Subsidence of Kolkata (Calcutta) City, India during the 1990s as observed from space by Differential Synthetic Aperture Radar Interferometry (D-InSAR) technique

Abstract

In Kolkata City, India potential land subsidence, due to over-drafting of groundwater under confined condition, has been reported by a number of previous workers and by the local media. The presence of a thick surface clay layer with an average thickness of more than 40 m over the aquifer sand layer raises questions on the possibility and doubtfulness of land subsidence in Kolkata City. In this work, D-InSAR based study has been undertaken to detect and measure land subsidence phenomenon in Kolkata City. For detecting slow land subsidence phenomenon in Kolkata City occurring primarily due to piezometric fall consequent to groundwater over-drafting, we have chosen the InSAR data pairs acquired during post-monsoon ( t 1)–pre-monsoon ( t 2) periods when the lowering of piezometric head was the maximum and therefore land subsidence would be more prominent than that of pre-monsoon ( t 1)–pre-monsoon ( t 2) and post-monsoon ( t 1)–post-monsoon ( t 2) periods of similar time intervals. For detecting slow land subsidence phenomenon essentially from long temporal baseline data pairs (usually in years), temporal decorrelation and atmospheric artefacts have been found to pose serious difficulties. Adaptive filtering of the noisy interferograms followed by summing of the independent interferograms was performed in complex domain in order to suppress noise from the fringes and highlight the deformation fringes over atmospheric artefacts respectively. Results indicate that an area in Kolkata City surrounded by Machhua Bazar, Calcutta University and Raja Bazar Science College had been undergoing subsidence during the observation period, i.e., 1992–1998 with an estimated rate of 5 to 6.5 mm/year.