利用永久散射體雷達干涉技術分析地表變形：以臺灣西南部平原為例

Abstract

近年來，差分合成孔徑雷達干涉技術已經成為地球科學研究上一項利於觀測地表變形的工具。透過所獲取的大量影像，有助於我們更能夠連續性的觀測地表變形。但是，雷達訊號隨著空間基線的增大和時間的增長，降低雷達訊號的相關性，限制了此項技術無法更廣泛的應用。永久散射體雷達干涉技術，利用萃取分散的穩定點目標，能有效克服時間以及空間上低相關的限制；因此，本研究將此項技術應用在所選取的研究區域內，以期能更清楚的了解地表變化的時空演變。 台灣位於歐亞大陸板塊與菲律賓海板塊之間的碰撞縫合帶，構造活動相當劇烈，提供了差分合成孔徑雷達干涉技術一個良好的材料來分析地表變形。本研究利用1996年至2000年間所接收的33張雷達影像，以永久散射體雷達干涉技術分析位於變形前緣的台南台地之地表變形；在144平方公里區域內，挑選約有1,650個永久散射點做為分析的點目標。分析結果顯示，台南台地上最大視衛星方向上的抬升速率為每年8公厘，將其座標轉換至垂直方向上的抬升速率，與層序沉積定年的結果相當接近，暗示此區長期抬升速率與短期抬升速率相近，屬於無震區。除此之外，靠近後甲里斷層的永久散射體的變形行為皆呈現出與季節性降雨有高度的相關性，其原因可能是斷層在雨季發生潛移活動的行為，呼應了此區為無震區的推論，但此推論還需要更多的測量結果才能確定。 除了構造活動外，人為活動也是造成地表位移的原因之ㄧ。台灣西南沿海的雲林縣是以農業為主的區域，因灌溉而長期抽取地下水造成此地區地層下陷的情況相當嚴重；因此，本研究再利用累加差分合成孔徑雷達干涉技術與永久散射體干涉技術，監測雲林地區1996年至1999年6月間的地層下陷時空演變。兩項結果都顯示出，位於內陸的褒忠鄉、土庫鄉以及麥寮鄉北部區域擁有最大的下陷量，約為每年6公分，沿海地區的台西鄉則呈現出每年1.58公分的下陷量，與精密水準測量結果相當吻合。此結果證明了利用此項技術，能夠有效的減少人力以及財力的花費，得到雲林地區地層下陷點狀以及面狀的資訊。

Differential Interferometric Synthetic Aperture Radar (DInSAR) technique has increasingly been applied in geosciences as a powerful tool to monitor land surface deformation. In addition, the large archives of SAR data enable us to monitor ground deformation continually. However, temporal and spatial decorrelations of radar signal have prevented this technique from more frequent utilization. An advanced InSAR processing technique that tracks the signals of discreted point-wise targets called Persistent Scatterers Interferometry Technique (PSInSAR) was herein applied to our research area in order to better understand the deformation patterns. Thus the PSInSAR method allows us to know the deformation of these discrete points that were minimally affected by decorrelation of radar signals through time. The active orogeny of Taiwan generated large amount of surface deformation that were partitioned throughout the island, which provides a great opportunity for analyzing tectonic displacements through interferometric methods. Here we present a PS-InSAR result deduced from 1996-1999 time spans in and around Tainan Tableland of southwestern Taiwan close to the deformation front. About 1,650 persistent scatterers are deduced from the processing of 33 SAR images (Tract: 232, frame, 3141) in an area of 144 km2. The maximum slant range displacement (SRD) rate along the radar line of sight (LOS) toward to satellite is about 8 mm which is close to the borehole data after transferring SRD rate to vertical deformation rate. Besides, the nonlinear deformation evolutions show a respectable correlation with seasonal rainfall on the Tainan Tableland near the Houchiali fault. One of the reasons might be the behaviors of fault creeping during the rainy seasons. These results indicate that the Tainan Tableland is an aseismic area. In addition, anthropogenic ground subsidence due to massive pumping of groundwater is one of problems in Taiwan. The Yunlin County located in the southwestern coastal region of Taiwan is one of the most counties with serious land subsidence because of the agricultural needs. Here, we also represented a both stacking DInSAR and PSInSAR results deduced from 1996-1999 time spans for monitoring of land subsidence in this area. Both DInSAR and PSInSAR results show the Baojhou, Tuku Township and northern Mailiao reveal a maximum subsidence rate of about 6 cm/yr along LOS and the Taishi Township located on the coastal area reveals a subsidence rate of 1.58 cm/yr, which is quite coincident with precise leveling result. These two results have proven that the effective reduction of labor and cost could be achieved by using this technique on monitoring land subsidence in Yunlin County.