Time Series Analysis of Hashtgerd Subsidence Using Radar Interferometry and Global Positioning System

A large area in Hashtgerd plain, in southwest of Tehran, is subject to the land subsidence induced by overexploitation of groundwater. In this paper, in order to study the subsidence SAR interferometry (InSAR) and global positioning system (GPS) are used. The small baseline subset (SBAS) algorithm is used for deformation time series analysis. Time series analysis is performed using 6 interferograms calculated from 4 ENVISAT ASAR data spanning 4 months in 2008. A smoothing constraint that reduces the atmospheric noise, unwrapping and orbital errors whereas it preserves the non-linear deformation features is added to the least-squares solution. The time series results revealed that the area is subsiding continuously. Mean LOS deformation velocity map obtained from time series analysis demonstrated a considerable subsidence rate of 47 (mm/month). In order to assess the time series analysis results a dense GPS network consisting of 18 measuring stations is then established. The network design is carried out based on the subsidence spatial pattern extracted from an interferogram calculated from radar data of 2003-2004. The GPS stations are collecting the data simultaneously with radar data acquisitions. Horizontal and vertical components of the subsidence are extracted from GPS measurements. The comparison of InSAR and GPS time series shows the high compatibility of the results demonstrating the high performance of InSAR technique.