SAR land subsidence monitoring

Images of the phase difference between two SAR (synthetic aperture radar) images acquired from two (nearly) repeated SAR coverages are called interferograms. The technique to use these data to generate digital elevation maps and/or deformation maps, is called SAR multi-pass interferometry.
At TNO-FEL the potential and limitations of these measurements for deformation mapping have been investigated in collaboration with Rijkswaterstaat and Delft University of Technology. A quantitative evaluation of the limitations and potential of the technique has been performed for multi-pass interferometry in general and for deformation mapping in particular. The research was focused at the mapping of the slow process of land subsidence due to natural gas extraction in Groningen.
Multi-pass interferometric radar measurements from space have a great potential for large scale deformation mapping with high accuracy and high resolution. SAR measurements of land subsidence can have an accuracy that is competitive with conventional levelling measurements. In comparison the method is cheaper, it allows for a true 2-dimensional overview and the data are abundantly available. Space platforms with SAR sensors will be available also in the future. Economic benefits are foreseen if conventional levelling measurements are partly replaced by SAR measurements.
The study showed that accuracies up to mm-level can be obtained. This accuracy can be obtained if an appropriate SAR data selection strategy is applied, based on meteorological conditions during the SAR coverages. In this way systematic errors in the deformation measurements caused by temporally and spatially varying electrical path lengths through the atmosphere are minimised. The maximum magnitude of these errors found in the data appears to be 2 mm. up to several centimetres depending on the meteorological conditions.
A special filtering technique has been developed by TNO to use long-term (> 1 year) coherent targets in the scenes. The phase information from these targets is isolated from the non-coherent information to create a 2-dimensional map of deformation.
It is advised to continue investigating ERS-1 interferometric SAR data of the Groningen subsidence area. The Groningen subsidence area will be revisited in the 35-days orbit by ERS-1 and (hopefully) ERS-2. Further research is needed to optimise the strategy and tools for selection, processing, filtering and summation of the interferometric data. It is advised to investigate the possibilities for operationalisation of the technique e.g. by combining SAR deformation maps with levelling measurements.