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

- Partially penetrating slug tests in a stratified formation involve measurement error.
- We propose an effective method for compensation with piezometer and tube methods.
- Numerical sensitivity analysis is performed to determine constants in the linear relations.
- Alternating both methods can efficiently provide vertical profiles of hydraulic conductivity.
- The profiles become more variable than only measurements as shown in a field example.

SummaryIn 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 1 m 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: KT, KU, and KL. The effect of vertical flow becomes significant when KT is small relative to KU or KL, and KL is more important than KU 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 KT/KU and KT/KL, 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.