Seismic velocity variations around an Underground Gas Storage (northern Italy) from ambient noise correlation measurements

We performed measurements of velocity variations using empirical Green's functions reconstructed by autocorrelation of seismic noise recorded during a period of 7 years in the proximity of an Underground Gas Storage, in northern Italy. Ambient seismic noise is a useful tool for investigating the shallowest layers of the earth’s crust. The coda of the correlation wavefield can be used to monitor variations in seismic velocity for detecting changes in the medium properties, with applications in different environments. We test the methodology for the estimation of seismic velocity variations associated with underground gas storage (UGS) activities. The injection and the withdrawal of gas in depleted hydrocarbon reservoirs are useful to meet fluctuations in gas demand and create a strategic reserve. We perform measurements of velocity variations using coda wave interferometry on 7 years of continuous data from a local seismic network deployed to monitor microseismicity around a UGS facility in northern Italy. We compute daily cross-correlation and auto-correlation functions of ambient noise recorded at 9 stations. We used different techniques to calculate daily velocity variations (dv/v), in different frequency ranges, and different time-lapses in the coda to estimate the depth dependence of the velocity variations. The results indicate the presence of measurable velocity changes. We discuss the possible effects of gas cyclical pressure variation, due to this storage activity as well as seasonal environmental influence.