Marc S. Boxberg

Abstract

The Federal Company for Radioactive Waste Disposal (BGE mbH) is tasked with the selection of a site for a high-level radioactive waste repository in Germany in accordance with the Repository Site Selection Act. In September 2020, 90 areas with favorable geological conditions were identified as part of step 1 in phase 1 of the Site Selection Act. Representative preliminary safety analyses are to be carried out next to support decisions on the question, which siting regions should undergo surface-based exploration. These safety analyses are supported by numerical simulations building on geoscientific and technical data. The models that are taken into account are associated with various sources of uncertainties. Addressing these uncertainties and the robustness of the decisions pertaining to sites and design choices is a central component of the site selection process. In that context, important research objectives are associated with the question of how uncertainty should be treated through the various data collection, modeling and decision-making processes of the site selection procedure, and how the robustness of the repository system should be improved. BGE, therefore, established an interdisciplinary research cluster to identify open questions and to address the gaps in knowledge in six complementary research projects. In this paper, we introduce the overall purpose and the five thematic groups that constitute this research cluster. We discuss the specific questions addressed as well as the proposed methodologies in the context of the challenges of the site selection process in Germany. Finally, some conclusions are drawn on the potential benefits of a large method-centered research cluster in terms of simulation data management.

Cite as

Kurgyis, K. and Achtziger-Zupancic, P. and Bjorge, M. and Boxberg, M. S. and Broggi, M. and Buchwald, J. and Ernst, O. G. and Flügge, J. and Ganopolski, A. and Graf, T. and Kortenbruck, P. and Kowalski, J. and Kreye, P. and Kukla, P. and Mayr, S. and Miro, S. and Nagel, T. and Nowak, W. and Oladyshkin, S. and Renz, A. and Rienäcker-Burschil, J. and Röhlig, K.-J. and Sträter, O. and Thiedau, J. and Wagner, F. M. and Wellmann, F. and Wengler, M. and Wolf, J. and Rühaak, W. (2024): Uncertainties and robustness with regard to the safety of a repository for high-level radioactive waste: introduction of a research initiative. Environmental Earth Sciences. https://doi.org/10.1007/s12665-023-11346-8

Abstract

Geophysical methods are widely used to gather information about the subsurface as they are non-intrusive and comparably cheap. However, the solution to the geophysical inverse problem is inherently non-unique, which introduces considerable uncertainties. As a partial remedy to this problem, independently acquired geophysical data sets can be jointly inverted to reduce ambiguities in the resulting multi-physical subsurface images. A novel cooperative inversion approach with joint minimum entropy constraints is used to create more consistent multi-physical images with sharper boundaries with respect to the single-method inversions. Here, this approach is implemented in an open-source software and its applicability on electrical resistivity tomography (ERT), seismic refraction tomography (SRT), and magnetic data is investigated. A synthetic 2D ERT and SRT data study is used to demonstrate the approach and to investigate the influence of the governing parameters. The findings showcase the advantage of the joint minimum entropy (JME) stabilizer over separate, conventional smoothness-constrained inversions. The method is then used to analyze field data from Rockeskyller Kopf, Germany. 3D ERT and magnetic data are combined and the results confirm the expected volcanic diatreme structure with improved details. The multi-physical images of both methods are consistent in some regions, as similar boundaries are produced in the resulting models. Because of its sensitivity to hydrologic conditions in the subsurface, observations suggest that the ERT method senses different structures than the magnetic method. These structures in the ERT result do not seem to be enforced on the magnetic susceptibility distribution, showcasing the flexibility of the approach. Both investigations outline the importance of a suitable parameter and reference model selection for the performance of the approach and suggest careful parameter tests prior to the joint inversion. With proper settings, the JME inversion is a promising tool for geophysical imaging, however, this work also identifies some objectives for future studies and additional research to explore and optimize the method.

