Florian M. Wagner

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

Abstract

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

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

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

Abstract

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

Abstract

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

Abstract

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

Abstract

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

Abstract

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

Abstract

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

Abstract

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

Abstract

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

Abstract

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Abstract

Electrical resistivity tomography (ERT) offers noninvasive monitoring capabilities for a wide range of environmentally relevant subsurface processes. Its sensitivity to fluid content and temperature changes positions it as an important tool for capturing dynamic processes such as the transport of groundwater pollutants, CO2 or radionuclides. Particularly crucial is its ability to achieve this without intrusively accessing to the site, making it highly valuable in closed repositories like high-level radioactive waste (HLW) storage sites. In highly sensitive and complex environments, as in the case of closed repositories, it is critical to maximize the information content of the planned (geo)physical measurements while keeping the costs to a minimum. Several past studies presented approaches to optimize both the sensor positions and the measurement configurations of ERT surveys for static or moving targets in the subsurface. This study extends Optimal Experimental Design (OED) strategies for geoelectrical measurements using information of active time-dependent transport processes in the subsurface. We present three different approaches for process monitoring and apply them to a simulated diffusive-advective transport process in a synthetic model over several time steps. The methods aim at focusing the survey only on the relevant part of the model, in this case the model region that is affected by the transport process. All presented approaches account for uncertain model input parameters by introducing an uncertainty factor in the ranking function. We present a purely model-driven and a purely data-driven active time-dependent OED approach. The first method utilizes the already acquired data from previous time steps to create predictive focusing masks for the next data set, the latter purely relies on model predictions to focus the survey. Moreover, we delineate a hybrid approach using both the simulated transport distance and the already acquired datasets. All three OED methods are compared to each other as well as to datasets that were acquired using standard electrode configurations. The results of our synthetic study show that the adaptively designed, time-dependent OED approaches result in increased image quality compared to both standard surveys as well as time-independent OED methods. For slow transport processes or small monitoring intervals, the purely data-driven approach is most suitable, since no model predictions, and thus no possible model parametrization uncertainties, are incorporated. For faster transport processes or monitoring strategies with larger acquisition intervals, the strategies that (partly) incorporate model predictions provide the most promising results.

Cite as

Menzel, N. and Uhlemann, S. and Wagner, F. M. (2024): Strategies for geoelectrical monitoring of subsurface fluid transport processes using Optimized Experimental Design. EGU General Assembly, Vienna, 14-19 April 2024.

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

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

Cite as

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

Abstract

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

Cite as

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

Abstract

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

Cite as

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

Abstract

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

Cite as

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

Abstract

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Abstract

Geothermal systems often occur in geologically complex structural environments with many closely spaced and intersecting faults. These commonly control the associated fluid flow needed for conventional geothermal reservoirs. One of the goals of the Innovative Training Network EASYGO - Efficiency and Safety in Geothermal Operations, aims to better characterize these systems in order to provide an initial assessment of geothermal potential in Europe. The Rhine-Ruhr region was selected as an area of interest for geothermal energy use in the context of the energy and heat transformation change in former coal mining areas. Here, Devonian carbonates and sandstones could play a role as potential reservoirs associated with karst systems or/and fracture zones. The magnetotelluric method has proven to be a useful tool in geothermal plays, where conductive bodies exist at depth. The goal of this study is to identify the structures and associated areas with enhanced fluid flow using structural analysis and magnetotelluric (MT) data. The initial areas chosen in the Rhine-Ruhr region were Rheindahlen, Lüdenscheid, and Aachen. Their local geology confirms favorable conditions for geothermal reservoir development. Additionally, these zones are strategic for MT data acquisition because of their distance from potential sources of anthropogenic noise. The study focuses on a quantitative method for fracture analysis attributes of potential reservoir rocks along with the integration of the geology, fault response modeling, and stress analysis. In addition, we plan to carry out an MT survey integrating the three areas of interest using prior geologic information. For this, we conducted a 3D forward modeling study to simulate the expected MT signals based on the initial structural analysis of the areas of interest. This was done as a feasibility study to predict if the calculated MT signal will be of sufficient signal-to-noise ratio to carefully design future MT acquisition campaigns. Results show favorable structural settings for the transport of fluids (e.g., fault intersection), where the structural component is marked by NW-SE striking normal faults and NE-SW oriented thrust faults with a strike slip-dilation component. Preliminary fracture analysis observed on the surface supports hints of density fracture zones for water circulation, but further studies should be conducted to see if these fractures propagate at depth. The synthetic MT study shows that a considerable signal is expected from conductive bodies within the range of 3,500 to 4,000 m depth. The characterization of the reservoir potential in these areas will facilitate similar studies in the entire Rhine-Ruhr region for a better understanding of the geothermal potential of North Rhine-Westphalia. This project has received funding from the European Unions Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 956965.

