions. 84th EAGE Annual Conference & Exhibition.

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Boxberg, M. S. and Simson, A. and Chen, Q. and Kowalski, J. (2022): Investigation of ice with geophysical measurements during the transit of cryobots. EGU General Assembly 2022, Vienna, Austria, 23-27 May 2022.

Abstract

In geodisciplines such as the cryosphere sciences, a large variety of data is available in data repositories provided on platforms such as Pangaea. In addition, many computational process models exist that capture various physical, geochemical, or biological processes at a wide range of spatial and temporal scales and provide corresponding simulation data. A natural thought is to hybridize measured and simulated data into comprehensive data sets that complement each other and provide a joint basis for subsequent model-based interpretation. Two aspects remain challenging, namely a) we are lacking a unified metadata approach that is ready to use for hybrid data compilations comprising both measured and simulated data each with their own characteristics and natural limitations, and b) we are not providing these data compilations in an ‘analysis-ready’ format, for instance, including uncertainties. In this contribution, we present an example from cryosphere science, where much potential remains in a joint interpretation of several field tests and simulation studies to generate an integrated, holistic representation of the ice body. Yet, to date, this joint interpretation is often not feasible because metadata of the measurements lack cross-repository consistency and completeness, and simulated data are often not equipped with metadata at all. We discuss these challenges in light of FAIR, while focusing on the example of sea ice core data. Specifically, we introduce our in-house Ice Data Hub (IDH) as a flexible data management tool that aims to overcome these challenges. We use the IDH to a) store measurement data sets together with enriched, consistent metadata, b) display, add, and plot data sets through its web browser-based GUI, and c) directly couple simulation environments to facilitate interdisciplinary dataflow and interoperability. Lastly, we present an example of an ‘analysis-ready’ sea ice core data set that is merged from individual ice cores stored in the IDH.

Cite as

Simson, A. and Boxberg, M. S. and Kowalski, J. (2022): Enriched metadata for hybrid data compilations with applications to cryosphere research. Helmholtz Metadata Collaboration Conference 2022.

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Chen, Q. and Boxberg, M. S. and Kowalski, J. (2022): Acoustic traveltime tomography of a cryobot's ambient ice at Langenferner Glacier, Italy. 82. Jahrestagung der Deutschen Geophysikalischen Gesellschaft.

Abstract

Geothermal systems often occur in geologically complex structural environments with many closely spaced and intersecting faults. These commonly control the associated fluid flow needed for conventional geothermal reservoirs. One of the goals of the Innovative Training Network EASYGO - Efficiency and Safety in Geothermal Operations, aims to better characterize these systems in order to provide an initial assessment of geothermal potential in Europe. The Rhine-Ruhr region was selected as an area of interest for geothermal energy use in the context of the energy and heat transformation change in former coal mining areas. Here, Devonian carbonates and sandstones could play a role as potential reservoirs associated with karst systems or/and fracture zones. The magnetotelluric method has proven to be a useful tool in geothermal plays, where conductive bodies exist at depth. The goal of this study is to identify the structures and associated areas with enhanced fluid flow using structural analysis and magnetotelluric (MT) data. The initial areas chosen in the Rhine-Ruhr region were Rheindahlen, Lüdenscheid, and Aachen. Their local geology confirms favorable conditions for geothermal reservoir development. Additionally, these zones are strategic for MT data acquisition because of their distance from potential sources of anthropogenic noise.

The study focuses on a quantitative method for fracture analysis attributes of potential reservoir rocks along with the integration of the geology, fault response modeling, and stress analysis. In addition, we plan to carry out an MT survey integrating the three areas of interest using prior geologic information. For this, we conducted a 3D forward modeling study to simulate the expected MT signals based on the initial structural analysis of the areas of interest. This was done as a feasibility study to predict if the calculated MT signal will be of sufficient signal-to-noise ratio to carefully design future MT acquisition campaigns.

Results show favorable structural settings for the transport of fluids (e.g., fault intersection), where the structural component is marked by NW-SE striking normal faults and NE-SW oriented thrust faults with a strike slip-dilation component. Preliminary fracture analysis observed on the surface supports hints of density fracture zones for water circulation, but further studies should be conducted to see if these fractures propagate at depth. The synthetic MT study shows that a considerable signal is expected from conductive bodies within the range of 3,500 to 4,000 m depth. The characterization of the reservoir potential in these areas will facilitate similar studies in the entire Rhine-Ruhr region for a better understanding of the geothermal potential of North Rhine-Westphalia.

This project has received funding from the European Unions Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 956965.

Cite as

Balza Morales, A. and Gomez Diaz, E. and Brehme, M. and Kukla P. A. and Wagner, F. M. (2022): Geothermal potential in the Rhine-Ruhr region - Integration of structural analysis and a preliminary magnetotelluric feasibility study. European Geothermal Congress, Berlin, 17.-21. Oct. 2022.

Abstract

Proper characterization of geologic structures that host geothermal systems is crucial for the efficiency and safety of their energy production. This includes estimating layer boundaries, complex geologic features, and lithology through means of inversion and its regularization. However, existing advanced regularization techniques (e.g., geostatistical regularization, minimum-gradient support, etc.) fail to capture the complexity of 3D geological models including fault networks, fault-surface interactions, unconformities, and dome structures. Förderer et al (2021) propose a solution by means of structure-based inversion, which implements implicit geological modeling and low-dimensional parametrization to produce sharp subsurface interfaces in 2D. This work aims to extend their approach to image realistic and complex geometries in 3D. We continue with the example of electrical resistivity tomography (ERT) and synthetic data; however, this approach is aimed towards independent and joint inversion of geophysical methods that are commonly used in geothermal exploration such as magnetotellurics, gravity, and seismic techniques.

The 3D geological model is created using GemPy, an open-source Python library, which constructs a structural geological model from interface points and orientations using an implicit approach based on co-kriging (de la Varga et al., 2019). Subsequently, the 3D model is discretized, and physical parameters are assigned using minimal pilot points that are then interpolated. We use pyGIMLi (Rücker et al., 2017), another open-source multi-method library for geophysical modelling and inversion, to perform a structure-based inversion, where we include the interface points in the primary model vector of the inversion to update these points iteratively to estimate a geological model in agreement with the geophysical observations.

In this work, special focus is placed on the sensitivity of each model parameter. To maintain low parametrization and account for the increase in computational power, the cumulative sensitivity is calculated and tested under criteria to optimize the model updates. This is relevant for geometries where the interface and pilot points are more influential in one dimension than others. The workflow has also been adapted to include more complex structures that can be defined in 3D, especially those that reflect geothermal systems. This work is part of the Innovative Training Network EASYGO (www.easygo-itn.eu), which aims to improve the efficiency and safety of geothermal operations but can be readily used in other applications.

References:

Förderer, A., Wellmann, F., and Wagner, F.M.: Geoelectrical imaging of subsurface discontinuities and heterogeneities using low-dimensional parameterizations, EGU General Assembly 2021, online, 19-30 Apr 2021, EGU21-10012, https//doi.org/10.5194/egusphere-egu21-10012, 2021.

de la Varga, M., Schaaf, A., and Wellmann, F., 2019. GemPy 1.0: open-source stochastic geological modeling and inversion, Geosci. Model Dev., 12, 1-32, doi 10.5194/gmd-12-1-2019.

Rücker, C., Günther, T., Wagner, F.M., 2017. pyGIMLi: An open-source library for modelling and inversion in geophysics, Computers and Geosciences, 109, 106-123, doi 10.1016/j.cageo.2017.07.011.

