With technology advancing at a rapid pace, scientists are now able to track even the slightest changes to the surface of the Earth with the help of advanced satellites. As a result, subsidence of only millimetres can be detected.
For example, experts have been able to discover that certain areas of the picturesque town of Assisi in Italy, which draws a large number of tourists, religious pilgrims and lovers of art, are sinking by 7.5 mm per year.
The above example is just one of the many areas around the world where satellites have detected surface changes down to the millimetre from their orbits hundreds of kilometres above.
Interferometric Synthetic Aperture Radar – or InSAR – is a remote sensing technique where two or more images over the same scene are combined to detect slight changes occurring between acquisitions.
Tiny changes on the ground cause changes in the radar signal and lead to rainbow-coloured interference patterns in the combined image, known as ‘a SAR interferogram’.
Precise measurements – down to a scale of a few millimetres – can be detected across wide areas. Tectonic plates grinding past one another, the slow ‘breathing’ of active volcanoes, the slight sagging of a city street due to groundwater extraction, even the thermal expansion of a building on a sunny day.
Just a few years ago, only dedicated areas could be mapped. But advances now allow for subsidence detection over large regions – even whole countries, like Italy – while maintaining a high resolution.