Interferometric Synthetic Aperture Radar (InSAR) can measure ground deformation over wide areas with millimetric precision. Real and perceived differences, between InSAR and ground-based measurements, have caused slow uptake of this technique within civil engineering. London’s sub-surface contains numerous geotechnical hazards, many of which are under-represented on geological maps due to the challenges of conventional geological mapping in urban areas; these present considerable risks for engineering projects.
Differences between InSAR and ground-based measurements have been addressed through validation with monitoring data from the Northern Line Extension and Thames Tideway East (TTE) tunnels. Comparisons of InSAR datasets derived from different satellites and processing chains have also been undertaken. In general, strong correlations were observed and the bigger the displacement measured, the stronger the correlation.
The benefits of InSAR during geotechnical site investigation are revealed through retrospective analysis of the Lee Tunnel; had InSAR data been analysed prior to tunnelling, an unusual displacement pattern caused by a drift-filled hollow may have been recognised. A TTE tunnel baseline study identified that much of the significant subsidence along its route is from tunnel settlement and dewatering induced for Crossrail.
InSAR data from ascending and descending passes can be decomposed into vertical and horizontal components, making it easier to distinguish anthropogenic from natural causes. Anthropogenically induced displacements were largely vertical, and the horizontal displacements reveal a ‘blocky’ pattern, likely due to faulting. Two-thirds of the probable faults had multiple lines of evidence when compared with published geological maps.
Active construction sites have a changing surface expression resulting in few persistent scatterers (PS). Corner reflectors, as artificial PS, were installed at nine locations across London to assess their utility in increasing PS density. Results show that the corner reflectors are readily identifiable in X-band SAR intensity data but less clear in C-band.
The findings of this research demonstrate the value, as well as limitations, of InSAR for geoscientists and civil engineers, at all stages of a project lifecycle.