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

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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 (t1)–pre-monsoon (t2) periods when the lowering of piezometric head was the maximum and therefore land subsidence would be more prominent than that of pre-monsoon (t1)–pre-monsoon (t2) and post-monsoon (t1)–post-monsoon (t2) 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.

Introduction

Among various aerial- and space-based techniques available to-date, D-InSAR appears to be an efficient technique for measuring spatially continuous land subsidence (Amelung et al., 1999, Carnec et al., 1995, Crosetto et al., 2002, Fruneau and Sarti, 2000, Strozzi et al., 2001, Tesauro et al., 2000). D-InSAR is used for two broad purposes: (i) identification of land subsidence phenomenon in an area, and (ii) quantitative analysis and modeling of deformation phenomenon with particular emphasis on precision of measurements. In the latter case, D-InSAR technique should be accompanied by precise GPS monitoring and/or ground-based levelling.

Kolkata is the second largest city in India after Mumbai and perhaps the ninth largest in the world with a total urban population of 13.217 million (Census of India, 2001) over an areal extent of 900 km2 (Kundu & Nag, 1996). It has been undergoing a tremendous growth during the last 315 years since its inception in 1690. Kolkata is located in eastern part of India in the state of West Bengal on the bank of the Hooghly River (the mighty Ganges River) as the chief commercial, financial, manufacturing and cultural centre of eastern India. We have confined our study on the city area which is falling under Kolkata Municipal Corporation (KMC), covering an areal extent of 185 km2 (Fig. 1) and a population of 4.58 million (Census of India, 2001).

In Kolkata City, India, potential land subsidence caused primarily by over-drafting of groundwater has been reported by a number of previous workers (Biswas and Saha, 1985, Sikdar et al., 1996) and by the local media. In a confined aquifer condition, as in the case of Kolkata City, the over-extraction of groundwater causes lowering of piezometric pressure. Groundwater contributes as support to the overlying confining layer material. The resulting reduction in artesian pressure causes tensional forces to develop in overlying confining layer material. As a result, compaction of overlying confining layer material occurs which prompts to land subsidence.

In this work, a D-InSAR based study has been attempted primarily to detect the subsiding areas in Kolkata City during the 1990s and estimate the average rate of subsidence, assuming a linearly uniform deformation over the period of observation (i.e., 1992–1998). For slowly subsiding areas, as in the present case, temporal decorrelation and atmospheric artefacts appear as major difficulties in the analysis of differential interferograms. We have attempted adaptive filtering of the noisy interferograms (Goldstein & Werner, 1998) to highlight the fringes from temporal decorrelation noise in the interferograms. To identify the deformation fringes from atmospheric artefacts, we have followed the approach of enhancing the deformation fringes and at the same time diluting the atmospheric fringes by summing independent interferograms in complex domain.

Section snippets

Kolkata City: Physiographic and geologic setting

Physiographically, the area represents a typical deltaic flat country with elevation ranging between 5.8 and 6.1 m above MSL (Chaterji et al., 1964). Several low lying depressions in the form of mashes or shallow lakes are seen in and around the area which represent river scars of past drainage channels in the area.

Geologically, the area around Kolkata City forms a part of Bengal Basin and is underlain by Quaternary sediments of fluvio-deltaic origin consisting of a succession of clay, silt and

Results

It is observed that in interferograms ‘a’, ‘b’ and ‘d’ (i.e., ERS1_7335 vs. ERS2_17064, ERS1_10842 vs. ERS1_24713 and ERS1_11343 vs. ERS1_25214 respectively), coherence is low to moderate and fringe(s) are fairly developed. In interferogram ‘c’ (i.e., ERS1_10842 vs. ERS2_6543), coherence is low and the fringe(s) are poorly developed.

Initially, the interferograms were studied to find out the fringe(s) with common geographic location in different interferograms so that the fringe(s) representing

Discussion

The phenomenon of land subsidence resulting from heavy withdrawal of groundwater for municipal supply, industrial or irrigational use has been observed in various parts of the world, viz., Venice and Bologna (Italy), Mexico City (Mexico), Tokyo and Osaka (Japan), Arizona, California and Nevada (United States), Ruhrgebiet (Germany) (Amelung et al., 1999, Galloway et al., 1998, Rahman, 1995, Sikdar et al., 1996, Strozzi et al., 2001, Strozzi et al., 2003). In Kolkata City, India, land subsidence

Acknowledgements

The work has been carried out under ESA Category-1 project with collaborative efforts from IIRS, NRSA, Dept. of Space, India and IFG, UMLV, France. The authors acknowledge European Space Agency for providing ERS SAR data at reproduction cost. The authors are thankful to CGWB, SWID (West Bengal), Metro Rail (Kolkata) and NATMO for providing collateral data and/or ancillary information in the project. The authors convey their sincere gratitude and thanks to the anonymous reviewers for their

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