Geophysical Imaging and Monitoring
Publications
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Prospection of faults on the Southern Erftscholle (Germany) with individually and jointly inverted refraction seismics and electrical resistivity tomography
2024 |
Journal of Applied Geophysics, doi:10.1016/j.jappgeo.2024.105549
PDFNote: This publication resulted from Nino Menzel's master thesis.Abstract
As part of the Lower Rhein Embayment (LRE), the Southern Erft block is characterized by a complex tectonic setting that influences hydrological and geological conditions on a local as well as regional level. The study area is located in the south of North Rhine-Westphalia and traversed by several NW-SE-oriented fault structures. Since the tectonic structures were located by past studies based on a sparse foundation of geological data, the positions 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. In addition to compiling individual resistivity and velocity models for all deduced measurements, ERT and SRT datasets were cooperatively 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. 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 cooperative inversions do not improve the geophysical models qualitatively, since the individually inverted datasets already provide results of good quality and resolution.
Cite as
Menzel, N. and Klitzsch, N. and Altenbockum, M. and Müller, L. and Wagner, F.M. (2024): Prospection of faults on the Southern Erftscholle (Germany) with individually and jointly inverted refraction seismics and electrical resistivity tomography. Journal of Applied Geophysics. https://doi.org/10.1016/j.jappgeo.2024.105549
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Probabilistic geophysical inversion of complex resistivity measurements using the Hamiltonian Monte Carlo method
2024 |
Geophysical Journal International, doi:10.1093/gji/ggae389
PDFAbstract
In this work, we introduce the probabilistic inversion of tomographic complex resistivity (CR) measurements using the Hamiltonian Monte Carlo (HMC) method. The posterior model distribution on which our approach operates accounts for the underlying complex-valued nature of the CR imaging problem accurately by including the individual errors of the measured impedance magnitude and phase, allowing for the application of independent regularization on the inferred subsurface conductivity magnitude and phase, and incorporating the effects of cross-sensitivities. As the tomographic CR inverse problem is non-linear, of high dimension and features strong correlations between model parameters, efficiently sampling from the posterior model distribution is challenging. To meet this challenge we use HMC, a Markov-chain Monte Carlo method that incorporates gradient information to achieve efficient model updates. To maximize the benefit of a given number of forward calculations, we use the No-U-Turn sampler (NUTS) as a variant of HMC. We demonstrate the probabilistic inversion approach on a synthetic CR tomography measurement. The NUTS succeeds in creating a sample of the posterior model distribution that provides us with the ability to analyze correlations between model parameters and to calculate statistical estimators of interest, such as the mean model and the covariance matrix. Our results provide a strong basis for the characterization of the posterior model distribution and uncertainty quantification in the context of the tomographic CR inverse problem.
Cite as
Hase, J. and Wagner, F.M. and Weigand, M. and Kemna, A. (2024): Probabilistic geophysical inversion of complex resistivity measurements using the Hamiltonian Monte Carlo method. Geophysical Journal International. https://doi.org/10.1093/gji/ggae389
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Uncertainties and robustness with regard to the safety of a repository for high-level radioactive waste: introduction of a research initiative
2024 |
Environmental Earth Sciences, doi:10.1007/s12665-023-11346-8
RWTH Publications PDFNote: This publication introduces the broader research vision behind our SmartMonitoring subproject.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
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Minimum entropy constrained cooperative inversion of electrical resistivity, seismic and magnetic data
2024 |
Journal of Applied Geophysics, doi:10.1016/j.jappgeo.2024.105490
RWTH Publications PDFNote: This publication resulted from the first M.Sc. graduate of GIM – Anton Ziegon.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
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Icy Ocean Worlds - Astrobiology research in Germany
2024 |
Frontiers in Astronomy and Space Sciences, doi:10.3389/fspas.2024.1422898
RWTH Publications PDFAbstract
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
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Electrical Conductivity of Hydrate-Bearing Rocks Studied by Pore-Scale Modeling
2024 |
Energy & Fuels, doi:10.1021/acs.energyfuels.4c01295
Abstract
Gas hydrates have the potential to significantly disturb global climate change and alter subsurface stability, particularly in the context of production due to their extensive presence and widespread distribution in marine deposits. The electrical conductivity of hydrate-bearing sediments (HBS) serves as a crucial parameter for hydrate reservoir prospection. However, the electrical conductivity of HBS is influenced not only by hydrate saturation but also by the hydrate distribution within the pore space. This study presents a numerical approach for quantifying the relationship between the hydrate volume, distribution, and conductivity of HBS using pore network modeling (PNM). We use two distinct hydrate distributions in pores, ideal grain-contacting and pore-filling. Their electrical conductivities, in relation to hydrate saturation, were simulated on the pore scale using the finite element method. Regardless of the hydrate distribution, the electrical conductivity of the pore network models decreases with increasing hydrate saturation. At the same saturation, the electrical conductivity of PNM with grain-contacting hydrates is higher than that of pore-filling hydrates. While the resistivity index of the hydrate-bearing PNM exhibits a variation pattern consistent with Archie’s formula, the saturation exponent is not a fixed value. The experimental samples represent a closed system where significant local fluid salinity changes would occur due to hydrate formation, strongly influencing the bulk conductivity. The numerical simulation results considering the salinity effect confirm the plausibility of grain-contacting hydrate while challenging the existence of an ideal pore-filling hydrate when compared to the measured data as the conductivity associated with such a uniformly distributed pore-filling hydrate contradicts the experimental measurements. Our research indicates that the variability of the saturation exponent, highlighting the complex nature of hydrate distributions within sediments, calls for refined electrical saturation models to enhance the evaluation of marine hydrate reservoirs.
Cite as
Zhang, Q. and Tao, H. and Wagner, F. M. and Klitzsch, N. and Zibulski, E. and Lu, H. and Zi, M. and Chen, D. (2024): Electrical Conductivity of Hydrate-Bearing Rocks Studied by Pore-Scale Modeling. Energy & Fuels. https://doi.org/10.1021/acs.energyfuels.4c01295
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Field-test performance of an ice-melting probe in a terrestrial analogue environment
2024 |
Icarus, doi:10.1016/j.icarus.2023.115852
RWTH Publications PDFNote: This publication resulted from Marc's time at MBD at RWTH Aachen, i.e. was prepared before GIM was founded.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
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Ice transit and performance analysis for cryorobotic subglacial access missions on Earth and Europa
2023 |
Astrobiology, doi:10.1089/ast.2021.0071
RWTH PublicationsNote: This publication resulted from Marc's time at MBD at RWTH Aachen, i.e. was prepared before GIM was founded.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
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Ice Melting Probes
2023 |
Handbook of Space Resources, doi:10.1007/978-3-030-97913-3_29
RWTH Publications PDFNote: This publication resulted from Marc's time at MBD at RWTH Aachen, i.e. was prepared before GIM was founded.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
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Characterization of rock glaciers environments combining structurally-coupled and petrophysically-coupled joint inversions of electrical resistivity and seismic refraction datasets
2023 |
Journal of Applied Geophysics, doi:10.1016/j.jappgeo.2023.105097
RWTH Publications PDFAbstract
The degradation of mountain permafrost is well documented at many Alpine sites. Geophysical techniques have been intensively used to monitor these sites, since permafrost degradation is not only a proxy of climate change and global warming but also a possible source of slope instabilities and triggering of mass movements. While the use of non-invasive geophysical techniques is promising, the interpretation of different geophysical results can introduce ambiguities in defining the investigated subsoil and often does not lead to a quantitative estimation of the internal permafrost constituents (rock matrix, ice, liquid water and air contents). To overcome these limitations, we applied an optimized joint inversion approaches of electrical resistivity and refraction seismic tomography datasets collected at two Swiss rock glaciers (Schafberg - Canton Grisons, and Ritigraben - Canton Valais). Firstly, to improve the structural interpretation of the frozen near subsurface, we performed a structurally coupled cooperative joint inversion, optimizing the coupling parameters. Subsequently, we used the petrophysical joint inversion to quantify the composition of these mountain permafrost substrates, optimizing the numerical and petrophysical parameters. The obtained results agree with field observations and the borehole data collected at these two sites, opening new perspectives for the future quantitative monitoring of permafrost constituents.
