ALBERT

Description: Highly fragmented rocks (i.e., pulverized rocks) in the fault damage zone presumably develop during co-seismic deformation processes. These pulverized rocks close to the fault core are generally thought to originate from high strain rates, whereas the genesis of pulverized rocks that can be found several hundred meters away from the fault core – where quasi-static conditions prevail – remains unclear. We thus conducted uniaxial cyclic loading experiments with axial strain rate of ∼10−3 s−1 on Leiyang marble in a stress-controlled manner in order to produce crushed rocks for analysis. We found that cyclic loading between 0.8 σc and 1.3 σc can simultaneously compact pre-existing cracks and generated new cracks in marble, which strengthened and stiffened the rock. The stiffened marble developed a higher crack density and energy density before rupture, thereby facilitating rock fragmentation compared with the reference sample, which was fractured monotonically in one cycle. Our results provide a plausible explanation for the genesis of pulverized marble at quasi-static strain rate in the field.

Description: This study investigates the optimization of Semi-Airborne Electromagnetic (SAEM) surveys for enhanced subsurface imaging in mineral exploration. It highlights the utility of multi-transmitter systems and explores real data utilization and the challenges of large-scale surveys. With emphasis on Data obtained from DESMEX project surveys. The use of multiple transmitters is crucial. Single transmitters can distort results and mask subsequent bodies. Employing two transmitters on both sides of the target enhances resolution and depth accuracy. results are based on finite element forward operator custEM and pyGIMLi’s inverse solver [1]. substantial advantages of combining single and multi-patch inversion data. This integration results in improved resolution, reduced artifacts, enhanced continuity of geological structures, superior anomaly detection, minimized edge effects, and improved depth penetration [2]. These findings open promising avenues for further exploration and research in geosciences, offering valuable insights into the Earth's subsurface and its intricate geological features. The next logical step involves expanding our methodology to large-scale inversion using more than three transmitters.

Description: The Atacama Desert along the Chilean Coastal Cordillera is a unique landscape to understand the Earth's evolution in hyper-arid and arid environments. The Paranal clay pan has studied by the CRC 1211 project to recover a continuous climate record for paleoclimate research. The goal is to provide the sedimentary architecture and bedrock topography of the Paranal site by interpreting multidimensional inversion of loop source transient electromagnetic (TEM) data. A total of 133 TEM soundings were carried out using a central loop configuration, with a transmitter loop size of 40×40 m2 and a receiver of about 10×10 m2. The TEM data was processed and analyzed, exhibiting high-quality data, with an average of noise level of about ηnoi = 3·10−10V/Am2. The 1D Occam inversion results exhibits a clear three-layered resistivitydepth structure with a second conductive layer of roughly 20 Ωm. The clay pan's resistivity distribution is well-resolved with a global misfit of around 1.1. However, the study site showed 2D effects that were stronlgy visible at the edges of the clay pan, leading to misinterpretations of the TEM data. This was confirmed based on 2D forward modelling. In this manner, to better deal with the observed 2D distortions in the TEM data and to derive a more accurate geometry of the clay pan, the recently developed Julia Package (3DTEMinv) for time-domain 3D inversion and modeling data was performed. The resulting 3D inversion presents a high convergence rate, and acceptable solutions are obtained after ten iterations with a good misfit of about 1.6. The 3D model exhibits a well-resolved geometry of the clay pan, with a high resolution of the derived conductive body. The drill core results confirm the 1D and 3D TEM models at the center of the clay pan, which is in good agreement with the resulting lithology with a maximum thickness of about 171 m depth and a weathered granodioritic bedrock below. These results agree with the local and regional geological context, improving the understanding of sediment deposition and transportation in this hilly and arid environment.

Description: Multi-dimensional inversion of Transient electromagnetic data is a computationally expensive task. Only few developments and practical interpretation tools exist. Here, we present a multidimensional inversion framework for loop source time-domain electromagnetic data. The developed algorithm is a robust, efficient, and user-oriented tool for the multi-dimensional inversion of typical loop source time-domain electromagnetic configurations. A time-domain finite volume discretization and the direct solver MUMPS are utilized to solve the 3D TEM forward problem. An iterative Gauss-Newton optimization method is implemented for the inversion kernel. The code is parallelized for calculating multiple sources simultaneously to accelerate the inversion. Based on exploration tasks, different configurations exist for commonly used loop source TEM configurations and typical field scales. Synthetic examples are used to verify the effectiveness and benchmark the developed 3D algorithm. Considering that TEM data is often gathered along profiles, adjusting the model roughness along the different modeling domain directions, sufficiently constrains to allow for 2D imaging. In addition to the vertical signal components, we also included horizontal components for large scale fixed loop applications. Subsequent to synthetic validation, the inversion algorithm is further verified using ~120 dense TEM soundings collected over a clay pan site in the Atacama Desert, Chile, to provide bedrock geometry information and suitable coring sites. The 3D inversion result provided an excellent depth estimate of sedimentary infill as well as the bedrock topography and was later confirmed by deep coring. Another interesting site is the Roter Kamm impact crater in Namibia. Our preliminary results obtained from largescale multicomponent fixed loop TEM data reveal a sedimentary infill down to ~300 m depth. In conclusion, our presented 3D inversion code is capable to handle data from various exploration scenarios and provides a robust tool for advanced EM interpretation.

