Comparison of slug and pumping tests for hydraulic tomography experiments: a practical perspective

Hydraulic tomography is the simultaneous analysis of several hydraulic tests performed in multiple isolated intervals in adjacent wells to image heterogeneous hydraulic property fields. In this study, we compare the resolutions associated with hydraulic tomography experiments carried out with slug tests and pumping tests for simple configurations with hydraulic property values representative of an extensively studied littoral aquifer. Associated test designs (e.g., pumping rates, test durations) and the validity of the principle of reciprocity are also assessed. For this purpose, synthetic tomography experiments and their associated sensitivity matrices are generated using a radial flow model accounting for wellbore storage. The resolution analysis is based on a pseudo-inverse analysis of the sensitivity matrix with a noise level representative of field measurements. Synthetic experiments used equivalent perturbations for slug tests and pumping tests. Even though pumping tests induce a drawdown in observation intervals that is three times larger than head changes due to slug tests, resolutions for hydraulic conductivities (horizontal and vertical) are similar for the two tests and slightly lower for specific storage with pumping. However, experiments with pumping require fifty times more water and are seven times longer to perform than experiments with slug tests. Furthermore, reducing pumping rates to limit disposal of water or test durations to decrease field data acquisition time would considerably lower resolutions for either scenario. Analyses are done using all available stressed and observation intervals as required by the non-applicability of the principle of reciprocity for slug tests and pumping tests with important wellbore storage. This study demonstrates concepts that have important implications for the performance and analysis of hydraulic tomography experiments.