Improvement of microseismic monitoring at the gas storage concession “Minerbio Stoccaggio” (Bologna, Northern Italy)

Abstract

The storage concession “Minerbio Stoccaggio” (Bologna, Northern Italy) covers a 69 km2 area, 65% of which is located in the Minerbio municipality. Since 1979, a microseismic network for the monitoring of seismicity, eventually induced by gas storage activities, has been installed in this area. The network was operated by Stogit S.p.A, a subsidiary company of Snam, which is the largest storage operator in Italy. In 2016, the microseismic network, consisting of three surface stations and one 100-m-deep borehole sensor with minimum inter-station distances of about 3.0 km, was integrated with 12 regional stations installed in an 80 × 80 km2 area centered on the surface projection of the reservoir. In 2018, the microseismic network was enhanced by adding one surface and three 150-m-deep borehole stations. In this work, we evaluate the detection improvement of the microseismic network, integrated with the regional stations. We define two crustal volumes for earthquake detection: the inner domain of detection, IDD (10 × 10 × 5) km3, within which we should ensure the highest network performance, and the extended domain of detection, EDD (22 × 22 × 11) km3. By comparing the simulated power spectral density of hypothetical seismic sources located in EDD with the average power spectra of ambient seismic noise observed at each station site, we calculate detection and localization thresholds for the two above-mentioned networks. Under unfavourable noise conditions, we find that the present operative seismic network allows locating earthquakes with ML ≥ 0.8 occurring at the depth of the reservoir and with ML ≥ 1.0 if located within IDD.