## 6 Conclusions  
This study proposes a fully integrated and open-source approach to monitoring ground movement associated with an underground gas storage system (north-eastern Madrid Basin, Yela facility). The reservoir is a deep aquifer that was converted into an underground gas storage reservoir in 2012 and consists of a carbonate succession (approximately 30 km\({}^{2}\)). Reservoir depth varies between 2300 and 2550 m from ground level and its geology is represented by the \"Santa Barbara\" formation consisting of Cretaceous carbonates (mainly dolomite) affected by a diffuse fracture network overlain and sealed with evaporitic facies. The trap exploited during the UGS activity is an anticline with two tops: (i) the reservoir, which is composed of dolomite, and (ii) the seal, consisting of anhydrites, both closed by a system of thrusts. The analysis and interpretation of InSAR data from the Yela case study revealed that:  
1. Both ascending and descending orbit mean velocity data demonstrate the overall stability of the field area;
2. UGS activity does not influence the average horizontal and vertical displacement velocities in the field area, aligning with the velocity range across the entire monitored domain;
3. Time series analysis of displacement is useful for identifying details that may be hidden in average motion signals computed over a given period. The mean velocity approach has useful applications in regions characterised by ground deformation with linear trends, but a lot of information on seasonal variability is lost. With regular acquisition of data from the Sentinel-1 satellite constellation, it is possible to monitor deformation over time, understand cyclical variations, and study related phenomena;
4. A strong correlation is evident between the gas volume curve in the reservoir and the time series of vertical ground displacements above the facility. This correlation was confirmed through the analysis of seasonal temperatures and precipitation and the presence of areas with different surface deformation behaviours;
5. Horizontal displacement is less pronounced than vertical displacement. Despite this, the horizontal motion shows a slight sinusoidal signal in the facility area, showing a positive trend during withdrawal periods (east direction) and a negative trend during injection periods (west direction);
6. Only a small deformation of the dolomite reservoir (4-6 mm amplitude) was evident on the surface during the injection and withdrawal of natural gas. The volume calculation performed in CC estimated a negative volumetric change of 1.074 m\({}^{3}\) in area A1 (see Figure 9) of about 1.5 km\({}^{2}\) located in the vicinity of the reservoir between October 2019 (maximum peak) and April 2020 (minimum peak). The short-term cyclic subsidence/uplift associated with UGS is limited to the field area and is maximum above the 11 wellheads (see Figure 7). 7. Several factors influence these results: the considerable depth of the reservoir (the shallower the storage depth, the more pronounced the surface deformation effect), the geotechnical properties of the reservoir (porous aquifers have better elastic properties than fractured ones), the geology of the area, the injection rate (affects the formation pressure and stress), and the amount of gas injected and withdrawn;
8. The southeastern area of the Yela gas storage facility exhibits a different ground displacement behaviour. The latter area shows the opposite phase of vertical movements analysed for the area above the reservoir. It also has a larger amplitude. This result is in accordance with a possible normal manifestation of seasonal cyclicity, showing a positive trend during the winter and a negative trend during the summer. The strategic significance of InSAR in the domain of underground gas storage (UGS) monitoring cannot be understated. While individual case studies offer invaluable insights, adopting a global perspective is paramount for a holistic understanding.