Estimation of Effective Hydrogeological Parameters in a Heterogeneous Aquifer
Date Issued
2012
Date
2012
Author(s)
Wu, Shih-Ching
Abstract
Spatial heterogeneity is ubiquitous in nature; and variability of parameter in subsurface is extensive. The effective parameters are obtained by conceptualizing the heterogeneous soil formation as an equivalent homogeneous medium that will discharge approximately the same flux as the ensemble flux of the heterogeneous formations. This study proposed a modified form of Tabu Search (TS) embedded Adjoint State Method (ASM), entitled “Modified Tabu Search Method (MTM)”, to estimate effective parameters for anisotropic, heterogeneous aquifers. MTM is validated by several numerical pumping tests. Comparisons are made to other well-known techniques, the type-curve method (TCM), the straight-line method (SLM), and Darcy’s law, to provide insight into the challenge of determining the most effective parameter for an anisotropic aquifer at different scales of heterogeneity with considering varying pumping time and pumping rates.
The results reveal that MTM can efficiently obtain the best representative and effective parameters in terms of the least mean square errors of the drawdown estimations. The use of MTM may involve less artificial errors than occur with TCM and SLM, and lead to better solutions. Therefore, effective transmissivity is more likely to be comprised of the geometric mean of all transmissivities within the cone of depression based on a precise estimation of MTM. Further investigation into the applicability of MTM shows: (1) While the duration of the pumping test is short, MTM proposed herein might successfully optimize the effective parameters batter than TCM and SLM. (2) With a larger variance of heterogeneity of an aquifer, the range of the pumping rate becomes smaller, leading to estimations by MTM closest to those estimated by Darcy’s law. (3) A higher level of heterogeneity in an aquifer can induce an uncertainty in estimations, while the changes in correlation length will affect the accuracy of MTM.
Subjects
effective parameters
heterogeneous aquifer
Tabu Search
Adjoint State Method
Modified Tabu Search Method
Type-curve Method
Straight-line Method
Darcy’s law
Type
thesis
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