Subsidence near two London tunnels constructed in the last decade is revealed in new images produced from satellite data. The ground movement appears near London Underground鈥檚 Jubilee line extension and a London Electricity tunnel.
鈥淲e are not saying this was definitely caused by the tunnels,鈥� says Mark Haynes, senior scientist for NPA Satellite Mapping, which created the images. But in terms of location, he says 鈥渢here is a definite correlation鈥�. Experts say more research is needed to make a link between the tunnel building and the subsidence.
NPA says a number of points near the line of these tunnels were found to have subsided by up to four millimetres per year. The company collected data from twin Synthetic Aperture Radar instruments, aboard the satellites ERS1 and ERS2 between 1995 and 2000.
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The technique used by NPA is called Permanent Scatterer Interferometry. This reduces noise in the gathered data by fixing measurements against thousands of permanent landmarks. The company says that the images highlight the commercial potential of Earth-monitoring satellites.
But Maurice Gooderham, an independent civil engineer, says this is an unproven method of measuring subsidence. 鈥淯p to now we haven鈥檛 used satellites to monitor settlement,鈥� he says.
London Electricity also questions the satellite results. A spokesman says there is no evidence of such substantial subsidence from ground-based measurements. He also notes that natural subsidence could be responsible for any ground movement. 鈥淭hat is a possibility,鈥� agrees Haynes.
London Underground, which constructed the Jubilee line extension, could not confirm whether there is any evidence of subsidence above the Jubilee line extension tunnel. But a spokeswoman says a small number of residents in the area have made insurance claims in relation to alleged subsidence.