| 摘要: | 衛星遙測在近十年來扮演一個重要的腳色，利用合成孔徑雷達觀測地表變形非常有 效。在 2014 年 ESA 發射 Sentinel-1 衛星之前，高解析度的合成孔徑雷達影像是很昂貴的， 這種影像可以提供一個很全面的地表變形觀測。
在這個研究中，利用合成孔徑雷達干涉用以觀測泰國曼谷市區的地表變形。合成孔 徑雷達干涉在時間上和空間上有高度的不確定性。這個研究顯示利用 Sentinel-1A 合成孔 徑雷達的短時間以及空間基線，可以得到很好的測量結果。在這研究中，利用 2017 年 3 月到 12 月，空間基線小於 50 公尺，時間基線小於 120 天的 Sentinel-1A 合成孔徑雷達影 像。因此空間和時間上的失相關性非常高。2015~2016 年的資料在配準影像上有一些問題， 因此這個研究只有用 2017 年的資料，因為精準的配準影像對於精確的量測相位差異及消 除造訊非常重要。
合成孔徑雷達量測的結果顯示，在曼谷市區，Samut Prakarn 有急遽的下陷，每年約 20~30 毫米，在曼谷中心的東側 Chao Phraya river 顯示相對緩慢的下陷速率約每年 15 毫 米。這結果也發現Bang Kho Laem, Sathon and Yannawa District下陷速率高達每年40毫米。 此外，對比 Royal Thai Survey Department 2016 年水準測量的結果顯示合成孔徑雷達干涉 的測量結果與其只有微小的差異。這個研究提供曼谷市區近期地層下陷速率，也說明 Sentinel-1 資料在地層下陷速度的應用，可以地層下陷區域的災害防治和土地管理有相當 的貢獻。;Satellite remote sensing plays an important role during the last decades. Its potential had been demonstrated to be an effective data for land deformation monitoring, by using the synthetic aperture radar (SAR) image. However, the most high-resolution SAR images are costly until Sentinel-1 SAR imagery freely offered by ESA since 2014, which covers wide-area coverage connected to a relatively high spatial resolution, therefore allowing obtains a comprehensive outlook of the deformation phenomena.
In this study, interferometric synthetic aperture radar (InSAR) technique were used to monitor land subsidence over Bangkok Metropolitan Area, Thailand. The application of InSAR technique in measuring land deformation has higher uncertainty when spatial and temporal decorrelation are involved. This study demonstrated that, with applying Sentinel-1A SAR data, image pairs can be collected with smaller perpendicular baselines and lower temporary discrepancy, leading to better measuring results. In this study, Sentinel-1A SAR data were collected from March to December in 2017 with the baseline within 50 m and 120 days. Therefore, spatial and temporal decorrelation have a negligible contribution. Because well co-registration is important for the accurate determination of phase difference and for noise reduction, data in 2017 was only used in this study, due to poor co-registration of data obtained in 2015-2016.
InSAR measuring results reveal that, in Bangkok Metropolitan Area, Samut Prakarn has been subsiding rapidly at the rate of 20-30 mm/yr, and Central Bangkok on the east of Chao Phraya river appears to be subsiding relatively slowly at rates around 15 mm/yr. Results also indicate fast subsiding areas in Bang Kho Laem, Sathon and Yannawa District, with strong subsidence around 40 mm/yr. In addition, the InSAR measurements were compared with Royal Thai Survey Department (RTSD) leveling rates in 2016, showing minor difference between the two data sets. This study maps the recent land subsidence rates over Bangkok Metropolitan Area, and also illustrates the applicability of Sentinel-1 data in land subsidence monitoring, which can be significantly contribute to the hazard prevention and land management for low land areas in Thailand. |
