Detection of Urban Land Subsidence in the City of Turku in Finland Using Differential Sar Interferometry

The centre of the Turku city by the river of Aurajoki in Finland has been built on clay and silt soil. The level of the ground water has been constantly lowering during the last few centuries due to the postglacial land uplift and extensive water usage leaving the old wooden pilework exposed to decaying. The decaying has led to the subsidence of some historical buildings in the old centre of Turku at the rate of few millimetres per year, thus, huge investments are needed to repair damages and to prevent any buildings from collapsing in the future. Nowadays, the subsidence monitoring is performed with traditional land surveying, namely by levelling the benchmarks in the area regularly. Recent advances in the SAR interferometry, especially with the permanent scatterers technique, has enabled a long-term study of vertical land movements using SAR images. In March 2003 our proposal for a Category 1 project ’Land subsidence and land uplift determination due to the postglacial rebound in the boreal forest zone using repeat pass satellite SAR interferometry and Permanent scatterers technique’ was accepted (CAT-1 1488) and we requested altogether 40 ERS-1 and ERS-2 single look complex SAR images covering our test area in the city of Turku. The reference data of the subsidence of individual buildings has been acquired from the Real Estate Department of Turku. This paper presents the first results of the long-term differential SAR analysis.