# Monitoring of Ground Movement and Groundwater Changes in London Using InSAR and GRACE
## 4 Results  
### InSAR Results  
Figure 3 shows the Line of Sight (LOS) average land deformation map for central London acquired from 47 ENVISAT-ASAR images from December 2002 to September 2010, where positive values suggest that the surface is moving towards the sensor (uplift) and the negative values suggest that the surface is moving away from the sensor (subsidence). To analyse the multi-temporal InSAR monitoring results, we can look at the statistical deformation results of the entire area through the number of persistent scatterer points (PSNs), mean rate of deformation (MRD), and standard deviation (SD) of deformation (Table 3). Figure 3: Line of Sight (LOS) average land deformation map for central London (blue areas depict uplift, while the red areas depict subsidence) with the location of observed boreholes (black dots), observed GNSS stations (red triangle) and four land movement sites (black boxes, detailed in Figure 8a–d). London has mostly urban areas, which makes it highly suitable to study land movement using PSinSAR data, but open areas such as land, parks, etc., are not captured by PSinSAR properly. The PS density depends on the coherence threshold, and the selection of optimum coherence threshold depends on the desired quality and number of persistent points in the resulting subsidence velocity map. A coherence threshold of 0.75 was chosen, which resulted in PS density of 497 PS/km\({}^{2}\). Locally, the displacement velocities and directions (towards or away from the satellite) obtained at PS points can differ because of construction, road resurfacing, localised settlement, etc., but small-scale regional patterns are discernible geographically regardless of these high magnitude changes. When land subsidence or uplift of an area is less than or equal to \(\pm\)10 mm/year, the area can be termed as stable according to the theory of mining subsidence ([PERSON], 2018). From Figure 3, it can be seen that the surface deformation varies from \(-\)6 to +6 mm/year, hence we can consider our study area to be stable during our observed time period. Additionally, it can be observed that uneven subsidence is prominent but that there are contiguous blocks of movement in a consistent direction (either uplift or subsidence). The area to the south of River Thames is mainly subsiding, while the area to the north is mainly rising. The main reason for this pattern can be attributed to the groundwater changes, sub-surface excavations for various construction activities and the subsurface geology. For example, there is uplift across the London clay adjacent to the south of River Thames, where white to grey chalk is present at the surface. Clays, some sands and gravels of the Lambeth Group and silts and sands of the Thanet Sand formation are present at the surface between the areas of London clay and chalk (London, 2018). The subsidence near the Thames can be mainly attributed to compressible ground material such as alluvium. The average temporal land movement for the entire study area (Figure 4) is showing continuous negative movement, so the area is subsiding slowly for our study period. It can also be verified by the fact that the average mean rate of deformation is \(-\)0.18 mm/year (Table 3). However, it can also be seen that there is an imperfect cyclic rise and fall of the land surface with a negative trend. This behaviour is probably because of seasonal and annual variations, which might be attributed to groundwater changes and the presence of compressible materials. This overall subsidence pattern can be attributed to the fact that the area has Holocene deposits of alluvium in the flood plains of the River Thames and the River Lea. These overlay deposits are of the London clay formation, Lambeth group, Thanet sand formation, Seaford chalk formation and Newhaven chalk formation. These Holocene alluviums are prone to slowly compact, dry and compress, if there is removal of groundwater, and thus can lead to subsidence.