This article on ESA's website has an interesting application of interferometry - Synthetic Aperture Radar (SAR), called SAR Interferometry, or InSAR. Essentially, they used data from multiple SAR images over time, and through interferometry, were able to detect surface changes of just a few millimeters. This was done using phasing in the timing of the return signal, compared to other images- the comparative interferometry creates ripples, which then can be translated into distance differentials.
This has a lot of interesting potential with respect to seismic activity and subsurface stability, for mapping and tracking of mine or other subsidence, lava domes, et cetera....
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