Cite as
Pavoni, M. and Boaga, J. and Wagner, F.M. and Bast, A. and Phillips, M. (2023): Characterization of rock glaciers environments combining structurally-coupled and petrophysically-coupled joint inversions of electrical resistivity and seismic refraction datasets. Journal of Applied Geophysics. https://doi.org/10.1016/j.jappgeo.2023.105097
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Mitigation of installation-related effects for small-scale borehole-to-surface ERT
2022 |
Journal of Applied Geophysics, doi:10.1016/j.jappgeo.2022.104530
RWTH Publications PDFAbstract
Small-scale resistivity inhomogeneities can result from the local distribution of water and the water and nutrient uptake of plants. Measuring small-scale Electrical Resistivity Tomography (ERT) in the field comes with a set of particularities, especially when including borehole electrodes for a better resolution with depth. We apply small-scale borehole-to-surface ERT over a palaeochannel. Combining surface ERT with detailed borehole-to-surface ERT profiles along the measurement line allows a delineation of finer layering within the coarser lithology. Our field setup includes a borehole electrode tool with 20 ring electrodes, electrically coupled to the ground via a conductive mud. Two main points are addressed in this publication: (1) In the field, we electrically coupled the borehole electrodes to the ground by filling the cavities around the tool with a soil mud, i.e., we need to account for the unknown conductive borehole filling in the inversion. If not incorporated, the mud has a considerable influence on the resistivities close to the borehole tool, but also on the region around the surface electrodes. Consequently, alongside with a 3D inversion scheme representing the electrodes with the Complete Electrode Model (CEM), we include the mud as a separate and uncoupled region. We model the geometry of the mud layer around the tool and do not allow an influence of this region on the rest of the model. (2) Due to the small electrode distances and the overall small-scale nature of the array, the depth of installation of the borehole electrode tool must be known accurately in the inversion model. However, it is not easy to measure the tool depth in the field with the required accuracy, due to small-scale surface roughness, e.g., from a weathered loose soil layer at the surface or from vegetation. We also investigate the influence of a tilted tool installation and optimise for the depth and installation angle of the borehole tool before inverting for resistivities. An accurate knowledge of the borehole electrode positions is crucial for a reliable and precise inversion result. The surface electrodes establish a coordinate system around the borehole tool on the surface, with an angle φ describing the direction around the tool in the top view. The sensitive plane (in-plane) is defined as the x-z plane cutting through φ = 0° and φ = 180°. A tilting of the tool from the vertical direction is described by a tilting angle θ. A tilting of the borehole tool within the sensitive plane manifests in an increased misfit between data points on both sides of the tool, i.e., at φ = 0° and φ = 180°. We use this difference to optimise on the tool angle. The true depth of the borehole tool is found by searching for a minimum of the objective function, describing the goodness of the found model, while assuming different tool depths in each inversion. We see a minimum of the objective functionwhich can be attributed to the correct depth range, as shown by a synthetic study. Through our optimisations, we can determine a tilting of the tool, i.e., the angle θ, with an accuracy of 2° to 3° and the tool depth with an accuracy of a few centimetres, depending partially on the subsurface resistivities, i.e., our optimisation works mainly in predominantly horizontally layered soils. A tilting in directions out of the sensitive plane (out-of-plane) can be projected onto the sensitive plane, since the out-of-plane tilt has a negligible influence on the data. After this optimisation, we can determine layer resistivities from our field data.
Cite as
Ochs, J. and Klitzsch, N. and Wagner, F. M. (2022): Mitigation of installation-related effects for small-scale borehole-to-surface ERT. Journal of Applied Geophysics. https://doi.org/10.1016/j.jappgeo.2022.104530
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Open AR-Sandbox: A haptic interface for geoscience education and outreach
2022 |
Geosphere, doi:10.1130/ges02455.1
RWTH Publications PDFAbstract
Virtual reality concepts have been widely adapted to teach geoscientific content, most notably in virtual field trips with increased developments due to recent travel restrictions and challenges of field access. On the spectrum between real and fully virtual environments are also combinations of digital and real content in mixed-reality environments. In this category, augmented-reality (AR) sandboxes have been used as a valuable tool for science outreach and teaching due to their intuitive and haptic interaction-enhancing operation. Most of the common AR-sandboxes are limited to the visualization of topography with contour lines and colors, as well as water simulations on the digital terrain surface. We show here how we can get beyond this limitation, through an open-source implementation of an AR-sandbox system with a versatile interface written in the free and cross-platform programming language Python. This implementation allows for creative and novel applications in geosciences education and outreach in general. With a link to a 3-D geomodelling system, we show how we can display geologic subsurface information such as the outcropping lithology, creating an interactive geological map for structural geology classes. The relations of subsurface structures, topography, and outcrop can be explored in a playful and comprehensible way. Additional examples include the visualizations of geophysical fields and the propagation of seismic waves, as well as simulations of Earth surface processes. We further extended the functionality with ArUco-marker detection to enable more precise and flexible interaction with the projected content. In combination, with these developments, we aim to make AR-sandbox systems, with the additional dimension of haptic interactions, accessible to a wider range of geoscientific applications for education and outreach.
Cite as
Wellmann, F. and Virgo, S. and Escallon, D. and de la Varga, M. and Jüstel, A. and Wagner, F. M. and Kowalski, J. and Zhao, H. and Fehling, R. and Chen, Q. (2022): Open AR-Sandbox: A haptic interface for geoscience education and outreach. Geosphere. https://doi.org/10.1130/ges02455.1
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Improved estimation of ice and water contents in alpine permafrost through constrained petrophysical joint inversion: The Hoher Sonnblick case study
2021 |
Geophysics, doi:10.1190/geo2020-0592.1
RWTH Publications PDFAbstract
Quantitative estimation of subsurface water and ice content values is critical for the understanding and modeling of permafrost evolution in alpine regions. Geophysical methods permit the assessment of subsurface conditions in a noninvasive and quasicontinuous manner; in particular, the combination of seismic refraction tomography (SRT) and electrical resistivity tomography (ERT) through a petrophysical model can quantitatively estimate ground water and ice content values. For the Hoher Sonnblick study area (3106 m.a.s.l., Austrian Alps), we have investigated the improved estimation of water and ice content values based on SRT, ERT, and ground-penetrating radar data collected in June and October 2019. We solve for the water and ice content values following different approaches, namely, (1) the independent inversion and subsequent transformation of the imaging results to the target parameters through a petrophysical model and (2) the petrophysical joint inversion (PJI) of the data sets. Supported by a synthetic study, we determine that the incorporation of structural and porosity constraints in the PJI allows for an improved quantitative characterization of subsurface conditions. For our measurements at Hoher Sonnblick, the constrained PJI resolves a shallow debris layer characterized by high air content and porosity, on top of a layer with lower porosity corresponding to fractured gneiss, and the bedrock layer with the lowest porosity. For both time steps, we find high water content at the lower end of the investigated area. Substantial variations in the subsurface ice content resolved between June and October 2019 indicate a correlation between the high water content and the meltwater discharge within the debris layer. Our results demonstrate that the constrained PJI permits an improved characterization of subsurface hydrologic parameters in alpine permafrost environments.
