Abstract
In this paper, Persistent Scatterer Interferometry was applied to ERS-1/2 and ENVISAT satellite data covering 1992–2000 and 2002–2010 respectively, to analyse the relationship between ground motion and hydraulic head changes in the London Basin, United Kingdom. The integration of observed groundwater levels provided by the Environment Agency and satellite-derived displacement time series allowed the estimation of the spatio-temporal variations of the Chalk aquifer storage coefficient and compressibility over an area of ∼1360km2. The average storage coefficient of the aquifer reaches values of 1×10−3 and the estimated average aquifer compressibility is 7.7×10−10Pa−1 and 1.2×10−9Pa−1 for the periods 1992–2000 and 2002–2010, respectively. Derived storage coefficient values appear to be correlated with the hydrogeological setting, where confined by the London Clay the storage coefficient is typically an order of magnitude lower than where the chalk is overlain by the Lambeth Group. PSI-derived storage coefficient estimates agree with the values obtained from pumping tests in the same area. A simplified one-dimensional model is applied to simulate the ground motion response to hydraulic heads changes at nine piezometers. The comparison between simulated and satellite-observed ground motion changes reveals good agreement, with errors ranging between 1.4 and 6.9mm, and being 3.2mm on average.
Highlights
Many cities rely on groundwater for water supply
The characterisation of the Chalk aquifer properties resulting from the presence of discontinuities ideally requires estimation in situ over wide areas, the high costs of such campaigns only permitted to carry out investigations at limited spatial or temporal scales
The combined analysis of hydrological information with displacement maps and time-series retrieved from multisensor and multi-temporal SAR images and Persistent Scatterer Interferometry (PSI) analysis has allowed the derivation of: (1) the Chalk aquifer storage coefficient maps over an area of $1360 km2 in the periods 1992–2000 and 2002–2010; and
Summary
Many cities rely on groundwater for water supply. In most parts of Europe over 40% of water supply comes from urban aquifers (Wolf et al, 2006). Changes to the aquifers and the geological strata overlying aquifers need to be understood and quantified, such that (i) changes in groundwater levels do not cause inundation of water into underground assets, (ii) differential changes in ground saturation do not significantly affect ground engineering properties, and (iii). With the vertical total stress unchanged, a variation in pore-fluid pressure causes a proportional change in effective stress within the aquifer, resulting in a volume change. The latter is influenced by the compressibility of the aquifer. When the effective stress does not exceed the maximum effective stress that the system has experienced in the past (i.e. pre-consolidation stress), the fluctuations in the water level create small elastic deformation of the aquifer-system and small land surface displacement. Vertical ground motion can be the effect of the elastic and/or inelastic compaction which depends on the hydraulic head changes and the thickness of the unconsolidated deposits (Riley, 1969; Helm, 1975, 1976)
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