Cite as

Balza Morales, A. and Gomez Diaz, E. and Brehme, M. and Kukla P. A. and Wagner, F. M. (2022): Towards structure-based joint geological-geophysical inversion for improved characterization of geothermal reservoirs. EGU General Assembly 2022, Vienna, Austria, 23-27 May 2022.

Abstract

The icy moons of our Solar System, such as the Saturnian moon Enceladus and the Jovian moon Europa, are scientifically highly interesting targets for future space missions, since they are potentially hosting extraterrestrial life in their oceans below an icy crust. Moreover, the exploration of these icy moons will enhance our understanding of the evolution of the Solar System. For their eventual in-situ exploration, novel technological solutions and simulations are necessary. This also includes model-based mission support to assist the development of future melting probes which comprise one option to access the subglacial water. Since 2012, several national projects under the lead of the DLR Explorer Initiatives develop key technologies to enhance our capability for the in-situ exploration of ice and to sample englacial or subglacial water. In 2020, the DLR Space Administration started the TRIPLE project (Technologies for Rapid Ice Penetration and subglacial Lake Exploration). This project develops an integrated concept for a melting probe that launches an autonomous underwater vehicle (nanoAUV) into a water reservoir and an AstroBioLab for in-situ analysis. All components are developed for terrestrial use while always having a future space mission with its challenges in mind. As part of a second project stage, it is envisioned to build the TRIPLE system and to access a subglacial lake in Antarctica in 2026. To deliver key parameters such as transit time and overall energy requirement, a virtual test bed for strategic mission planning is currently under development. This consists of the Ice Data Hub that combines available data from Earth and other planetary bodies - measured or taken from the literature - and allows the visualization, interpretation and export of data, as well as trajectory models for the melting probe. We develop high-fidelity thermal contact models for the phase change as well as macroscopic trajectory models that consider the thermodynamic melting process and the convective loss of heat via the melt-water flow. In this contribution, we present previous field test data obtained with the melting probe EnEx-IceMole from field deployments on temperate glaciers in the Alps and on Taylor Glacier in Antarctica together with the thermal contact models. We explore the validity and accuracy of the models for different terrestrial environments and use the findings to predict the melting probe behaviour in extraterrestrial locations of future space missions.

Cite as

Boxberg, M. S. and Baader, F. and Boledi, L. and Chen, Q. and Dachwald, B. and Francke, G. and Kerch, J. and Plesa, A. C. and Simson, A. and Kowalski, J. (2021): Concepts to utilize planetary analogue studies for icy moon exploration missions. EGU General Assembly 2021, online, 19-30 Apr 2021.

Abstract

Modelling the propagation of seismic waves in porous media gets more and more popular in the seismological community since it is an important but challenging task in the field of computational seismology. The fluid content of, for example, reservoir rocks or soils, and the interaction between the fluid and the rock or between different immiscible fluids has to be taken into account to accurately describe seismic wave propagation through such porous media. Often, numerical models are based on the elastic wave equation and some might include artificially introduced attenuation. This simplifies the problem but only approximates the true physics involved. Hence, the results are also simplified and could lack accuracy or miss phenomena in some applications. The aim of the conducted work was the consistent derivation of a theory for seismic wave propagation in porous media saturated by two immiscible fluids and the accompanying numerical solution for the derived wave equation. The theory is based on Biot's theory of poroelasticity. Starting from the basic conservation equations (energy, momentum, etc.) and generally accepted laws, the theory was derived using a macroscopic approach which demands that the wavelength is significantly larger than the size of the heterogeneities in the medium due to the size of the grains and pores or due to effects on the mesoscopic scale. This condition is usually fulfilled for seismic waves since the typical wavelength of seismic waves is in the order of 10 m to 10 km. Fluid flow is described by a Darcy type flow law and interactions between the fluids by means of capillary pressure curve models. In addition, consistent boundary conditions on interfaces between poroelastic media and elastic or acoustic media are derived from this poroelastic theory itself. The nodal discontinuous Galerkin method is used for the numerical modelling. The poroelastic solver is integrated into the 1D and 2D codes of the larger software package NEXD that uses the nodal discontinuous Galerkin method to solve wave equations. The implementation has been verified using symmetry tests and the method of exact solutions. This work has potential for applications in various scientific fields like, for example, exploration and monitoring of hydrocarbon or geothermal reservoirs as well as CO₂ storage sites.

Cite as

Boxberg, M. S. and Friederich, W. (2021): Simulation of Seismic Wave Propagation in Porous Rocks Considering the Exploration and the Monitoring of Geological Reservoirs. 81. Jahrestagung der Deutschen Geophysikalischen Gesellschaft.

Abstract

The ocean worlds of our Solar System, like Saturn's moon Enceladus and Jupiter's moon Europa are covered with ice. Recently, these icy moons gained further scientific interest, as they are attributed some potential to sustain or host extraterrestrial life in a subglacial ocean. The investigation of these moons will also help to understand the evolution of the Solar System. The in-situ exploration of these moons requires novel technological solutions as well as intelligent data acquisition and interpretation tools. In 2020, the DLR Space Administration started the TRIPLE project (Technologies for Rapid Ice Penetration and subglacial Lake Exploration) which develops an integrated concept for a melting probe that launches an autonomous underwater vehicle (nanoAUV) into a scientifically interesting water reservoir and an AstroBioLab for in-situ analysis. These three components build up the TRIPLE system. As part of a second project stage, it is envisioned to build the TRIPLE system and test it in Antarctica in 2026. In this contribution, we are going to present the general concept of TRIPLE with a focus on the geophysically most relevant aspects. To navigate the melting probe through the ice, a forefield reconnaissance system (TRIPLE-FRS) based on combined radar and sonar techniques is designed. This will include radar antennas directly integrated into the melting head combined with a pulse amplifier and a piezoelectric acoustic transducer just behind the melting head. In addition, an in-situ permittivity sensor will be implemented to account for the ice structure dependent propagation speed of electromagnetic waves. With this system, obstacles as well as the ice-water interface at the bottom of the icy shell could be detected. To deliver key parameters such as transit time and overall energy requirement, a virtual test bed for strategic mission planning is currently under development. This consists of the Ice Data Hub that combines available data from Earth or any other planetary body – measured or taken from the literature – and allows display, interpretation and export of data, as well as trajectory models for the melting probe. We develop high-fidelity thermal contact models for the phase change as well as macroscopic trajectory models that consider the thermodynamic melting process and the convective loss of heat via the melt-water flow.

Cite as

Boxberg, M. S. and Audehm, J. and Becker, F. and Boledi, L. and Burgmann, B. and Chen, Q. and Friend, P. and Haberberger, N. and Heinen, D. and Nghe, C. T. and Simson, A. and Stelzig, M. and Kowalski, J. (2021): TRIPLE - Ice Data Hub, Model-based Mission Support and Forefield Reconnaissance System. 81. Jahrestagung der Deutschen Geophysikalischen Gesellschaft.

