# Monitoring of Ground Movement and Groundwater Changes in London Using InSAR and GRACE  
[PERSON]  
1 Faculty of Engineering, University of Nottingham, Nottingham NG7 2 RD, UK;  
[EMAIL_ADDRESS] (R.L.G. ); [EMAIL_ADDRESS] (S.M.) 2 School of Geography, University of Nottingham, Nottingham NG7 2 RD, UK;  
[EMAIL_ADDRESS]  
Correspondence: [EMAIL_ADDRESS]; Tel. : +44-7436035061  
2  
[PERSON]  
Additionally, the ground movement in the vertical direction results from elastic or inelastic compression, which relies on variation in the hydraulic head and thickness of the unconsolidated deposits [5; 6]. Chalk is the most vital aquifer in the United Kingdom (UK) as it provides approximately 60% of the groundwater consumed in the UK [7] and accounts for nearly 80% of water supply in River Thames catchment and 20% in London alone [8]. Chalk is one of the most supervised and controlled aquifer systems in the UK and has been providing water in London for domestic and industrial use since the 19 th century [9; 10]. During the early and mid-twentieth century, a lot of groundwater abstraction took place because of anthropogenic development around central London. This abstraction led to a fall of water level to 88 m below sea level in the 1960s, which resulted in a depression in the water table. Due to the de-industrialization since the 1980s, the abstraction was reduced, and the groundwater recovered by 3 m/year in the 1990s, which lead to the rise of groundwater level. This rise in groundwater level caused a threat to infrastructure in the London basin. To check this rise, the General Aquifer Research, Development and Investigation Team (GARDIT) strategy was implemented in 1992 [11]. The GARDIT strategy was implemented using groundwater withdrawal licensing and monitoring, which stabilized the groundwater around the year 2000. Subsequently, the study of groundwater and related subsidence during the period 2002 to 2010 has been carried out here using different geospatial tools. To map the surface movement, traditional methods like levelling, Global Navigation Satellite System (GNSS), 3D laser scanning, and similar provide accurate information, but are tedious, expensive, have low spatial extent and; therefore, are unsuited to surveying large (basin level) areas [3; 12]. A better alternative is to use the Persistent Scatterer Interferometric Synthetic Aperture Radar (PSInSAR) technique, which can identify coherent and stable scattering targets and use their phase history to measure land movement. Moreover, PSInSAR can carry out independent Phase Unwrapping for each permanent scatterer and provide weather independency, sunlight independency (active sensor) and high (basin-level or greater) spatial coverage. Therefore, PSInSAR provides better accuracy and spatial resolution as compared to other traditional methods of land surveying. It has also been used by many researchers to study land movement resulting from earthquakes [13], glacial movements [14] and subsidence [12; 15; 16; 17]. It is not convenient to directly estimate regional-scale water storage as it is labour intensive and expensive work. Many researchers have shown that Gravity Recovery and Climate Experiment (GRACE) gravimetry offers an alternative remote sensing tool [1; 18; 19; 20; 21], which is capable of monitoring water storage. This water may include surface water, groundwater, soil moisture, snow and ice. GRACE has emerged as a vital information source to hydrologists, as it can be used for optimal groundwater observations at a regional scale. When there is a large change in groundwater storage volume in a small area, GRACE can then detect the corresponding mass change because of its high signal-to-noise ratio [22; 23; 24]. Therefore, GRACE data can be an excellent source of monitoring the variation in groundwater storage. To the best of our knowledge, very few attempts have been made to study PSInSAR derived land subsidence and GRACE gravity derived groundwater together [22; 25; 26]. For London, this is the first InSAR-GRACE study.