Natural and Anthropogenic Geohazards in Greater London Observed from Geological and ERS-1/2 and ENVISAT Persistent Scatterers Ground Motion Data: Results from the EC FP7-SPACE PanGeo Project

Abstract

Abstract-We combine geological data and ground motion estimates from satellite ERS-1/2 and ENVISAT persistent scatterer interferometry (PSI) to delineate areas of observed natural and anthropogenic geohazards in the administrative area of Greater London (United Kingdom). This analysis was performed within the framework of the EC FP7-SPACE PanGeo project, and by conforming to the interpretation and geohazard mapping methodology extensively described in the Production Manual (cf. http://www. pangeoproject.eu). We discuss the results of the generation of the PanGeo digital geohazard mapping product for Greater London, and analyse the potential of PSI, geological data and the PanGeo methodology to identify areas of observed geohazards. Based on the analysis of PSI ground motion data sets for the years 1992-2000 and 2002-2010 and geology field campaigns, we identify 25 geohazard polygons, covering a total of *650 km 2 . These include not only natural processes such as compaction of deposits on the River Thames flood plain and slope instability, but also anthropogenic instability due to groundwater management and changes in the Chalk aquifer, recent engineering works such as those for the Jubilee Line Extension project and electricity tunnelling in proximity to the River Thames, and the presence of made ground. In many instances, natural and anthropogenic observed geohazards overlap, therefore indicating interaction of different processes over the same areas. In terms of ground area covered, the dominant geohazard is anthropogenic land subsidence caused by groundwater abstraction for a total of *300 km 2 , followed by natural compression of River Thames sediments over *105 km 2 . Observed ground motions along the satellite line-of-sight are as high as ?29.5 and -25.3 mm/year, and indicate a combination of land surface processes comprising ground subsidence and uplift, as well as downslope movements. Across the areas of observed geohazards, urban land cover types from the Copernicus (formerly GMES) EEA European Urban Atlas, e.g., continuous and discontinuous urban fabric and industrial units, show the highest average velocities away from the satellite sensor, and the smallest standard deviations (*0.7-1.0 mm/year). More rural land cover types such as agricultural, semi-natural and green areas reveal the highest spatial variability (up to *4.4 mm/year), thus suggesting greater heterogeneity of observed motion rates within these land cover types. Areas of observed motion in the PSI data for which a geological interpretation cannot be found with sufficient degree of certainty are also identified, and their possible causes discussed. Although present in Greater London, some geohazard types such as shrink-swell clays and ground dissolution are not highlighted by the interpretation of PSI annual motion rates. Reasons for absence of evidence of the latter in the PSI data are discussed, together with difficulties related to the identification of good radar scatterers in landsliding areas.

Methodology for Detection and Interpretation of Ground Motion Areas with the A-DInSAR Time Series Analysis

“…The PS-DS detected over these units is around 100 PS-DS/km 2 In addition, comparisons between the engineering geological units and the areas of ground motion detected using the principal component analyses were carried out for the monitored periods (Figure 7c,d). The results of the analyses show that the area of ground motion reaches around 23 and 17 km 2 , respectively, in the periods 1992-2000 and 2003-2010.…”

“…In many cases, the movements are due to the complex interactions of multi-driving forces, which act at various spatial and temporal scales [1][2][3]. The interference and/or overlap makes the effective decomposition of the components of motion difficult.…”

Current and future role of instrumentation and monitoring in the performance of transport infrastructure slopes

“…Remote sensing techniques using airborne and satellite-based sensors can provide a very cost-effective means of acquiring highresolution information for the ground surface over very large areas Miller et al 2012;Castagnetti et al 2013;Cigna et al 2015;Wasowski et al 2014;Hugenholtz et al 2015), but are generally limited in terms of temporal resolution (which is based on satellite orbits or flight schedules) and provide only surface or very near-surface information. For smaller infrastructure slopes (v. large landslides) spatial resolution may also be insufficient, and remote sensing techniques can also be impeded by the dense vegetation cover present on some infrastructure slopes (e.g.…”

Accounting for groundwater in future city visions

“…The extent of groundwater abstraction can be monitored from the induced ground level change, seen in remotely sensed radar interferometry (e.g. Cigna et al, 2015;Boni et al, 2016;Pratesi et al, 2016).…”