Multiscale modelling of Magnetic field at Mefite D'Ansanto (Southern Apennines, Italy)

The Mefite site in the Ansanto Valley (Southern Apennines, Italy) is a unique geological system characterized by intense low‑temperature gas emissions, primarily carbon dioxide (CO₂) and hydrogen sulfide (H₂S), emanating from a small sulfurous pond. Unlike typical geothermal or volcanic settings, these emissions occur in a non-volcanic environment and are associated with pseudo-volcanic processes linked to deposits formed during the Messinian salinity crisis. Mefite d’Ansanto is considered the largest natural source of low‑temperature CO₂-rich gases in a non‑volcanic setting on Earth, with an estimated daily emission of around 2000 tons. The gas discharge is supplied by a deep reservoir consisting of permeable limestone sequences overlain by low‑permeability clay layers, which help channel fluids toward the surface. In May 2025, we conducted a UAV‑based LiDAR survey followed by a magnetic survey to better characterize the subsurface structures guiding fluid flow in the Mefite area. The LiDAR dataset produced high‑resolution Digital Terrain (DTM) and Digital Surface Models (DSM) over 1.2 km², providing detailed topographic information essential for planning a terrain‑following magnetic survey with constant altitude relative to the ground. The UAV used for both LiDAR and magnetic acquisitions was a DJI Matrice 300 RTK. Magnetic data were collected using the Geometrics MagArrow magnetometer, equipped with Micro Fabricated Atomic Magnetometer (MFAM) sensors. These sensors have a sensitivity of 1 pT/√Hz and operate at a 1000 Hz sampling rate. MFAM technology is affected only by a polar dead zone, where the signal weakens when the sensor aligns within ±35° of the Earth’s magnetic field vector. A drone‑based magnetic surveys were performed on the main Mefite emission pond, was surveyed in May 2025, covering 350 × 450 m with 10 m line spacing; here, the MagArrow was suspended 3 m below the UAV. The surveys maintained an altitude of 35 m above ground level and a flight speed of 4 m/s. Magnetic data processing included corrections for heading errors and high-frequency rotor-induced noise, ensuring the isolation of true geophysical signals. The local geomagnetic parameters (declination 4°, inclination 57°) were used for reduction‑to‑the‑pole processing. The final magnetic map revealed an ellipsoidal anomaly (60–70 nT) centered on the CO₂ pond and a second, stronger anomaly northeast of the main vent. These anomalies are modelled to investigate the responsible source may related to magnetic minerals transported by deep fluids and precipitated near the emission vents.

Acknowledgments

The activities are partially supported by the projects “Relation between 3D Thermo-Rheological Model and Seismic Hazard for Risk Mitigation in the Urban Areas of Southern Italy”, funded under the PRIN2022 PNRR initiative (code: P202299L2C) and FRACTURES PRIN-MUR 2022 (grant no. 2022BEKFN2), both supported by the European Union-Next Generation EU.