Abstract

NEXD is an open source software package for the simulation of seismic waves in complex geological media. This includes elastic, viscoelastic, porous and fractured media with complex geometries. For the computation of the wave fields, the nodal discontinuous Galerkin approach (NDG) is used. The NDG approach combines unstructured tetrahedral meshes with an element-wise, high-order spatial interpolation of the wave field based on Lagrange polynomials. NEXD offers capabilities for modeling wave propagation in one-, two- and three-dimensional settings of very different spatial scale with little logistical overhead. It allows the import of external triangular (2D) and tetrahedral (3D) meshes provided by independent meshing software and can be run in a parallel computing environment. The computation of adjoint wavefields and an interface for the computation of waveform sensitivity kernels are offered. The method is verified by means of symmetry tests and the method of exact solutions. The capabilities of NEXD are demonstrated through, for example, a 2D synthetic survey of a geological carbon storage site. The most recent developments have been the inclusion of porous media in 2D and the inversion capabilities to the latest release versions of the 2D and 3D codes as well as the release of the 1D code. NEXD is available on GitHub: https://github.com/seismology-RUB.

Cite as

Boxberg, M. S. and Lamert, A. and Möller, T. and Friederich, W. (2021): NEXD: A Software Package for Seismic Wave Simulation in Complex Geological Media - New Developments. 81. Jahrestagung der Deutschen Geophysikalischen Gesellschaft.

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Uhlemann, S. and Isabelle, A. and Wagner, F. M. and Dafflon, B. and Ulrich, C. and Hubbard, S. (2021): Integrated geophysical imaging of permafrost distribution across an Arctic watershed. SEG Image '21 - First International Meeting for Applied Geoscience & Energy Expanded Abstracts.

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Klahold, J. and Hauck, C. and Wagner, F. M. (2021): Ice or rock matrix? Improved quantitative imaging of Alpine permafrost evolution through time-lapse petrophysical joint inversion. EGU General Assembly 2021, online, 19–30 Apr 2021.

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Wellmann, F. and Virgo, D. and Escallon, D. and de la Varga, M. and Jüstel, A. and Wagner, F. M. and Kowalski, J. and Fehling, R. (2021): Open AR-Sandbox: a Haptic Interface for Geoscience Education and Outreach. EGU General Assembly 2021, online, 19–30 Apr 2021.

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Förderer, A. and Wellmann, F. and Wagner, F. M. (2021): Geoelectrical imaging of subsurface discontinuities and heterogeneities using low-dimensional parameterizations. EGU General Assembly 2021, online, 19–30 Apr 2021.

Abstract

Determining elastic wave velocities and intrinsic attenuation of cylindrical rock samples by transmission of ultrasound signals appears to be a simple experimental task, which is performed routinely in a range of geoscientific and engineering applications requiring characterization of rocks in field and laboratory. P- and S-wave velocities are generally determined from first arrivals of signals excited by specifically designed transducers. A couple of methods exist for determining the intrinsic attenuation, most of them relying either on a comparison between the sample under investigation and a standard material or on investigating the same material for various geometries. Of the three properties of interest, P-wave velocity is certainly the least challenging one to determine, but dispersion phenomena lead to complications with the consistent identification of frequency-dependent first breaks. The determination of S-wave velocities is even more hampered by converted waves interfering with the S-wave arrival. Attenuation estimates are generally subject to higher uncertainties than velocity measurements due to the high sensitivity of amplitudes to experimental procedures. The achievable accuracy of determining S-wave velocity and intrinsic attenuation using standard procedures thus appears to be limited. We pursue the determination of velocity and attenuation of rock samples based on full waveform modeling and inversion. Assuming the rock sample to be homogeneous - an assumption also underlying standard analyses - we quantify P-wave velocity, S-wave velocity and intrinsic P- and S-wave attenuation from matching a single ultrasound trace with a synthetic one numerically modelled using the spectral finite-element software packages SPECFEM2D and SPECFEM3D. We find that enough information on both velocities is contained in the recognizable reflected and converted phases even when nominal P-wave sensors are used. Attenuation characteristics are also inherently contained in the relative amplitudes of these phases due to their different travel paths. We present recommendations for and results from laboratory measurements on cylindrical samples of aluminum and rocks with different geometries that we also compare with various standard analysis methods. The effort put into processing for our approach is particularly justified when accurate values and/or small variations, for example in response to changing P-T-conditions, are of interest or when the amount of sample material is limited.

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Boxberg, M. S. and Duda, M. and Löer, K. and Friederich, W. and Renner, J. (2020): Determining P- and S-wave velocities and Q-values from single ultrasound transmission measurements performed on cylindrical rock samples: it's possible, when.... EGU General Assembly 2020, online, 4-8 May 2020.

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Hase, J. and Wagner, F. M. and Weigand, M. and Kemna, A. (2020): Probabilistic geophysical inversion using the Hamiltonian Monte Carlo No-U-Turn sampler. 80. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), München, 23.-26.03.2020.

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Wagner, F. M. and Rücker, C. and Günther, T. and Dinsel, F. and Skibbe, N. and Weigand, M. and Hase, J. (2020): Open-source hydrogeophysical modeling and inversion with pyGIMLi 1.1: Recent advances and examples in research and education. EGU General Assembly 2020, Online Meeting.

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Mollaret, C. and Wagner, F. M. and Hilbich, C. and Hauck, C. (2020): Quantification of ground ice through petrophysical joint inversion of seismic and electrical data applied to alpine permafrost. EGU General Assembly 2020, Online Meeting.

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Mollaret, C. and Wagner, F. M. and Hilbich, C. and Hauck, C. (2019): Alpine permafrost field applications of a petrophysical joint inversion of refraction seismic and electrical resistivity data to image the subsurface ice content. EGU General Assembly 2019, Vienna.

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Wagner, F. M. and Mollaret, C. and Günther, T. and Uhlemann, S. and Dafflon, B. and Hubbard, S. and Hauck, C. and Kemna, A. (2019): Characterization of permafrost systems through petrophysical joint inversion of seismic and geoelectrical data. EGU General Assembly 2019, Vienna.

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Wagner, F. M. and Mollaret, C. and Günther, T. and Kemna, A. and Hauck, C. (2019): Quantitative Bildgebung von Permafrostsystemen mittels petrophysikalisch gekoppelter Inversion von seismischen und geoelektrischen Messdaten. 79. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), Braunschweig, 04.-07.03.2019.

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Uhlemann, S. and Dafflon, B., Michail, S. and Wagner, F. M. and Shirley, I. and Peterson, J. and Ulrich, C. and Hubbard, S. (2019): Imaging Spatial and Temporal Subsurface Variability in a Discontinuous Permafrost Environment. AGU Fall Meeting, San Francisco, 9-13 Dec 2019, Geophysical Advances in Cryospheric Processes, Structure, and Environmental Change II (NS14A).

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Hauck, C. and Kemna, A. and Weigand, M. and Wagner, F. M. and Pellet, C. and Mollaret, C. and Hoelzle, M. and Hilbich, C. (2018): Monitoring spatio-temporal infiltration pattern and its interaction with permafrost thaw using electrical resistivity and self-potential measurements at Schilthorn, Swiss Alps. EGU General Assembly 2018, Vienna.

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Mollaret, C. and Wagner, F. M. and Hilbich, C. and Hauck, C. (2018): Ice and liquid water saturations jointly inverted from electrical and refraction seismic datasets in mountain permafrost. 5th European Conference on Permafrost (EUCOP 2018), Chamonix-Mont Blanc, France, 23th June - 1^st July 2018.

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Manhaeghe, T. and Wagner, F. M. and Dumont, G. and Garré, S. (2018): Evaluation of the Effect of Micro-Topography of a Potato Field on ERT to Assess Soil Moisture Patterns in Sandy Soil. Near Surface Geoscience 2018 - the 24th European Meeting of Environmental and Engineering Geophysics, Near Surface Geoscience (9-13 September, Porto, Portugal).

