利用Jacob近似法探討空間異質場對抽水試驗之影響

Abstract

本研究的目的，是探討在異質含水層進行抽水試驗時，能否由觀測點的洩降數據，推估出具有足夠代表性的含水層特性參數。研究中對Jocab近似法及Theis法進行重新檢討，利用不同方法分析抽水試驗時詮釋資料的特性。 研究目標包括：(1)均質含水層中，可由總水頭梯度推估代表含水層特性的流通係數；異質含水層中，不同總水頭梯度所得到的流通係數，與其內部各點不同之含水層特性參數的關係，存在何種平均方式最能夠表示異質含水層之流通特性。(2)以往利用抽水試驗洩降數據進行含水層特性分析時，多採用Jacob近似法或是Theis法，理論上僅能應用於含水層幾近於均質的情況，應用於異質含水層時，是否仍能推估出具代表性的含水層參數。(3)利用均質場的模擬洩降資料進行以上方法的推估，推估得到的均質含水層特性與實際之差異。(4)藉由抽水試驗分析結果比較與實際流場之差異，並探討若採用不同抽水量進行抽水，是否會產生不同的含水層特性推估結果。 本研究選用VSAFT2數值模式進行含水層試驗的模擬，以得出各種不同條件下的數值試驗結果。並根據數值試驗的模擬數據，比較各種不同模擬結果之間，對於推求含水層特性產生的影響。研究結果顯示平行流場之有效流通係數，最接近隨機場網格點T值的幾何平均，而徑向流抽水試驗利用Jacob近似法與Theis法推求之流通係數與均質場之有效流通係數Teffp相當接近，但隨著異質度增加誤差隨之增大，利用 Jacob近似法與Theis法推求之流通係數對於真實含水層特性的描述能力大幅下降。利用全域最佳套配時段的方式，無法建立觀測點與抽水井距離和最佳套配時間區段的關係。利用區段u套配尋找最佳套配T，雖然在異質性含水層模擬試驗中並未呈現明顯的趨勢，但在均質場中最佳套配T的區間落於u = 0.01~0.02之間，顯示利用晚期抽水洩降資料確可獲得較佳的推估流通係數。

The purpose of this study is to estimate representative aquifer parameters via drawdown data collected during pumping tests in heterogeneous fields. Jacob approximation and Theis method are reexamined along with Best-fit methods for pumping test analysis. The objects include；(1)searching for the representative average for aquifer parameters under different hydraulic gradients, (2)examining if the Jacob approximation and Theis method remain available in heterogeneous fields for estimating representative parameters, (3)comparing the differences between the estimated parameters and the effective transmissivity, (4)differentiating the estimates derived from pumping tests and natural flow of radial flow and parallel flow respectively, and then discussing the results derived under different variances and pumping rates. A numerical model, VSAFT2, is used to simulate the aquifer tests to derive effective transmissivities under different conditions. The results showed that the effective transmissivities come most closely to the generated points’ geometric mean. Transmissivities estimated by Jacob approximation and Theis method under radial flow are representative in homogeneous field, however, the deviation may increase with variance thus decrease the describing abilities for actual characteristics. The relation between the distance from observation point to pumping well and the best-fit section is not able to be developed so far. It can be found that a high percentage of the best-fit sections fall at u=0.01~0.02 though there’s not an obvious trend to determine the representative transmissivities according to the pumping test data.