SAR interferometry has proven to be a feasible tool for detecting ground deformations caused by earthquakes or volcano activity. However, the application of the technique for deformation processes which take place on a longer time scale remains difficult for a number of reasons. Limiting factors for monitoring slow deformation processes are mainly temporal decorrelation and atmospheric delay differences within a SAR scene. Especially areas with agricultural activity lose coherence typically within an interval of days, which is too short to unravel the slow deformation rates. The systematical analysis of series of interferograms seems to be the only possibility for extracting feasible subsidence rates from SAR interferometry. In this paper we will give an overview of the possible error sources and propose a method for the analysis of severely decorrelated phase interferograms.

The monitoring of slow deformation processes by means of SAR Interferometry requests the observed area to mantain its correlation for more years. This usually happens only for those areas having particularly favourable characteristics, such as poor vegetation and dry and no windy climate. However, even in an area which decorrelates in a few weeks, a certain number of features has been observed, which appear to mantain high coherence values. The aim of this paper is therefore to investigate to which extent these structures, mainly man-made features, remain coherent on long time scales. For this purpose, a time series in a test area in the Northern Netherlands has been generated, with a maximum temporal extension of 3 1/2 years. Even on such a long time span, a considerable number of highly coherent features has been found. A sample of such features has then been selected and the coherence values have been traced in the whole time series. This technique has revealed to be useful in order to check the "goodness" in terms of coherence of an interferogram: in particular, in this way an interferogram could be identified having generally lower coherence, probably due to different seasonal conditions. Other tests have been performed in order to study the coherence as a function of time and of the baseline length.