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Student Spotlight Spring 2022

Andrew Fore

The newest L3Harris Geospatial student spotlight, Vivek Agarwal, is passionate about civil and survey engineering, water resource engineering and remote sensing. Vivek combined his passions into his Ph.D. studies as a scholar in both the Nottingham Geospatial Institute (NGI) and the Food Water Waste (FWW) Department at The University of Nottingham in England.

His Ph.D. work focused on studying groundwater properties and behaviors using geospatial tools like Permanent Scatterers Interferometric Synthetic Aperture Radar (PSInSAR), GRACE gravity anomalies, in-situ observations and data analysis visualization. Vivek relies on ENVI® software because “it is a useful platform for analyzing remote sensing data, especially Synthetic Aperture Radar (SAR) analysis in ENVI SARscape.” While he received his Ph.D. this past January from the University of Nottingham, he is continuing his work as a research associate on intelligent resource use to deliver net zero plus through geospatial data mining in the FWW department.

Vivek has a master’s degree in civil engineering (Geomatics) from the Indian Institute of Technology (IIT) Roorkee, India. His Ph.D. is under the supervision of Professor Stuart Marsh and Professor Rachel Gomes at the University of Nottingham. Not one to take all the credit for his accomplishments, Vivek adds “I am very thankful to my parents, family, teachers and especially my uncle (Mr. Hariom Agarwal) for their constant blessings and support throughout my career.”

Vivek’s recent research paper titled ‘Comparative Study of Groundwater-Induced Subsidence for London and Delhi Using PSInSAR’ was published in the journal, Remote Sensing (https://doi.org/10.3390/rs13234741). The InSAR processing was done using ENVI SARscape processing and analysis software. Vivek secured the funding for data and ENVI SARscape by collaborating with the European Space Agency (ESA) Earth Observation data hosted processing for science (EOhopS), a cooperative program between ESA and L3Harris Geospatial. A part of Vivek’s research is discussed here.

PROBLEM:

For decades, groundwater has been extensively exploited for domestic, agricultural and industrial purposes; this has necessitated subsequent artificial recharge to balance the groundwater depletion and control land subsidence. Long-term groundwater exploitation and recharge in confined aquifers alter the piezometric and pore pressures in aquifers.

According to the effective-stress principle, aquifer systems consolidate owing to these changes in their properties, leading to land subsidence. Thus, groundwater variation can cause land-surface movement, which in turn can cause significant and recurrent harm to infrastructure and the water storage capacity of aquifers. Therefore, it is necessary to understand land subsidence and the compaction process caused by groundwater exploitation and recharge.

The capital cities in England (London) and India (Delhi) are witnessing an ever-increasing population that has resulted in excess pressure on groundwater resources, thus posing a threat of land subsidence. However, the subsurface geology and infrastructure of the two cities are different, and yet the responses of groundwater to anthropogenic activities and its subsequent effect on land movement is not well understood.

To the best of our knowledge, no previous attempts have been made to simultaneously examine the specific causes of groundwater-induced land subsidence for London alongside that of NCT-Delhi using PSInSAR. In this study, the groundwater-induced land subsidence for the two major cities is studied between October 2016 and October 2020 using a variety of geospatial techniques such as PSInSAR, GIS, spatio-temporal analysis, mathematical modelling and statistical analysis.

STUDY AREA AND METHOD:

Figure 1: Study area showing the administrative boundary of (a) London and (b) NCT-Delhi. The red boundary shows the extent of Sentinel-1 data processed, and the dots show the location of observed groundwater wells.

Figure 2: Methodology

RESULTS:

Figures 3 (a) and 3 (b) display the land displacement velocity map for London and NCT-Delhi obtained using 95 and 99 Sentinel-1 SAR images, respectively. The movement of land in the direction of sensor (uplift) and away from the sensor (subsidence) is represented by positive (green color) and negative values (red color), respectively.

