# Monitoring of Ground Movement and Groundwater Changes in London Using InSAR and GRACE
## 2 Study Area  
In the study, the area chosen is the administrative area of Greater London (black boundary in the Figure 1). The area is bounded by 0\({}^{\circ}\)30\({}^{\prime}\) W and 0\({}^{\circ}\)20\({}^{\prime}\) E longitudes and 51\({}^{\circ}\)42\({}^{\prime}\) N and 51\({}^{\circ}\)17\({}^{\prime}\) N latitudes, covering an area of approximately 1600 km\({}^{2}\) in the southern part of England. The London basin is infilled by younger Paleogene deposits and the chalk group forms a rim around it and has a depth exceeding 200 m in central London [27,28]. Table 1 summarizes the geology of the London Basin, which shows that the Bagshot formation and the Claygate member are present above the London clay. These are young deposits and form nearly 50% of the geology of the basin. London clay is densely fissured and has a blue-grey texture, and beneath this is the Harwich formation which is mainly made of sand and gravels. Then comes the Lambeth group, which consists of sands, silts and clays. The Cretaceous era is mainly marked by chalk deposits, which forms one of the most important geological formations in the UK, as it accounts for the majority of the groundwater used in the country. The chalk aquifer provides approximately 60% of the groundwater used in the country [29] and also is a source for nearly 80% of public water supply in the catchment area of the River Thames and approximately 20% in London [30]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Era** & **Group** & **Formation** & **Thickness (m)** \\ \hline \multirow{4}{*}{Palaeogene} & \multirow{2}{*}{Thames} & Bagshot Formation & 10–25 \\  & & Claygate Member London Clay & 30–90 \\ \cline{2-3}  & & Harwich Formation & 0–10 \\ \cline{2-3}  & \multirow{2}{*}{Lambeth} & Woolwich and Reading Beds & 10–20 \\  & & Upnor Formation & 5–7 \\ \cline{2-3}  & & Thanet Sands & 0-30 \\ \hline Cretaceous & & Chalk & 180–245 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Geology of the London Basin (adapted from EA (Environment Agency) status report 2010 [11]). Figure 1: Study Area: Red box, blue box and green box show the extent of ENVISAT-ASAR (Environmental Satellite-Advanced Synthetic Aperture Radar) image, the extent of GRACE (Gravity Recovery and Climate Experiment) data used and InSAR (Interferometric Synthetic Aperture Radar) study area, respectively. Black boundary shows the administrative area of London. Purple dots, pink triangles and brown lines show the location of key boreholes, GNSS (Global Navigation Satellite System) stations and faults in the study area, respectively. The water supply in London for public and industrial use [10] is mainly provided by the chalk group aquifer and it also provides significant base flow to the Thames, with a base flow index of 0.63 [8]. Even though the Paleogene group are not the main deposits, they confine the underlying chalk aquifer around central London. These provide a heterogeneous nature to the hydrogeology of the region and a good amount of groundwater can be present in sand-rich horizons. The study of the hydrogeology of central London and the response of the chalk aquifer to faulting by different researchers have highlighted that different permeability of faults results in irregular groundwater flow [9; 31]. Th infrastructure and lithology can cause ample pressure on the properties of the chalk aquifer in the Thames basin [32], and chalk has a porosity of dual-medium with matrix and fracture [33; 34]. When the groundwater level declines then most flow occurs in fractures and storage occurs in a matrix. The chalk aquifer's primary porosity is between 0.25-0.40, which is due to the coccoliths found in the aquifer, which does not allow water to drain easily as the pore throats are tiny [35; 36] and hence groundwater storage of the Thames aquifer is mainly secondary porosity along faults.