GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.1 551 several environmental risks among which seismic risk. Recently, Obermann et al. (2015) have highlighted the potential of seismic noise techniques to monitor the St. Gallen geothermal site in Switzerland. Application of efficient methods to reconstruct the elastic properties of the propagation medium can improve our knowledge of the basic physics of triggering natural or induced earthquakes processes and therefore can enhance our ability to monitor and reduce seismic risks. The area under study is a shale gas extraction site in Wysin which belongs to the Stara Kiszewa concession of Polish Oil and Gas Company (PGNiG) in Pomerania region, northern Poland (López Comino et al. , 2018) (Fig. 1). Fig. 1 - Seismic network geometry and ray coverage used in this study. Wellhead (blu dot) and horizontal boreholes (blu lines) are shown. In this area exploration and exploitation of shale gas are carried out through hydro-fracturing (HF) operations. The information obtained from the cross-correlation analysis can be used in two main applications: imaging and monitoring. While the former allows us to construct group and phase velocity maps of the Rayleigh and Love surface waves at local, regional, and global scale, the latter allows to asses the temporal seismic velocity variations (Vassallo et al. , 2016). Method . It has been shown (Weaver and Lobkins, 2001) that – under the condition of energy equipartition – the response of the Earth after the action of an impulsive source, the so-called Green’s function, can be estimated by computing cross-correlation (c-c) function of continuous ambient noise data recorded at couples of synchronized stations. From a physical point of view the c-c represents the Green’s function, i.e. the seismic signal that would be recorded at one of the receivers if an impulsive source were acting at the second. From a mathematical point of view the c-c is a function equivalent to the convolution with no time reversal action, which provides a measure of similarity between two waveforms in time