You may have seen old buildings that appear distorted from the outside. Where a portion of the building has caved in and hence it 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 impacted from Subsidence due to our burgeoning Ground Water needs and erratic Construction methods. As a result, Subsidence comes with severe cost & risk implications.
With Radar Remote Sensing, it is possible to determine the changes in Ground Level over a period of time. The process to extract this information from Satellite Imagery is called Differential Interferometry. To explain it simply, electromagnetic waves transmitted by the Satellite's Transmitter reflect back to the Receiver in the form of Backscatter Reflectance after hitting the earth's surface / objects on it. The time it takes for the reflected waves to return, and its intensity among other factors helps us to interpret the properties of the surface and its objects that was exposed to these waves. In our case, we use the same principle to understand the relative changes in Ground level over a period of time i.e. determine the presence and extent of Subsidence. Differential Interferometry would generate any of the three possible outputs for each imagery pixel - a) Subsidence, b) no change in Ground Level or c) Uplift.
To learn about Radar Remote Sensing and Interferometry you may pursue this informative and free online course.
While I have not elaborated the processing steps involved to compute Subsidence from Raw Satellite Imagery in this article, you can observe the final output below - The Subsidence data over Kolkata (2020 vs 2019) is overlaid on a Google Earth Basemap -
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, Kolkata city is sinking (i.e. experiencing Land Subsidence) and at a discomforting rate (up to 6 cm y-o-y). There is a visible pattern to it (higher Subsidence in North and Central parts of the city) and historical research lends credence to it. The Central / Southern extent of the city has relatively less Subsidence at 1-3 cm, however, it is of concern too. Recent Mega Urban Development projects such as New Town and Rajarhat also have moderately high Subsidence levels of around 3 cm. That being said, certain sections on the Southern edge of of the city (Maheshtala / Berisha) have experienced an Uplift in the year of observation as well (between 0-3 cm).
The Map extent doesn't depict Subsidence further north of Barrackpore as it is technically located beyond the Kolkata city extent - Nonetheless, you'll be alarmed to know that the Subsidence recorded there is >10 cms between 2019 and 2020.
You may wonder why there are blank spaces in-between the shaded Subsidence pixels. This is due to low Coherence levels of < 0.4. Simply put, both the Imagery datasets being studied (from 2020 & 2019 respectively) did not have high enough correlation for the Backscatter Reflectances for the said pixels and hence, determining Subsidence by comparing the readings of the affected pixels wouldn't be appropriate and hence these pixels have not been interpreted i.e. shaded. Nonetheless, one can mentally interpolate the absent values from the trend observed in the surrounding shaded pixels.
Can you spot any additional patterns or have local area knowledge explaining this Subsidence phenomenon? Would you be aware the reason behind excessive Groundwater Extraction where the highest Subsidence readings have been observed? Would you like for me to study Subsidence over a larger period of time instead of the 1-year time period demonstrated in this study?
I hope this exercise helped you to be aware of the risk arising out of reckless Ground Water Consumption - 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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