以模擬退火演算法推求定流量試驗受壓含水層徑向雙層系統之參數

Abstract

在鑿井或洗井的過程可能會產生井膚層，水井若含有膚層，則水層特性將有非均質的現象。此外，若水層為補綴含水層(patchy aquifer)，而抽水井恰好在補綴層的中心，則此問題與水井含有膚層相似，這二種異質的水層可視為徑向雙層系統。過去在檢定地下水含水層的參數，大多利用抽水試驗，再以圖解法或數值方法來分析洩降數據以推求參數值，但這些方法皆假設水層為均質（單層）。若忽略徑向雙層的異質效應，貿然用傳統的抽水試驗數據分析，結果會錯估所推求的水層參數值。 本文利用模擬退火演算法，以定流量抽水試驗數據，結合拉普拉氏域定流量完全貫穿井受壓含水層之徑向雙層的解，來推求雙層水層的第一層厚度及雙層的水文地質特性參數。在膚層及補綴層兩個案例分析中，雖初始猜值均設定為為單層值，但模擬退火演算法均能求得良好的雙層參數值。此外，本研究發現選擇適當的觀測井位置及初期洩降數據數目，才能夠檢定出較好的結果。

The drilling or development in well construction may create a skin zone around the well. A patchy aquifer is an aquifer which has a disc of anomalous hydraulic properties differing from adjacent formation. A patchy aquifer or an aquifer with a skin can be considered as a radial two-zone aquifer system. A two-zone aquifer may have five aquifer parameters, i.e., two transmissivities (one for the skin and another for the formation), two storage coefficients (one for the skin and another for the formation), and one skin thickness. The hydrologists often use a graphical (type-curve) or numerical method to analyze pumping test data for estimating aquifer parameters based on the homogeneity assumption. Yet, a two-zone aquifer is actually heterogeneous. Erroneous results may be incurred if one uses those traditional methods to estimate parameters for a two-zone aquifer. The objective of this study is to use the simulated annealing (SA) integrated with the Laplace-domain solution for a confined two-zone aquifer formation under constant-flux pumping to determine the aquifer parameters and the thickness of the first zone. The estimated results for those five parameters are reasonable good even using homogeneous parameter values as the initial guesses. Further, the locations of the observation well and the number of early time drawdown data are studied. The results indicate that good results are obtained if choosing a suitable location of the observation well and the number of early time drawdown data.