Abstract
The magnetotelluric (MT) method is useful for monitoring geophysical processes because of a large dynamic depth range. In this paper, a feasibility study of employing continuous MT observations to monitor hydrothermal systems for both volcanic hazard assessment and geothermal energy exploitation is presented. Sensitivity of the MT method has been studied by simulating spatial and temporal evolution of temperature and gas saturation distributions in a hydrothermal system, and by calculating the MT response at different time steps. Two possible scenarios have been considered: the first is related to an increase in the fluid flow rate at the system source, the second is associated to an increase in the permeability of the rocks hosting the hydrothermal system. Numerical simulations have been performed for each scenario, and the sensitivity of the MT monitoring has been analyzed by evaluating the time interval needed to observe significant variations in the MT response. This study has been applied to the hydrothermal system of the Campi Flegrei (southern Italy) and it has shown that continuous MT measurements are not sensitive enough to detect a significant increase in the source fluid flow rate over time intervals less than ten years. On the contrary, if the permeability of the upwelling zone increases, a measurable change in the MT response occurs over a time interval ranging from six months to three years, depending on the extent of the permeability increase. Such findings are promising and suggest that continuous MT observations in active volcanic areas can be useful for imaging volcano-hydrothermal system activity.
Highlights
Monitoring a hydrothermal system involves the recognition of time variations in chemical, physical, and thermal parameters of the fluids circulating through pores and fractures of the host rocks
Starting from the model described in section “The Campi Flegrei Model,” two sets of simulations, which we assume as representative of possible evolutions of active hydrothermal systems, have been performed
The MT monitoring sensitivity has been assessed by assuming the presence of seven magnetotelluric stations, distant 500 m each other, along a profile crossing the center of the Campi Flegrei (CF) caldera, whose orientation was chosen based on the outer caldera major axis (Acocella, 2010) and the direction of the main faults in the area, whose strike is predominantly NW−SE
Summary
Monitoring a hydrothermal system involves the recognition of time variations in chemical, physical, and thermal parameters of the fluids circulating through pores and fractures of the host rocks. Ground deformation measurements are commonly used to obtain information on inflation or deflation of the hydrothermal system, possibly due to variations in its pressure and fluid content, and to infer the probability of explosion/eruption (e.g., Bartel et al, 2003; Puskas et al, 2007; Chiodini et al, 2010) These techniques are very effective in monitoring hydrothermal systems, they have some limitations. The underlying idea is that, since the Earth subsurface resistivity distribution is sensitive to presence, composition, and temperature of the fluids along permeable fracture systems (Hersir and Björnsson, 1991; Ussher et al, 2000; Hersir and Árnason, 2009), observing resistivity changes over time may give useful insights on possible variations in the physical properties of the system. It is worth noting that the observed resistivity variations can reflect stress-induced structural changes
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