Cite as
Steiner, M. and Wagner, F. M. and Maierhofer, T. and Schöner, W. and Flores Orozco, A. (2021): Improved estimation of ice and water contents in alpine permafrost through constrained petrophysical joint inversion: The Hoher Sonnblick case study. Geophysics. https://doi.org/10.1190/geo2020-0592.1
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An overview of multimethod imaging approaches in environmental geophysics
2021 |
Advances in Geophysics, Vol. 44, doi:10.1016/bs.agph.2021.06.001
RWTH Publications PDFAbstract
Quantitative characterization of subsurface properties is critical for many environmental applications and serves as the basis to simulate and better understand dynamic subsurface processes. Geophysical imaging methods allow to image subsurface property distributions and monitor their spatio-temporal changes in a minimally invasive manner. While it is widely agreed upon that models integrating multiple independent data sources are more reliable, the number of approaches to do so is increasing rapidly and often overwhelming for researchers and, particularly, novices to the field. With this work, we aim to contribute to the development multimethod imaging through (1) an overview of, and didactic introduction to, existing inversion approaches for the integration of multiple geophysical data sets with other measurement types (e.g., hydrological observations), petrophysical models, and process simulations, (2) a state-of-the-art review on the use and potentials of these approaches in various environmental applications, and (3) a discussion on new frontiers and remaining challenges in the field. We hope that this chapter provides an entry point to recent developments in multimethod geophysical imaging, clarifies similarities, differences, and development potentials of existing approaches, and ultimately helps practitioners to choose the optimum one to integrate their data sets.
Cite as
Wagner, F. M. and Uhlemann, S. (2021): An overview of multimethod imaging approaches in environmental geophysics. Advances in Geophysics, Vol. 44. https://doi.org/10.1016/bs.agph.2021.06.001
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Integrating time-lapse gravity, production, and geological structure data in a gas reservoir study
2020 |
Interpretation, doi:10.1190/int-2019-0272.1
Note: This publication resulted from Andrea's master thesis i.e. was prepared before GIM was founded.Abstract
Time-lapse gravity is most commonly used to monitor fluid movement and is especially useful when monitoring water encroachment in a gas reservoir. Although time-lapse gravity data are directly sensitive to the fluid saturation changes in reservoirs, it is still necessary to integrate multiple types of data with complementary information to enhance the time-lapse gravity interpretation. When monitoring water-influx in a reservoir, the changes in water yield in production wells may directly indicate saturation changes with time and provide such complementary information about the areas of fluid movement. We present a workflow to invert a time-lapse gravity data set and production data to help monitor the edge water encroachment through a case study at the Sebei gas field in Western China. Three time-lapse gravity surveys were acquired between 2011 and 2013 and production data were also collected from 286 wells during the same period of time. We integrate the two data sets and the structural information in the reservoir through a framework of constrained time-lapse gravity inversion. In this workflow, we incorporate the information from the production data into the inversion by converting the gas and water yield into a reference model. We also incorporate geological structural information through spatially varying bound constraints. Through this approach, we construct a set of time-lapse density contrast models that are consistent with the time-lapse gravity data, production data, and structural information. The resultant density contrast models better delineate the regions of the reservoir with increased water influx and also enable us to produce improved porosity estimations in the reservoir.
Cite as
Balza Morales, A. and Li, Y. (2020): Integrating time-lapse gravity, production, and geological structure data in a gas reservoir study. Interpretation. https://doi.org/10.1190/int-2019-0272.1
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Groundwater Throughflow and Seawater Intrusion in High Quality Coastal Aquifers
2020 |
Scientific Reports, doi:10.1038/s41598-020-66516-6
RWTH Publications PDFAbstract
High quality coastal aquifer systems provide vast quantities of potable groundwater for millions of people worldwide. Managing this setting has economic and environmental consequences. Specific knowledge of the dynamic relationship between fresh terrestrial groundwater discharging to the ocean and seawater intrusion is necessary. We present multi- disciplinary research that assesses the relationships between groundwater throughflow and seawater intrusion. This combines numerical simulation, geophysics, and analysis of more than 30 years of data from a seawater intrusion monitoring site. The monitoring wells are set in a shallow karstic aquifer system located along the southwest coast of Western Australia, where hundreds of gigalitres of fresh groundwater flow into the ocean annually. There is clear evidence for seawater intrusion along this coastal margin. We demonstrate how hydraulic anisotropy will impact on the landward extent of seawater for a given groundwater throughflow. Our examples show how the distance between the ocean and the seawater interface toe can shrink by over 100% after increasing the rotation angle of hydraulic conductivity anisotropy when compared to a homogeneous aquifer. We observe extreme variability in the properties of the shallow aquifer from ground penetrating radar, hand samples, and hydraulic parameters estimated from field measurements. This motived us to complete numerical experiments with sets of spatially correlated random hydraulic conductivity fields, representative of karstic aquifers. The hydraulic conductivity proximal to the zone of submarine groundwater discharge is shown to be significant in determining the overall geometry and landward extent of the seawater interface. Electrical resistivity imaging (ERI) data was acquired and assessed for its ability to recover the seawater interface. Imaging outcomes from field ERI data are compared with simulated ERI outcomes derived from transport modelling with a range of hydraulic conductivity distributions. This process allows for interpretation of the approximate geometry of the seawater interface, however recovery of an accurate resistivity distribution across the wedge and mixing zone remains challenging. We reveal extremes in groundwater velocity, particularly where fresh terrestrial groundwater discharges to the ocean, and across the seawater recirculation cell. An overarching conclusion is that conventional seawater intrusion monitoring wells may not be suitable to constrain numerical simulation of the seawater intrusion. Based on these lessons, we present future options for groundwater monitoring that are specifically designed to quantify the distribution of; (i) high vertical and horizontal pressure gradients, (ii) sharp variations in subsurface flow velocity, (iii) extremes in hydraulic properties, and (iv) rapid changes in groundwater chemistry. These extremes in parameter distribution are common in karstic aquifer systems at the transition from land to ocean. Our research provides new insights into the behaviour of groundwater in dynamic, densely populated, and ecologically sensitive coastal environments found worldwide.