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Wagner, F. M. and Uhlemann, S. and Dafflon, B. and Ulrich, C. and Peterson, J. and Akins, H. and Kemna, A. and Hubbard, S. (2018): Permafrost characterization near Teller, Alaska, using petrophysical joint inversion of seismic and geoelectrical data. AGU Fall Meeting, Washington, D.C., 10-14 Dec 2018, Advances and Revelations from Geophysical Exploration and Observation in the Cryosphere I (NS42A).

Abstract

We present a nodal discontinuous Galerkin scheme for solving the poroelastic wave equation for materials saturated by one or two immiscible fluids. The presented wave equation is based on Biot's theory and accounts for macroscopic flow. Using an example of a numerical simulation we show the existence of the third P-wave. The velocity and amplitude of this wave are significantly smaller than the velocities and amplitudes of the first and second P-wave. The numerical codes can be applied to various scientific questions related to unsaturated soils or rocks like exploration and monitoring of hydrocarbon or geothermal reservoirs or CO₂ storage sites.

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Boxberg, M. S. and Heuel, J. and Friederich, W. (2017): A Nodal Discontinuous Galerkin Solver for Modeling Seismic Wave Propagation in Porous Media. Poromechanics VI.

Abstract

Numerical simulations are a key tool to improve the knowledge of the interior of the earth. For example, global simulations of seismic waves excited by earthquakes are essential to infer the velocity structure within the earth. Numerical investigations on local scales can be helpful to find and characterize oil and gas reservoirs. Moreover, simulations help to understand wave propagation in boreholes and other complex geological structures. Even on laboratory scales, numerical simulations of seismic waves can help to increase knowledge about the behaviour of materials, e.g., to understand the mechanisms of attenuation or crack propagation in rocks. To deal with highly complex heterogeneous models, the Nodal Discontinuous Galerkin Method (NDG) is used to calculate synthetic seismograms. The main advantage of this method is the ability to mesh complex geometries by using triangular or tetrahedral elements together with a high order spatial approximation of the wave field. The presented simulation tool NEXD has the capability of simulating elastic, anelastic, and poroelastic wave fields for seismic experiments for one-, two- and three-dimensional settings. In addition, fractures can be modelled using linear slip interfaces. NEXD also provides adjoint kernel capabilities to invert for seismic wave velocities. External models provided by, e.g., Trelis can be used for parallelized computations. For absorbing boundary conditions, Perfectly Matched Layers (PML) can be used. Examples are presented to validate the method and to show the capability of the software for complex models such as the simulation of a tunnel reconaissance experiment. The software is available on GitHub: https://github.com/seismology-RUB

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Boxberg, M. S. and Lamert, A. and Möller, T. and Lambrecht, L. and Friederich, W. (2017): NEXD: A Software Package for High Order Simulation of Seismic Waves using the Nodal Discontinuous Galerkin Method. EGU General Assembly 2017, Vienna, Austria, 23-28 April 2017.

Abstract

We developed a tool for automatic determination of first arrivals in large active seismic datasets, subsequent tomographic inversion and velocity model visualization. A graphical user interface (GUI) is provided to allow easy application also to non-expert users. Due to an efficient interface between picking and tomographic inversion, together with a parallelization of the code, it is possible to calculate preliminary 3D models already in the field. The software package ActiveSeismoPick3D is written in Python. For picking first arrivals, the software searches the maximum of a characteristic function which is calculated from the unfiltered waveforms and measures the deviation of the frequency distribution of the data from a Gaussian normal distribution. Picks may be refined by evaluating the Akaike information criterion in the vicinity of this maximum. The first arrival data, together with geometry information, are prepared in a way to be directly fed into the tomographic inversion code FM-TOMO. Output files are converted into the VTK format, allowing the 3D visualization of the resulting velocity models. Additionally, the software offers tools for interactive quality control and postprocessing, e.g. various visualization and repicking functionalities. For flexibility, the tool also includes methods for the preparation of geometry information of large seismic arrays. The tool was applied to two different 3D field data sets, with the larger survey consisting of almost 36.000 traces gathered from 97 shots, recorded at 369 receivers, deployed in a regular 2D array at the Rockeskyller Kopf volcanic complex in the Eifel, Germany. A three-dimensional P-velocity model of the subsurface was generated from these data and compared to a previous model generated from geomagnetic data.

Cite as

Paffrath, M. and Wehling-Benatelli, S. and Küperkoch, L. and Hauburg, N. and Boxberg, M. S. and Friederich, W. (2017): ActiveSeismoPick3D - a tool for automatic picking of 3D active seismic data,fast refraction tomography and velocity model visualization. 77. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Potsdam, 27.-30. März 2017.

Abstract

In der Westeifel werden bei Rockeskyll außergewöhnlich große Sanidine gefunden, welche vermutlich in phonolithischen Tuffen aus einer stark differenzierten Magmenkammer stammen. Zur Lokalisierung des Diatrems, welcher das Material gefördert hat, wurde eine detaillierte magnetische Kartierung, sowie eine 2D elektrische Tomographie und eine 3D-refraktionsseismische Tomographie durchgeführt. Für die magnetische Kartierung wurde die Totalintensität des Magnetfeldes an etwa 300 Messpunkten auf einer Fläche von etwa 200 m x 200 m gemessen. Die Messpunkte wurden mithilfe eines Lasertachymeters mit höchster Genauigkeit eingemessen. Es wurde eine Anomalie von knapp 1500 nT detektiert. Die 2D elektrische Tomographie wurde an einem 100m langen Profil mit 50 Elektroden quer zur gemessenen Magnetfeldanomalie durchgeführt. Es wurden sowohl eine Dipol-Dipol-Auslage, als auch eine Schlumberger-Auslage gemessen. Für die 3D-refraktionsseismische Tomographie wurden 97 Schüsse an 369 Geophonen auf einer Fläche von 120 m x 120 m im Bereich der Magnetfeldanomalie aufgezeichnet. Um die daraus resultierenden 35.793 Einsatzzeiten zu picken und eine Tomographie durchzuführen wurde das Tool ActiveSeismoPick3D in Kombination mit dem Programm FMTOMO (Fast Marching Tomography) verwendet. Die drei Verfahren wurden zunächst einzeln ausgewertet. Aus der magnetischen Kartierung wurde ein Modell des Untergrundes erstellt. Das Ergebnis dieser Modellierung ist ein annähernd zylinderförmiger Körper, dessen Oberkante sich etwa 10 m unter der Geländeoberkante befindet. Der Körper hat eine maximale Ausdehnung in Ost-West Richtung von 75 m und in Nord-Süd Richtung von 80 m. Die Auswertung der Geoelektrik liefert im Bereich des vermuteten Diatrems deutlich erhöhte elektrische Widerstände. Die Ergebnisse der Refraktionstomographie decken sich sehr gut mit der Modellierung aus der Magnetik und zeigen deutlich erniedrigte Geschwindigkeiten im Bereich des vermuteten Diatrems. Die Lage des Diatrems wurde durch die Kombination der drei geophysikalischen Messverfahren mit hoher Wahrscheinlichkeit bestimmt. Die Bestätigung der Ergebnisse durch eine Kleinbohrung (z.B. Rammkernsondierung) steht noch aus.

Cite as

Boxberg, M. S. and Hauburg, N. and Plumpe, N. and Paffrath, M. and Friederich, W. (2017): Magnetische Kartierung sowie 3D-refraktionsseismische und elektrischeTomographie zur Untersuchung eines Phonolith-Diatrems bei Rockeskyll,Westeifel. 77. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Potsdam, 27.-30. März 2017.

