Combined use of Sentinel-1 and SMOS data with the BSS subsurface database to better estimate the shrink-swell hazard in Southwestern France

Abstract : Acquisitions of the Sentinel-1 satellite are processed and comprehensively analyzed to investigate the ground displacement during a three-year period above two gas storage sites (Lussagnet and Izaute) in Southwestern France (Figure 1). Despite quite low vertical displacements (between 4 and 8 mm) compared to the noise level, the local displacements reflect 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. Nonetheless, despite a general consistency, some features in the Lussagnet case that cannot be explained by the reservoir pressure variations are further investigated. The 10-day SMOS surface soil moisture (SSM) products for descending overpasses between 18 October 2014 and 26 October 2017 in the grid cell of the studied zone (Figure 1) are used to calculate the average of the median, minimum and maximum SSM values for each Sentinel-1 period (12 days). 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 related both to the rainfall and to the surface soil moisture. This transient surface deformation has a similar amplitude than the cyclic deformation due to gas exploitation (about 5 mm). This last result is consistent with the swelling of the clay layer described in the geological logs extracted from the BRGM subsurface database (BSS). This work provides therefore new insights on the ground deformation using a three-year integrated monitoring of a gas storage site. As a perspective of this work in the framework of future European Ground Motion Services (EU-GMS), this method coupling both satellite acquisitions (Sentinel-1 and SMOS) and the BSS database may be generalized to improve the global French shrink/swell hazard map based on the 1:50 000 geological map.