Saeed Nazari, PhD

The growing demand for critical raw materials necessitates efficient exploration techniques capable of imaging deep-seated mineral deposits over large areas. Semi-airborne electromagnetics (SAEM) combines the rapid data acquisition capabilities of airborne receivers with the strong signal penetration of grounded bipole transmitters. We applied a multi-patch 3D inversion strategy based on Gauss-Newton minimization and explicit sensitivity calculation to a large-scale SAEM dataset collected in 2022 across 130 km² in the Upper Harz Mountains, a historic mining region known for Zn-Pb-(Cu) mineralization. The survey included 11 overlapping flight patches corresponding to 11 transmitters of 2-4 km length and currents up to 23 A. To handle the inversion of all data, we developed a workflow that begins with creating a high-resolution super-mesh that includes all survey patches. Single-patch inversions were then performed for each subarea to evaluate the data quality and consistency. We clustered groups of neighboring patches afterwards to run inversions of the combined data. However, due to computational limitations, it was not possible to combine more than three or four patches in single inversion runs. Therefore, we interpolated the results from the clustered data set inversions into the super-mesh to obtain a complete resistivity model of the subsurface. Overlapping areas were combined using coverage-weighted means. Finally, we used iterative refinements to optimize the subsurface model by exchanging or expanding the clusters. We were able to resolve complex three-dimensional geological features, such as small-scale faults, folded structures, and mineralized zones, at depths up to 1 km. The final regional resistivity model covering an area of more than 130 km² showed clear correlations with known geological structures and mineral vein networks and revealed important conductive anomalies. Conductive zones, which align with known faults and mineral veins, suggest potential extensions of mineralization at greater depths and offer the potential for identifying new mineral targets.