Figure 3: Land movement map obtained using Sentinel-1 data for (a) London and (b) NCT-Delhi. The green areas depict uplift, while the red areas depict subsidence. The black rectangular boxes show the selected sites for case studies.

Both London and NCT-Delhi are heavily built-up urban areas and therefore prove to be ideal sites for PSInSAR analysis. For both cities, even though the land movement is stable overall, it is predominantly spatially variable with distinct chunks of displacement in the form of either uplift or subsidence. Even though the land movement is small (mm-level), it is identifiable from the Sentinel-1 data

Several interesting features are identified from the Sentinel PSInSAR measurements for both cities. These land movement features are identified as case studies, the locations of which are marked with black rectangular boxes in Figure 3. Only results for the area marked as L1 are discussed here. For all the case studies please refer to the full paper (https://doi.org/10.3390/rs13234741).

Figure 4: For Northern Line Extension: (a) PSInSAR land movement map, (b) groundwater change map, (c) time-series of land uplift (L1-b), (d) time-series of land subsidence (L1-a). The black triangles in (a), represents the location of the two main dewatering shafts required for placing the tunnel boring machine.

The Northern Line Extension (NLE) between Kennington and Battersea was proposed to help regenerate the Vauxhall, Nine Elms, and Battersea areas and was scheduled to be completed by the end of 2021. Figure 4 (a) shows the displacement velocity map obtained from Sentinel-1 images between October 2016 and October 2020. It highlights distinct subsidence and uplift pattern near Battersea Power Station and Kennington, respectively. To help analyze these patterns, the time series of surface deformation for both the uplifting and subsiding areas were also extracted (Figure 4 (c) and 4 (d)).

The main tunnelling work of the NLE consisted of creating two tunnels between Battersea and Kennington Park. The construction for the NLE began in July 2016 and required dewatering of the ‘deep’ aquifer, which includes the lower part of the Lambeth group. The dewatering shafts for the NLE are located on the northern edge of Kennington Park, and the location of the two main shafts required for placing the tunnel boring machines are shown in Figure 4 (a) (black triangles). The geology of the area is relatively complex, with several faults, buried hollows and laterally discontinuous superficial strata. The underground construction work, tunnelling shafts, and groundwater extraction contributed to the land motion pattern observed in this area.

In Kennington, the average land motion trend during the observed time-period is that of uplift (Figure 4 (a)), which is concurrent with the change in groundwater level (Figure 4 (b)). However, the deformation time series exhibits phases of both subsidence and uplift during this period (Figure 4(c)). Specifically, the ground subsided during 2016-2017, before continuously uplifting since November 2017. This motion corresponds to subsidence due to dewatering during the construction of the tunnels, followed by groundwater rebound (heave) once the dewatering ceased in November 2017.

Around Battersea Power Station, the time series (Figure 4(d)) shows a linear trend of land subsidence between 2016 and 2020. The surface displacement here is consistent with the decrease in the groundwater level during this period (Figure 4(b)). It is most likely due to the groundwater abstraction that was undertaken to dewater the locality for the NLE tunnelling. The construction activities around Battersea Power Station are still ongoing; hence, the groundwater extraction and associated ground deformation can be seen to continue beyond the observed time-period.

OUTCOMES:

While London and Delhi are superficially different in terms of civil engineering, the response of their groundwater to engineering decisions (such as underground metro construction) and how that is reflected in the change in surface-level, tells a similar story. This suggests that groundwater change and resulting subsidence may be a universal effect, which we might anticipate observing in other major cities worldwide that are subjected to similar engineering decisions.

Some InSAR deformation relating to groundwater results have been presented before for London, but not in NCT-Delhi. The purpose of the study was to demonstrate that a groundwater and subsidence inter-relationship generally holds true. The study also demonstrates that ENVI SARscape is a powerful, user-friendly tool to analyze SAR remote sensing data, and this study can serve as a guideline to government agencies to identify the areas and extent of groundwater-induced land subsidence and take proper steps to mitigate them.

 

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