## 4 Results  
The displacement maps in the ascending (Figure 6A) and descending (Figure 6B) orbits show the overall stability of the study area, with local zones showing positive values indicating motion towards the satellite, e.g., a slight uplift, and negative values representing motion away from the satellite, e.g., a subsidence trend. The vertical velocity map (Figure 6C) shows the overall stability of the field area, as does the horizontal velocity map (Figure 6D). Although the area of interest does not show vertical or horizontal velocity on ground deformation maps in LoS, a useful time series analysis of the UGS site can be conducted since it enables the detection of additional information on seasonal variability as non-linear and seasonality trends. Figure 6: Displacement velocity rates calculated using (**A**) ascending, (**B**) descending, (**C**) vertical, and (**D**) Horizontal Sentinel-1 data collected from January 2015 to December 2021. The grey polygon, elongated in a NE–SW direction, depicts the deep structural trap where natural gas is stored. The dotted polygon represents the Yela facility. Thanks to the regular acquisition of data from the Sentinel-1 constellation, it is possible to monitor deformation over time, understand cyclical variations, and investigate related phenomena. Time series analysis showed cyclical trends, with no general linear trend, explaining why the average velocity of the areas of interest remained within the \(\pm\)2 mm/year stability range. In fact, no trends, excluding the cyclicity of the withdrawal and injection phases, were detected over long periods. This demonstrates the importance of analysing time series MP data from areas involved in UGS activities to detect details that may be hidden in the average velocity computed for a given period. The mean velocity approach is beneficial in continuously deforming regions. However, by merely fitting a line through a time series to calculate it, most of the information on temporal variability is lost. Figure 7 shows interpolation maps of the vertical ground information on the MP, constructed using IDW interpolation [38]. These maps correspond to (A) the maximum peak (October 2019) at the end of the injection period and (B) the minimum peak (April 2020), indicating the conclusion of the withdrawal period over the Yela field. The temporal evolution of the vertical displacement component at the Yela site follows a pronounced sinusoidal trend. This trend correlates strongly with the volume of the gas in storage, both in amplitude and periodicity, as illustrated in Figure 7C-E. It is worth noting that a slight shift of approximately 15 days can be recognized, probably due to the time necessary for the fluid to flow everywhere and create the subsurface response to the overpressure. This cyclical behaviour is closely linked to the periodic injection and withdrawal of natural gas into and from the reservoir. Vertical displacement in years with matching data (from the end of 2019 to 2021, see Figure 7E) shows the maximum and minimum peaks at the end of each injection and withdrawal period (October/November and March/April). In this case study, the prominence of vertical displacement over horizontal displacement aligns with the expectations, taking into account the depth of the Yela gas storage, which is between 2300 and 2550 m deep. Deformations triggered by natural gas injection and extraction must be progressively relayed from the topmost layer of the reservoir to the surface, as detailed in [41]. Thus, theoretically, surface deformation effects become more pronounced the shallower the storage depth is. However, determining this \"sealing depth\" or effective depth of influence is not straightforward and several geotechnical factors come into play, including reservoir porosity, saturation, and other intrinsic properties, as well as the cap rock's integrity and thickness [10].