Analysis of partially penetrating slug tests in a stratified formation by alternating piezometer and tube methods

In partially penetrating slug tests, hydraulic conductivity (K) estimates might not necessarily be valid because of vertical flows in heterogeneous formations. We assess the error in hypothetical stratified formations by numerical sensitivity analysis, and propose an effective method for compensation by incorporating two types of casing configuration (piezometer and tube). The hypothetical stratified formation consists of completely horizontal layers, each 1m thick; the permeability is different between, but not within, layers. In this study, conductivity estimates in the piezometer and tube methods are calculated by assigning various patterns of conductivity to the test, upper, and lower layers: K_T, K_U, and K_L. The effect of vertical flow becomes significant when K_T is small relative to K_U or K_L, and K_L is more important than K_U because the base of the borehole is open to the lower formation. The conductivity ratios (estimate over actual value) are treated as approximately linearly dependent on logarithms of K_T/K_U and K_T/K_L, so that conductivity estimates can be straightforwardly derived from one piezometer measurement and two tube measurements at the top and bottom of the screen. The linear relations are evaluated and constant parameters are determined under specific conditions. This study also recommends alternating piezometer and tube methods in the drilling procedure because the actual variation of K with depth is larger than that found using isolated measurements, as shown in a field study of alluvial fan gravel deposits in Sapporo, Japan.