台灣西南部,處於一個由隱沒構造帶轉為弧陸碰撞帶之板塊斜聚合區。屏東平原,尤其是在沿海的地區,快速的沉陷情形造成許多土地利用上的問題。 合成孔徑雷達干涉技術(InSAR)以其高解析度及公分級精度的優勢被廣泛的使用在大地測量上。而差分合成孔徑雷達(D-INSAR)則是衛星利用重複軌道的方式,量測地表變形之差分干涉技術。本研究中利用DIAPASON軟體進行相關處理,使用二軌跡法之差分干涉技術觀測屏東平原區地層下陷的情形。二軌跡法是使用研究區中既有的DEM,當作地形對;另外使用ESA所發射兩顆載有SAR系統的ERS1/2衛星所攝得的兩張影像作干涉處理,並轉換與DEM座標一致,作為變形對,兩者差分相減之後得到我們所需的變形相位資訊。 研究結果顯示,靠近林邊溪的出海口為屏東地區主要的下陷地區,由差分干涉圖與地下水位的分析可得知,當地下水位低的時候,干涉環愈明顯。從1996年1月31日至5月16日,平均月下陷量速率約為2.9 cm。而1995年至1999年的乾季平均每月下陷速率則為2.9至0.57 cm,下降速率有明顯趨緩的趨勢。而由其他四幅時間在溼季的差分干涉結果顯示,平均每月下降速率則為0.11~0.15 cm。 由D-InSAR結果顯示,1996及1999年1月至5月影像對中顯示為下陷部分的差分干涉環與細顆粒沉積物的厚度百分比等值線相比較,兩者之間具有很大的相似性,指示著在研究區域中細顆粒沉積物的壓密作用及超抽地下水的行為在地層下陷的情形中扮演著重要的角色。
Abstract Southwestern Taiwan is located in the transition zone between subduction to the south and collision to the north. The rapid subsidence of the Pingtung Plain, especially near the coast, has caused many land-use problem. InSAR (SAR Interferometry) is a geodetic tool now widely applied in earth surface deformation, which has the benefits of its high spatial resolution and centimetric accuracy. D-InSAR (Differential SAR Interferometry ) aims at the measurement of ground deformation using repeat-pass interferometry. We apply D-InSAR geodetic technique between 1995 and 2000 for land subsidence in the Pingtung Plain and perform the two-pass method by using an external DEM (Digital Elevation Model), in which we can subtract from the interferogram that is converted into DEM coordinates. We construct a differential interferogram using the DIAPASON software based on two SAR images from ERS-1 and/or ERS-2 satellites. The result of D-InSAR reveals that the critical subsidence region was located near the mouth of Linbien River. The analysis of D-InSAR result shows the fringe is obvious when the groundwater level is lowest during one year. The subsidence rate is about 2.9 cm/month in the dry season from January 31 to May 16, 1996. The subsidence rate decrease to about 0.57 cm/month form 2.9 cm/month in the dry season from 1995~1999. However, in the wet season , the subsidence rate is about 0.11~0.55 cm/month shown by 4 D-InSAR pairs. Our 4-month duration (January ~May) D-InSAR pattern is highly consistent with the isopach of fine-grained sediment thickness of core data from the Groundwater Monitoring Network Plan in Pingtung Plain (Huang, et. al., 1998) that the compaction of fine-grained sediments and the overdraft of groundwater play important roles of observed subsidence.
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