Investigation of a graphite deposit with drone-based semi-airborne electromagnetics

Abstract The shift towards renewable energy sources necessitates a stable supply of essential raw materials and calls for advanced, non-invasive techniques to explore deep-seated mineral resources. In the last years, the semi-airborne electromagnetic (sAEM) method has been further developed to efficiently investigate the subsurface down to ~1 km depth. The Kropfmühl graphite deposit in Lower Bavaria comprises high-grade metamorphic graphite-bearing gneisses. While mining reaches depths of 200 m, the western extent of the deposit remains unknown. To verify the western extension and continuity of the Kropfmühl graphite deposit, a drone-based semi-airborne electromagnetic survey was conducted, combining a dense spatial sampling of magnetic fields using an UAV system and a high signal strength with controlled-source electromagnetic (CSEM) transmitters on the ground. The survey area spanned approximately 1.8 × 2.5 km2, using four galvanically coupled dipole transmitters and multiple UAV flights over four days. To increase depth sensitivity, a total-field magnetometer alongside a three-component induction coil system was utilized. To derive a reliable subsurface conductivity model, a 3D inversion of multi-frequency, multi-source sAEM data of both vector and scalar magnetic field data was conducted. The obtained model revealed three east-west trending conductive anomalies, interpreted as graphite-rich zones. The main anomaly (C1), ~200 m south of the active mine, indicates possible fault-controlled displacement. Borehole resistivity logs and available HEM data correlate well with conductive zones and graphite enrichment. The results highlight the potential of UAV-based sAEM to image complex subsurface structures. The method delivers high-resolution, cost-effective data acquisition, with a high data coverage and only the transmitters requiring ground installation. This renders the method suitable for imaging geologically complex settings in hard-to-access areas.