Cite as

Ziegon, A.H. and Boxberg, M.S. and Wagner, F.M. (2024): Minimum entropy constrained cooperative inversion of electrical resistivity, seismic and magnetic data. Journal of Applied Geophysics. https://doi.org/10.1016/j.jappgeo.2024.105490

Abstract

Icy bodies with subsurface oceans are a prime target for astrobiology investigations, with an increasing number of scientists participating in the planning, development, and realization of space missions to these worlds. Within Germany, the Ocean Worlds and Icy Moons working group of the German Astrobiology Society provides an invaluable platform for scientists and engineers from universities and other organizations with a passion for icy ocean worlds to share knowledge and start collaborations. We here present an overview about astrobiology research activities related to icy ocean worlds conducted either in Germany or in strong collaboration with scientists in Germany. With recent developments, Germany offers itself as a partner to contribute to icy ocean world missions.

Cite as

Klenner, F. and Baqué, M. and Beblo-Vranesevic, K. and Bönigk, J. and Boxberg M.S. and Dachwald, B. and Digel, I. and Elsaesser, A. and Espe, C. and Funke, O. and Hauber, E. and Heinen, D. and Hofmann, F. and Hortal Sánchez, L. and Khawaja, N. and Napoleoni, M. and Plesa, A.C. and Postberg, F. and Purser, A. and Rückriemen-Bez, T. and Schröder, S. and Schulze-Makuch, D. and Ulamec, S. and Paul de Vera, J.P. (2024): Icy Ocean Worlds - Astrobiology research in Germany. Frontiers in Astronomy and Space Sciences. https://doi.org/10.3389/fspas.2024.1422898

Abstract

Melting probes are a proven tool for the exploration of thick ice layers and clean sampling of subglacial water on Earth. Their compact size and ease of operation also make them a key technology for the future exploration of icy moons in our Solar System, most prominently Europa and Enceladus. For both mission planning and hardware engineering, metrics such as efficiency and expected performance in terms of achievable speed, power requirements, and necessary heating power have to be known. Theoretical studies aim at describing thermal losses on the one hand, while laboratory experiments and field tests allow an empirical investigation of the true performance on the other hand. To investigate the practical value of a performance model for the operational performance in extraterrestrial environments, we first contrast measured data from terrestrial field tests on temperate and polythermal glaciers with results from basic heat loss models and a melt trajectory model. For this purpose, we propose conventions for the determination of two different efficiencies that can be applied to both measured data and models. One definition of efficiency is related to the melting head only, while the other definition considers the melting probe as a whole. We also present methods to combine several sources of heat loss for probes with a circular cross-section, and to translate the geometry of probes with a non-circular cross-section to analyse them in the same way. The models were selected in a way that minimises the need to make assumptions about unknown parameters of the probe or the ice environment. The results indicate that currently used models do not yet reliably reproduce the performance of a probe under realistic conditions. Melting velocities and efficiencies are constantly overestimated by 15 to 50 % in the models, but qualitatively agree with the field test data. Hence, losses are observed, that are not yet covered and quantified by the available loss models. We find that the deviation increases with decreasing ice temperature. We suspect that this mismatch is mainly due to the too restrictive idealization of the probe model and the fact that the probe was not operated in an efficiency-optimized manner during the field tests. With respect to space mission engineering, we find that performance and efficiency models must be used with caution in unknown ice environments, as various ice parameters have a significant effect on the melting process. Some of these are difficult to estimate from afar.

