**1. Introduction**

Among the various natural risks in France, the risk due to shrinking and swelling of subsurface clays is the second most important cause of financial compensation from insurance companies behind the flooding risk. In 2010, a first shrink/swell hazard map of metropolitan France, based on 1:50,000 geological maps, geotechnical data and spatial distribution of building damages has been published by the French Geological Survey (BRGM). So far, in situ monitoring of soil moisture and ground movements and ex situ clay characterization are traditionally used to assess this risk (e.g., the Mormoiron site with a Mediterranean climate [1] or at the Pessac site with an oceanic climate [2]). Since 2016, a new site characterized by a high shrink/swell hazard, level and located at Chaingy (France) has been instrumented by BRGM.

Standard ground displacement monitoring techniques (e.g., extensometers and GNSS) provide information on a very limited number of points within an area. Allowing a higher density of measurement points, the monitoring using Multi-Temporal Synthetic Aperture Radar Interferometry (MT-InSAR) techniques has been intensively developed in the last decades to track land subsidence or uplift related to groundwater extraction or recharge of aquifers around large cities [3–5]. Conversely, the monitoring of expansive clays based on InSAR has been the subject of very few studies until now [6–8]. The major limiting factor is the non-availability of relatively high-temporal resolution remote sensing datasets. There is indeed the requirement for fine temporal sampling due to the non-linear behavior of the shrink/swell cycles [9]. The launch by the European Space Agency (ESA) of the

**Citation:** Burnol, A.; Foumelis, M.; Gourdier, S.; Deparis, J.; Raucoules, D. Monitoring of Expansive Clays over Drought-Rewetting Cycles Using Satellite Remote Sensing. *Atmosphere* **2021**, *12*, 1262. https://doi.org/ 10.3390/atmos12101262

Academic Editors: Andrzej Walega and Agnieszka Ziernicka-Wojtaszek

Received: 21 July 2021 Accepted: 24 September 2021 Published: 28 September 2021

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Copernicus Sentinel-1A/1B satellites enables the systematic data provision, with a 6-day repeat cycle at the equator.

The aim of this article is to present an investigation of the shrink/swell behavior of a clay soil in relation to drought-rewetting cycles using both in situ and satellite remote sensing monitoring techniques over an instrumented site. The ground displacement measured by InSAR is compared to the in situ measurements in the studied zone during a three-year period. We will analyze how accurate Sentinel-1 data can measure the ground displacement due to the shrink/swell process, when the Parallel Small Baseline Subset (P-SBAS) technique is applied, and how well P-SBAS results and precise extensometers agree in validation for our study area. Our hypothesis is that the vertical displacement captured by the P-SBAS technique using a 90 m by 90 m cell is an average vertical displacement in that cell. Moreover, we show the time lag between the in-ground soil moistures at 1.2 m depth and the surface soil moisture acquired by the SMOS satellite. Finally, we propose for a first time a methodology for evaluating the depth and the thickness of subsurface clay layers relying fully on remote sensing observations, namely the use of the relative phase difference between Sentinel-1 InSAR displacement and SMOS surface soil moisture time series. In order to further validate this new approach, we deployed an electric tomography survey providing insights on the subsurface structure.

#### **2. Materials and Methods**
