Determining vertical anisotropy ratio using a graphical, iterative procedure based on the Hantush equation

A method is presented for determining the transmissivity, storage coefficient, and vertical anisotropy ratio of a confined aquifer using constant rate pumping test data obtained from a system of partially penetrating wells. The vertical anisotropy ratio is defined as the ratio of vertical to horizontal hydraulic conductivity. Knowing the anisotropy ratio is essential for computing drawdown and capture zones around partially penetrating pumping structures such as vertical wells, horizontal wells, or trenches. In the method, drawdown data obtained from the observation wells are corrected for partial penetration using the Hantush equation and analyzed using conventional distance-drawdown methods. Because the correction is dependent on knowing the aquifer coefficients, an iterative procedure is used in which each successive iteration improves on the estimates of transmissivity, storage coefficient and, ultimately, the vertical anisotropy ratio. Computer calculations are used to determine the drawdown correction factors but graphical methods are used to determine the aquifer coefficients so that the analyst can judge the quality of the data fit and become aware of any anomalies that may exist. Although no mathematical proof is offered herein, the iterative procedure in practice has always converged to produce a unique solution for the vertical anisotropy ratio. Based on the author's experience, accurate determination of the aquifer coefficients can be obtained when both deep and shallow observation wells are available