The analysis of pulse interference tests conducted in a fractured rock aquifer bounded by a constant free surface

Abstract

An analytical model is presented for the analysis of pulse interference tests conducted in a double porosity medium. The special case of a horizontal fracture zone in a fractured rock environment with vertical connection to a high permeability zone at the water table is considered. The high permeability zone is modeled as a hydraulic boundary of constant head and the vertical fractures are modeled using a formulation based on equivalent porous media. Wellbore storage at the source and observation wells is accounted for using an approximate superposition technique. The solution is found using the Laplace transform method and numerical inversion into real space. The derivation is presented in dimensioned terms and a method for estimating the hydraulic conductivity of the vertical fractures is developed. Several alternate solutions describing differing system geometry and boundary conditions are presented for comparative purposes. A sensitivity analysis shows that the new model predicts unique values for horizontal transmissivity and vertical hydraulic conductivity over a range of realistic storage term values for a given distance to the constant head boundary. Storage values were not found to be uniquely determined with this method. Several field examples are presented in order to validate the applicability of the analysis to real data. The effects of the connection to the water table through double porosity were found to be significant, resulting in an observation well response that cannot be simulated using a single porosity model. Thus, the method is very useful for uniquely estimating the vertical hydraulic properties of fractured rock aquifers, parameters that are often difficult to measure in this setting.