Abstract

Numerical simulations are a key tool to improve the knowledge of the interior of the earth. For example, global simulations of seismic waves excited by earthquakes are essential to infer the velocity structure within the earth. Numerical investigations on local scales can be helpful to find and characterize oil and gas reservoirs. Moreover, simulations help to understand wave propagation in boreholes and other complex geological structures. Even on laboratory scales, numerical simulations of seismic waves can help to increase knowledge about the behaviour of materials, e.g., to understand the mechanisms of attenuation or crack propagation in rocks. To deal with highly complex heterogeneous models, the Nodal Discontinuous Galerkin Method (NDG) is used to calculate synthetic seismograms. The main advantage of this method is the ability to mesh complex geometries by using triangular or tetrahedral elements together with a high order spatial approximation of the wave field. The presented simulation tool NEXD has the capability of simulating elastic, anelastic, and poroelastic wave fields for seismic experiments for one-, two- and three-dimensional settings. In addition, fractures can be modelled using linear slip interfaces. NEXD also provides adjoint kernel capabilities to invert for seismic wave velocities. External models provided by, e.g., Trelis can be used for parallelized computations. For absorbing boundary conditions, Perfectly Matched Layers (PML) can be used. Examples are presented to validate the method and to show the capability of the software for complex models such as the simulation of a tunnel reconaissance experiment. The software is available on GitHub: https://github.com/seismology-RUB

Cite as

Boxberg, M. S. and Lamert, A. and Möller, T. and Lambrecht, L. and Friederich, W. (2017): NEXD: A Software Package for High Order Simulation of Seismic Waves usingthe Nodal Discontinuous Galerkin Method. 77. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Potsdam, 27.-30. März 2017.

Abstract

Modeling the propagation of seismic waves in porous media gets more and more popular in the seismological community. However, it is still a challenging task in the field of computational seismology. Nevertheless, it is important to account for the fluid content of, e.g., reservoir rocks or soils, and the interaction between the fluid and the rock or between different immiscible fluids to accurately describe seismic wave propagation through such porous media. Often, numerical models are based on the elastic wave equation and some might include artificially introduced attenuation. This simplifies the computation, because it only approximates the physics behind that problem. However, the results are also simplified and could lack accuracy or miss phenomena in some applications. We present a numerical solver for wave propagation in porous media saturated by two immiscible fluids. It is based on Biot's theory of poroelasticity and accounts for macroscopic flow that occurs on the same scale as the wavelength of the seismic waves. Fluid flow is described by a Darcy type flow law and interactions between the fluids by means of capillary pressure curve models. In addition, consistent boundary conditions on interfaces between poroelastic media and elastic or acoustic media are derived from this poroelastic theory itself. The poroelastic solver is integrated into the larger software package NEXD that uses the nodal discontinuous Galerkin method to solve wave equations in 1D, 2D, and 3D on a mesh of linear (1D), triangular (2D), or tetrahedral (3D) elements. Triangular and tetrahedral elements have great advantages as soon as the model has a complex structure, like it is often the case for geologic models. We illustrate the capabilities of the codes by numerical examples. This work can be applied to various scientific questions in, e.g., exploration and monitoring of hydrocarbon or geothermal reservoirs as well as CO 2 storage sites.

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Boxberg, M. S. and Friederich, W. (2017): Working towards a numerical solver for seismic wave propagation inunsaturated porous media. 77. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Potsdam, 27.-30. März 2017.

Abstract

Introduction Modeling the propagation of seismic waves in porous media gets more and more popular in the seismological community. Though it is still a challenging task, increasing computational power allows for more advanced and complex numerical simulations. Using poroelastic equations is important since the fluid content of, e.g., reservoir rocks or soils, and the interaction between the fluid and the rock or between different immiscible fluids has to be taken into account to accurately describe seismic wave propagation through such porous media. Often, numerical models are based on the elastic wave equation and some might include artificially introduced attenuation. This simplifies the computation, because it approximates the physics behind that problem. However, the results are also simplified and could lack accuracy or miss phenomena in some applications. Therefore, we want to extend the software package neXd (Lambrecht et al. submitted) to include an extended version of the poroelastic wave equation presented by Boxberg et al. (2015).

Methodology The presented numerical solver uses a theory that describes wave propagation in porous media saturated by two immiscible fluids. The theory is based on Biot’s theory of poroelasticity (Biot 1956) and accounts for macroscopic flow that occurs on the same scale as the wavelength of the seismic waves. Fluid flow is described by a Darcy type flow law and interactions between the fluids by means of capillary pressure curves. In addition, consistent boundary conditions on interfaces between poroelastic media and elastio or acoustic media are derived from this poroelastic theory itself. For the numerical modeling, the nodal discontinuous Galerkin method is used. A comprehensive description of this method and its application to seismic wave propagation is presented by Käser and Dumbser (2007) and Igel (2016). A more detailed description can be found in Hesthaven and Warburton (2008). The poroelastio solver is integrated into the larger software package NEXD (Lambrecht et al. submitted) that uses the nodal discontinuous Galerkin method to solve wave equations in ID, 2D, and 3D on a mesh of linear (ID), triangular (2D), or tetrahedral (3D) elements. Triangular and tetrahedral elements have great advantages as soon as the model has a complex structure, like it is often the case for geologic models.

Numerical Example We illustrate the capabilities of the code by a one dimensional example, i.e., harmonic plane wave propagation in x-direction. For this case, we can easily calculate the velocities of the three P-waves by choosing a plane wave ansatz and solve the equation for the velocities. For a synthetical set of parameters representing a rock filled with two fluids, we obtain the following velocities: cP1 = 4463.03 m/s, cP2 = 2279.73 m/s, and cP3 = 4.2327 m/s. We used a 4 km long model with constant parameters on the whole domain and a 40 Hz Ricker Wavelet as a source located at xs = 50 m. The source time is 0 s. Figure shows a snapshot of the partial pressure field of one fluid, p1(t), after t = 0.856 s.

Conclusions Our numerical example shows that there are indeed three types of P-waves in unsaturated porous media. The P1 and P2 waves are clearly visible, but the P3 wave has a very low amplitude, but it exists and it is possible to simulate this third type P-wave. In addition, it is easy to see that the calculated velocities in fact match the velocities that can be observed in the snapshot. This work can be applied to various scientific questions in, e.g., exploration and monitoring of hydrocarbon or geothermal reservoirs as well as CO₂ storage sites.

Cite as

Boxberg, M. S. and Friederich, W. (2017): Working towards a numerical solver for seismic wave propagation in unsaturated porous media. 8^th European Geothermal PhD Day, Bochum, 1-3 March 2017.

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Kemna, A. and Weigand, M. and Wagner, F. M. and Hilbich, C. and Hauck, C. (2017): Monitoring the Dynamics of Water Flow at a High-Mountain Permafrost Site Using Electrical Self-Potential Measurements. 77. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), Potsdam, 27.-30.03.2017.

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Zoporowski, A. and Wagner, F. M. and Kemna, A. (2017): Programmieren mit Python - Einbindung in Bachelor- und Mastermodule. 77. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), Potsdam, 27.-30.03.2017.

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Kemna, A. and Weigand, M. and Flores-Orozco, A. and Wagner, F. M. and Hilbich, C. and Hauck, C. (2017): Use of geoelectrical monitoring methods for characterizing thermal state, ice content and water flow in permafrost environments. 4th International Workshop on Geoelectrical Monitoring, Nov. 22-24, Vienna.