Cite as

Baader, F. and Boxberg, M. S. and Chen, Q. and Förstner, R. and Kowalski, J. and Dachwald, B. (2024): Field-test performance of an ice-melting probe in a terrestrial analogue environment. Icarus. https://doi.org/10.1016/j.icarus.2023.115852

Abstract

Ice-covered ocean worlds, such as the Jovian moon Europa, are some of the prime targets for planetary exploration due to their high astrobiological potential. While upcoming space exploration missions, such as the Europa Clipper and JUICE missions, will give us further insight into the local cryoenvironment, any conclusive life detection investigation requires the capability to penetrate and transit the icy shell and access the subglacial ocean directly. Developing robust, autonomous cryorobotic technology for such a mission constitutes an extremely demanding multistakeholder challenge and requires a concentrated interdisciplinary effort between engineers, geoscientists, and astrobiologists. An important tool with which to foster cross-disciplinary work at an early stage of mission preparation is the virtual testbed. In this article, we report on recent progress in the development of an ice transit and performance model for later integration in such a virtual testbed. We introduce a trajectory model that, for the first time, allows for the evaluation of mission-critical parameters, such as transit time and average/overall power supply. Our workflow is applied to selected, existing cryobot designs while taking into consideration different terrestrial, as well as extraterrestrial, deployment scenarios. Specific analyses presented in this study show the tradeoff minimum transit time and maximum efficiency of a cryobot and allow for quantification of different sources of uncertainty to cryobot's trajectory models.

Cite as

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. Astrobiology. https://doi.org/10.1089/ast.2021.0071

Abstract

The exploration of icy environments in the solar system, such as the poles of Mars and the icy moons (a.k.a. ocean worlds), is a key aspect for understanding their astrobiological potential as well as for extraterrestrial resource inspection. On these worlds, ice melting probes are considered to be well suited for the robotic clean execution of such missions. In this chapter, we describe ice melting probes and their applications, the physics of ice melting and how the melting behavior can be modeled and simulated numerically, the challenges for ice melting, and the required key technologies to deal with those challenges. We also give an overview of existing ice melting probes and report some results and lessons learned from laboratory and field tests.

Cite as

Dachwald, B. and Ulamec, S. and Kowalski, J. and Boxberg, M. S. and Baader, F. and Biele, J. and Kömle, N. (2023): Ice Melting Probes. . https://doi.org/10.1007/978-3-030-97913-3_29

Abstract

In December 2018, the National Aeronautics and Space Administration (NASA) Interior exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission deployed a seismometer on the surface of Mars. In preparation for the data analysis, in July 2017, the marsquake service initiated a blind test in which participants were asked to detect and characterize seismicity embedded in a one Earth year long synthetic data set of continuous waveforms. Synthetic data were computed for a single station, mimicking the streams that will be available from InSight as well as the expected tectonic and impact seismicity, and noise conditions on Mars (Clinton et al., 2017). In total, 84 teams from 20 countries registered for the blind test and 11 of them submitted their results in early 2018. The collection of documentations, methods, ideas, and codes submitted by the participants exceeds 100 pages. The teams proposed well established as well as novel methods to tackle the challenging target of building a global seismicity catalog using a single station. This article summarizes the performance of the teams and highlights the most successful contributions.

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van Driel, M. and Ceylan, S. and Clinton, J. F. and Giardini, D. and Alemany, H. and Allam, A. and Ambrois, D. and Balestra, J. and Banerdt, B. and Becker, D. and Böse, M. and Boxberg, M. S. and Brinkman, N. and Casademont, T. and Cheze, J. and Daubar, I. and Deschamps, A. and Dethof, F. and Ditz, M. and Drilleau, M. and Essing, D. and Euchner, F. and Fernando, B. and Garcia, R. and Garth, T. and Godwin, H. and Golombek, M. P. and Grunert, K. and Hadziioannou, C. and Haindl, C. and Hammer, C. and Hochfeld, I. and Hosseini, K. and Hu, H. and Kedar, S. and Kenda, B. and Khan, A. and Kilchling, T. and Knapmeyer-Endrun, B. and Lamert, A. and Li, J. and Lognonne, P. and Mader, S. and Marten, L. and Mehrkens, F. and Mercerat, D. and Mimoun, D. and Möller, T. and Murdoch, N. and Neumann, P. and Neurath, R. and Paffrath, M. and Panning, M. P. and Peix, F. and Perrin, L. and Rolland, L. and Schimmel, M. and Schröer, C. and Spiga, A. and Stähler, S. C. and Steinmann, R. and Stutzmann, E. and Szenicer, A. and Trumpik, N. and Tsekhmistrenko, M. and Twardzik, C. and Weber, R. and Werdenbach-Jarklowski, P. and Zhang, S. and Zheng, Y. (2019): Preparing for InSight: Evaluation of the Blind Test for Martian Seismicity. Seismological Research Letters. https://doi.org/10.1785/0220180379

