Assessing the Accuracy of ALOS/PALSAR-2 and Sentinel-1 Radar Images in Estimating the Land Subsidence of Coastal Areas: A Case Study in Alexandria City, Egypt

Abstract

Recently, the Differential Interferometric Synthetic Aperture Radar (DInSAR) technique is widely used for quantifying the land surface deformation, which is very important to assess the potential impact on social and economic activities. Radar satellites operate in different wavelengths and each provides different levels of vertical displacement accuracy. In this study, the accuracies of Sentinel-1 (C-band) and ALOS/PALSAR-2 (L-band) were investigated in terms of estimating the land subsidence rate along the study area of Alexandria City, Egypt. A total of nine Sentinel-1 and 11 ALOS/PALSAR-2 scenes were used for such assessment. The small baseline subset (SBAS) processing scheme, which detects the land deformation with a high spatial and temporal coverage, was performed. The results show that the threshold coherence values of the generated interferograms from ALOS-2 data are highly concentrated between 0.2 and 0.3, while a higher threshold value of 0.4 shows no coherent pixels for about 80% of Alexandria’s urban area. However, the coherence values of Sentinel-1 interferograms ranged between 0.3 and 1, with most of the urban area in Alexandria showing coherent pixels at a 0.4 value. In addition, both data types produced different residual topography values of almost 0 m with a standard deviation of 13.5 m for Sentinel-1 and −20.5 m with a standard deviation of 33.24 m for ALOS-2 using the same digital elevation model (DEM) and wavelet number. Consequently, the final deformation was estimated using high coherent pixels with a threshold of 0.4 for Sentinel-1, which is comparable to a threshold of about 0.8 when using ALOS-2 data. The cumulative vertical displacement along the study area from 2017 to 2020 reached −60 mm with an average of −12.5 mm and mean displacement rate of −1.73 mm/year. Accordingly, the Alexandrian coastal plain and city center are found to be relatively stable, with land subsidence rates ranging from 0 to −5 mm/year. The maximum subsidence rate reached −20 mm/yr and was found along the boundary of Mariout Lakes and former Abu Qir Lagoon. Finally, the affected buildings recorded during the field survey were plotted on the final land subsidence maps and show high consistency with the DInSAR results. For future developmental urban plans in Alexandria City, it is recommended to expand towards the western desert fringes instead of the south where the present-day ground lies on top of the former wetland areas.

Land Subsidence Monitoring Method in Regions of Variable Radar Reflection Characteristics by Integrating PS-InSAR and SBAS-InSAR Techniques

“…The coherence calculating, flatting, filtering, and minimum phase unwrapping based on minimum cost flow (MCF) [34][35][36] are carried out for 272 interferometric pairs. Or-bital refinement and residual phase removal are performed on the unwrapping results, and the GCP points are used to accurately estimate the orbital parameters and deduct the residual flat phase and orbit errors.…”

A deep learning-based interferometric synthetic aperture radar framework for abnormal displacement deformation prediction of bridges

“…Additionally, the max normal baseline was set as 45% because the Sentinel-1A satellite has a stable orbit and produces less deviation (P Zhang et al, 2022). In this study, the minimum cost flow (MCF) method (Darwish et al, 2021: 2), is used for phase unwrapping. The phase unwrapping results are then subjected to orbital refinement and residual phase removal.…”

Coastal Environment Impact on the Construction Materials of Anfushi’s Necropolis (Pharos’s Island) in Alexandria, Egypt

“…Additionally, Alexandria's land subsidence ranges from 0 to −5 mm yearly. From 2017 to 2020, the vertical displacement/deformations in Alexandria were −60 mm with an average of −12.5 mm and a mean displacement rate of −1.73 mm/yr [62]. Moreover, another study suggested that Alexandria was subjected to 0.4 mm/yr on average and up to 2 mm/yr locally over the last decade after analysis using GPS and persistent scattered interferometry data [63].…”