Cite as
Costall, A. R. and Harris, B. D. and Teo, B. and Schaa, R. and Wagner, F. M. and Pigois, J. P. (2020): Groundwater Throughflow and Seawater Intrusion in High Quality Coastal Aquifers. Scientific Reports. https://doi.org/10.1038/s41598-020-66516-6
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Petrophysical Joint Inversion Applied to Alpine Permafrost Field Sites to Image Subsurface Ice, Water, Air, and Rock Contents
2020 |
Frontiers in Earth Sciences, doi:10.3389/feart.2020.00085
RWTH Publications PDFAbstract
Quantification of ground ice is crucial for understanding permafrost systems and modeling their ongoing degradation. The volumetric ice content is however rarely estimated in permafrost studies, as it is particularly difficult to retrieve. Standard borehole temperature monitoring is unable to provide any ice content estimation, whereas non-invasive geophysical techniques, such as refraction seismic and electrical resistivity measurements can yield information to assess the subsurface ice distribution. Electrical and seismic data are hereby complementary sensitive to the phase change. A petrophysical joint inversion was recently developed to determine volumetric water, air, ice and rock contents from electrical and seismic data using a petrophysical model, but was so far only tested on synthetic data and one proof-of-concept field example. In order to evaluate its applicability on different types of permafrost materials and landforms (bedrock, rock glacier, talus slope), we apply this petrophysical joint inversion scheme to five profiles located in the northwestern Alps. The electrical mixing rule (Archie's second law) was hereby identified as a source of model uncertainty, as it applies only when the electrolytic conduction is the dominating process. We therefore investigate and compare four petrophysical models linking the electrical resistivity with the ground constituents: Archie's law, Archie's law with an additional surface conduction factor, a model considering only surface conduction, and the geometric mean model. In most cases, the three first resistivity relations yield largely comparable results, whose reliability is discussed. The geometric mean model better resolve high ice content, as it is less influenced by the ice-rock ambiguity. We perform a systematic analysis of the regularization parameters and then evaluate our results with validation data including thaw depths and ice contents derived from borehole measurements. Geophysical surveys have generally a lower resolution than borehole data, but have the advantage of providing spatio-temporal information in 2D or 3D. The joint inversion results are in relatively good agreement with the validation data for all sites from ice-poor to ice-rich conditions, when choosing the most adequate resistivity model and porosity initial value. Additional forcing constraints (e.g., porosity range constraint) based on site knowledge can improve the model parameter estimation.
Cite as
Mollaret, C. and Wagner, F. M. and Hilbich, C. and Scapozza, C. and Hauck, C. (2020): Petrophysical Joint Inversion Applied to Alpine Permafrost Field Sites to Image Subsurface Ice, Water, Air, and Rock Contents. Frontiers in Earth Sciences. https://doi.org/10.3389/feart.2020.00085
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Imaging of plant current pathways for non-invasive root Phenotyping using a newly developed electrical current source density approach
2020 |
Plant and Soil, doi:10.1007/s11104-020-04529-w
RWTH Publications PDFAbstract
The flow of electric current in the root-soil system relates to the pathways of water and solutes, its characterization provides information on the root architecture and functioning. We developed a current source density approach with the goal of non-invasively image the current pathways in the root-soil system. A current flow is applied from the plant stem to the soil, the proposed geoelectrical approach images the resulting distribution and intensity of the electric current in the root-soil system. The numerical inversion procedure underlying the approach was tested in numerical simulations and laboratory experiments with artificial metallic roots. We validated the method using rhizotron laboratory experiments on maize and cotton plants. Results from numerical and laboratory tests showed that our inversion approach was capable of imaging root-like distributions of the current source. In maize and cotton, roots acted as leaky conductors, resulting in successful imaging of the root crowns and negligible contribution of distal roots to the current flow. In contrast, the electrical insulating behavior of the cotton stems in dry soil supports the hypothesis that suberin layers can affect the mobility of ions and water. The proposed approach with rhizotrons studies provides the first direct and concurrent characterization of the root-soil current pathways and their relationship with root functioning and architecture. This approach fills a major gap toward non-destructive imaging of roots in their natural soil environment.
Cite as
Peruzzo, L. and Chou, C. and Wu, Y. and Schmutz, M. and Mary, B. and Wagner, F. M. and Petrov, P. and Newman, G. and Blancaflor, E. B. and Liu, X. and Ma, X. and Hubbard, S. (2020): Imaging of plant current pathways for non-invasive root Phenotyping using a newly developed electrical current source density approach. Plant and Soil. https://doi.org/10.1007/s11104-020-04529-w
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Imaging plant responses to water deficit using electrical resistivity tomography
2020 |
Plant and Soil, doi:10.1007/s11104-020-04653-7
RWTH Publications PDFAbstract
Monitoring root water uptake dynamics under water deficit (WD) conditions in fields are crucial to assess plant drought tolerance. In this study, we investigate the ability of Electrical Resistivity Tomography (ERT) to capture specific soil water depletion induced by root water uptake. A combination of surface and depth electrodes with a high spatial resolution (10 cm) was used to map 2-D changes of bulk soil electrical conductivity (EC) in an agronomic trial with different herbaceous species. A synthetic experiment was performed with a mechanistic model to assess the ability of the electrode configuration to discriminate abstraction patterns due to roots. The impact of root segments was incorporated in the forward electrical model using the power-law mixing model. The time-lapse analysis of the synthetic ERT experiment shows that different root water uptake patterns can be delineated for measurements collected under WD conditions but not under wet conditions. Three indices were found (depletion amount, maximum depth, and spread), which allow capturing plant-specific water signatures based moisture profile changes derived from EC profiles. When root electrical properties were incorporated in the synthetic experiments, it led to the wrong estimation of the amount of water depletion, but a correct ranking of plants depletion depth. When applied to the filed data, our indices showed that Cocksfoot and Ryegrass had shallower soil water depletion zones than white clover and white clover combined with Ryegrass. However, in terms of water depletion amount, Cocksfoot consumed the largest amount of water, followed by White Clover, Ryegrass+White Clover mixture, and Ryegrass. ERT is a well-suited method for phenotyping root water uptake ability in field trials under WD conditions.
Cite as
Rao, S. and Lesparre, N. and Flores Orozco, A. and Wagner, F. and Kemna., A. and Javaux, M. (2020): Imaging plant responses to water deficit using electrical resistivity tomography. Plant and Soil. https://doi.org/10.1007/s11104-020-04653-7
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A monitoring system for spatiotemporal electrical self-potential measurements in cryospheric environments
2020 |
Geoscientific Instrumentation, Methods and Data Systems, doi:10.5194/gi-9-317-2020
RWTH Publications PDFNote: This publication resulted from Florian's time at the University of Bonn, i.e. was prepared before GIM was founded.Abstract
Climate-induced warming increasingly leads to degradation of high-alpine permafrost. In order to develop early warning systems for imminent slope destabilization, knowledge about hydrological flow processes in the subsurface is urgently needed. Due to the fast dynamics associated with slope failures, non- or minimally invasive methods are required for inexpensive and timely characterization and monitoring of potential failure sites to allow in-time responses. These requirements can potentially be met by geophysical methods usually applied in near-surface geophysical settings, such as electrical resistivity tomography (ERT), ground-penetrating radar (GPR), various seismic methods, and self-potential (SP) measurements. While ERT and GPR have their primary uses in detecting lithological subsurface structure and liquid water/ice content variations, SP measurements are sensitive to active water flow in the subsurface. Combined, these methods provide huge potential to monitor the dynamic hydrological evolution of permafrost systems. However, while conceptually simple, the technical application of the SP method in high-alpine mountain regions is challenging, especially if spatially resolved information is required. We here report on the design, construction, and testing phase of a multi-electrode SP measurement system aimed at characterizing surface runoff and meltwater flow on the Schilthorn, Bernese Alps, Switzerland. Design requirements for a year-round measurement system are discussed; the hardware and software of the constructed system, as well as test measurements are presented, including detailed quality-assessment studies. On-site noise measurements and one laboratory experiment on freezing and thawing characteristics of the SP electrodes provide supporting information. It was found that a detailed quality assessment of the measured data is important for such challenging field site operations, requiring adapted measurement schemes to allow for the extraction of robust data in light of an environment highly contaminated by anthropogenic and natural noise components. Finally, possible short- and long-term improvements to the system are discussed and recommendations for future installations are developed.
