Magnetotelluric and gravity surveys for constraining reservoir and fluid pathways in non-volcanic CO2 degassing areas: a study case in the Southern Appenines (Italy)

In this contribution we present a multidisciplinary investigation that combines geophysics and geological observations to characterize active degassing areas in the Southern Appenines (Italy). Our focus is on two target areas of Contursi-Oliveto Citra and Mefite d’Ansanto (MdA); MdA, in particular, is one of the largest non-volcanic emission sites worldwide with more than 2000 t of CO₂ emitted per day.

Both areas have been the subject of several previous studies that have revealed a classic trap-reservoir structure at depth, connected to the surface by a complex network of fractures that control the gas outflows. However, critical uncertainties remain regarding the precise locations of the reservoirs, the volumes of gas involved and the detailed definition of the structural controls on underground gas pathways.

To address these questions, advanced geophysical surveys were carried out as part of the MEFITIS project. In the MdA area a high-resolution 3D Magnetotelluric (MT) survey was conducted alongside a gravimetric campaign. Over 50 new gravity stations, which were co-located with AMT soundings and precisely positioned via GNSS RTK, enabled us the to produce a Free-Air anomaly map. Residual gravity anomalies were computed after calculating the Bouguer anomalies and removing long wavelength trends. Two relevant gravity highs bordering the area of maximum emission were identified and interpreted as depth variations of the basement rocks hosting the reservoir. MT surveys delineated several low-resistivity anomalies linked to fault system and gas-saturated formations and allowed to define key structural traps and leakage pathways. In the Contursi-Oliveto Citra area, a deep 2D MT profile explored the reservoir geometry at depths over of 5 km and was integrated with a novel analysis of gravity anomalies available from the Italian gravimetric map.

Through this multidisciplinary approach we demonstrate how the combination of geophysical measurements and geological observations can provide valuable insights into the storage capacity and leakage potential of these natural CO₂ systems. This information is essential for assessing and characterizing the state of the reservoir; sites candidate for CCS, hydrocarbon reservoir systems are analogous contexts that could benefit from a similar combination of geophysical methods.