GNGTS 2022 - Atti del 40° Convegno Nazionale

460 GNGTS 2022 Sessione 3.2 Stratigraphical and structural survey. The study area is located along the Matese Mts and the Morcone ridge (Fig. 1b). Here, an Upper Cretaceous-lower Miocene margin to slope succession is exposed, corresponding to the Sepino-Mt Moschiaturo unit (Vitale and Ciarcia, 2018). We performed a geological survey of the area that allowed us to draw a geological map (Fig. 1c). The rocks exposed in the study area are formed at the base by margin calcirudites frequently recrystallized (Calcari Cristallini Fm), passing upward to slope Scaglia-type deposits, including calcarenites with Nummulitids, reddish argillites and clays, and finally to deep basin reddish and greyish clays and argillites. In the study area, the scaglia- type deposits are unconformably covered by sandstones and clays of the Castelvetere Fm. Finally, terraced Quaternary conglomerates, arenites and clays cover the whole succession. The area is characterized by the presence of a degassing spring, defined by aligned with the main centre forming a degassing pool of ca 50 cm in diameter. Generally, sulfur mineralizations are widespread around the gas emissions. Other minor gas vents are scattered in the area, marked by the absence of vegetation and circular halos. The entire succession is crosscut by normal faults. N-S and ENE-WSW normal faults are dislocated by NW-SE faults defined by dominant normal kinematics. Geoelectrical survey. In the last decades, several stratigraphic and structural studies devoted to the understanding of the processes governing the fluids flow along active fault systems have proven the potentiality of the geophysical methods in reconstructing the architecture of geological contexts characterized by strong emissions of CO 2 (e.g. Byrdina et al. , 2009; Pettinelli et al. , 2010; Di Maio et al. , 2021). In particular, electrical resistivity tomography (ERT) is shown to be among the most appropriate methods for identifying the underground spatial distribution of CO 2 , whose migration towards the surface in non-volcanic areas is essentially controlled by fractures, faults and/or structural highs of the carbonate reservoir (e.g. Rogie et al. , 2000). In order to obtain a very high-resolution resistivity distribution of the investigated underground volume, a non-conventional 3D ERT data acquisition technique was used, which proved to be very effective in geological contexts similar to the one under study (e.g. Di Maio et al. , 2021). The survey was realized in an area of size 20 m x 235 m by using 144 electrodes (Fig. 1c). The apparent resistivity dataset was collected by the IRIS Syscal Pro Switch multi-channel georesistivimeter (IRIS Instruments) using the pole-dipole electrode array, which allowed a measurement sequence consisting of 1414 current injections and 13368 measurement quadrupoles, resulting in an investigation depth of about 45 m. The acquired data were inverted by the ERTLab64 software (Geostudi Astier Srl, Livorno, Italy, & Multi-Phase Technologies LLC, Nevada, USA), which is based on a smoothness- constrained least-squares approach that uses tetrahedral meshes to correctly account for surface topography (for details, the reader is referred to Morelli and LaBrecque, 1996 and references therein). The final 3D inversion model of the investigated volume was attained with a root mean square (RMS) error of less than 5%. As can be seen in Fig. 2, the resistivity values range from a few Wm to about 80 Wm, which, as inferred from the geological survey and the stratigraphic logs available in the neighbouring of the study area, are consistent with sedimentary formations consisting of calcirudites, calcarenites, argillites, clays and sandstones. In particular, the lowest resistivity values (blue zones in Fig. 2) are reasonably correlated to the occurrence of significant fluid circulation; whilst, the relatively high resistive zones (red areas in Fig. 2) are ascribable to the presence of a mixture of water and CO 2 , the latter being responsible for the increase in the resistivity values. The net contrast between the resistive and conductive zones, which is observed approximately in the central portion of the investigated volume (black dashed line in Fig. 2), could be associated with the presence of NW-SE striking fault systems that would favour the uprising of CO 2 gas flow.