以時域相關點雷達干涉量測研究彰化、雲林與嘉義地區之地層下陷

Abstract

台灣西部過去因不當抽用地下水造成大範圍的地層下陷情形，傳統的監測方法採用點狀式的大地監測方法（包括水準測量、GPS），上述方法會因監測密度的高低影響監測的精度。本研究以時域相關點雷達干涉技術（Temporarily Coherent Point SAR Interferometry, TCPInSAR）對地表進行監測，該方法不僅可獲取大範圍地表變形資訊，同時因為不需經由相位解纏（Phase Unwrapping），避免可能存在相位模糊（Phase Ambiguities）的錯誤解算。 本研究使用了ALOS軌道446從2006至2011年19幅衛星影像獲取彰化地區TCP密度約為361 pixel/km2，高於水準點密度0.2 point/km2，以及軌道447從2007至2011年15幅衛星影像獲取雲林與嘉義地區TCP密度分別為196、250 pixel/km2，而該區域水準點密度分別為0.3與0.2 point/km2。本研究將所獲得衛星視距方向（Line of Sight, LOS）變化量，結合GPS資料簡單內插模擬區域內水平速度場影響後，予以扣除水平向變化，轉換成垂直向變化，比對TCP成果與水準量測成果差異之均方根誤差（Root Mean Square Error）分別為0.9、0.6、0.8 cm/year，顯示該方法可有效地應用在地表的變形監測。

Western Taiwan is suffering severe subsidence caused by groundwater withdrawal. The conventional geodetic sensors, including leveling and GPS, have been used to monitor land subsidence. Those sensors can only deliver point-wise displacements and the monitoring capacity of displacement is limited by point density. In this study, we applied temporarily coherent point SAR interferometry (TCPInSAR) to derive long-term deformation rates over Changhua, Yunlin, and Chiayi. The proposed method can estimate deformation parameters without the effect of unresolved phase ambiguity resolut. The study uses 19 ALOS PALSAR images along track 446 from 2006 to 2011 to derive line of sight (LOS) deformation in Changhua. The pixel density over Changhua is 361 pixel/km2, compared to 0.2 point/km2 of the leveling benchmarks. The study also utilized 15 ALOS PALSAR images at 447 track from 2007 to 2011 derive LOS deformation in Yunlin and Chiayi. The pixel density over Yunlin and Chiayi are 196 and 250 pixel/km2, compared to 0.3 and 0.2 point/km2 of the leveling benchmarks respectively. We use GPS data to reduce the effect of the horizontal displacement on the LOS deformation to generate vertical displacement. The vertical displacements from TCPInSAR matches the leveling result to 0.9, 0.6, and 0.8 cm/year (RMSE) in Changhua, Yunlin, and Chiayi. This research suggests that TCPInSAR with ALOS PALSAR data can effectively monitor land deformation in a large area like western Taiwan.