Monitoring volcano-hydrothermal systems by continuous magnetotelluric data: a synthetic study on the Campi Flegrei area (Southern Italy)

Monitoring hydrothermal systems is of crucial interest for different applications, such as volcanic hazard assessment and geothermal energy exploitation. In this work, the feasibility of using continuous magnetotelluric (MT) measurements for monitoring hydrothermal systems developing within active volcanoes is investigated. The MT method is indeed a good candidate for characterizing the dynamics of a volcano-hydrothermal system as it is suitable for deep exploration of the subsoil (from one hundred meters to hundreds of kilometers) in terms of electrical resistivity values, which are strongly sensitive to variations in temperature and underground fluids distributions. To test the proposed methodology, a reliable model of the Campi Flegrei (Southern Italy) volcanic complex based on geochemical and geophysical data is built and a sensitivity study is carried out by changing the fluid flow rate of the hydrothermal source and the permeability distribution of the system. In particular, for each evolution scenario the MT sensitivity has been studied by evaluating the time intervals needed for observing significant resistivity variations. The simulations have shown that the MT monitoring is much more sensitive to changes in rock permeability rather than in the fluid flow rate emitted by the source. In general, time intervals not useful for volcano monitoring purposes (i.e., longer than 10 years) are found if only changes in fluid flow rate are assumed to govern the hydrothermal system dynamics. Conversely, by increasing the permeability of the hosting rocks up to about one order of magnitude, consistent resistivity variations are observed over a period ranging from one year and a half to three months. Such findings are promising and encourage the use of the continuous MT measurements in active volcano-hydrothermal areas.