含水層參數檢定與洩降敏感度分析

Abstract

抽水試驗是調查含水層水文地質參數的重要方法，傳統的分析方式，都應用圖解法或數值方法中的梯度法求解，然而，這些方法都有其限制。本研究建立一參數檢定模式，應用模擬退火演算法結合水層抽水洩降的解析模式，推求滲漏及自由含水層的參數。結果顯示此參數檢定模式較傳統圖解法，更能準確求出水層參數值，且與擴展式卡門濾波和牛頓法的結果精度相同。此外，本研究對模擬退火演算法的控制參數進行敏感度分析，結果顯示本方法是可信賴、且穩定的。 一般而言，進行一個抽水試驗需要花費許多人力及資源，包括井的設置、數據的量測及分析等。若洩降數據不足，使用傳統圖解分析方法，往往無法得到正確的答案。長時抽水及人力需求的問題，可透過於現地量測洩降數據的同時，即時檢定含水層的參數。然而，在滲漏及自由水層部分的水文地質特性，需要抽水一段時間後，才會充分的反應於洩降數據上。因此在應用參數檢定模式即時推求參數時，很難決定停止參數檢定的時間。本研究使用敏感度分析，尋找滲漏與自由水層的參數，在進行抽水試驗時，反應於洩降的影響時程。同時，應用建立的參數檢定模式，即時推求參數值。結果顯示當含水層參數開始影響洩降時，即時參數檢定模式立刻正確的檢定出水層參數值。這個發現可作為停止參數檢定時間的重要參考。此外，本研究透過敏感度分析，進一步分析、探討不同的比出水率，及不同的抽水井與觀測井間的距離，對於試驗停止時間的影響。

The pumping test is a very important method in investigating the aquifer hydrogeologic characteristics. Conventional graphical or computer methods for identifying aquifer parameters have their own inevitable limitations. This study applies the parameter estimation model (PEM) based on the simulated annealing (SA) and analytical models to estimate the parameters of leaky and unconfined aquifers. The estimated results of proposed method have better accuracy than those of the graphical methods and agree well with those of the computer methods based on the extended Kalman filter and Newton’s method. Moreover, the sensitivity analyses for the control parameters of SA indicate that the proposed method is very robust and stable in parameter estimation procedures. Generally, a pumping test requires a lot of effort and expense to perform the test and the drawdown data are measured and analyzed for determining the aquifer parameters. The estimated aquifer parameters obtained from graphical approaches may not be in good accuracy if the pumping time is too short to give a good visual fit to the type curves. The problems of long pumping time and required efforts can be reduced if the drawdown data are measured and the parameters are simultaneously estimated on-line. However, the drawdown behavior of the leaky and unconfined aquifers in response to the pumping may have a time lag. The time to terminate the estimation may not be easily and quickly to decide when applying a PEM on-line to analyze the parameters. This study employs the sensitivity analysis to analyze the influence period of parameters in response to the pumping in both leaky and unconfined aquifers. In the meanwhile, a PEM based on the SA is used to determine the parameters of these two aquifers on-line. The results indicate that the aquifer parameters can be accurately estimated when they start to influence the drawdown. This finding can be used as a guide in terminating the estimation. Moreover, the sensitivity analysis is also used to study the effects of different values of specific yield Sy and the distance between pumping well and observation well on the influence time of Sy during the pumping.