Monitoring Land Subsidence in Kolkata using Remote Sensing
Updated: Jun 13
You may have seen old buildings that appear distorted from the outside. Where a portion of the building has caved in compared to the other and hence the building does not appear flat / leveled to the eye. This phenomena can be attributed to Land Subsidence i.e. lowering of the ground level due to excessive pumping of water underneath, among other factors. India is much exposed to the Subsidence problem due to our burgeoning ground water needs and erratic construction methods. As a result, Subsidence comes with severe risk & cost implications.
With Radar Remote Sensing, it is possible to determine the changes in ground level over a period of time. The process to do so is called Differential Interferometry. Simply put, electromagnetic waves transmitted by the satellite's transmitter reflect back to the receiver in the form of Backscatter after hitting the earth's surface or the objects on it. The time it takes for the waves to return, among other factors, helps us to understand and interpret the properties of the surface and its objects, in our case, the relative changes in the ground level over a period of time, to determine subsidence. There could be three possible outputs - Subsidence, no change in Ground Level or an Uplift.
To learn about radar remote sensing and interferometry you may opt for this informative online course.
While I have not elaborated the processing steps involved to compute Subsidence from Raw Satellite Imagery in this article, please find below the output generated as part of the 'Land Subsidence Mapping for Kolkata' study, overlayed on a Google Earth basemap below -
The output has been extracted using two Sentinel-1A, SLC images roughly one year apart (January 2020 v/s January 2019). The Kolkata city extent is approximated - includes outskirts too.
Much thanks to RUS Copernicus for the training material.
To summarize the result as depicted in Figure 1 - Yes, the city is sinking and at a discomforting rate. And there is a visible pattern to it with supporting historical evidence. The highest subsidence is towards the northern suburbs of the city - maximum subsidence recorded is 6 cm. The central / main city area has relatively less subsidence at 1-3 cm, nonetheless it is discomforting too. Recent urban developments such as New Town and Rajarhat localities have moderately high subsidence levels at around 3 cm. Certain sections of the southern tips of the city (Maheshtala/Berisha) have an uplift at 0-3 cm.
The map doesn't depict subsidence further north of Barrackpore as it is technically much outside the Kolkata city extent - Nonetheless, the Subsidence recorded there is >10 cms. Quite alarming!
You may wonder why there are blank spaces in-between the colored Subsidence pixels over the basemap imagery. These were so due to low Coherence levels of < 0.4. Simply put, both the imagery products being studied (from 2020 & 2019 respectively) did not have comparable reflectance correlation and hence, determining subsidence from two readings which aren't apple-to-apple would be inaccurate. Nonetheless, the trend of subsidence can be easily discerned despite the presence of blank spaces. Our brains are very good at deducting patterns using visual interpolation.
Can you spot any additional patterns or have local area knowledge explaining this phenomenon? Why is there excessive groundwater extraction at the regions affected by the highest subsidence? Would you like me to study subsidence over a larger period of time instead of the 1-year time period utilized for this study?
I hope this exercise helped you to be aware of the risk arising out of reckless water usage - how we consume and manage water resources would play an important role in determining the sustainability of our planet. Read my elaborate case study on Drought Monitoring at Indirasagar Reservoir here.
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