以多時間尺度重力變化估計含水層比出水率

Estimation of aquifer specific yields by gravity changes at multiple time scales

陳冠宏(KUAN-HUNG CHEN)

指導教授 ： 黃金維

國立交通大學/工學院/土木工程系所/博士(2019年)

摘要

比出水率(Specific yield，Sy)是未受壓含水層的儲水比例參數，也是評估地下水庫儲量的重要參數。傳統上，比出水率只能透過人為主動引起的小規模水位變化之抽水試驗獲得，而昂貴的鑿井費用是比出水率參數很少的主要原因。地下水位變化會引起重力變化與地表變形，可以利用大地測量方法估計Sy與儲水係數(Storage coefficient，S)。前人研究多以模擬方式指出抽水期間的重力與地表變形，相較於此，本研究著重利用精密水準與重力測量方法，在野外實際量測不同時間尺度(自小時到年)，由地下水位變化引起的重力變化與地表變形，並藉此估計含水層的儲水係數。本研究於台灣數個短期的抽水試驗，以精密水準測儀與絕對重力儀，觀測抽水期間的地表變形與重力變化。內城抽水試驗期間，以大地測量方法估計之S=2.97×10-4、Sy =0.20，與水力試驗結果一致。在中時間尺度的Sy參數估計，本研究以新民地區，大雨事件後含水層快速退水與重力變化之一致性結果，推算新民地區之Sy =0.16，與最鄰近的抽水試驗之結果近似。在較大尺度的跨季節試驗，本研究在2015至2017年間，設立10個絕對重力測站，於濁水溪沖積扇之扇頂地區與名竹盆地地區，一共量測40組重力變化值。跨季節量的測結果顯示，重力方法估計之Sy值介於0.04至0.28之間，與水力試驗結果之Sy為0.03至0.24的結果一致。本研究結果建議，利用地下水位的連續上升與下降，即可達到在相同的含水層區間，重複進行抽水與注水試驗，而利用重力方法，可以在跨季節試驗中，獲得穩定且經濟的的比出率參數。本研究以重力方法在濁水溪沖積扇與名竹盆地，跨季節與多站點的Sy估計結果與水力試驗結果一致，足以顯示重力方法的可操作性與穩定性。異質性地層會影響影響抽水試驗結果，但重力方法估計之比出水率參數，更具有區域代表性。本研究將討論在實際野外試驗時，大地測量方法用於估計儲水系數的限制與優勢。本研究的相關經驗將有助於類似實驗前的可行性評估。

關鍵字

比出水率 ； 地下水 ； 重力

並列摘要

Specific yield (Sy) is the ratio of drainable groundwater in an unconfined aquifer and also a key parameter for evaluating the storage of an underground reservoir. Conventionally, Sy is obtained by pumping tests which manually create a small scaled groundwater level change, but the high cost of well construction restricts the well numbers in the field. Groundwater level change will introduce gravity and land surface change, which can be used to estimate Sy and storage coefficient (S) by the geodetic method. Earlier studies for the geodetic method are largely based on simulated gravity and surface changes. In comparison, this study focuses on field measurements of gravity and surface change to determine these aquifer coefficients using precision gravimeters and levels at time scales ranging from hours to years. For short-time scales, we used a precise level and absolute gravimeter to detect the land surface change and gravity change during several pumping tests in Taiwan. At the Neicheng pumping test site, the geodetic method results in S=2.97×10-4 and Sy =0.20, which are consistent with the pumping test result. For the median time scale of Sy determination, we used a post-rain aquifer recession that introduced declines in groundwater levels and in gravity values at Shinming. These groundwater level and gravity change result in Sy =0.16, which is close to the Sy value at a nearby pumping test site. For the long time scale, we established 10 absolute gravity sites over the upper Choushui River Alluvial Fan (CRAF) and Mingchu basin (MCB), where we measured 40 successive gravity changes to estimate Sy values over several seasons from 2015 to 2017. The estimated Sy values range from 0.04 to 0.28, which are consistent with those determined by the hydraulic method which ranges from 0.03 to 0.24. The result from this study suggests that the sequentially uplift and decline of groundwater levels work much like pumping and injection test at the same segment of an aquifer, which can be sensed by a precise gravimeter. This gravity method (the geodetic method that uses gravity changes) is cost-effective and can produce reliable Sy values using successive groundwater and gravity changes between seasons. At most of the gravity sites in the CRAF and MCB, the gravity method produced almost the same Sy values from groundwater and gravity changes spanning wet and dry seasons in different years, suggesting that this method is operational and reliable for Sy determination, as the hydraulic method. The gravity method is not affected by the heterogeneity of an aquifer that can damage the result from the hydraulic method. The study discusses the limitations and advantages of the geodetic method in estimating aquifer storage coefficients in the field. The lessons of the gravity field work from this study contribute to assessing the feasibility of the geodetic method at a given site.

並列關鍵字

Specific yield ； Groundwater ； Gravity

參考文獻

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