GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.1 397 CHARACTERIZATION OF SEISMIC NOISE AMPLITUDE RECORDED BY A VERTICAL ARRAY INSTALLED IN A DEEP BOREHOLE AT THE S. POTITO-COTIGNOLA GAS STORAGE FIELD (NORTHERN ITALY) C. Rossi 1,2,3 , F. Grigoli 4 , P. Gasperini 2 , S. Gandolfi 3 , T. Gukov 5 , C. Cocorullo 1,6 , P. Macini 3 1 Seismix srl, Palermo, Italy 2 Dipartimento di Fisica e Astronomia, Alma Mater Studiorum Università di Bologna, Italy 3 Dipartimento di Ingegneria Civile, Chimica, Ambientale e dei Materiali, Alma Mater Studiorum Università di Bologna, Italy 4 Dipartimento di Scienze della Terra, Università di Pisa, Italy 5 Edison Stoccaggio S.p.A., Italy 6 Solgeo srl, Bergamo, Italy With the growing number of industry-related seismicity events and their effects, the monitoring andanalysis of suchevents havebecome increasingly important. Seismicmonitoring network in areas interested by underground fluids production activities has a fundamental role to characterize seismicity, to control the industrial operation and to assess the seismic hazard. Furthermore, a proper seismic network allows to detect even low magnitude events occurring in the area surrounding the industrial site. The capability to detect weak events can provide useful information about the reservoir condition, as well as the distribution and geometry of active faults and geo(mechanic) discontinuities even close to the industrial site. After the natural M6 earthquake occurred in Emilia Romagna region (2012), the Italian Ministry for Economic Development (MiSE) published a reference document “Guidelines for monitoring seismicity, ground deformation and pore pressure in subsurface industrial activities” (Dialuce et al. , 2014). According to such document, the monitored area is divided into an inner survey domain, which consists of a volume around the reservoir, properly sized depending on the industrial activity type, and an extended survey domain surrounding the inner part. For the inner survey domain, seismic monitoring network should be able to locate and size seismic events starting fromML (local magnitude) ranging between 0 and 1 (0≤ML≤1). However, such low magnitude detection capability is affected by the level of anthropogenic noise at the site as well as by the sedimentary cover rocks type and thickness. As suggested in the above guidelines, the reduction of high level of noise and thus the improvement of sensitivity and capability of the seismic network, can be achieved by installing seismic stations at depth in boreholes. Furthermore, the analysis of amplitude noise reduction as function of depth can be useful to design a seismic network able to reduce the signal to noise ratio (SNR) and thus to detect and locate such low-magnitude events. Here we analyse continuous seismic noise from January 2019 to December 2021 recorded at underground gas storage site of S. Potito-Cotignola in the Po Valley (Italy). We use a broadband (BB) seismic station at surface and a vertical array composed by 6-short period 3-components seismometers installed at depth ranging between 35 to 285 m in borehole at the same location of the BB sensor, installed at industrial site. We aim to characterize the seismic noise by computing power spectral density (PSD) and quantify the amplitude noise reduction in terms of dB as a function of depth for different frequencies. Furthermore, for each sensor-depth, we even compute the SNR selecting 3 seismic events with different magnitude and epicentral distance recorded. We found that 1) the maximum difference between the deepest instrument and the one at the surface is about -33 dB at 5Hz, while for the most extreme frequencies of the investigated range the noise reduction ranging from -10 to -15; 2) the dependence of noise levels (in dB) with depth follows a logarithmic empirical law; 3) the lower magnitude event shows the maximum difference of SNR between the deepest sensor and the one at surface of about 20 dB, in agreement with the noise amplitude level reduction range. References Dialuce G., Chiarabba C., Di Bucci D., Doglioni C., Gasparini P., and Lanari R., Priolo E., Zollo A.; 2014: Guidelines for monitoring seismicity, ground deformation and pore pressure in subsurface industrial activities , Italian Ministry for the economic development (MiSE), 34 pp.