Monitoring surface deformation of deep salt mining in Vauvert (France), combining InSAR and levelling data for multi-sources inversion

Abstract

<jats:p>Abstract. The salt mining industrial exploitation located in Vauvert (France) has been injecting water at high pressure in wells to dissolve salt layers at depth. The extracted brine is used in chemical industry for more than thirty years, inducing a subsidence of the surface. Yearly levelling surveys monitor the deformation since the 1996. This dataset is supplemented by synthetic aperture radar images, and since 2015, GNSS data are also continuously measuring the deformation. New wells are regularly drilled to carry on with the exploitation of the salt layer, maintaining the subsidence. We make use of this careful monitoring by inverting the geodetic data to constrain a model of deformation. As InSAR and levelling are characterized by different strengths (spatial and temporal coverage for InSAR, accuracy for levelling) and weaknesses (various biases for InSAR, notably atmospheric, very limited spatial and temporal coverage for levelling), we choose to combine SAR images with levelling data, to produce a 3-D velocity field of the deformation. To do so, we develop a two-step methodology which consists first by estimating the 3-D velocity from images in ascending and descending acquisition of Sentinel 1 between 2015 and 2017, and second by applying a weighted regression kriging to improve the vertical component of the velocity in the areas where levelling data are available. GNSS data are used to control the resulting velocity field. We design an analytical model based on the geological and geophysical data. The model is made of 21 planes of dislocation with fixed position and geometry. We invert the combined geodetic dataset to estimate the slip and tensile motions of each plane. The results of the inversion highlight two behaviours of the salt layer: a major collapse of the salt layer beneath the extracting wells and a salt flow from the deepest and most external zones towards the centre of the exploitation. </jats:p>

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“…Both source models can be associated with the withdrawal of fluids and/or removal of sub-ground solid materials 29 , 30 . In the case of salt mining, the volume loss might be attributed to the extraction of salt 31 . The search for the best modelling parameters was performed in a non-linear inversion scheme (see “ Data and methods ”) by repeating hundreds of simulations, until the misfit between the data and model is minimized.…”