Ground displacement, or the variations in altitude of the land surface, is a potentially damaging hazard, mainly resulting from a diversity of natural or man-made triggers. Natural events such as earthquakes, soil compaction, isostatic glacial adjustment, erosion, tectonic movements, sea level rise, and permafrost thawing, but also human activities such as excessive pumping of groundwater and geothermal fluids, underground mining, construction, and reclamation processes(Zhao, Ma et al. 2019, Li, Zhou et al. 2022). Coastal areas of any country are significant regions due to fluctuations in hydrological, geological, climatic, and economic conditions; the shoreline's position is dynamic in both space and time. Due to ground subsidence from natural causes such as typhoons, storm surges, and global sea level rise, these low-lying coastal communities are now more susceptible to flooding risks (Tessler, Vörösmarty et al. 2015). In addition, extensive human activities have altered the geomorphology of coastal regions significantly, endangering coastal security (Temmerman and Kirwan 2015). In several coastal regions, such as Charlestown, Rhode Island (RI), USA, the Italian coastal plains, the coastal zones of Poland, southeast Queensland, USA, New York, USA, and Shanghai, China, coastal inundation threats have been extensively analyzed and mapped (Wang, Gao et al. 2012, Mills, Mutafoglu et al. 2016, Antonioli, Anzidei et al. 2017, Grilli, Spaulding et al. 2017, Lin and Shullman 2017).
The urban coastal area of Pakistan is also prone to land deformation disasters (Kanwal, Ding and Zhang 2018, Ali, Shahzad et al. 2021, Amin, Shahzad et al. 2022, Tirmizi, Khan et al. 2023), which pose a serious threat to lives and properties. A network of barrier islands, oriented NW-SE, surrounds Karachi city with about 270 km of coastline which lies in the Indus Delta Region, which is currently home to 6.8% of Pakistan's total population, making it the fifth largest coastal metropolis (Waqas, Nazeer et al. 2019). Because of its proximity to a plate border and numerous surrounding tectonically active features, the city is vulnerable to earthquakes. It is conceivable to make geological and demographic comparisons with another large metropolis with a high seismic risk, such as Los Angeles, California (Bilham, Lodi et al. 2007). On the other hand, the reclamation of this sea trade and industrial center region is accelerating, and within eight years, urban areas have undergone a significant transformation and grown by 40% (2009–2017) (Rizvi, Fatima et al. 2020). Global sea level rise leads to erosion activities in coastal areas as they are mutually interconnected. Globally, the average sea level has reportedly risen at a rate of 3.2 mm/year during the past ten years, up from 1.7 mm/year between 1900 and 2010 (Hay, Morrow et al. 2015). Similarly, according to the Intergovernmental Panel on Climate Change (IPCC), by 2100, the average global sea level will rise by 0.43 to 0.84 meters in the present situation and by 0.61 to 1.10 meters in the worst-case scenario (Oppenheimer, Glavovic et al. 2019). Excessive groundwater extraction is also a significant contributing factor in land subsidence (Belperio 1993), which was assessed in the area under investigation in this research (Amin, Shahzad et al. 2022, Hussain, Chen et al. 2022).
The benefits of Interferometric Synthetic Aperture Radar (InSAR) include extensive area coverage, high temporal resolution, low cost, and capabilities for all-weather monitoring. InSAR demonstrates major benefits in a variety of geoscience disciplines, including the study of seismicity (Béjar-Pizarro, Álvarez Gómez et al. 2018), the investigation of volcanoes (Hooper, Segall and Zebker 2007), the analysis of artificial construction deformation (Zhang, Gong et al. 2008), and studies on the surface deformation caused by mining for underground resources (Zerbini, Richter et al. 2007). In the case of investigation of coastal urban areas, different researchers have applied several InSAR techniques to estimate and map the ground displacement at several different major locations (Grilli, Spaulding et al. 2017, Emil, Sultan et al. 2021, An, Jiang et al. 2023) tested Small Baseline Subset (SBAS), (Ali, Shahzad et al. 2018, Zhao, Ma et al. 2019) incorporated differential SAR interferometry (DInSAR) (Sun, Zhang et al. 2017, Li, Zhou et al. 2022, Areggi, Pezzo et al. 2023), employed persistent scatterer (PSInSAR) (Cavalié, Sladen and Kelner 2015), used New Small Baseline Subset (NSBAS) and (Wu, Yang et al. 2020) used a variety of SAR sensors and a more advanced Multi-temporal (MT-InSAR) technology to track and measure deformation.
Time-series InSAR methods were carefully studied, and PSInSAR approach was opted for this research work, because the PSI method avoids the conventional unwrapping error by carrying out the inversion model directly on the differential interferometric phase that has not been wrapped. Permanent scatterers (PS) are the primary target of the PSI method, which primarily focuses on targets that maintain high coherence in the interferometric data set (Hu, Chen and Zhang 2019, Ramzan, Fan et al. 2022). It has been demonstrated that PSI can correctly describe linear deformation, whether it is fast or slow, in areas with a lot of "permanent scatters" (Hooper, Bekaert et al. 2012, Yan, Doin et al. 2012, Chaussard, Wdowinski et al. 2014).
Similar to the standard PSInSAR, this method provides for the estimation of velocity but is only applicable to the specified moving window. Consequently, the method makes it possible to estimate a general non-linear motion, which is key for this extensive study (Hu, Chen and Zhang 2019). A non-linear approach also detects minute motions on the structures that were previously unnoticed by traditional monitoring methods (Bakon, Perissin et al. 2014). The area under investigation is evolving exponentially, the population and building area have doubled in the last 20 years, where non-linear deformation is expected because of high load on the ground and uncontrolled extraction of groundwater prone the area to subsidence. After careful analysis of the nature of the study area, this approach was preferred in this research work. Lake of GPS, building, and large-scale groundwater table data made our analysis challenging for us which could be overcome in future research works.
This study employed PSInSAR processing of freely available Sentinel-1 data to monitor non-linear ground subsidence in the urban coastline zone of Southern Pakistan from February 2017 to June 2023 with both ascending and descending tracks. PSInSAR results provided a promising basement for our comparative analysis with the most effective parameters assessed in this study. The reclaimed coastal area owns a huge number of PS points, which was helpful to analysis and high populated zones were marked in optical imagery and PS outcomes were overlaid to extract the ground deformation activities. Areas in the premises of fault lines and the zones where the groundwater table is depleting continuously were investigated carefully, which provided a positive correlation with InSAR results as also found by other researchers. A long-term method can evaluate general deformation patterns and causes of deformation more extensively than a short-term sequence analysis, which is important in precisely pinpointing the settlement region.