GNGTS 2013 - Atti del 32° Convegno Nazionale

Continuous monitoring of such phenomena is on-going, by permanent networks for seismic, ground deformation and geochemical measurements. Geophysical surveys have so far allowed a quite good knowledge of the subsurface structure of the CFc volcanic system. Electromagnetic evidence. A 1 km long, nearly W-E directed CSAMT-MT profile cross- ing the fumaroles field was realised (Troiano et al. , submitted ), carried out with the aim of deducting an EM model of the structural setting of the hydrothermal system in the first 3 km depth of the Solfatara-Pisciarelli area. The results allow us to identify three EM zones (Fig. 1). The first EM zone (A) is characterized by a very shallow, electrically conductive body localized beneath the westernmost segment of the profile, which, within a short distance of about 100 m, dips westwards from near surface down to some hundred metres depth. This shallow zone has been ascribed to a water-saturated, high-pressurized geothermal reservoir. The second EM zone (B), which has been localized below the west-central portion of the EM transect, appears as a composite body made of a nearly vertical plume-like structure arising from about 2.25 km depth to the top edge of the east side of a presumably horizontal plate-like body. Such plume-like structure, centered in correspondence of the Solfatara fumaroles field, rises up to the free surface whereas the plate-like structure deepens at least down to the 3 km of maximum EM exploration depth. The plume-like portion is likely associated with a steam/ gas-saturated column and the plate-like portion to a high temperature (>300°C), over-pressur- ized, gas-saturated reservoir. Finally, a third EM zone (C), which has been localized beneath the eastern half of the EM transect, corresponding to the Pisciarelli area, is also characterized by the lowest resistivity values (1-10 Ωm) from about 1.2 km down to about 3 km of depth. As it is known, in a volcano-geothermal coastal environment a highly conductive body can indicate either a hydrothermally mineralized, clay-rich layer (e.g. Keller and Frischknecht, 1966; Ward, 1990; Parasnis, 1997), or a cold seawater-bearing layer (e.g. Goldman et al. , 1991; Frohlich et al. , 1994; Di Maio et al. , 1997; Mauriello and Patella, 1999), or a highly hydrothermalized wa- ter-bearing rock (e.g. Patella et al. , 1979; Detwiler and Roberts, 2003). In order to decide which of this hypothesis is the most reliable, we consider that in all of the deep wells drilled by AGIP at the west border (Mofete area) and north border (San Vito area) of the caldera, the effects of a strong hydrothermal paragenesis have been detected. Abundance of semiconducting minerals ( e . g . pyrrhotite, pyrite, magnetite) and presence of thick argillitic layers, are, in fact, documented at temperatures ranging between 250°C and 350°C, in the depth range between 1 and 3 km, which was the maximum depth reached by the wells (Chelini and Sbrana, 1987; Mormone et al ., 2011). Therefore we are tentatively allowed to associate the very low resistivity zone (C), under the Pisciarelli area, with a hydrothermally mineralized, clay-rich body. Alternatively, we cannot exclude the presence of a deep hydrothermal aquifer, although we know from previous drillings that critical temperature is reached in the whole Fig. 2 – On the left CO2/CH4 ratio from 16/05/2012 to 05/06/2012 measured by Quadrupole Mass Spectometer. On the right ground deformation (from Osservatorio Vesuviamo website). 290 GNGTS 2013 S essione 1.3