GNGTS 2015 - Atti del 34° Convegno Nazionale

24 GNGTS 2015 S essione 1.1 it would be necessary the installation of at least three stations, properly distributed around the surface projection of the reservoir, at the same distance of FIU. Conclusions. The goal of the paper is to summarize the analysis carried out in the area of the “Sant’Alberto” gas reservoir (Po Plain, Italy), concerning the presence of possible active faults nearby the reservoir, and the feasibility of a microseismic network that satisfies the features required in the MiSE document: “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). Since the fifties of the last century, the great amount of data collected for hydrocarbons exploration purposes allowed a refined interpretation of the underground structures involved in area of the reservoir. These are mostly represented by blind ramp anticlines and anticlinal stacks belonging to the northern Apennines arcs, currently buried below the Plio–Quaternary sequence. Furthermore, the 2012 Emilia seismic sequence has allowed to outline with great detail the geometry of northern Apennine fold-and-thrust outer belt. In particular, recent works, concerning the relocation of the sequence, highlight that the whole structure of the “Sant’ Alberto” reservoir, located on the back-limb of a ramp anticline, does not seem to be affected by the events of the 2012 Emilia sequence. As regard to the planning of a microsesmic network in accordance with the criteria specified in MiSE-DGRME (2014), the present analysis shows that: 1. The area to be monitored is characterized by a strong anthropization, that produce high levels of ambient noise. The observed PSD of seismic noise are comparable with the NHNM curves of Peterson (1993). 2. According to the guidelines specified in MiSE-DGRME (2014), the internal domain of detection (DI) and the extended domain of detection (DE) correspond to crustal volumes of 8.6 x 8.6 x 4.0 km 3 and 18.6 x 18.6 x 9.0 km 3 , respectively. 3. Due to the elevated levels of the observed ambient seismic noise, in order to ensure adequate detection thresholds in the whole DI, the network should include at least one borehole (200 m depth) station. 4. The installation of a borehole station (200 m depth), localized near the SM1d well, and an accurate selection of sites characterized by favorable levels of ambient noise, considerably improve the detection. Hypothesizing favorable levels of the surface noise and a decreasing rate of 0.1 dB/m of the mean noise level recorded at depth, a minimal network composed of 5 stations installed within DI, integrated with a neighbor station of the RSN, allows to achieve M L detection thresholds of about 0.2 unit at the bottom of DI. In particular, in this case, it is possible to obtain a detection threshold M L -0.3 near the reservoir. 5. The above mentioned minimal network, is capable to localize M L 0.9 events occurring up to 4 Km depth, thus fulfilling the requirements specified in MiSE-DGRME (2014) for what concerns the internal domain of detection. 6. The above mentioned minimal network, shows a minimum localization magnitude M L 1.1 at the bottom of DE (9 km depth). The requested decreasing of about one unit of the completeness magnitude obtained with the regional networks operating in this area, can be allowed by installing at least three additional stations within the surface projection of DE. References Anderson J. G. and Hough S. E.; 1984: A model for the shape of the Fourier amplitude spectrum of acceleration at high frequencies , Bull. Seism. Soc. Am. 74, 1969-1993. Bertello F., Fantoni R., Franciosi R., Gatti V., Ghielmi M. and Pugliese A.; 2010: From thrust and fold belt to foreland: hydrocarbon occurrences in Italy, in B.A. Vining and S.C. Pickering (Eds.). Petroleum Geology: From Mature Basin to New Frontiers, Proceedings of the 7th Petroleum Geology Conference, Geol. Soc., p. 113-126. Boccaletti M., Bonini M., Corti G., Gasperini P., Martelli L., Piccardi L., Tanini C. and Vannucci G.; 2004: Carta simotettonica della regione Emilia Romagna, 1:250.000 e note illustrative, SELCA Editore, Firenze.