ALBERT

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 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: 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: Transient electromagnetic (TEM) data can be significantly distorted by induced polarization (IP) effect, leading to a sign reversal feature and, if overlooked, false geological interpretation. The aim of this paper is to incorporate IP effects in the forward modelling and recover the distorted TEM data using an efficient inversion algorithm. To achieve this aim, we developed a 1D forward solver to incorporate the IP effects using various IP parameterizations including Cole-Cole, maximum phase angle (MPA), maximum imaginary conductivity (MIC) (Fiandaca et al., 2018) and the Jeffrey transform of Cole-Cole parameters (Ghorbani et al., 2007). For 1D inversion of distorted TEM data we used Levenberg-Marquardt and very fast simulated annealing algorithms. The result of 1D forward calculation and inversion of synthetic IPdistorted TEM data revealed that, for incorporation the IP effects into the TEM data, the Cole- Cole parametrization is more robust and reliable than MPA, MIC, and Jeffrey transform. Moreover, the result of inversion using Levenberg-Marquardt algorithm is strongly depends on the starting model. We successfully implemented these algorithms for 1D inversion of synthetic IP-affected TEM data (Fig. 1 ). For synthetic data generation, a 3-layered half space model with the thickness of the first and second layers of 5 m was considered. The resistivities of the layers from top to bottom are 10, 5 and 300 Ωm, respectively. To include the IP effect, second layer considered to be chargeable with Cole-Cole parameters of m = 0.5, τ = 0.01 s and c = 0.5. TEM central-loop configuration with a loop size of 50*50 m2 and step-off current of 1 A with a zero ramp time was used for data simulation. We evaluated the performance of our algorithm using field data, successfully.

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.