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.

中文翻译：

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评估ALOS / PALSAR-2和Sentinel-1雷达图像在估计沿海地区地面沉降中的准确性：以埃及亚历山大市为例

近年来，差分干涉合成孔径雷达（DInSAR）技术被广泛用于量化土地表面变形，这对于评估对社会和经济活动的潜在影响非常重要。雷达卫星以不同的波长工作，并且每个卫星提供不同水平的垂直位移精度。在这项研究中，对Sentinel-1（C波段）和ALOS / PALSAR-2（L波段）的精度进行了研究，以估计埃及亚历山大市研究区域沿岸的地面沉降率。总共使用了9个Sentinel-1和11个ALOS / PALSAR-2场景进行了评估。执行了小的基线子集（SBAS）处理方案，该方案可以检测具有高时空覆盖率的土地变形。结果表明，从ALOS-2数据生成的干涉图的阈值相干值高度集中在0.2到0.3之间，而较高的阈值0.4则表明约80％的亚历山大市区没有相干像素。但是，Sentinel-1干涉图的相干值在0.3到1之间，亚历山大市的大部分市区显示​​相干像素为0.4。此外，使用相同的数字高程模型（DEM），这两种数据类型都产生了不同的接近0 m的残余地形值，Sentinel-1的标准偏差为13.5 m，ALOS-2的标准偏差为-20.5 m（33.24 m）和小波数。因此，使用Sentinel-1的阈值为0.4的高相关像素估计最终变形，该阈值可与约0的阈值相媲美。使用ALOS-2数据时为8。从2017年到2020年，沿研究区域的累积垂直位移达到-60 mm，平均为-12.5 mm，平均位移速率为-1.73 mm /年。因此，发现亚历山大海岸平原和市中心相对稳定，地面沉降速率范围为0到-5 mm /年。沿马里奥特湖和前阿布奇尔礁湖的边界发现最大下沉速率达-20毫米/年。最后，在实地调查中记录的受影响建筑物被绘制在最终的地面沉降图上，并与DInSAR结果高度吻合。对于亚历山大市未来的发展城市规划，建议将其扩展到西部沙漠边缘，而不是现在的地面位于以前的湿地地区之上的南部。从2017年到2020年，沿研究区域的累积垂直位移达到-60 mm，平均为-12.5 mm，平均位移速率为-1.73 mm /年。因此，发现亚历山大海岸平原和市中心相对稳定，地面沉降速率范围为0到-5 mm /年。沿马里奥特湖和前阿布奇尔礁湖的边界发现最大下沉速率达-20毫米/年。最后，在实地调查中记录的受影响建筑物被绘制在最终的地面沉降图上，并与DInSAR结果高度吻合。对于亚历山大市未来的发展城市规划，建议将其扩展到西部沙漠边缘，而不是现在的地面位于以前的湿地地区之上的南部。从2017年到2020年，沿研究区域的累积垂直位移达到-60 mm，平均为-12.5 mm，平均位移速率为-1.73 mm /年。因此，发现亚历山大海岸平原和市中心相对稳定，地面沉降速率范围为0到-5 mm /年。沿马里奥特湖和前阿布奇尔礁湖的边界发现最大下沉速率达-20毫米/年。最后，在实地调查中记录的受影响建筑物被绘制在最终的地面沉降图上，并与DInSAR结果高度吻合。对于亚历山大市未来的发展城市规划，建议将其扩展到西部沙漠边缘，而不是现在的地面位于以前的湿地地区之上的南部。