Cite as
Weigand, M. and Wagner, F. M. and Limbrock, J. K. and Hilbich, C. and Hauck, C. and Kemna, A. (2020): A monitoring system for spatiotemporal electrical self-potential measurements in cryospheric environments. Geoscientific Instrumentation, Methods and Data Systems. https://doi.org/10.5194/gi-9-317-2020
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Quantitative imaging of water, ice and air in permafrost systems through petrophysical joint inversion of seismic refraction and electrical resistivity data
2019 |
Geophysical Journal International, doi:10.1093/gji/ggz402
RWTH Publications PDFNote: This publication resulted from Florian's time at the University of Bonn, i.e. was prepared before GIM was founded.Abstract
Quantitative estimation of pore fractions filled with liquid water, ice and air is crucial for a process-based understanding of permafrost and its hazard potential upon climate-induced degradation. Geophysical methods offer opportunities to image distributions of permafrost constituents in a non-invasive manner. We present a method to jointly estimate the volumetric fractions of liquid water, ice, air and the rock matrix from seismic refraction and electrical resistivity data. Existing approaches rely on conventional inversions of both data sets and a suitable a priori estimate of the porosity distribution to transform velocity and resistivity models into estimates for the four-phase system, often leading to non-physical results. Based on two synthetic experiments and a field data set from an Alpine permafrost site (Schilthorn, Bernese Alps and Switzerland), it is demonstrated that the developed petrophysical joint inversion provides physically plausible solutions, even in the absence of prior porosity estimates. An assessment of the model covariance matrix for the coupled inverse problem reveals remaining petrophysical ambiguities, in particular between ice and rock matrix. Incorporation of petrophysical a priori information is demonstrated by penalizing ice occurrence within the first two meters of the subsurface where the measured borehole temperatures are positive. Joint inversion of the field data set reveals a shallow air-rich layer with high porosity on top of a lower-porosity subsurface with laterally varying ice and liquid water contents. Non-physical values (e.g. negative saturations) do not occur and estimated ice saturations of 050 per cent as well as liquid water saturations of 1575 per cent are in agreement with the relatively warm borehole temperatures between −0.5 and 3 ° C. The presented method helps to improve quantification of water, ice and air from geophysical observations.
Cite as
Wagner, F. M. and Mollaret, C. and Günther, T. and Kemna, A. and Hauck, C. (2019): Quantitative imaging of water, ice and air in permafrost systems through petrophysical joint inversion of seismic refraction and electrical resistivity data. Geophysical Journal International. https://doi.org/10.1093/gji/ggz402
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Preparing for InSight: Evaluation of the Blind Test for Martian Seismicity
2019 |
Seismological Research Letters, doi:10.1785/0220180379
RWTH PublicationsNote: This publication resulted from Marc's time at RUB, i.e. was prepared before GIM was founded.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.
Cite as
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
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Optimized survey design for electrical resistivity tomography: Combined optimization of measurement configuration and electrode placement
2018 |
Geophysical Journal International, doi:10.1093/gji/ggy128
RWTH Publications PDFNote: This publication resulted from Florian's time at the University of Bonn, i.e. was prepared before GIM was founded.Abstract
Within geoelectrical imaging, the choice of measurement configurations and electrode locations is known to control the image resolution. Previous work has shown that optimized survey designs can provide a model resolution that is superior to standard survey designs. This paper demonstrates a methodology to optimize resolution within a target area, while limiting the number of required electrodes, thereby selecting optimal electrode locations. This is achieved by extending previous work on the ‘Compare-R’ algorithm, which by calculating updates to the resolution matrix optimizes the model resolution in a target area. Here, an additional weighting factor is introduced that allows to preferentially adding measurement configurations that can be acquired on a given set of electrodes. The performance of the optimization is tested on two synthetic examples and verified with a laboratory study. The effect of the weighting factor is investigated using an acquisition layout comprising a single line of electrodes. The results show that an increasing weight decreases the area of improved resolution, but leads to a smaller number of electrode positions. Imaging results superior to a standard survey design were achieved using 56 per cent fewer electrodes. The performance was also tested on a 3-D acquisition grid, where superior resolution within a target at the base of an embankment was achieved using 22 per cent fewer electrodes than a comparable standard survey. The effect of the underlying resistivity distribution on the performance of the optimization was investigated and it was shown that even strong resistivity contrasts only have minor impact. The synthetic results were verified in a laboratory tank experiment, where notable image improvements were achieved. This work shows that optimized surveys can be designed that have a resolution superior to standard survey designs, while requiring significantly fewer electrodes. This methodology thereby provides a means for improving the efficiency of geoelectrical imaging.
Cite as
Uhlemann, S. and Wilkinson, P. B. and Maurer, H. and Wagner, F. M. and Johnson, T. C. and Chambers, J. E. (2018): Optimized survey design for electrical resistivity tomography: Combined optimization of measurement configuration and electrode placement. Geophysical Journal International. https://doi.org/10.1093/gji/ggy128
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Fully coupled inversion on a multi-physical reservoir model Part II: The Ketzin CO2 storage reservoir
2018 |
International Journal of Greenhouse Gas Control, doi:10.1016/j.ijggc.2018.04.009
RWTH Publications PDFNote: This publication resulted from Florian's time at GFZ Potsdam, i.e. was prepared before GIM was founded.Abstract
Reliable monitoring of CO2 storage reservoirs requires a combination of different observation methods. However, history matching is typically limited to CO2 pressure data alone. This paper presents a multi-physical inversion of hydraulic pressure, CO2 pressure, CO2 arrival time and geoelectrical crosshole observations of the Ketzin pilot site for CO2 storage, Germany. Multi-physical inversion has rarely been reported for CO2 storage reservoirs. In contrast to previous studies, there is no need for pre-inversion of geophysical datasets as these are now directly included in a fully coupled manner. The deteriorating impact of structural noise is effectively mitigated by preconditioning of the observation data. A double regularisation scheme provides stability for insensitive parameters and reduces the number of required model runs during inversion. The model shows fast and stable convergence and the results provide a good fit to the multi-physical observation dataset. It has certain predictive power as the known migration direction of the CO2 plume is captured. These results clarify two long discussed issues of the Ketzin CO2 storage reservoir: (1) The pre-existing hypothesis of an existing hydraulic barrier became unsubstantial as the data series suggesting weak hydraulic communication are identified as erroneous. (2) Salt precipitation around the injection well doubles the injection overpressure compared to salt free conditions, which is equivalent to a well skin of 10. The presented framework allows to integrate various types of observations into a single multi-physical model leading to an increased confidence in the spatial permeability distribution and, in perspective, to improved predictive assessments of CO2 storage reservoirs.