Description: Carrying out laboratory experiments is usually a time-consuming process. In addition, the options for varying parameter studies are limited and adjustments to the design of the measuring equipment are often not possible at all. In order to circumvent these limitations, we supplement our laboratory experiments with virtual experiments as best as possible. For this purpose, we have expanded our finite element library FEMALY [1] to include the so-called complete electrode model [2], which allows us to simulate electrodes of any shape for DC and IP applications and also provides us with explicit mathematical expressions for calculating sensitivities [3]. As a first case study, we consider an IP measurement on a measuring cylinder with embedded ring electrodes to virtually reproduce the time-varying change of the apparent resistivity for laboratory tracer experiments (Figure 1). We present the real and imaginary part of the sensitivity distribution of the underlying measurement configuration that confirms our initial assumption that the actual electrode surface shape has a relatively small influence on the observed measurement quantities.

Description: The transition towards renewable energies demands secure supply with critical raw material and requires efficient non-invasive methods for deep earth resources exploration. The novel DESMEX (Deep electromagnetic sounding for mineral exploration) semi-airborne electromagnetic (semi-AEM) exploration concept aims at efficient exploration of resources down to 1 km depth. Here we present a large-scale semi-AEM exploration study in a graphite mining district in eastern Bavaria, Germany. At the ground, several horizontal electrical dipole transmitters were deployed and helicopter-towed magnetic field sensors measure the EM fields along flight lines within several overlapping flight areas, providing a fast data acquisition and a high spatial coverage. Imaged shallow high conductivity structures can be correlated with graphite-rich zones and match well with existing helicopter-borne EM results. The presence of graphite leads to significant induced polarization (IP) effects with considerably high chargeabilities superposing electromagnetic induction. We include these effects in a realistic 3D inversion using a synthetic data study to analyse, if the IP effect alters the overall conductivity structure and demonstrate that the obtained 3D model is reliable.

Description: We present a finite element software library written in Matlab for the numerical simulation and inversion of electromagnetic fields in two and three dimensions. It is designed in a modular way to easily plug together fundamental building blocks for various electromagnetic applications from DC to the inductive range in the frequency and even time domain. External modules comprise the mesh generator and the equation solver library. Through its homogeneous software concept the adoption to any field application is relatively simple and makes the code suitable to open source distribution. We introduce the key features of this library including higher-order Lagrange and Nédélec finite elements formulated on unstructured tetrahedral grids, a Gauss- Newton inversion approach using linear Raviart-Thomas elements for H1 regularization, and the ability to incorporate any geometric feature such as topography, bathymetry and internal voids like caves, tunnels and mine buildings. The library is currently being tested with large real data sets to confirm its usefulness as a tool for practical data interpretation. Therefore, case studies for the magnetotelluric, direct current resistivity, controlled source electromagnetic and induced polarization methods in the field and laboratory are briefly outlined as examples with challenging geometric features.

Description: In petrophysics, physical rock properties are typically established through laboratory measurements of individual samples. These measurements predominantly relate to the specific sample and can be challenging to associate with the rock as a whole since the physical attributes are heavily reliant on the microstructure, which can vary significantly in different areas. Thus, the obtained values have limited applicability to the entirety of the original rock mass. To examine the dependence of petrophysical measurements based on the variable microstructure, we generate sets of random microstructure representations for a sample, taking into account macroscopic parameters such as porosity and mean grain size. We show that the methodology can adequately mimic the physical behavior of real rocks, showing consistent emulation of the dependence of electrical conductivity on connected porosity according to Archie's law across different types of pore space (micro-fracture, inter-granular, and vuggy, oomoldic pore space). Furthermore, properties such as the internal surface area and its fractal dimension as well as the electrical tortuosity are accessible for the random microstructures and show reasonable behavior. Finally, the possibilities, challenges and meshing strategies for extending the methodology to 3D microstructures are discussed.