Use of microgravity for identification of delayed gravity drainage and conceptual model selection in unconfined aquifers

The estimation of hydraulic parameters at the aquifer scale typically relies in the analysis of pumping tests. Drawdown data, from a frequently limited number of boreholes and piezometers, is used in conjunction with analytical formulas derived for conceptually simplified models in a history matching procedure. In unconfined aquifers it is well known that the drainage process is controlled by the unsaturated zone. Several models have been proposed in the literature for the calculation of drawdown caused by pumping. Approaches range from the assumption of instantaneous and complete drainage to the inclusion of a delay term or the full implementation of the unsaturated zone to represent the delayed drainage in the unsaturated zone. Because borehole drawdown data is not informative about the unsaturated zone processes, it is common for practitioners to rely in the simplest models available, usually in some pumping test interpretation software. For example, Neuman's instantaneous drainage model is still widely used in the industry even when it has been demonstrated that under some circumstances provides incorrect estimations of key aquifer parameters. Additionally, as boreholes are scarce, spatial information is ignored and conceptual assumptions of axial-symmetry are a normal practice. In this work we show how the use of microgravity instruments, sensitive to storage variations in the near subsurface, could be a simple, cheap and convenient tool for the identification of delayed drainage processes in unconfined aquifers. The joint use of drawdown and gravity data can be utilized to select the most preferable conceptual model for the parameter estimation problem.