II cycle compressibility from satellite measurements

Knowledge of ground compressibility in unloading/reloading conditions is of paramount importance in several geomechanical processes, but its estimate is often affected by large uncertainties, especially for deep rocks. Satellite measurements of the land motion can help improve this information. The present study investigates the ground response due to underground gas storage activities as revealed by satellite interferometric measurements in three gas fields of the Po river basin, Italy. The ground proves to behave elastically, moving upwards and downwards in response to gas injection and extraction, respectively. The accurate calibration of a detailed geomechanical, transversely isotropic, finite-element model allows for an indirect estimate of the vertical ground compressibility in unloading/reloading conditions, which proves consistent with the results already available from the same basin. Assuming a hypo-plastic constitutive model for the compressibility in virgin loading conditions, the ratio between I cycle and II cycle compressibility at the load inversion turns out to be between 3.5 and 4.0 in the depth range 1100-1500 m, again proving to be in good agreement with other results obtained in the same basin.