| 摘要: | 臺灣位於菲律賓海板塊及歐亞大陸板塊的聚合邊界，複雜的板塊運動使得臺灣有相當高的變形速率及常有斷層活動造成的地震。此外，西南部地區由於農業灌溉及養殖魚業所需大量用水，超抽地下水造成地層下陷，進而危害到建築物、公路及管線的安全。因此，若能有效地大範圍地偵測地表變形，可有助於減少災害的發生。合成孔徑雷達差分干涉技術(Differential Interferometric Synthetic Aperture Radar, DInSAR) 常被利用來偵測地表變形，在過去許多研究中皆證明它能觀測地表大範圍且精準的三維變動。由歐洲太空總署(European Space Agency, ESA)在2014年發射的Sentinel-1A提供C波段合成孔徑雷達(SAR)影像，並使用Interferometric Wide Swath (IW)模式作為陸地上的觀測模式，IW 模式擁有長170公里及寬250公里的大範圍覆蓋面積，而且Sentinel-1A的短再訪週期每12天可提供一幅臺灣地區的影像。因此利用Sentinel-1A的大覆蓋面積及再訪週期間隔短的特性，可快速且有效地取得整個臺灣的變形圖。
然而，臺灣位於電離層赤道異常地區(Equatorial Ionization Anomaly, EIA)，不同時間的全電子含量(Total Electron Content, TEC)會隨著EIA強弱而改變，兩幅影像中若帶有TEC差異將會在干涉圖(interferogram)上產生額外的相位條帶，進而影響相位解纏及變形圖的結果。此研究比較了全球電離層模式(Global Ionosphere Map, GIM)及臺灣區域電離層模式(Taiwan Regional Ionospheric Map, TRIM)之TEC，模擬電離層在干涉圖中產生的額外相位，並在原始的干涉圖中進行修正，得到電離層修正後的干涉圖。利用2016及2017年Sentinel-1A升軌影像所產生的47對干涉圖中，有2對明顯受到電離層效應並可以利用TRIM的TEC進行消除。經本研究統計，若兩幅影像TEC值在350公里穿透層於南北向的梯度差超過16 TECU (TECU=1016電子數/m2)，則會在干涉圖中產生明顯的電離層效應。最後，利用Global Positioning System (GPS)地面站的時間序列變化比較電離層修正前及修正後的結果，結果顯示經電離層修正後的變形量與地面觀測較為一致，顯示電離層確實會造成變形量的誤差。經修正後，地表垂直向變動的均方根誤差(Root Mean Square Error, RMSE)由修正前的42公厘減少為11公厘。
;Taiwan is located at the convergent boundary between the Eurasian Plate and the Philippine Sea Plate. This tectonically active region produces high surface deformation rate and fault activities. In addition, over-pumping of groundwater for agriculture and aquaculture needs in the southwestern part of Taiwan has induced extra land subsidence in the past 20 years. A systematic monitoring method for land deformation using advanced remote sensing technology is thus needed. The Differential Interferometric Synthetic Aperture Radar (DInSAR) technique has been proven useful to measure surface deformation in an effectively way. Sentinel-1A satellite launched by the European Space Agency (ESA) is one of them acquiring images with various pre-defined scenarios to serve different thematic domains. It operates mostly in the Interferometric Wide Swath (IW) mode with dual polarization in east Asia, which provides a broad coverage with 250 km in range and 170 km in azimuth direction. It orbit characteristics allow users to produce interferograms for most of terrestrial areas every 12 days. However, Taiwan is situated on the north edge of the Equatorial Ionization Anomaly (EIA) Region. The Total Electron Content (TEC) irregularity between snapshots causes ionospheric effect on the interferograms. The ionospheric residuals induce extra fringes in the interferograms that further affect unwrapping and displacement result.
In this study, we utilize global and regional ionospheric vertical TEC maps, namely the Global Ionosphere Map (GIM) released by the International GNSS Service and the Taiwan Regional Ionospheric Map (TRIM) provided by the Ionospheric Radio Science Laboratory of National Central University, to compensate ionospheric phase in the interferograms. Our results show that TRIM is capable to reduce most of extra fringes caused by the ionospheric effect. Comparing these two types of TEC map, TRIM presents more accurate and detailed ionospheric patterns than GIM. In a series of 47 interferograms generated from Sentinel-1A ascending mode in 2016–2017, we conclude that when the TEC gradient in south-north direction exceeds 16 TECU (1016 electrons/m2), it will cause extra fringes in the interferogram. Finally, the Global Positioning System (GPS) continuous station measurements are used to compare with DInSAR displacement for the performance of ionospheric correction. The results show that after correction the displacement is more consistent with ground truth. It also confirms that the ionosphere may cause serious error in the displacement estimates. After ionospheric correction, the root-mean-square error (RMSE) of estimated vertical motion is reduced from 42 mm to 11 mm. |
