# Monitoring of Ground Movement and Groundwater Changes in London Using InSAR and GRACE
## 6 Conclusions  
In this study, we explored the suitability of monitoring ground movement using PSInSAR subsidence and GNSS data for London between December 2002 and September 2010. We also  
Figure 11: Subsidence and groundwater time-series for boreholes on either side of the northern boundary fault (i.e., TQ38_241 (45–47 Gresham Street) on the southern side and TQ38_263a (Victoria House, Southampton Row) on the northern side). Figure 10: **(a–d):** Time-series variation of groundwater change (from borehole) and subsidence (from InSAR) for **(a)** TQ2788a (Ashley Gardens, Westminster), **(b)** TQ27_159 (Ram Brewery, High street, Wandsworth), **(c)** TQ28_153 (Kensington Gardens) and **(d)** TQ27_282 (I Stratford Grove, Putney). monitored groundwater variation using GRACE gravity anomalies and observed boreholes for London for the same time period. We observed that the ground movement in London is distinctly uneven and non-linear. The ground movement in the north-western part of the study area is positive, implying land uplift. On the contrary, the ground movement in the south-eastern part of the study area is negative, implying land subsidence. The average subsidence in the study area is found to be \(-0.176\) mm/year. By statistically analysing the average subsidence velocities, it is observed that the standard deviation of \(98.22\%\) of the PS points are less than \(5\) mm/year, and that for overall PSInSAR subsidence is \(0.665\) mm/year. The InSAR subsidence values were validated using GNSS data. The two-sample \(t\)-test provides enough evidence that GNSS up values are in close agreement with the obtained InSAR subsidence values, thus demonstrating that the PSInSAR is an efficient way to survey ground movement in London. The groundwater changes obtained from GRACE demonstrates a decreasing tendency and the average groundwater loss from GRACE was found to be \(9.003\) MCM/year. The best-fit line for the temporal groundwater variation from GRACE has a negative slope, indicating groundwater depletion. The volumetric change of groundwater derived from GRACE data accounted for \(60\%\) of the total change in the report from the Environment Agency. In addition, the GRACE-derived temporal time-series showed seasonal and annual variations. Different reasons for site-specific land movement patterns were discussed. The major factor which was consistent with spatial ground movement is the spatial change in the groundwater. In addition, ground movement was studied in conjunction with groundwater variation obtained from various boreholes. We suggest that when a large volume of groundwater is extracted from an aquifer, it results in compaction of underlying aquifer material and, in turn, is responsible for land subsidence, and, in contrast, recharge or rebound of groundwater results in land upliftment. The spatial resolution for GRACE is low, while that of InSAR is high. The PSInSAR method is excellent for studying spatial variation of groundwater, and GRACE is excellent for studying temporal variation of groundwater. With new processing algorithms and the GRACE follow-on mission, the resolution is expected to improve. In addition, the increasing coverage and availability of SAR data should enhance the accuracy of subsidence mapping. With these updated data sets, it will be suitable to read InSAR and GRACE results at the same resolution and make a direct statistical comparison for future studies. Overall, the study of InSAR and GRACE in conjunction provides an interesting technology for spatial and temporal mapping of groundwater and ground movement. Conceptualization, V.A., R.L.G. and S.M. ; data curation, V.A., R.L.G. and S.M. ; formal analysis, V.A. and A.K. ; funding acquisition, V.A., R.L.G. and S.M. ; investigation, V.A., R.L.G. and S.M. ; methodology, V.A., A.K., R.L.G. and S.M. ; resources, V.A. ; software, V.A. and A.K. ; supervision, R.L.G. and S.M. ; validation, R.L.G. and S.M. ; visualization, V.A., A.K., R.L.G. and S.M. ; writing--original draft, V.A. ; writing--review and editing, V.A., R.L.G. and S.M.