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Wagner, F. M. and Weigand, M. and Kemna, A. (2017): Identification of outliers, electrode effects and process dynamics in electrical self-potential monitoring data. 4th International Workshop on Geoelectrical Monitoring, Nov. 22-24, Vienna.

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Weigand, M. and Wagner, F. M. (2017): Towards unified and reproducible processing of geoelectrical data. 4th International Workshop on Geoelectrical Monitoring, Nov. 22-24, Vienna.

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Mollaret, C. and Wagner, F. M. and Hilbich, C. and Hauck, C. (2017): Joint inversion of electric and seismic data applied to permafrost monitoring. 4th International Workshop on Geoelectrical Monitoring, Nov. 22-24, Vienna.

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Günther, T. and Rücker, C. and Wagner, F. M. (2017): Advanced ERT inversion strategies with BERT & pyGIMLi. 4th International Workshop on Geoelectrical Monitoring, Nov. 22-24, Vienna.

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Wagner, F. M. and Rücker, C. and Günther, T. (2017): Reproducible hydrogeophysical inversions through the open-source library pyGIMLi. AGU Fall Meeting, New Orleans, 11-15 Dec 2017, Open-Source Software in the Geosciences (NS41B-0016).

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Wagner, F. M. and Weigand, M. and Kemna, A. (2017): Removal of outliers and electrode effects from spatial self-potential monitoring data to elucidate subsurface process dynamics. AGU Fall Meeting, New Orleans, 11-15 Dec 2017, Data Integration, Inverse Methods, and Data Valuation Across a Range of Scales in Hydrogeophysics (H31B-1502).

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Heinze, T. and Limbrock, J. and Weigand, M. and Wagner, F. M. and Kemna, A. (2017): Self-potential monitoring of landslides on field and laboratory scale. AGU Fall Meeting, New Orleans, 11-15 Dec 2017, Landslide Geophysics: Advances in the Characterization and Monitoring of Unstable Slopes (NS43A-02).

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Boxberg, M. S. and Friederich, W. (2016): Modelling of Seismic Wave Propagation in Porous Media Using a NodalDiscontinuous Galerkin Method. 76. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Münster, 14.-17. März 2016.

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Bergmann, P. and Schmidt-Hattenberger, C. and Labitzke, T. and Wagner, F. M. and Just, A. and Flechsig, C. and Rippe, D. (2016): Fluid injection monitoring using electrical resistivity tomography - Five years of CO₂ injection at Ketzin, Germany. 76. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), Münster, 14.-17.03.2016.

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Bergmann, P. and Schmidt-Hattenberger, C. and Labitzke, T. and Wagner, F. M. and Just, A. and Flechsig, C. and Rippe, D. (2016): Five Years of CO₂ Injection Monitoring at Ketzin, Germany, Using Electrical Resistivity Tomography. 78th EAGE Conference & Exhibition, 30 May - 2 June 2016, Vienna.

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Rücker, C. and Günther, T. and Wagner, F. M. (2016): Lösung gekoppelter Inversionsprobleme mit pyGIMLi. 76. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), Münster, 14.-17.03.2016.

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Rücker, C. and Günther, T. and Wagner, F. M. (2016): PyGIMLi - An Open Source Python Library for Inversion and Modelling in Geophysics. 78th EAGE Conference & Exhibition, 30 May - 2 June 2016, Vienna, WS08-Open Source Software in Applied Geosciences.

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Schmidt-Hattenberger, C. and Bergmann, P. and Labitzke, T. and Rippe, D. and Wagner, F. M. (2016): CO₂ Reservoir Monitoring Using a Permanent Electrode Array - The Ketzin Case Study. 78th EAGE Conference & Exhibition, 30 May - 2 June 2016, Vienna.

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Rippe, D. and Bergmann, P. and T. Labitzke and Wagner, F. M. and Schmidt-Hattenberger, C. (2016): Surface-downhole and crosshole geoelectrics for monitoring of brine injection at the Ketzin CO₂ storage site. Geophysical Research Abstracts, Vol. 18, EGU2016-15388, EGU General Assembly 2016.

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Wagner, F. M. and Wiese, B. and Schmidt-Hattenberger, C. and Maurer, H. (2016): Estimating permeability of a CO₂ storage reservoir based on multi-physical observations. 76. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), Münster, 14.-17.03.2016.

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Kemna, A. and Weigand, M. and Wagner, F. M. and Hilbich, C. and Hauck, C. (2016): Monitoring the Dynamics of Water Flow at a High-Mountain Permafrost Site Using Electrical Self-Potential Measurements. AGU Fall Meeting, 12-16 December, 2016, San Francisco, USA.

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Wagner, F. M. and Wiese, B. and Schmidt-Hattenberger, C. and Maurer, H. (2016): Insights on CO₂ Migration Based on a Multi-physical Inverse Reservoir Modeling Framework. 78th EAGE Conference & Exhibition, 30 May - 2 June 2016, Vienna, WS10-Quantitative Data Integration and Joint Inversion from Surface to Reservoir.

Abstract

Wave propagation in porous rocks gets more and more attention in computational seismology since the description of wave propagation in homogeneous and isotropic solid media is not sufficient to explain all important details of observed waveforms. Often, it is necessary to artificially introduce viscoelastic attenuation to fit synthetic waveforms to observed waveforms. However, this just explains that the wave loses energy but not how. Therefore, several numerical codes that are capable of describing wave propagation in saturated porous media have been de-veloped. This work is a step to a more accurate description of wave propagation in unsaturated porous media, i.e. porous media saturated by more than one fluid. This case is highly relevant for the exploration of oil-/gas-fields or the geological storage of CO₂, where at least two im-miscible fluids (e.g. oil and water or CO₂ and water) share the pore space. The derived wave equation in a velocity-stress formulation includes a Darcy-type flow law to describe the fluid flow and accounts for the interactions between two different fluids which are described by the capillary pressure. It is found that there are three P-waves and two S-waves for porous media saturated by two immiscible fluids. The wave speed of the first mode P-wave is in the usual range of wave speeds for elastic P-waves in solids and it is slightly attenuated. The two additional P-waves are significantly slower and highly attenuated. This makes them difficult to observe. However, the existence of these two additional waves significantly affects the wave speed and the attenuation of the first mode P-wave and the S-wave. The nodal discontinuous Galerkin method is used for numerical simulations of wave propagation described by the derived equations. It is a finite-element technique that has, amongst others, great advantages as soon as the model has a complex structure, like it is often the case for geologic models. This work has potential for applications in exploration and monitoring of reservoirs like hydrocarbon or geothermal reservoirs as well as CO₂ storage sites.

Cite as

Boxberg, M. S. and Friederich, W. (2015): Working towards modelling of seismic wave propagation in unsaturated porous media. 75. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Hannover, 23.-26. März 2015.

Abstract

When it comes to geological storage of CO₂, monitoring is crucial to detect leakage in the caprock. In our study, we investigate the wave speeds of porous rocks filled with CO₂ and water, as well as with water only, in order to determine reservoir changes. We focus on deep storage sites where CO₂ is in a supercritical state. In case of a leak, CO₂ rises and eventually starts to boil as soon as it reaches temperatures or pressures below the critical point. At this point, there are two distinct phases in the pore space. We derived the necessary equations to calculate the wave speeds for unsaturated porous media and tested these equations for a representative storage scenario. We found that there are three modes of P-waves instead of two for the saturated case. The new mode has a very small wave speed and is highly attenuated. In practice, this mode will most likely be very hard to detect directly and therefore, to detect leakage, it may be necessary to use time-lapse seismic migration to detect changes in the first mode P-wave.