Abstract

A nodal discontinuous Galerkin (NDG) approach is developed and implemented for the computation of viscoelastic wavefields in complex geological media. The NDG approach combines unstructured tetrahedral meshes with an element-wise, high-order spatial interpolation of the wavefield based on Lagrange polynomials. Numerical fluxes are computed from an exact solution of the heterogeneous Riemann problem. Our implementation offers capabilities for modelling viscoelastic wave propagation in 1-D, 2-D and 3-D settings of very different spatial scale with little logistical overhead. It allows the import of external tetrahedral meshes provided by independent meshing software and can be run in a parallel computing environment. Computation of adjoint wavefields and an interface for the computation of waveform sensitivity kernels are offered. The method is validated in 2-D and 3-D by comparison to analytical solutions and results from a spectral element method. The capabilities of the NDG method are demonstrated through a 3-D example case taken from tunnel seismics which considers high-frequency elastic wave propagation around a curved underground tunnel cutting through inclined and faulted sedimentary strata. The NDG method was coded into the open-source software package NEXD and is available from GitHub.

Cite as

Lambrecht, L. and Lamert, A. and Friederich, W. and Möller, T. and Boxberg, M. S. (2018): A nodal discontinuous Galerkin approach to 3-D viscoelastic wave propagation in complex geological media. Geophysical Journal International. https://doi.org/10.1093/gji/ggx494

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.

Cite as

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. https://doi.org/10.1061/9780784480779.185

Abstract

When it comes to geological storage of CO₂, monitoring is crucial to detect leakage in the caprock. In our study, we investigated the wave speeds of porous media filled with CO₂ and water in order to determine reservoir changes. We focused 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 the equations for a representative storage scenario. We found that there are three modes of pressure waves instead of two for the saturated case. The new mode has a very small wave speed and is highly attenuated. This mode will most likely be very hard to detect in practice and therefore it may be necessary to use time-lapse seismic migration to detect leakage.

Cite as

Boxberg, M. S. and Prevost, J. H. and Tromp, J. (2015): Wave Propagation in Porous Media Saturated with Two Fluids. Transport in Porous Media. https://doi.org/10.1007/s11242-014-0424-2

Abstract

Interpreting independent geophysical data sets can be challenging due to ambiguity and non-uniqueness. To address this, joint inversion techniques have been developed to produce less ambiguous multi-physical subsurface images. Recently, a novel cooperative inversion approach that uses minimum entropy constraints has been proposed. The major feature of this approach is that it can produce sharper boundaries inside the model domain. We implemented this approach in an open-source software framework and systematically investigated its capabilities and applicability on electrical resistivity tomography (ERT), seismic refraction tomography (SRT), and magnetic data. First, we conducted a synthetic 2D ERT and SRT data study to demonstrate the approach and investigate the influence of the equations parameters that must be calibrated as well as to justify extensions of the method. The results show that the use of the joint minimum entropy (JME) stabilizer outclasses separate, conventional smoothness-constrained inversions and provides improved images. Next, we used the method to analyze 3D ERT and magnetic field data from Rockeskyller Kopf, Germany. Independent inversion of the magnetic field data already suggested a subsurface volcanic diatreme structure, but the joint inversion using JME not only confirmed the expected structure, but also provided improved details in the subsurface image. The multi-physical images of both methods are consistent in many regions of the model as they produce similar boundaries. Due to the sensitivity of the ERT measurements to hydrogeological conditions in the subsurface, some structures are only visible in the ERT data. These features seem not to be enforced on the magnetic susceptibility model, which highlights another advantage and the flexibility of the approach. However, the results of both the synthetic and field data use cases suggest that careful parameter tests are required prior to cooperative inversion to obtain a suitable hyperparameters and reference model. Our work implies that minimum entropy constrained cooperative inversion is a promising tool for geophysical imaging provided that proper settings are chosen while it also identifies some objectives for future research to improve the approach.

