Jana H. Börner

Workflows for generating tetrahedral meshes for finite element simulations on complex geological structures

Subsurface processing numerical simulations require accurate discretization of the modeling domain such that the geological units are represented correctly. Unstructured tetrahedral grids are particularly flexible in adapting to the shape of geo-bodies and are used in many finite element codes. In order to generate a tetrahedral mesh on a 3D geological model, the tetrahedrons have to belong completely to one geological unit and have to describe geological boundaries by connected facets of tetrahedrons. This is especially complicated at the contact points between several units and for irregular sharp-shaped bodies, especially in case of faulted zones. This study develops, tests and validates three workflows to generate a good tetrahedral mesh from a geological basis model. The tessellation of the model needs (i) to be of good quality to guarantee a stable calculation, (ii) to include certain nodes to apply boundary conditions for the numerical solution, and (iii) support local mesh refinement. As a test case we use the simulation of a transient electromagnetic measurement above a salt diapir. We can show that the suggested workflows lead to a tessellation of the structure on which the simulation can be run robustly. All workflows show advantages and disadvantages with respect to the workload, the control the user has over the resulting mesh and the skills in software handling that are required. HighlightsWe developed, tested and validated three workflows to generate a good tetrahedral mesh from a geological basis model.We used a simulation of a transient electromagnetic measurement as a testcase.We can show that the suggested worklfows lead to a tessellation on which the simulation can be run robustly.

Multi-method virtual electromagnetic experiments for developing suitable monitoring designs: A fictitious CO2 sequestration scenario in Northern Germany

We present a numerical study for 3D time-lapse electromagnetic monitoring of a fictitious CO2 sequestration using the geometry of a real geological site and a suite of suitable electromagnetic methods with different source/receiver configurations and different sensitivity patterns. All available geological information is processed and directly implemented into the computational domain, which is discretized by unstructured tetrahedral grids. We thus demonstrate the performance capability of our numerical simulation techniques. The scenario considers a CO2 injection in approximately 1100 m depth. The expected changes in conductivity were inferred from preceding laboratory measurements. A resistive anomaly is caused within the conductive brines of the undisturbed reservoir horizon. The resistive nature of the anomaly is enhanced by the CO2 dissolution regime, which prevails in the high-salinity environment. Due to the physicochemical properties of CO2, the affected portion of the subsurface is laterally widespread but very thin. We combine controlled-source electromagnetics, borehole transient electromagnetics, and the direct-current resistivity method to perform a virtual experiment with the aim of scrutinizing a set of source/receiver configurations with respect to coverage, resolution, and detectability of the anomalous CO2 plume prior to the field survey. Our simulation studies are carried out using the 3D codes developed in our working group. They are all based on linear and higher order Lagrange and N´ed´elec finite-element formulations on unstructured grids, providing the necessary flexibility with respect to the complex real-world geometry. We provide different strategies for addressing the accuracy of numerical simulations in the case of arbitrary structures. The presented computations demonstrate the expected great advantage of positioning transmitters or receivers close to the target. For direct-current geoelectrics, 50% change in electric potential may be detected even at the Earth’s surface. Monitoring with inductive methods is also promising. For a well-positioned surface transmitter, more than 10% difference in the vertical electric field is predicted for a receiver located 200 m above the target. Our borehole transient electromagnetics results demonstrate that traditional transient electromagnetics with a vertical magnetic dipole source is not well suited for monitoring a thin horizontal resistive target. This is due to the mainly horizontal current system, which is induced by a vertical magnetic dipole.

Transient electromagnetic fields: their efficient three‐dimensional simulation and application to borehole‐based observation techniques

GEM Beijing 2011: International Workshop on Gravity, Electrical & Magnetic Methods and Their Applications Beijing, China. October 10-13, 2011. Transient electromagnetic fields: their efficient three-dimensional simulation and application to borehole-based observation techniques Martin Afanasjew, Jana Borner, Ralph-Uwe Borner, Michael Eiermann, Oliver Ernst, Klaus Spitzer, 1 Institute of Geophysics and Geoscience Informatics, TU Bergakademie Freiberg, Germany 2 Institute of Numerical Analysis and Optimization, TU Bergakademie Freiberg, Germany

Anomalous Complex Electrical Conductivity of a Graphitic Black Schist From the Himalayas of Central Nepal

We analyzed in the laboratory the frequency-dependent, complex-valued, electrical conductivity of a graphitic black schist and an augen gneiss, both collected in the Main Central Thrust shear zone in the Himalayas of central Nepal, which was heavily affected by the deadly Mw7.8 Gorkha earthquake in 2015. We focused on anisotropy and salinity dependence of both cores and crushed material as well as the impact of CO2 on conductivity. This black schist possesses an extraordinarily high polarizability and a highly frequency-dependent conductivity. Its anisotropy is very pronounced. The investigations can relate the main polarization feature to disseminated, aligned plates of graphite. By contrast, the augen gneiss shows low polarizability and a moderately anisotropic conductivity dominated by the pore-filling brine. We further demonstrate that neglecting the complex and frequency-dependent nature of conductivity can lead to serious misinterpretation of magnetotelluric data during inversion if highly polarizable rocks are present. Plain Language Summary We investigated the electrical properties of a graphitic black schist and an augen gneiss, both collected in a shear zone in the Himalayas of central Nepal, which was heavily affected by the deadly Ghorka earthquake in 2015 (moment magnitude Mw7.8). We focused on electrical resistivity, polarization, anisotropy, and the influence of pore water salinity. Both cores and crushed material were analyzed, which allows for a more detailed understanding of the mechanisms of electric conduction in such rocks. The black schist shows a strongly frequency-dependent resistivity, which is associated with an extraordinarily high polarization. Its anisotropy is very pronounced. Scanning electron microscope images confirm that this behavior is due to disseminated, aligned plates of graphite. The augen gneiss on the other hand shows a regular electrical resistivity, which is dominated by the pore-filling brine. Besides the new insights in the mechanisms of electric conduction in these unusual, highly metamorphic rocks, our investigations bear relevance for large-scale geophysical surveys aiming at revealing the internal structure of the Himalayas and understanding the occurrence of large earthquakes in the area. We demonstrate that neglecting the unusual electrical properties of the black schist during interpretation of influenced data can lead to serious misinterpretation.