André Burnol, Hideo Aochi, Daniel Raucoules, Fernanda M.L. Veloso, Fifamè Koudogbo, Alfio Fumagalli, Pierre Chiquet, Christophe Maisons

Acquisitions of the Sentinel-1 satellite are processed and comprehensively analyzed to investigate the ground displacement during a 3-year period above a gas storage site in Southwestern France. Despite quite low vertical displacements (between 4 and 8 mm) compared to the noise level, the local displacements reflects the variations due to charge and discharge during summer and winter periods, respectively. A simplified mechanical model can explain these displacements at both storage sites (Lussagnet and Izaute). However, these low-magnitude ground movements may be also controlled by natural factors, like the temperature or the surface soil moisture (SSM). Using an additive decomposition, we show first that the temperature is only a second order triggering factor compared to the reservoir pressure. Using a wavelet-based analysis, we show there is an uplift in the Lussagnet zone that contrasts both in phase and period with the seasonal deformation due to the gas exploitation and that is linked to the SSM measured by the SMOS satellite. This other displacement is consistent with the water infiltration in the unsaturated zone followed by the swelling of a clay layer. This work provides therefore new insights on the ground deformation using a three-year integrated monitoring of a gas storage site.