Cite as

Boxberg, M. S. and Prevost, J. H. and Tromp, J. (2015): Is it possible to detect leakage of a CO₂ storage site using seismic waves?. 75. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Hannover, 23.-26. März 2015.

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Rücker, C. and Günther, T. and Wagner, F. M. (2015): Coupled hydrogeophysical modelling and ERT monitoring using pyGIMLi. 3^rd International Workshop on Geoelectrical Monitoring - GELMON (Vienna 2015).

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Rücker, C. and Günther, T. and Wagner, F. M. (2015): PyGIMLi - Eine Open Source Python Bibliothek zur Inversion und Modellierung in der Geophysik. 75. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), Hannover 2015.

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Schmidt-Hattenberger, C. and Bergmann, P. and Labitzke, T. and Rippe, D. and Wagner, F. M. (2015): Technical and methodological requirements for a permanent downhole geoelectrical measurement system as CO₂ monitoring tool - A review from the Ketzin pilot site. 3^rd International Workshop on Geoelectrical Monitoring - GELMON (Vienna 2015).

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Rippe, D. and Bergmann, P. and Labitzke, T. and Wagner, F. M. and Schmidt-Hattenberger, C. (2015): Surface-downhole geoelectrics for post-injection monitoring at the Ketzin pilot site. 3^rd International Workshop on Geoelectrical Monitoring - GELMON (Vienna 2015).

Abstract

The electrical resistivity tomography (ERT) is part of the geophysical measurement program at the Ketzin CO₂ storage site. Designed as permanent downhole electrode array, the ERT monitoring system contributes to the observation of the pore fluid changes due to the CO₂/brine displacement process in the target reservoir zone. A sequence of suitable data evaluation tools has been developed and enables this permanent reservoir monitoring system to support subsurface management operations.

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Rippe, D. and Bergmann, P. and Labitzke, T. and Wagner, F. M. and Schmidt-Hattenberger, C. (2015): A Permanent Downhole Electrode Array as Valuable Tool for CO₂ Monitoring at the Ketzin Pilot Site. 8th Trondheim Conference on CO₂ Capture, Transport and Storage, 16-18 June 2015 (Trondheim, Norway).

Abstract

The electrical resistivity tomography (ERT) is part of the geophysical measurement program at the Ketzin CO₂ storage site. Designed as permanent downhole electrode array, the ERT monitoring system contributes to the observation of the pore fluid changes due to the CO₂/brine displacement process in the target reservoir zone. A sequence of suitable data evaluation tools has been developed and enables this permanent reservoir monitoring system to support subsurface management operations.

Cite as

Schmidt-Hattenberger, C. and Bergmann, P. Labitzke, T. and Wagner, F. M. (2015): A Permanent Downhole Electrode Array as Valuable Tool for CO₂ Monitoring at the Ketzin Pilot Site. Third EAGE Workshop on Permanent Reservoir Monitoring 2015 (Stavanger, Norway 2015).

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Wagner, F. M. and Bergmann, P. and Labitzke, T. and Wiese, B. and Schmidt-Hattenberger, C. and Rücker, C. and Maurer, H. (2015): Effekte und Korrektur von Bohrloch bedingten Fehlern bei der permanenten geoelektrischen Überwachung von geologischen Speichern. 75. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), Hannover 2015.

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Wagner, F. M. and Wiese, B. and Schmidt-Hattenberger, C. and Maurer, H. (2015): Insights on CO₂ migration by means of a fully-coupled hydrogeophysical inversion. 3^rd International Workshop on Geoelectrical Monitoring - GELMON (Vienna 2015).

Abstract

Seismic waves are attenuated in most rocks. Besides the rock itself, reasons for this attenuation are the contents of pores, joints and fissures. For many applications it is important to quantify the attenuation. The quality factor Q is commonly utilized for this quantification. Previous laboratory experiments have shown that the determination of the attenuation using ultrasound measurements is not trivial. Using different methods described in literature, unreliable quality factors can be obtained. This study intends to assist the analysis of ultrasonic measurements with numerical methods. It aims to calculate more precise quality factors as well as accurate P-and S-wave velocities. Both two-dimensional and three-dimensional models of the cylindrical samples used in the laboratory are created and used for the calculation of synthetic seismograms. The underlying method is the spectral element method (SEM). First, it is checked whether it is generally possible to imitate the measured ultrasound seismograms by using this method. Because of the attenuation, the amplitude is smaller for long cylinders than for short cylinders. This decay is exploited to calculate the quality factor. For the practical use as a tool for analyzing laboratory measurements, it is desirable to apply an automated process. Therefore, an inversion procedure based on the Conjugate Gradient method which allows to invert for the seismic velocities is developed. It was found that it is generally possible to reproduce the seismograms using the SEM. However, it is necessary to use three-dimensional models to reproduce the whole waveform. Whereas two-dimensional models may only yield sufficient P-wave speeds. It is shown using synthetic data that good estimates of quality factors can be obtained by utilizing the amplitude decay. Though it is still difficult to obtain accurate values for real data. Furthermore, P-wave velocities from the inversion are more reliable than the S-wave velocities.

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Boxberg, M. S. and Lambrecht, L. and Friederich, W. (2014): Seismic Velocities and Attenuation of Rock Samples from Inversion of Ultrasonic Waveforms. 74. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Karlsruhe, 10.-13. März 2014.

Abstract

Since wave propagation in homogeneous and isotropic media is not sufficient to explain all important details in many fields of geosciences, including exploration geophysics, hydrology and geothermal energy, wave propagation in porous and fractured rocks became a main focus of attention in computational seismology. The nodal discontinuous Galerkin method (henceforth NDG) (Hesthaven and Warburton 2008) is a finite-element technique that has great advantages as soon as the model has a complex structure, because it uses tetrahedral (3D) or triangular (2D) elements. The NDG is also capable of using high-order approximations. At the seismology working group at Ruhr-University a NDG software (2D and 3D) has been developed recently. This software is currently able to calculate seismograms at any number of stations for any number of different sources (single force as well as moment tensor) in isotropic media. Absorbing boundaries are realised either by perfectly matched layers (PML) or by suppressing incoming fluxes at the boundaries. The most popular theory for wave propagation in saturated porous and elastic media was developed by Biot (1955, 1956a,b). This theory has already been used for simulations with several numerical methods (Carcione et al. 2010). Moreover, extensions to unsaturated porous media have been suggested, amongst others, by Santos et al. (1990), Albers (2009) and Boxberg et al. (submitted). In order to model the response of seismic waves in fractured media an approach called "equivalent medium theory" (EMT) is used. This process mathematically replaces a heterogeneous medium with fractures and cracks with a homogeneous medium that has the same macroscopic properties. Two concepts are currently used: The linear slip interface model by Schoenberg and Sayers (Schoenberg 1980, Schoenberg and Sayers 1995) and the inclusion-based model (e.g. Hudson 1980, 1981). This current work aims to efficiently implement the formulas for unsaturated porous media and seismic anisotropy using EMT in the existing NDG software. In addition it is planned to apply the above described methods to different aspects of geothermal energy systems including exploration of geothermal reservoirs, seismic-while-drilling (SWD) and monitoring of induced seismicity. One main focus of the applications will be the test drilling site at International Geothermal Center Bochum (GZB).

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Boxberg, M. S. and Möller, T. (2014): Numerical Simulation of Wave Propagation in Porous and Fractured Rocks Using a Nodal Discontinuous Galerkin Method with Regard to Exploration and Monitoring of Geothermal Reservoirs. 5^th European Geothermal PhD Day, Darmstadt, 31.03.-02.04.2014.

