Large-scale 3D inversion of semi-airborne electromagnetic data — Topography and induced polarization effects in a graphite exploration scenario

Abstract

The transition toward renewable energies demands a secure supply of critical raw materials and requires efficient noninvasive methods for deep earth resource exploration. The novel, deep electromagnetic (EM) sounding for the mineral exploration semi-airborne EM (semi-AEM) exploration concept aims at the efficient exploration of resources down to a depth of 1 km. Here, we evaluate a large-scale semi-AEM exploration study in a graphite mining district in eastern Bavaria, Germany. The derived 3D semi-AEM model is based on approximately 70,000 complex-valued data points recorded using seven transmitters. Strong conductivity contrasts with dominant east–west-trending anomalies are visible. Shallow high-conductivity structures correlate well with the known occurrence of graphite and match existing helicopter-borne EM results. The main anomaly reaches down to several hundred meters depth. To investigate different interpretation scenarios for large-scale semi-AEM data, we determine the effect of topography and analyze the feasibility of fast 2D inversion applications. To validate the robustness of the 3D semi-AEM model, the data set is inverted with two different 3D inversion algorithms and the results are compared. The presence of graphite leads to significant induced polarization (IP) effects with considerably high chargeabilities superposing EM induction. We include these effects in a realistic 3D inversion using a synthetic data study to analyze if the IP effect alters the overall conductivity structure and demonstrate that the obtained 3D model is reliable.