Characterization of horizontal transmissivity anisotropy using cross-hole slug tests

Traditional methods for characterizing horizontally anisotropic aquifers are all based on pumping tests. In this paper, we present a new method for the identification of horizontal anisotropy using cross-hole slug tests, which is much more easily accessible comparing to pumping tests. Through scaler transform, an anisotropic medium was converted into an equivalent isotropic medium. When applying the analytical solutions derived for isotropic media to interpret cross-hole slug tests performed in anisotropic media, the estimated transmissivity is the geometrical mean of the anisotropic transmissivity tensor, regardless of sampling direction. However, the apparent storativity calculated from different observation wells, is equal to the true storativity scaled by a factor defined by the formation anisotropy. Thus, anisotropy can be resolved through apparent storativity that bears directional information. The proposed method was first validated by a numerical experiment and then applied to field data. It was found that the tensor results obtained by this new method with cross-hole slug tests are consistent with that obtained by applying the classical Papadopulos' method with cross-hole pumping tests conducted in the same wells. When applied to heterogeneous media, tensor results produced by the new method may be subject to considerable errors. This is because the results are strongly sensitive to the connectivity between measurement boreholes and the formation medium. To reduce the error level, responses from a large number of observation wells located in various directions around the source well are needed. In addition to the adopted KGS solution for confined aquifers, the proposed methodology can be applied to incorporate other analytical methods for a variety of aquifer types, as long as the characteristic relationship between the measure scale and the aquifer storativity can be established.