You may have seen those distorted old houses where a portion of the structure has sunk - akin to the Leaning Tower of Pisa. Unstable foundation tends to be the leading cause of this distortion, which itself is often affected by Subsidence i.e. the lowering of the ground level due to excessive removal of groundwater underneath which vertically compresses the sediments carrying this water, among other factors. That India exploits its groundwater resources in an unplanned manner due to burgeoning population and erratic construction practises is publicized. As a result, Subsidence is common and it comes with significant cost & risk implications.
With Radar Remote Sensing, it is possible to determine the changes in ground level i.e. Deformation, over a period of time. The method to extract this information, which is a telltale sign of Displacement or Subsidence, is called Differential SAR Interferometry or DinSAR Interferometry. To explain this method simply, it involves measuring the time it takes for Microwaves of a particular wavelength, also known as Synthetic Aperture Radar or SAR, emitted by the satellite's transmitter to return to the receiver onboard upon interacting with the ground/surface features on earth. By comparing this radar backscatter information across two or more points time (the differential in DinSAR), one can ascertain if the ground level has changed i.e. whether it has subsided, has not changed, or has uplifted. Besides time differential, the intensity differential of the radar backscatter also helps to know whether the structural properties of the ground/surface feature has changed - a more technical explanation can be accessed in my post on Deformation mapping at volcanic sites. In case you wish to know more, and practise tutorials on Radar Remote Sensing and Interferometry, you may pursue this wonderful online course.
While the demonstration of performing DinSAR Interferometry can be viewed here, my output for Subsidence Mapping in Kolkata between January 2019 and January 2020 is enclosed below. I have used SAR data from Sentinel-1A satellite's Single Look Complex imagery product to derive this information. The geographic extent of Kolkata has been approximated - it includes outskirt areas too.
So what can one infer/observe from this output?
The output indicates that large swathes of Kolkata have experienced Subsidence between 2019 and 2020 and at a discomforting rate of as high as 6 centimeters. There appears to be a pattern to it as well - the north-to-central stretch of the city has sunk the most - as it has historically. The central-to-south extent of the city has subsided too, albeit less so (between 1-3 centimeters).
The sites of mega urban development projects such as New Town and Rajarhat also display moderate-to-high Subsidence levels of around 3 centimeters. To my surprise, certain sections on the southern edge of of the city such as Maheshtala and Berisha have experienced ground uplift during the year (between 0-3 centimeters).
The map extent doesn't depict Subsidence further north of Barrackpore as it is located outside the Kolkata city extent, however, you'll be alarmed to know that the Subsidence that I derived for this region is greater than 10 centimeters during the same year!
The blank pixels in the output have low Coherence levels of below 0.4 which means that the degree of similarity i.e. correlation between the surface features of both the imagery datasets is less. This could very well signal displacement, however, it could also be due to volume scattering of backscatter - which can happen if the surface is vegetated (wind and crop growth does change the orientation of the leaves and plant between both the imagery acquisitions). The latter could very well be the case because the area to the west of the Hooghly river is comparatively not as urban as the area to the east. That being said, one can mentally interpolate the presence and extent of subsidence from the surrounding shaded pixels.
Did you draw any other observations from the output or have local area knowledge explaining the reason behind the subsidence? Would you like me to analyse Subsidence over a larger period of time? Let me know.
I hope this study made some of you aware about the literal pitfalls of reckless Ground Water mining, invariably for construction purposes - how we consume and manage Water resources would play such an important role in determining the sustainability of our planet - I have written extensively on Water Management and you may access the catalogue here.
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