An analytical procedure for estimating land subsidence parameters using field data and InSAR images in Neyshabur plain

Groundwater exploitation, when the aquifer discharge is greater than its recharge, changes the balance of the aquifer and results in temporal and spatial decline of hydraulic heads in the aquifer. Decline in hydraulic head or water level, decreases pore water pressure and increases effective stress on aquifer sediments. This phenomenon leads to consolidation of interbed layers which consequently results in land subsidence. In recent years, over-withdrawals from aquifer of Neyshabur plain in the North East Part of Iran have been reported. As a result, an average of 90 cm/year decline in water level and maximum displacement rate of 19 cm/year in ground level have been recorded. This study focuses on the estimation of effective parameters related to the land subsidence in Neyshabur Plain. These parameters include the thickness of the fine compressible sediments and inelastic skeletal specific storage (Sskv), which are directly involved in inelastic displacement computations. In the first step, using the geologic log of 231 boreholes and Kriging interpolation method, a map showing the distribution of fine sediments in Neyshabur plain was prepared. This map was used to investigate for reasonable relationships between annual subsidence rate and groundwater yield with the thickness of the fine sediment. Comparisons of maps indicate that the least amount of discharge is seen in regions where the fine layers have the most thickness. In the second step, subsidence rate, extracted from InSAR images, and water level decline at several observation wells located in the study area were analyzed. Investigation of the trend of land subsidence during the recharge and discharge seasons at observation wells showed that the compaction of the aquifer is majorly due to plastic deformation and that the share of elastic deformation is significant. Then, by using the fine sediments distribution map prepared in the first step, the Sskv values were calculated at different locations. The end result of the proposed procedure was a map showing the spatial distribution of inelastic skeletal specific storage (Sskv) in Neyshabur plain.