InSAR monitoring of progressive land subsidence in Neyshabour, northeast Iran

The area of Neyshabour, a small historical city located in Northeast Iran, is subject to land subsidence. To monitor the temporal evolution of the subsidence, the small baseline subset (SBAS) algorithm is used for interferometric synthetic aperture radar (SAR) time-series analysis. To limit the spatial and temporal decorrelation phenomena, the interferograms produced from the raw ENVISAT ASAR data are characterized by small spatial and temporal baselines. Accordingly, four independent SAR acquisition data sets separated by large spatial and temporal baselines are used in the time-series analysis. To link the separate data sets, a smoothing constraint that minimizes the curvature of the subsidence temporal evolution is added to the least-squares method. The optimum smoothing factor estimated in the smoothed time-series analysis reduces the atmospheric noise, unwrapping and orbital errors whereas it preserves the non-linear seasonal deformation features. The time-series results show an incremental lowering of the ground surface, accompanied by small seasonal effects. The mean LOS deformation velocity map computed from the time-series analysis demonstrates a considerable subsidence rate of up to 19 cm yr(-1). Comparison between the InSAR time-series and continuous GPS measurements verifies the accuracy of the obtained results. Moreover, the quantitative integration of the InSAR-derived displacement measurements with observations of the hydraulic head fluctuations causing these displacements yields information about the compressibility and storage properties of the aquifer system.