Even in the densest of urban environments, there are places that the most common InSAR methods cannot penetrate. Take London as an example. Approximately 33% of Greater London is green space and an additional 14% is estimated to be vegetated private, domestic garden land. Therefore, if we consider mapping land subsidence in London using a satellite-based method like Persistent Scatterers InSAR (PSInSAR) which relies upon radar reflections from buildings and hard infrastructure, the best coverage that can be achieved, even with the highest resolution possible, is never going to be better than 53%!
Identifying buildings and infrastructure that are moving does address a number of applications, but in the estimation of risk and planning for future developments, it is not enough.
As an example, let’s consider the recent tunnelling work done in London as a part of the Crossrail project, a project which has caused some levels of subsidence that can be detected by InSAR. The rail line built runs from east to west through part of the most densely urbanised part of Central London. An example is shown below where we can see that an optical image shows lots of buildings but with some green spaces at various places. We have overlaid the image with the Crossrail route, in white.
We can adequately map the subsidence of this entire area with free Sentinel-1 data, as shown, and provide almost complete coverage with our APSIS© system at 20m, demonstrating clear subsidence along the Crossrail path. However, 20m is often not considered a good enough resolution for urban areas and commonly premium satellite data from elsewhere, in this case from the COSMO-Skymed (CSM) mission, is applied which can provide 3m of resolution.
If we apply a conventional method, such as PSInSAR, to CSM data we see that we achieve a result that provides measurements over all of the buildings and some of the infrastructure. This is very pleasing to the eye but what is clear is that there are lots of gaps between these measurements and it’s sometimes not so easy to see the pattern of deformation. With our APSIS© system, we can provide measurements across almost the entire scene, identifying the full pattern of the ground motion with ease, achieving a coverage of much better than 95%.