DEM derivation and subsidence detection on Hanoi from ERS SAR Interferometry

Flooding risk is a real problem in Vietnam leading the population to build dykes. When focusing on Hanoi and its surrounding, the flooding risk is even higher than in other parts of the country as most of areas in and around Hanoi are below the level of the river. In this framework, subsidence phenomenon which is the result of ground water pumping, is considered critical because of its close relationship with water supply issue. SAR Interferometry is an efficient alternative method to obtain information on topography and vertical ground movements, specially on urbanised zones where the backscatter characteristics of the targets produce a more reliable signal. The main shortcomings we had to cope with, were related to the availability of the optimal interferometric pairs in consideration of the smoothness of the relief. For DEM derivation, the compromise had to be realised between time interval separating the images, which has as consequence temporal decorrelation, and ambiguity height in relation with the small altitude variations. The access to additional ground levelling measurements was difficult. We had to extract the information from a SPOT orthoimage in order to obtain a corrected geocoding model issued from the interferogram (phases and orbits). This model was used both for DEM derivation and geocoding of subsidence related interferograms. Though we focused on areas strongly urbanised, the phase was still noisy for time intervals allowing the expected detection of movement (few centimetres per year). Neverthless, signatures of ground movements have been detected. In addition, the amplitude of the detected movements stays smaller than half wavelenght therefore the estimations have to be handled prudently as they are close to the precision limit of the method. The presented study highlights the performances and limits of the technique in the region and the interest of a long-term application to subsidence monitoring. Introduction The framework of the presented work is the study of subsidence resulting from ground water pumping and flooding risks on the area of the city of Hanoi (Vietnam). The information which can be derived from SAR interferometry for this objective concerns the vertical movements detection by differential interferometry and the production of Digital Elevation Models. The aim is therefore to use this information as an alternative to ground measures in the perspective of a long-term application to subsidence monitoring. The paper discribe the procedure carried out to obtain the information and the limitations of the technique. In our case, the limitations were essentially due to the unavailability of the appropriate data (SAR and ground information) in relation with the objectives of the study. In particular, the small amplitude of the expected movements (about 1 cm/year) and the high DEM precision needed for flooding risks management purpose induce constrains on the SAR data concerning the amount of data, their acquisition dates and their orbital characteristics.