## 4 Results
Despite this, the horizontal motion shows a slight sinusoidal signal in the MP located within the structural trap in the Yela area (MP1 in Figure 8C), showing a positive trend during withdrawal periods (east direction) and a negative trend during injection periods (west direction). The behaviour of ground displacement of a wide area is similar to a bowl both in uplift and subsidence, showing a small component in the centre and outwards during the withdrawal and injection phases, respectively. Increasing the scale of the investigation showed two areas with different surface displacement time series behaviours. Three representative MPs in the InSAR-monitored area were selected inside (MP1 in Figure 8A,B) and outside (MP2 and MP3 in Figure 8A,B) the structural trap area where the natural gas is stored (represented by the grey polygon). The analysis of these MPs is useful for analysing the time series of ground displacement along the vertical and east-west directions. The southeastern area of the Yela underground gas storage facility (MP2 and MP3) has a phase that is exactly opposite to the vertical movements observed in MP1. Consequently, as the terrain above the reservoir rises due to the injection of natural gas, the adjacent areas subside, and vice versa. In contrast to MP1, the two MPs selected outside the structural trap show no seasonality in horizontal displacement. In addition, it is possible to recognise a slight shift of approximately 20-30 days between the time series of MP1 and those of MP2 and MP3 (Figure 8C). The interpolation maps of the basin scale of the vertical ground information of the MPs generated using IDW [38] on the vertical motion data (Figure 9A) for the maximum peak (October 2019) at the end of the injection period (Figure 9B) and the minimum peak (April 2020) at the end of the extraction period over the Yela field shows a different response on the surface (Figure 9). Typically, during the injection and withdrawal processes, the change in reservoir formation pressure results in noticeable surface deformation. However, when the reservoir is very deep, the surface ground deformation caused by UGS activities, which is recognisable in seasonal trend variations, is confined close to the injection and withdrawal wells [10]. Furthermore, as indicated earlier, the Yela facility area (A1 in Figure 9) and the external south-eastern area (A2 in Figure 9) show different ground deformation behaviours. The external area shows the opposite phase of vertical movement, thus the A2 area shows subsidence during the injection of natural gas, and vice versa during the withdrawal, with an amplitude value larger than that of area A1. This effect is evident when comparing the time series of MPs in the two selected areas (see Figure 8C). Figure 7: Vertical ground displacement maps of Yela facility constructed using IDW interpolation of motion data, referring to (**A**) the period of maximum injection in 2019 (October) and (**B**) the period of maximum extraction in 2020 (April). (**C**) Average time series analysis of selected MP (polygon with blue dashed border in (**A**,**B**)) compared with the curve of gas in storage (at the top). (**D**) 3D analysis of ground displacement occurring in the maximum (October 2019-2020) and minimum (April 2020) peaks at the end of each injection and withdrawal period. (**E**) Focus on average time series in the years with data matching the curve of gas in storage (at the top). Figure 8: Vertical (**A**) and horizontal (**B**) displacement velocity rates. At the bottom is the time series comparison between vertical and horizontal displacements (**C**) of three MPs (MP1, MP2, and MP3) in the study area. The grey polygon, elongated in a NE–SW direction, depicts the deep structural trap where natural gas is stored.