Design and realization of a seismic monitoring system for the geothermal production site of Torre Alfina (Italy)

Abstract

The geothermal field of Torre Alfina is located in Central Italy, at the boundary between the regions Tuscany and Latium, at the northern extremity of the Vulsini quaternary volcanic complex. Geochemical and geophysical wells drilled in the 1970s and 1980s down to depths ranging from 563 m to 2710 m revealed that Torre Alfina is a medium- enthalpy (T = 140$\,^{\circ}$ C) geothermal field, hosted in buried fractured Mesozoic limestones. Recently a multi-national industrial company received the license for the production of geothermal energy up to a maximum of 5 MW. In 2013 the INGV was commissioned to realize a monitoring system that includes the observation of gas emissions, microseismicity and ground deformation. National guidelines published recently by the Italian Ministry of Economic Development (MISE) regulate in detail the governmental monitoring ordinances exclusively regarding the extraction of hydrocarbons and the fluid injection due to wastewater and CO2 storage but concerning geothermal energy production appropriate regulations are missing. Following the recommendations, described in the Ministerial Decree that regulates the geothermal production activity, the seismic monitoring system should be capable to record (i) the local microseismicity during the phase of geothermal energy production and (ii) the natural seismicity already 12 months before the beginning of the production operations. In 2014 we started to install a seismic network consisting of 5 short period stations within a radius of 5 km (Inner Domain) and thereafter further 5 stations inside an area with a 10 km radius (Outer Domain). The seismic noise level recorded at the single stations is unfavorably high, due to an intensive colonization and numerous settlements of small and medium industries. Transients as well as persistent monochromatic disturbances are recorded very well especially at the southern stations, probably due to a low wave energy dissipation inside the layer of quaternary volcanic rocks.