Interferometric measurements of ground surface subsidence induced by overexploitation of groundwater

Precise leveling surveys across southwest of Tehran have revealed a significant subsidence due to the overexploitation of groundwater. In order to monitor the temporal evolution of the deformation, Interferometric SAR time series analysis was applied using ENVISAT ASAR images recorded between 2003 and 2005. Only Interferograms with small temporal baselines are processed to decrease the temporal decorrelation effect caused by the agricultural fields. However, the spatial baselines of the processed interferograms are not as small as in the conventional Small Baseline Subset (SBAS) method. Coherence analysis reveals that the spatial decorrelation is insignificant. However, since the constructed interferograms are affected by topographic artifacts caused by the large spatial baselines, a multi-step procedure was used in order to refine the interferometric phase. Smoothed time series analysis was then carried out to retrieve the atmospheric-error free deformation corresponding to every acquisition time. The mean displacement velocity map extracted from the time series results indicates a maximum subsidence rate of 24 cm/yr. Chronological sequence of the computed deformations for several points located in the subsidence area shows the permanent aquifer system compaction at a long-term constant rate on which the seasonal effects are superimposed. Sustained hydraulic head declines reveal a relatively low correlation with InSAR derived information. Comparison of the subsidence rate to soil type profiles in different parts of the subsidence area was then used to interpret the deformation signal.