GNGTS 2022 - Atti del 40° Convegno Nazionale

430 GNGTS 2022 Sessione 3.2 ACTIVE AND PASSIVE SEISMIC AND ELECTRICAL METHODS FOR IMAGING GAS UPWELLING FLOWS AT THE FIUMICINO COASTAL AREA (CENTRAL ITALY) G. Penta de Peppo 1 , G. De Donno 1 , A. Bosman 2 , E. Cardarelli 1 , M. Cercato 1 1 “Sapienza” University of Rome - DICEA, Italy 2 Istituto di Geologia Ambientale e Geoingegneria (IGAG), Consiglio Nazionale delle Ricerche s.c. DICEA, Italy Introduction. The accurate location of gas upwelling flows is still an open problem for non- invasive imaging techniques. Gas blowouts may represent a serious threat to human health in urban areas or when planning future urbanization. Natural gases found in the shallow subsurface can have different causes, even if the most abundant are undoubtedly carbon dioxide (CO 2 ) and methane (CH 4 ). These gases are generally of deep origin and rise toward the surface through faults, cracks and voids but also due to anthropogenic causes, such as borehole drilling. For these reasons, previous works have been focused on locating gas in the shallow subsurface (e.g. Carcione et al. , 2011), even if a codified procedure for the identification of these gases through non-invasive investigations has not been found. In such geological scenarios, where borehole drilling is not recommended, non-destructive geophysical surveys are the only cost-effective choice for imaging the gas pathways. Geoelectrical methods can be diagnostic for gas detection, as they can highlight the resistivity contrast between a water- saturated (conductive) and a gas-saturated (more resistive) medium. Seismic reflection methods can complement and validate the electrical models, by inferring the subsoil layering down to significant depths as well to locate the gas upwelling flows often visible on the seismic sections as blank zones of low-amplitude signal levels (e.g. Riedel et al. , 2002). This work aims to combine high-resolution seismic and electrical methods to retrieve an accurate image of the gas upwelling flows in the coastal area of Fiumicino, 25 km western of Rome. To these aims, we performed active (multibeam bathymetry, high-resolution seismic sub-bottom profiler, electrical resistivity tomography) and passive (ambient noise recordings and self-potential) geophysical methods, to reduce the ambiguities often arising when geophysical techniques are applied standalone. Study area, data acquisition and processing. The study area is located on the Tiber delta, made by Pleistocene-Holocene sediments. The near-surface layering consists of three main geological units separated by unconformity surfaces (Milli et al. , 2013), from the bottom to the top: i) Lower Pleistocene clay and silty clay, ii) Middle Pleistocene gravels and sandy gravels (PGS), iii) Upper Pleistocene to Holocene clay and peaty clay. The top of the PGS formation is the most permeable layer hosting a ground aquifer where gas rising from depth may accumulate. Conversely, the uppermost clays act as an impervious cap rock for PGS gravels allowing gas pressurization (Carapezza et al. , 2015). Previous investigations included boreholes (see Sella et al. , 2013) and soil gas surveys (Bigi et al. , 2014). PGS unit was found in these boreholes between 42 and 52 m b.s.l., while the maximum gas concentrations CO 2 and CH 4 were found at the “Capo due Rami” site, on the right bank of the river (Fig. 1). We carried out geoelectrical investigations along four different profiles (L1-L4), almost parallel to the Tyrrhenian coast. At the “Capo due Rami” site, we used also passive geophysical methods (self-potential and ambient noise recordings) to improve the diagnostic capability of the non-invasive surveys. A high-resolution (HR) seismic reflection (sub-bottom) profile was acquired along the Tiber river at the “Capo due Rami” site (Fiumara Grande branch), together with a multibeam investigation of the river bed. The multibeam surveys were performed using the Teledyne Reson SeaBat 7125 echo- sounder (400 kHz) using the multibeam transducers in standardmode (look down) and rotating