## 5 Discussion  
Underground gas storage serves as a vital mechanism for establishing national strategic reserves and ensures a consistent and dependable supply of natural gas. These storage facilities play a crucial role in maintaining stability in gas networks, allowing for quick compensation during consumption peaks, especially in winter. Compared with storage options on the surface, underground gas storage facilities offer the advantage of storing significantly larger quantities of gas, leveraging natural geological formations for capacity. However, gas storage activities, which induce continuous stress changes in the reservoir, can threaten caprock integrity. Since 2012, the north-eastern Madrid Basin area, where the Yela facility is located, has been exploited for Underground Gas Storage activities. Its strategic location in central Spain makes Yela a key infrastructure for guaranteeing energy supply for the country. The UGS Yela site provides great flexibility to the gas system by exploiting a deep fractured aquifer with an active water system, making it possible to switch almost seamlessly from injection to extraction, and vice versa. An appropriate monitoring system and a risk management plan can ensure the safety and stability of local environments. In this regard, InSAR data can play a key role thanks to the high-precision measurements of ground deformations. In addition, the huge spatial and temporal ex  
Figure 10: Analysis of volumetric changes using CC. In **(A)**, an extended view of all the MPs relative to the period of maximum injection in October 2019 (Red) and the period of maximum extraction in April 2020 are shown together. It is evident that inside the white square (Yela UGS site), the green dots are higher than the red ones. In **(B)**, the green dots represent the MPs used for CC volume computation, and in **(C)**, the height differences between the maximum injection in 2019 and the maximum extraction in 2020, assigned to each 125 \(\times\) 125 m\({}^{2}\) grid centres (represented by the coloured dots), are shown. tension of the InSAR products allows the investigation of the UGS at a low cost (EGMS data are completely free). Analysis of InSAR measurements can provide accurate surface deformation data over wide areas, enabling continuous monitoring of UGS activities over time. While additional data is needed to establish comprehensive guidelines for UGS management, InSAR technology appears to be a key tool for complementing in situ data, enhancing facility safety and minimising the risk of gas leaks. One of the main limitations of the InSAR approach is land cover. Rural areas, with rough terrain and vegetated areas, have associated low-phase coherence; this results in limited spatial coverage of InSAR measurement points [42]. Another limitation is the reduced presence of reliable and durable PS in heavily vegetated and complex mountainous areas [9] where many UGS sites are located. Despite these issues, InSAR technology is one of the best methods for monitoring UGS projects. Furthermore, InSAR holds significant potential for future developments. For example, by comparing ground deformation data with pressure and temperature changes in the reservoirs, optimal thresholds of injection and withdrawal rates can be identified, thereby improving the overall management and longevity of the storage system. InSAR results show a strong correlation between periodic natural gas injection/withdrawal in the reservoir and corresponding vertical ground deformation around the Yela facility. Time series results, obtained by comparing the vertical time series of displacements with the volume of the stored gas, highlight that ground movements in the Yela UGS site are not associated with seasonal temperature changes in the area. The temperature variation should show the same behaviour in all MPs, whereas in this case, the seasonality associated with UGS operations is only recognisable in those up to a maximum distance of approximately 2 km from the injection and extraction wells.