Fluid conduits and shallow-reservoir structure defined by geoelectrical tomography at the Nirano Salse (Italy)

Abstract

Abstract. Mud volcanoes are fluid escape structures allowing for surface venting of hydrocarbons (mostly gas but also liquid condensates and oils) and water–sediment slurries. For a better understanding of mud volcano dynamics, the characterization of the fluid dynamics within mud volcano conduits; the presence, extent, and depth of the fluid reservoirs; and the connection among aquifers, conduits, and mud reservoirs play a key role. To this aim, we performed a geoelectrical survey in the Nirano Salse Regional Nature Reserve, located at the edge of the northern Apennines (Fiorano Modenese, Italy), an area characterized by several active mud fluid vents. This study, for the first time, images the resistivity structure of the subsoil along two perpendicular cross sections down to a depth of 250 m. The electrical models show a clear difference between the northern and southern sectors of the area, where the latter hosts the main discontinuities. Shallow reservoirs, where fluid muds accumulate, are spatially associated with the main fault/fracture controlling the migration routes associated with surface venting and converge at depth towards a common clayey horizon. There is no evidence of a shallow mud caldera below the Nirano area. These findings represent a step forward in the comprehension of the Nirano Salse plumbing system and in pinpointing local site hazards, which promotes safer tourist access to the area along restricted routes.

Gravity Data Allow to Image the Shallow‐Medium Subsurface Below Mud Volcanoes

“…These gryphons form above shallow (down to a few tens meters) mud/gas accumulations just below the vents whose plumbing system is connected to the low-density zone along the fault. Such a scenario was also suggested by hydrogeological and georesistivity surveys (Giambastiani et al, 2022;Romano et al, 2023). This information could be used to better plan access routes to the reserve and to fence off potential areas prone to sinking and subsidence.…”

“…The implications for unraveling the subsurface structure are important to evaluate potential hazards at different spatial scales connected to the fluid emissions. Short scale resistivity surveys were carried out in the Nirano area (e.g., Accaino et al., 2007; Lupi et al., 2015; Romano et al., 2023) also integrated with geochemical methods (e.g., Martinelli et al., 2012; Oppo et al., 2017; Sciarra et al., 2019). To better understand the conceptual model of fluids, rise and storage as well as the subsurface structural setting of Nirano a new data set of gravimetric data is here presented.…”

Structural Control on Carbon Emissions at the Nirano Mud Volcanoes – Italy

“…7). The NW-SE structural is associated with normal faulting as imaged in the geoelectric tomography (Romano et al, 2023), the trend of the geomorphological benches south of the Nirano Salse crater (Fig. 1), and the gravimetry (Fig.…”

“…Structural data from our geologic survey, surface gas emissions, and available geologic maps (Gasperi et al, 2005) as well as recently published dipole-dipole electric tomography (see Figs. 8, 9, and 10 in Romano et al, 2023) and gravimetry data by Nespoli et al (2023) are presented in the map of Fig. 7 and in Fig.…”

“…In this paper, we investigate the relationships between gas emissions and local structures in the Nirano Salse (Italy), which is a well-studied site where geochemical, geophysical, and structural data are available (Bonini, 2007(Bonini, , 2008(Bonini, , 2009Giambastiani et al, 2022;Heller et al, 2011Heller et al, , 2012Lupi et al, 2016;Martinelli and Rabbi, 1998;Nespoli et al, 2023;Romano et al, 2023;Sciarra et al, 2019). We examine both natural gas macro-, mini-and micro-seepages for relating their venue to structural elements such as faults and fractures.…”