以持久性散射體雷達干涉分析抽取地下水引起之地表變形──以臺北盆地捷運工程為例

隨著臺北盆地的人口密度提高，導致民生用水的需求增加，在臺北盆地有約2,200口抽水井。由於過去過度抽取地下水，盆地的地表沉降曾高達5公分/年。在1970年，臺灣政府開始控制地下水用量，而翡翠水庫在1987年完工，也成功減緩了臺北盆地限制抽水後供水不足的問題。雖然政府開始限制地下水抽取，但臺北盆地的地下水位仍然波動變化，肇因於工程大型深開挖或其他工程用途的抽水。近年盆地內發展的各種基礎設施和密集的交通網絡，需短時間內集中抽水來避免地下水的上浮力影響工程安全。因此，臺北盆地的地下水在工程抽水的時間段內都有明顯的下降現象。本研究的目的是利用持久性散射體雷達干涉分析在短時間內大量抽水是否影響盆地的表面變形。在本研究中，利用持久性散射體雷達干涉技術分析2011年5月18日至2015年4月2日間共計37幅X波段雷達圖像的相位差。並利用水利署30口地下水觀測井的水位紀錄與雷達影像計算出的視衛星方向地表變形，對比結果發現影響臺北盆地表面變形的主要因素是水位高程變化。此外，以桃園捷運臺北站的共構大樓開挖工程為例，根據雷達圖像所計算出的地表變形時間序和施工區域監測的地下水位井分析，開挖工程前大量的抽水量將引起地面變形。由於景美及五股地層主要由高孔隙率的沉積物組成，抽水會使得孔隙水壓降低，並造成上層覆蓋的黏土層孔隙水壓力隨著時間消散造成地表變形。本研究發現抽水的影響範圍大於工區面積的五倍，但其中地表變形與地下水位並沒有計算到時間延遲效應，推測短期間內大量抽水所造成的土壤壓密尚未完成，目前所觀察到的地表變形量，來自於地層材料的彈性變形。因此透過彈性變化量以及現地土壤參數來計算，發現所計算的地表變形量大於現地地表變形，可能源自於現地工程工法降低部分地下水對於鄰近地區地表變形的影響。

關鍵字

持久性散射體雷達干涉；地下水；地表變形

並列摘要

With high population density, there were 2,200 wells in the Taipei basin used for pumping. The overuse of ground water lead to the land subsidence rate up to ~5 cm/yr. In 1970, the government in Taiwan started to control the extraction of groundwater in Taipei city. Followed by the completion of Feitsui Reservoir in 1987, the problem of insufficient water supply in the Taipei basin was solved. Although the government had already begun to limit groundwater pumping since 1968, the groundwater in the Taipei Basin was still undulating. This is caused by pumping for large deep excavation site or other engineering use. Taipei, capital of Taiwan, is filled with a variety of infrastructure and intensive transport networks. Before these structures are built, intensive pumping is needed to remove the underground water and cancel the floating force of water. Therefore, the groundwater level changes in the Taipei Basin indicates that there were significant drops in water level in many time intervals. Whether intensive pumping within a short period will affect the surface deformation is the objective of this study. In this study, persistent scatterer interferometry technique is used for processing 37 high resolution X-band radar images to characterize deformation map in the period between May 2011 to April 2015. From the ground table records of 30 wells in the Taipei basin, the results indicated that the main factor to the surface deformation of the Taipei basin is the elevation change of water table. Moreover, in the case of the Taipei metro construction, according to the analytical results of radar image and the monitoring well of the construction, the high water pumping before the underground construction project will inflict surface deformation. It is noticeable that, the Jingmei and the Wuku formation are composed of sediments with high porosity. The pore water pressure would disappear during water pumping, which dissipates the pore water pressure of the clay layer above. Thus, the actual land subsidence caused by water pumping would be five times than the underground construction areas. In conclusion, the values calculated by the elasticity theory is greater than that of the monitoring values, because the soil compaction is not yet completed.

並列關鍵字

PS-InSAR ； Groundwater ； Surface deformation

參考文獻

Chiang, C. J., Chang, M. H., Lin, T. C., Huang, C. C., Su, T. W. and Chen, J. E., 2012. A Primary Study on the Hydrogeology of the Taipei Basin.

Teng, L. S., Yuan, P. B., Chen, P. Y., Peng, C. H., Lai, T. C., Fei, L. Y., and Liu, H. C., 1999. Lithostratigraphy of Taipei Basin deposits. Cent. Geol. Surv. Spec. Publ, 11, pp. 41-66. (in Chinese with English abstract)

Angelier, J., Lee, J. C., Chu, H. T., Hu, J. C., Lu, C. Y., Chan, Y. C., Lin, T. J., Font, Y., Deffontaines, B., Tsai, Y. B., 2001. Le séisme de Chichi (1999) et sa place dans l'orogène de Taiwan. Comptes Rendus de l'Académie des Sciences-Series IIA-Earth and Planetary Science, 333(1), pp. 5-21.

Hu, J. C., Yu, S. B., Chu, H. T. & Angelier, J., 2002. Transition tectonics of Taiwan. Geol. Soc. Am. Special Papers 358, pp. 149-162.

Amelung, F. S., Jónsson, H., Zebker, H. and Segall, P., 2000. Widespread uplift and "trapdoor" faulting on Galapagos volcanoes observed with radar interferometry. Nature, 407(6807), pp. 993-996, doi:10.1038/35039604.

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以持久性散射體雷達干涉分析抽取地下水引起之地表變形──以臺北盆地捷運工程為例

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