Cite as
Wagner, F. M. and Wiese, B. U. (2018): Fully coupled inversion on a multi-physical reservoir model Part II: The Ketzin CO2 storage reservoir. International Journal of Greenhouse Gas Control. https://doi.org/10.1016/j.ijggc.2018.04.009
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Fully coupled inversion on a multi-physical reservoir model Part I: Theory and concept
2018 |
International Journal of Greenhouse Gas Control, doi:10.1016/j.ijggc.2018.05.013
RWTH Publications PDFNote: This publication resulted from Florian's time at GFZ Potsdam, i.e. was prepared before GIM was founded.Abstract
State of the art reservoir monitoring delivers numerous property data with high resolution. Especially the consistent interpretation of pressure data with different geophysical methods requires multi-physical modelling and inversion workflows. Such a workflow is developed based on the reservoir monitoring concept of the Ketzin pilot site for CO2 storage, Germany. The workflow consists of three physical models, (i) a single phase hydraulic model, (ii) a multiphase CO2 migration model and (iii) a geoelectrical model. Calibration is carried out to match observation data groups hydraulic pressure, CO2 pressure, CO2 arrival time and geoelectrical cross-hole observations. Calibration parameters are spatially distributed hydraulic permeability and porosity, compressibility, the relative permeability function and the geoelectrical saturation exponent. Geoelectrical measurements with low coverage that cannot be inverted with traditional methods could be included, since the multiphysical reservoir model acts as physical regularisation. The indirect nature of geophysical data is overcome by implementation of petrophysical relations between permeability and porosity and between CO2 saturation and electrical resistivity. Stability against field data is increased by reducing the impact of structural noise through preprocessing the observation data. Stability against the overparameterisation is added by Tikhonov regularisation and singular value decomposition, the latter combined with super parameter definition reducing the problem dimensions and simulation time by three quarters. A synthetic case study demonstrates that the model resolves the spatial permeability and identifies the petrophysical relation between CO2 saturation and electrical resistivity. The weighting scheme balances different observation data groups and measurement intervals. The model to measurement misfit is reduced proprotionally for all observation data groups, while the geoelectrical data are most difficult to match.
Cite as
Wiese, B. U. and Wagner, F. M. and Norden, B. and Maurer, H. and Schmidt-Hattenberger, C. (2018): Fully coupled inversion on a multi-physical reservoir model Part I: Theory and concept. International Journal of Greenhouse Gas Control. https://doi.org/10.1016/j.ijggc.2018.05.013
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A nodal discontinuous Galerkin approach to 3-D viscoelastic wave propagation in complex geological media
2018 |
Geophysical Journal International, doi:10.1093/gji/ggx494
RWTH Publications PDFNote: This publication resulted from Marc's time at RUB, i.e. was prepared before GIM was founded.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
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pyGIMLi: An open-source library for modelling and inversion in geophysics
2017 |
Computers & Geosciences, doi:10.1016/j.cageo.2017.07.011
RWTH Publications PDFNote: This publication marks version 1.0 of pyGIMLi and resulted from collaboration with Carsten Rücker and Thomas Günther during Florian's time at GFZ Potsdam and the University of Bonn, i.e. was prepared before GIM was founded.Abstract
Many tasks in applied geosciences cannot be solved by single measurements, but require the integration of geophysical, geotechnical and hydrological methods. Numerical simulation techniques are essential both for planning and interpretation, as well as for the process understanding of modern geophysical methods. These trends encourage open, simple, and modern software architectures aiming at a uniform interface for interdisciplinary and flexible modelling and inversion approaches. We present pyGIMLi (Python Library for Inversion and Modelling in Geophysics), an open-source framework that provides tools for modelling and inversion of various geophysical but also hydrological methods. The modelling component supplies discretization management and the numerical basis for finite-element and finite-volume solvers in 1D, 2D and 3D on arbitrarily structured meshes. The generalized inversion framework solves the minimization problem with a Gauss-Newton algorithm for any physical forward operator and provides opportunities for uncertainty and resolution analyses. More general requirements, such as flexible regularization strategies, time-lapse processing and different sorts of coupling individual methods are provided independently of the actual methods used. The usage of pyGIMLi is first demonstrated by solving the steady-state heat equation, followed by a demonstration of more complex capabilities for the combination of different geophysical data sets. A fully coupled hydrogeophysical inversion of electrical resistivity tomography (ERT) data of a simulated tracer experiment is presented that allows to directly reconstruct the underlying hydraulic conductivity distribution of the aquifer. Another example demonstrates the improvement of jointly inverting ERT and ultrasonic data with respect to saturation by a new approach that incorporates petrophysical relations in the inversion. Potential applications of the presented framework are manifold and include time-lapse, constrained, joint, and coupled inversions of various geophysical and hydrological data sets.
Cite as
Rücker, C. and Günther, T. and Wagner, F. M. (2017): pyGIMLi: An open-source library for modelling and inversion in geophysics. Computers & Geosciences. https://doi.org/10.1016/j.cageo.2017.07.011
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Monitoring the Complete Life-cycle of a CO2 Storage Reservoir Demonstration of Applicability of Geoelectrical Imaging
2017 |
Energy Procedia, doi:10.1016/j.egypro.2017.03.1526
RWTH Publications PDFNote: This publication resulted from Florian's time at GFZ Potsdam, i.e. was prepared before GIM was founded.Abstract
In this paper, the applicability of deep downhole geoelectrical monitoring for detecting CO2 related signatures is evaluated after a nearly ten year period of CO2 storage at the Ketzin pilot site. Deep downhole electrode arrays have been studied as part of a multi-physical monitoring concept at four CO2 pilot test sites worldwide so far. For these sites, it was considered important to implement the geoelectrical method into the measurement program of tracking the CO2 plume. Analyzing the example of the Ketzin site, it can be seen that during all phases of the CO2 storage reservoir development the resistivity measurements and their corresponding tomographic interpretation contribute in a beneficial manner to the measurement, monitoring and verification (MMV) protocol. The most important impact of a permanent electrode array is its potential as tool for estimating reservoir saturations.
Cite as
Schmidt-Hattenberger, C. and Bergmann, P. and Labitzke, T. and Pommerencke, J. and Rippe, D. and Wagner, F. M. and Wiese, B. (2017): Monitoring the Complete Life-cycle of a CO2 Storage Reservoir Demonstration of Applicability of Geoelectrical Imaging. Energy Procedia. https://doi.org/10.1016/j.egypro.2017.03.1526
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Fully Coupled Hydrogeophysical Inversion of Hydraulics, Gas Pressure and Geoelectrics
2017 |
Energy Procedia, doi:10.1016/j.egypro.2017.03.1490
RWTH Publications PDFNote: This publication resulted from Florian's time at GFZ Potsdam, i.e. was prepared before GIM was founded.Abstract
The comprehensive interpretation of different data types becomes increasingly challenging, even more if the integration should be carried out in a quantitative manner. A hydrogeophysical reservoir model is set up that links the single phase hydraulics, multiphase behaviour and geoelectrical properties of a CO2 storage reservoir. The model is embedded into a fully coupled inversion framework that explicitly honours the physical processes underlying the different types of measurement data. The calibrated model provides a comprehensive representation of all data, with an excellent accuracy for hydraulic and gas pressure data and a satisfactory accuracy of arrival times and geoelectrical data. The permeability is within reasonable bounds but the spatial distribution shows several indications for overfitting. The model reproduces the main migration direction of the plume.