Cite as

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

Abstract

Proper characterization of geologic structures that host geothermal systems is crucial for the efficiency and safety of their energy production. This includes estimating layer boundaries, complex geologic features, and lithology through means of inversion and its regularization. However, existing advanced regularization techniques (e.g., geostatistical regularization, minimum-gradient support, etc.) fail to capture the complexity of 3D geological models including fault networks, fault-surface interactions, unconformities, and dome structures. Förderer et al (2021) propose a solution by means of structure-based inversion, which implements implicit geological modeling and low-dimensional parametrization to produce sharp subsurface interfaces in 2D. This work aims to extend their approach to image realistic and complex geometries in 3D. We continue with the example of electrical resistivity tomography (ERT) and synthetic data; however, this approach is aimed towards independent and joint inversion of geophysical methods that are commonly used in geothermal exploration such as magnetotellurics, gravity, and seismic techniques. The 3D geological model is created using GemPy, an open-source Python library, which constructs a structural geological model from interface points and orientations using an implicit approach based on co-kriging (de la Varga et al., 2019). Subsequently, the 3D model is discretized, and physical parameters are assigned using minimal pilot points that are then interpolated. We use pyGIMLi (Rücker et al., 2017), another open-source multi-method library for geophysical modelling and inversion, to perform a structure-based inversion, where we include the interface points in the primary model vector of the inversion to update these points iteratively to estimate a geological model in agreement with the geophysical observations. In this work, special focus is placed on the sensitivity of each model parameter. To maintain low parametrization and account for the increase in computational power, the cumulative sensitivity is calculated and tested under criteria to optimize the model updates. This is relevant for geometries where the interface and pilot points are more influential in one dimension than others. The workflow has also been adapted to include more complex structures that can be defined in 3D, especially those that reflect geothermal systems. This work is part of the Innovative Training Network EASYGO (www.easygo-itn.eu), which aims to improve the efficiency and safety of geothermal operations but can be readily used in other applications. References: Förderer, A., Wellmann, F., and Wagner, F.M.: Geoelectrical imaging of subsurface discontinuities and heterogeneities using low-dimensional parameterizations, EGU General Assembly 2021, online, 19-30 Apr 2021, EGU21-10012, https//doi.org/10.5194/egusphere-egu21-10012, 2021. de la Varga, M., Schaaf, A., and Wellmann, F., 2019. GemPy 1.0: open-source stochastic geological modeling and inversion, Geosci. Model Dev., 12, 1-32, doi 10.5194/gmd-12-1-2019. Rücker, C., Günther, T., Wagner, F.M., 2017. pyGIMLi: An open-source library for modelling and inversion in geophysics, Computers and Geosciences, 109, 106-123, doi 10.1016/j.cageo.2017.07.011.

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

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. 76. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), Münster, 14.-17.03.2016.

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): Five Years of CO₂ Injection Monitoring at Ketzin, Germany, Using Electrical Resistivity Tomography. 78th EAGE Conference & Exhibition, 30 May - 2 June 2016, Vienna.

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Abstract

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

Cite as

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

Abstract

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Abstract

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

Cite as

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

Cite as

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

Abstract

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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

Cite as

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