A New Approach for Longterm Monitoring of Deformations by Differential SAR Interferometry

SAR Interferometry is a radar technique which allows to measure terrain deformations by comparing the phase information of two SAR images taken at different times. The result is an image of the deformations occurred in the meantime, with an accuracy of the order of a few millimetres. The major drawback of this technique is temporal decorrelation, i.e. the loss of information due to the change of the backscattering properties of the terrain in the time between the two acquisitions. On time intervals of more years the presence of heavy decorrelation makes it extremely difficult, if not impossible, to apply SAR Interferometry in the conventional way. Alternative methods have therefore been searched for the retrieval of deformation information in such extreme conditions. Two different test sites were considered, and for each of them a different, new approach for the measurement of deformations with interferometry has been developed. The first test site under study was the province of Groningen, in the Netherlands, where subsidence due to extraction of natural gas occurs. Long term interferograms of this site contain only highly coherent, mostly pointwise phase information from anthropogenic features. Despite their small spatial scale with respect to the image resolution, such features resulted to be particularly stable in time, in many cases even at the level of few millimetres. An unconventional approach has then been used to measure the deformations using the phase information from these objects. The second test site was Campi Flegrei, a densely populated area located near the Vesuvius volcano in Naples (Italy). This high volcanic-risk area is currently subsiding with a mean rate of 4mm per month. Also in this case a new approach has been developed to monitor the deformations occurring in the last seven years. The result is a time series of deformation figures describing, for the first time in a two-dimensional picture, the evolution in time of the subsidence from 1993 to 1999