Electromagnetic outline of the Solfatara-Pisciarelli hydrothermal system, Campi Flegrei (Southern Italy)

Abstract

We describe the results from a combined controlled source audio magnetotelluric (CSAMT) and natural source magnetotelluric (MT) survey carried out in the Solfatara–Pisciarelli (S–P) area, located in the central part of the Campi Flegrei (CF) composite caldera, west of Naples, Southern Italy. The S–P area represents the most active zone within the CF caldera, in terms of hydrothermal manifestations and local seismicity. Since 1969, the CF caldera is experiencing ground deformation, seismicity and geochemical fluid changes, which are particularly ev- ident in the S–P area. A 1 km long, nearly W–E directed CSAMT–MT profile crossing the fumarole field was carried out in the S–P area with the aim of deducting a resistivity model of the structural setting of the hydrothermal system in the first 3 km depth. An interpretation of the modelled section across the profile is given in this paper, taking advantage from already existing seismic, gravity and geochemical data in the same area. Three well distinct zones have been outlined. The first zone is a very shallow, electrically conductive body localized be- neath 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 in depth. Mostly accounting for the very low resistivity (1–10 Ω m) and the exceedingly high values of vP/vS (N4), this shallow zone has been ascribed to a water- saturated, high-pressurized geothermal reservoir. The second zone, which has been localized below the west- central portion of the CSAMT–MT transect, appears as a composite body made up of a nearly vertical plumelike structure that escapes at about 2.25 km depth from the top edge of the east side of a presumably horizontal plate- like body. The plumelike structure rises up to the free surface in correspondence of the fumarole field, whereas the platelike structure deepens at least down to the 3 km of maximum exploration depth. The combined interpretation of resistivity (50–100 Ω m), body wave velocity ratio (vP/vS b 2.0), mass density contrast (Δσ b 0 g/cm3), and geochemical data indicates that the plumelike portion can likely be associated with a steam/gas- saturated column and the platelike portion with a high temperature (N300 °C), over-pressurized, gas- saturated reservoir. Finally, the third zone, which has been localized beneath the eastern half of the transect, from about 1.2 km down to about 3 km of depth, is also characterized by very low resistivity values (1–10 Ω m). Jointly interpreted with seismic (vP/vS b 1.73) and gravity (⨂ N 0 g/cm3) data, this last electrically conduc- tive structure appears to be associated with a hydrothermally mineralized, clay-rich body.