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Boxberg, M. S. and Friederich, W. (2014): Numerical simulations of wave propagation in porous rocks saturated with a number of immiscible fluids. Der Geothermie Kongress 2014, Essen, 11.-13. November 2014.

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Wagner, F. M. and Bergmann, P. and Labitzke, T. and Schmidt-Hattenberger, C. and Rücker, C. and Maurer, H. (2014): Accounting for complex borehole completion in crosshole resistivity monitoring. 4th Helmholtz-Alberta Initiative (HAI) Science Forum, September 29, 2014, Edmonton, Canada.

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Wagner, F. M. and Bergmann, P. and Labitzke, T. and Schmidt-Hattenberger, C. and Günther, T. and Maurer, H. (2014): High-Resolution Monitoring of CO₂ Injection with Permanent Electrodes: A 5-Year Retrospect from the Ketzin Site and Design Recommendations for Future Projects. AGU Fall Meeting, 15-19 December, 2014, San Francisco, USA.

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Wagner, F. M. and Bergmann, P. and Labitzke, T. and Deisman, N. and Schmidt-Hattenberger, C. and Maurer, H. and Chalaturnyk, R. (2014): Paving the way to estimate CO₂ saturation from geoelectrical data. 4th Helmholtz-Alberta Initiative (HAI) Science Forum, September 29, 2014, Edmonton, Canada.

Abstract

The electrical resistivity tomography (ERT) is part of the geophysical measurement program at the Ketzin CO₂ storage site. Designed as permanent downhole electrode array, the ERT monitoring system contributes to the observation of the pore fluid changes due to the CO₂/brine displacement process in the target reservoir zone. A sequence of suitable data evaluation tools has been developed and enables this permanent reservoir monitoring system to support subsurface management operations.

Cite as

Schmidt-Hattenberger, C. and Bergmann, P. and Bösing, D. and Labitzke, T. and Möller, M. and Schröder, S. and Wagner, F. M. and Schütt, H. (2013): Permanent Downhole Geoelectrical Monitoring at the Ketzin CO₂ Pilot Site. Second EAGE Workshop on Permanent Reservoir Monitoring 2013 - Current and Future Trends (Stavanger, Norway 2013).

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Schmidt-Hattenberger, C. and Bergmann, P. and Labitzke, T. and Wagner, F. M. (2013): Electrical Resistivity Tomography (ERT) as a permanent monitoring tool to image the CO₂ migration at the Ketzin pilot site - Experiences from more than five years of operation. 2^nd Internat. Workshop on Geoelectrical Monitoring, GELMON 2013, Vienna, 04.-06.12.2013, Berichte Geol. B.-A., 104, ISSN 1017-8880.

Abstract

Electrical resistivity tomography (ERT) has received consideration as a tool for permanent monitoring of saline storage reservoirs due to its high sensitivity to compositional pore fluid changes. The information content offered by geoelectrical data is ultimately limited by the electrode arrangement, and consequently, its full exploitation requires a well-conceived experimental design. We present a methodology to estimate an optimum number of electrodes as well as their specific locations along the borehole trajectories based on a maximization of the respective model resolution. Using a synthetic example analogous to the Ketzin site, Germany, we demonstrate that relatively sparse optimized setups with a refinement of the electrode spacings in the target horizon can offer comparable tomographic imaging capabilities with regard to rather dense arrays. The approach presented can assist practitioners with the design of high-resolution and cost-effective down-hole installations at future CO₂ storage sites.

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Wagner, F. M. and Günther, T. and Schmidt-Hattenberger, C. and Maurer, H. (2013): On the Design of Cross-hole Resistivity Arrays for High-resolution and Cost-effective Storage Reservoir Monitoring. Near Surface Geoscience 2013 - the 19th European Meeting of Environmental and Engineering Geophysics of the Near Surface Geoscience (Bochum 2013).

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Wagner, F. M. and Günther, T. and Schmidt-Hattenberger, C. and Maurer, H. (2013): Optimized crosshole resistivity monitoring strategies for geological carbon dioxide storage reservoirs. 3^rd Helmholtz-Alberta Initiative (HAI) Science Forum, September 2013, Edmonton, Canada.

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Wagner, F. M. and Günther, T. and Schmidt-Hattenberger, C. and Maurer, H. (2013): Estimating optimum electrode locations for high-resolution cross-hole resistivity monitoring. 2^nd Internat. Workshop on Geoelectrical Monitoring, GELMON 2013, Vienna, 04.-06.12.2013, Berichte Geol. B.-A., 104, ISSN 1017-8880.

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Wagner, F. M. and Schmidt-Hattenberger, C. and Bergmann, P. and Labitzke, T. and Chalaturnyk, R. and Giroux, B. (2013): Towards quantitative monitoring of CO₂ with time-lapse electrical resistivity tomography (ERT): Experiences from the Ketzin pilot site, Germany. 3^rd Annual Conference of Carbon Management Canada, Calgary, Canada.

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Kempka, T. and Endler, R. and Eydam, D. and Herd, R. and Huenges, E. and Jahnke, C. and Jolie, E. and Janetz, S. and Krause, Y. and Kühn, M. and Magri, F. and Moeck, I. and Möller, M. and Muñoz, G. and Nakaten, B. and Ritter, O. and Schafrik, W. and Schmidt-Hattenberger, C. and Schöne, E. and Tillner, E. and Voigt, H. J. and Wagner, F. M. and Zimmermann, G. (2012): CO₂ storage in eastern Brandenburg: Implications for geothermal heat provision and conception of a salinisation early warning system - Review of current progress of the joint-project brine. .

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Möller, M. and Schmidt-Hattenberger, C. and Wagner, F. M. and Schröder, S. (2012): Hochauflösende Geoelektrik als Teil eines Frühwarnsystems zur Überwachung einer möglichen Grundwasserversalzung bei der CO₂-Speicherung. 72. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), Hamburg 2012.

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Wagner, F. M. and Hosseini, B. K. and Kempka, T. and Schmidt-Hattenberger, C. and Chalaturnyk, R. (2012): Optimized resistivity monitoring strategies for geological carbon dioxide storage based on reservoir simulations. 2^nd Science Forum of the Helmholtz-Alberta-Initiative, Potsdam Sep. 2012.

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Wagner, F. M. and Möller, M. and Schmidt-Hattenberger, C. and Kempka, T. and Maurer, H. (2012): Monitoring brine migration in analog transport models using surface-to-hole ERT. EGU General Assembly 2012, Vienna.

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Wagner, F. M. and Schmidt-Hattenberger, C. and Bergmann, P. and Labitzke, T. and Möller, M. and Schröder, S. (2012): Quantitative CO₂ monitoring via time-lapse electrical resistivity tomography (ERT): From tool development to advanced inversion strategies. 3^rd Annual Meeting, Helmholtz Alberta Initiative (Edmonton, Alberta, Canada 2012).

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Möller, M. and Schmidt-Hattenberger, C. and Wagner, F. M. and Schröder, S. (2011): Development of an integrated monitoring concept to detect possible brine migration. 1^st International Workshop on Geoelectrical Monitoring - GELMON (Vienna 2011).

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Wagner, F. M. and Möller, M. and Schmidt-Hattenberger, C. and Kempka, T. and Maurer, H. (2011): Detection of groundwater salinisation by geoelectric measurements. EGU General Assembly 2011, Vienna.