Cite as
Wiese, B. U. and Wagner, F. M. and Norden, B. and Schmidt-Hattenberger, C. (2017): Fully Coupled Hydrogeophysical Inversion of Hydraulics, Gas Pressure and Geoelectrics. Energy Procedia. https://doi.org/10.1016/j.egypro.2017.03.1490
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A Nodal Discontinuous Galerkin Solver for Modeling Seismic Wave Propagation in Porous Media
2017 |
Poromechanics VI, doi:10.1061/9780784480779.185
RWTH PublicationsNote: This publication resulted from Marc's time at RUB, i.e. was prepared before GIM was founded.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
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Fluid injection monitoring using electrical resistivity tomography - five years of CO2 injection at Ketzin, Germany
2016 |
Geophysical Prospecting, doi:10.1111/1365-2478.12426
RWTH Publications PDFNote: This publication resulted from Florian's time at GFZ Potsdam, i.e. was prepared before GIM was founded.Abstract
Between the years 2008 and 2013, approximately 67 kilotons of CO2 have been injected at the Ketzin site, Germany. As part of the geophysical monitoring programme, time-lapse electrical resistivity tomography has been applied using crosshole and surface-downhole measurements of electrical resistivity tomography. The data collection of electrical resistivity tomography is partly based on electrodes that are permanently installed in three wells at the site (one injection well and two observation wells). Both types of ERT measurements consistently show the build-up of a CO2-related resistivity signature near the injection point. Based on the imaged resistivity changes and a petrophysical model, CO2 saturation levels are estimated. These CO2 saturations are interpreted in conjunction with CO2 saturations inferred from neutron-gamma loggings. Apart from the CO2brine substitution response in the observed resistivity changes, significant imprints from the dynamic behaviour of the CO2 in the reservoir are observed.
Cite as
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. Geophysical Prospecting. https://doi.org/10.1111/1365-2478.12426
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Permanent crosshole electrical resistivity tomography (ERT) as an established method for the long-term CO2 monitoring at the Ketzin pilot site
2016 |
International Journal of Greenhouse Gas Control, doi:10.1016/j.ijggc.2016.07.024
RWTH Publications PDFNote: This publication resulted from Florian's time at GFZ Potsdam, i.e. was prepared before GIM was founded.Abstract
At the Ketzin pilot site, a permanent downhole electrode array has been utilized as part of the multi-disciplinary monitoring concept during all phases of the CO2 storage project. After more than seven years under the present CO2/brine prevailing subsurface conditions, including the five years of regular CO2 injection, a first long-term evaluation of the general technical performance of the electrode array can be given. This article reports on the application of the crosshole electrical resistivity tomography (ERT) technique considering the site specific requirements and challenges. It describes the installation procedure of the electrode array, field data acquisition schemes and automated pre-processing routines applied to the continuously growing data archive. It has been found that the evaluation of contact resistance measurements provides useful information about the current condition of the downhole installation. The 3D time-lapse inversion of one of the major observation planes yields resistivity distributions from various operational stages during the injection and post-injection phases of the CO2 storage reservoir. A simple saturation approach converts averaged resistivity signatures from the target reservoir zone into CO2 saturation estimates, which are in good agreement with saturation results measured by borehole logging campaigns. The permanent ERT array has shown a promising lifespan under downhole conditions, and is able to provide complementary information in conjunction with other monitoring systems. For further practical applications, a more standardized workflow for data acquisition and processing might be beneficial.
Cite as
Schmidt-Hattenberger, C. and Bergmann, P. and Labitzke, T. and Wagner, F. and Rippe, D. (2016): Permanent crosshole electrical resistivity tomography (ERT) as an established method for the long-term CO2 monitoring at the Ketzin pilot site. International Journal of Greenhouse Gas Control. https://doi.org/10.1016/j.ijggc.2016.07.024
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Review on geophysical monitoring of CO2 injection at Ketzin, Germany
2016 |
Journal of Petroleum Science and Engineering, doi:10.1016/j.petrol.2015.12.007
RWTH Publications PDFNote: This publication resulted from Florian's time at GFZ Potsdam, i.e. was prepared before GIM was founded.Abstract
Geophysical monitoring activities were an important component of the CO2 injection program at the Ketzin site, Germany. Here we report on the seismic and electrical resistivity tomography (ERT) measurements performed during the period of the site development and CO2 injection. Details on the site geology, the history of the CO2 injection operation, and petrophysical models relevant for the interpretation of the geophysical data are presented. The seismic measurements comprise 2D and 3D surface seismic surveys, vertical seismic profilings, and crosshole measurements. Apart from the measurements, results from advanced processing methods, such as impedance inversion and full-waveform inversion are also presented. In addition, results from crosshole ERT and surface-downhole ERT are presented. If operational efforts are taken into consideration we conclude that a combination of several geophysical methods is preferable given the demands of a spatiotemporally comprehensive monitoring program. We base this conclusion on that the different imaging characteristics and petrophysical sensitivities of different methods can complement each other. An important finding is, based on signal quality and reduced operational costs, that the use of permanent installations is promising. Generally, specific monitoring layouts will depend on site-specific characteristics, such as reservoir depth, availability of wells, petrophysical characteristics, and accessibility of surface locations. Given the comprehensive range of methods applied, the reported results are not only relevant to the operation of CO2 storage sites, but are also of interest to other monitoring projects dealing with fluid injection or production.
Cite as
Bergmann, P. and Diersch, M. and Götz, J. and Ivandic, M. and Ivanova, A. and Juhlin, C. and Kummerow, J. and Liebscher, A. and Lüth, S. and Meekes, S. and Norden, B. and Schmidt-Hattenberger, C. and Wagner, F. M. and Zhang, F. (2016): Review on geophysical monitoring of CO2 injection at Ketzin, Germany. Journal of Petroleum Science and Engineering. https://doi.org/10.1016/j.petrol.2015.12.007
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Impact and mitigation of borehole related effects in permanent crosshole resistivity imaging: An example from the Ketzin CO2 storage site
2015 |
Journal of Applied Geophysics, doi:10.1016/j.jappgeo.2015.10.005
RWTH Publications PDFNote: This publication resulted from Florian's time at GFZ Potsdam, i.e. was prepared before GIM was founded.Abstract
Geoelectrical methods are particularly suited for CO2 injection monitoring due to their high sensitivity to fluid displacement processes in porous rock formations. The use of borehole electrodes is favorable for deep storage horizons. Yet data acquisition based on permanently installed borehole electrodes can be challenged by the finite extent of the electrodes, unintended borehole deviation and complex borehole completion. Such conditions can lead to systematic errors in the electrical data sets, distortions of tomograms, and ultimately misinterpretations. We systematically analyze the effects of different borehole related error sources on tomographic inversion results and present respective methods for mitigation. Specifically, we incorporate the finite extent of the ring electrodes and the borehole completion into the electrical finite-element models and discuss the opportunity to infer borehole deviations solely based on geoelectrical data by means of a coupled inversion. While the finite extent of ring electrodes can be neglected if the electrode spacing is sufficiently large (> 5 m), different borehole completion materials used to fill the well annulus can cause potentially strong resistivity contrasts between the borehole completion and the rock formation, i.e., close to the electrodes. Resulting inversion artifacts are generally less severe when the borehole completion is more resistive compared to the surrounding rock. It is also shown that 2.5D inversion approaches are not adequate for imaging injection experiments in the presence of borehole completion. Unintended borehole deviation can result in geometric errors. Especially, vertical electrode shifts cause strong and localized inversion artifacts. Coupled inverse schemes potentially provide the opportunity to infer electrode shifts solely based on geoelectrical data provided the availability of high quality measurements (< 5% data error). After discussing the effects of the different borehole related error sources, the mitigation methods are validated using synthetic data sets. Subsequently, relevant methods are applied to a field data set from the Ketzin CO2 storage site, Germany, where crosshole electrical resistivity imaging is used for CO2 migration monitoring. The mitigation methods presented can improve estimates of the subsurface resistivity distribution, which, in our particular example, is an essential basis for the quantification of CO2 saturation from time-lapse geoelectrical measurements.
