Geophysical characterization of the Nirano Salse (Fiorano Modenese, Italy): A discussion on the possible explanation for the observed extremely low resistive values.

Mud volcanoes are fluid escape structures allowing for surface venting of hydrocarbons and water/sediment slurries. For a better understanding of mud volcanoes dynamics, the characterization of the fluid dynamics within mud volcanoes conduits, the presence, extent, and depth of the fluid reservoirs as well as the connection among aquifers, conduits, and mud reservoirs play a key role. We present the results of a geoelectrical survey performed in the Regional Nature Reserve of the Nirano Salse, aimed to image the resistivity structure of the subsoil down to a depth of 250 m. The electrical models show the presence of shallow reservoirs, that are spatially coincident with the main fault/fracture controlling the migration routes associated with surface venting and converge at depth towards a common clayey horizon. In the geoelectrical sections, conductive zones can be associated to fluid accumulation areas. Fluid presence, however, is not sufficient to justify the extremely low resistivity values observed. We suggest that these values may be associated with i) clay content and surface conductivity phase and/or ii) presence of iron sulphides due to precipitation of hydrogen sulphide.