GNGTS 2015 - Atti del 34° Convegno Nazionale

18 GNGTS 2015 S essione 1.1 “San Pietro in Casale” gas field, in which 24 wells have been drilled. The historic production of the field, ended in 1995, amounts to 500 MSm 3 from 14 wells. In 2011, PVE started a new exploration phase by acquiring a 31-km line of 2D seismic survey. The results from these new seismic lines, confirmed the availability of a re-usable reservoir of about 50 MSm 3 , deliverable in 12-15 years, from the exploration well named “Santa Maddalena 1 dir (SM1d)”. The development plan for the exploitation of the “Sant’Alberto” concession, currently under review by the Italian “Ministero dell’Ambiente e della Tutela del Territorio e del Mare (MATTM)” (http://www.va.minambiente.it ), provides for the evaluation of various aspects related to the impact of this facility on the territory. As an example, detailed investigation concerning the possible presence of active faults near the reservoir and the possible presence of archaeological, historical or cultural sites located near the production facilities, have been required. In particular, in order to verify the possible relationships between the exploitation activities planned and induced seismicity, the INGV was commissioned to perform a feasibility study of a network for the monitoring of the local seismicity. �� ��� ���� �������� ���� ��� It has been required that the seismic network meets the monitoring criteria specified in the recent MiSE report: “Indirizzi e linee guida per il monitoraggio della sismicità, delle deformazioni del suolo e delle pressioni di poro nell’ambito delle attività antropiche” (MiSE-DGRME, 2014). �� ���� ������ �� ������� ��� In this paper, we present the preliminary results obtained by the INGV team concerning the investigation about the presence of possible active faults nearby the reservoir and the planning of a microseismic network that allows to reach the detection thresholds specified in MiSE-DGRME (2014). Monitoring areas and network geometry. Two crustal volumes of earthquakes detection are specified in MiSE-DGRME (2014): the inner domain of detection (DI) and the extended domain of detection (DE). DI is the crustal volume within which we expect to detect some type of induced seismicity so, within this volume, the planned monitoring network must reach the highest detection capability. According to MiSE-DGRME (2014), the surface projection of DI must be defined by extending horizontally the area corresponding to the surface projection of the reservoir. For oil and gas extraction activities without re-injection, a 3 km extension is required, and the same distance must be adopted in order to define the bottom of DI, by moving vertically, starting from the depth of the reservoir. The definition of DE is required in order to contextualize the monitoring performed by the microseismic network. In this volume, the planned network should improve the magnitude of completeness of the regional networks operating in the area of about 1 degree. For the “Sant’Alberto” reservoir, DE can be defined by extending DI both horizontally and vertically, for a distance of 5 km. We thus defined the surface projections of DI and DE as squared areas centered to the position of the SM1d well, and with areas of 8.6 x 8.6 km 2 and 18.6 x18.6 km 2 , respectively (Fig.1, right panel). Moreover, being the reservoir located between 850 and 900 m depth, we defined the bottoms of DI and DE at 4.0 and 9.0 km depth, respectively. At the present, in an area of 100x100 km 2 centered to the “Sant’Alberto” concession, the Italian National Seismic Network (RSN) features 12 stations. The minimum inter-station distance is about 21 km (Fig.1, left panel). In this region, the RSN shows a magnitude of completeness of about 2.0, and as an example, a M L 1.5 event has a 50% probability of detection (Shorlemmer et al. , 2010). In order to meet the requirements specified in MiSE-DGRME (2014), it is thus necessary to install in the area a microseismic network characterized by a higher detection capability. In this paper, we examine the possibility to detect and locate seismic events occurring in DI, with local magnitude of at least 0.5 unit, and seismic events occurring in DE, with local magnitude of at least 1.0 unit. Ahypothetical network consisting of five stations, with an average inter-station distance of 4-5 km, has been considered (Fig.1, right side). The detection threshold is established by comparing simulated power spectra of earthquakes characterized by different values of magnitude and distance, with the power spectrum of the observed ambient noise. In order to minimize the influence of anthropic noise and to improve the network detection, we also consider the possibility to install at least one borehole sensor at 200 m depth.