Cite as
Wagner, Florian M. and Bergmann, P. and Rücker, C. and Wiese, B. and Labitzke, T. and Schmidt-Hattenberger, C. and Maurer, H. (2015): Impact and mitigation of borehole related effects in permanent crosshole resistivity imaging: An example from the Ketzin CO2 storage site. Journal of Applied Geophysics. https://doi.org/10.1016/j.jappgeo.2015.10.005
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Constructive optimization of electrode locations for target-focused resistivity monitoring
2015 |
Geophysics, doi:10.1190/geo2014-0214.1
RWTH Publications PDFNote: This publication resulted from Florian's time at GFZ Potsdam, i.e. was prepared before GIM was founded.Abstract
Crosshole resistivity tomography has received consideration as a tool for quantitative imaging of carbon dioxide stored in deep saline aquifers. With regard to the monitoring responsibility of site operators and the substantial expenses associated with permanent downhole installations, optimized experimental design gains particular importance. Based on an iterative appraisal of the formal model resolution matrix, we developed a method to estimate optimum electrode locations along the borehole trajectories with the objective to maximize the imaging capability within a prescribed target horizon. For the presented crosshole case, these layouts were found to be symmetric, exhibiting refined electrode spacings within the target horizon. Our results revealed that a sparse but well conceived set of electrodes can provide a large part of the information content offered by comparably dense electrode distributions. In addition, the optimized layout outperformed equidistant setups with the same number of electrodes because its resolution was focused on the monitoring target. The optimized electrode layouts presented provided a powerful and cost-efficient opportunity to complement permanent installations, particularly at, but not limited to, future CO2 storage sites. Although preliminarily developed to support the design of crosshole resistivity layouts, our approach is directly applicable to other survey geometries including surface and surface-to-hole acquisitions.
Cite as
Wagner, F. M. and Günther, T. and Schmidt-Hattenberger, C. and Maurer, H. (2015): Constructive optimization of electrode locations for target-focused resistivity monitoring. Geophysics. https://doi.org/10.1190/geo2014-0214.1
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Wave Propagation in Porous Media Saturated with Two Fluids
2015 |
Transport in Porous Media, doi:10.1007/s11242-014-0424-2
RWTH PublicationsNote: This publication resulted from Marc's time at Princeton Univeristy, i.e. was prepared before GIM was founded.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
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CO2 Migration Monitoring by Means of Electrical Resistivity Tomography (ERT) Review on Five Years of Operation of a Permanent ERT System at the Ketzin Pilot Site
2014 |
Energy Procedia, doi:10.1016/j.egypro.2014.11.471
RWTH Publications PDFNote: This publication resulted from Florian's time at GFZ Potsdam, i.e. was prepared before GIM was founded.Abstract
At the Ketzin pilot site, Germany, electrical resistivity tomography (ERT) is a substantial component in a multi-disciplinary monitoring concept established in order to image CO2 injected in a saline aquifer. Since more than five years, crosshole ERT data sets have repeatedly been collected using a borehole electrode array acting as a permanent reservoir monitoring tool. This contribution summarizes the aspects being essential for a successful deployment and operation of such a downhole installation. It is shown that the presented installation can facilitate stable and reliable data collection at least throughout the investigated five- year period of ongoing CO2 injection. Based on the experiences being gained so far, it is concluded that a properly calibrated and integrated downhole ERT system allows for mapping of quantitative CO2 saturation estimates in the subsurface.
Cite as
Schmidt-Hattenberger, C. and Bergmann, P. and Labitzke, T. and Wagner, F. (2014): CO2 Migration Monitoring by Means of Electrical Resistivity Tomography (ERT) Review on Five Years of Operation of a Permanent ERT System at the Ketzin Pilot Site. Energy Procedia. https://doi.org/10.1016/j.egypro.2014.11.471
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Electrical Resistivity Tomography (ERT) for Monitoring of CO2 Migration - from Tool Development to Reservoir Surveillance at the Ketzin Pilot Site
2013 |
Energy Procedia, doi:10.1016/j.egypro.2013.06.329
RWTH Publications PDFNote: This publication resulted from Florian's time at GFZ Potsdam, i.e. was prepared before GIM was founded.Abstract
Since more than four years of operation, the Ketzin pilot site is successfully demonstrating a multi-disciplinary monitoring concept for detecting and tracking the CO2 distribution in the subsurface. In this research frame, the electrical resistivity tomography (ERT) is part of the geophysical measurement program and contributes to the observation of the pore fluid changes due to the CO2/brine displacement process in the reservoir zone. Our work demonstrates the feasibility of a permanently installed geoelectrical array and its potential for providing frequently acquired time-lapse results as well as for supporting periodical surface-downhole surveys. Based on standardized technical components and equipped with a sequence of suitable data evaluation tools, this permanent reservoir monitoring system aims to support subsurface management solutions.
Cite as
Schmidt-Hattenberger, C. and Bergmann, P. and Bösing, D. and Labitzke, T. and Möller, M. and Schröder, S. and Wagner, F. and Schütt, H. (2013): Electrical Resistivity Tomography (ERT) for Monitoring of CO2 Migration - from Tool Development to Reservoir Surveillance at the Ketzin Pilot Site. Energy Procedia. https://doi.org/10.1016/j.egypro.2013.06.329
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Monitoring freshwater salinization in analog transport models by time-lapse electrical resistivity tomography
2013 |
Journal of Applied Geophysics, doi:10.1016/j.jappgeo.2012.11.013
RWTH Publications PDFNote: This publication resulted from Florian's time at GFZ Potsdam, i.e. was prepared before GIM was founded.Abstract
Deep saline aquifers are target formations both for the geological storage of carbon dioxide as well as for geothermal applications. High pressure gradients, resulting from fluid or gas injection processes, provide a potential driving force for the displacement of native formation waters, implicating a potential salinization of shallow freshwater resources. Geoelectrical monitoring techniques are sensitive to compositional changes of groundwater resources, and hence capable to detect salinization processes at an early stage. In this context, numerical simulations and analog modeling can provide a valuable contribution by identifying probable salinization scenarios, and thereby guiding an optimum sensor network layout within the scope of an early warning system. In this study, coupled numerical flow and transport simulations of a laterally uniform salinization scenario were carried out and used to support a subsequent realization in a laboratory sandbox model. During the experiment, electrical resistivity tomography (ERT) was applied in a practical surfaceborehole setup in order to determine the spatio-temporal variations of electrical properties influenced by saltwater intrusion. Inversion results of different electrode configurations were evaluated and compared to numerical simulations. With regard to surfaceborehole measurements, good results were obtained using crossed bipoles, while regular bipole measurements were more susceptible to noise. Within the scope of a single-hole tomography, the underlying resistivity distribution was best reproduced using the Wenner configuration, which was substantiated by synthetic modeling.
Cite as
Wagner, F. M. and Möller, M. and Schmidt-Hattenberger, C. and Kempka, T. and Maurer, H. (2013): Monitoring freshwater salinization in analog transport models by time-lapse electrical resistivity tomography. Journal of Applied Geophysics. https://doi.org/10.1016/j.jappgeo.2012.11.013