Cite as

Ziegon, A. H. and Boxberg M. S. and Wagner, F. M. (2024): Minimum entropy constrained cooperative inversion with application to electrical resistivity, seismic and magnetic field and synthetic data. EGU General Assembly, Vienna, 14-19 April 2024.

Abstract

Given the importance of ensuring the safe disposal of radioactive waste, it is vital to understand the targeted subsurface systems and to build physics-based models to predict their dynamic responses to human interventions. Constructing robust predictive models, however, is very challenging due to the systems' complexity as well as the scarcity and cost of geophysical data acquisition. Optimal matching of data acquisition and predictive simulations is therefore necessary and can be achieved via integrating predictive process modeling, Bayesian parameter estimation, and optimal experimental design into a modular workflow. This allows to quantify the information content of measurement data and therefore enables optimal planning of data acquisition and monitoring strategies. Conducting such data-integrated simulation studies, however, requires a robust workflow management that ensures reproducibility, error management, and transparency. To meet this demand, we established a data-centric approach to workflow control combining error-managed simulations with a functional data hub, providing simulations with direct access to a database of essential material properties. The latter are being made available as site specific scenario compilations along with uncertainty margins and meta information. The data hub serves as an interface facilitating seamless data and simulation exchange to support subsequent model-driven decision-making processes and guarantees that simulations are conducted using manageable, comparable, and reproducible test cases. Furthermore, it ensures that the simulation results can be readily transferred to a designated repository allowing for real-time updates of the model. The implementation of the data hub is based on a Python-based framework for two different use cases: 1) GUI-based use case: The graphical user interface (GUI) facilitates data import, export, and visualization, featuring distinct sections for geographic data representation, structured table organization, and comprehensive visualization of physical properties in varying dimensions. 2) Module-based use case: Built on the YAML-based data-hub framework, it enables direct integration of simulation modules storing measurements and model parameters in the YAML data format. The data is systematically organized to furnish a versatile data selection framework that allows information to be extracted from a variety of references, including specific on-site measurements, laboratory measurements and other references, thereby enabling a comprehensive exploration of different reference-oriented scenarios. This study showcases the data hub as a management infrastructure for executing a modular workflow. Multiple models—such as process and impact models as well as their surrogates and geophysical inverse models—are generated within this workflow utilizing scenarios provided by the data hub. Our study shows that adopting a data-centric approach to control the simulation workflow proves the feasibility of conducting different data-integrated simulations and enhances the interchangeability of information across different stages within the workflow. The paradigm of sustainable model development ensures reproducibility and transparency of our results, while also offering the possibility of synergetic exchange with other research areas.

Cite as

Chen, Q. and Boxberg, M. S. and Menzel, N. and Morales Oreamuno, M. F. and Nowak, W. and Oladyshkin, S. and Wagner, F. M. and and Kowalski, J (2024): The site selection data hub: a data-centric approach for integrated simulation workflow management in radioactive waste disposal site selection. EGU General Assembly, Vienna, 14-19 April 2024.

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.

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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.

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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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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.

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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

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.

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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.

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.

Cite as

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.

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 using the Nodal Discontinuous Galerkin Method. EGU General Assembly 2017, Vienna, Austria, 23-28 April 2017.

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.

Cite as

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

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.

Cite as

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.

Cite as

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.

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.

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.

Cite as

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).

Cite as

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.

Cite as

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.