Cross-well slug interference tests: An effective characterization method for resolving aquifer heterogeneity

In this study the potential of cross-well slug interference tests for high resolution aquifer characterization of hydraulic heterogeneity was assessed. The cross-well slug interference tests were performed at the research site “Stegemühle”, located in the Leine River valley near Göttingen, Germany. The geological composition of the subsurface, consisting mainly of 3.5 m silt and clay overlying 2.5 m sand and gravel, was determined by geophysical well logging and bore core data. To account for lateral changes a refraction seismic survey was conducted. Based on these data an area, characterized by an aquifer thickness of approximately 2 m and an average hydraulic conductivity of 5.0 × 10−4 m/s (determined by pumping tests), most appropriate for cross-well slug interference tests, was chosen. Altogether 196 cross-well slug interference tests were performed using a tomographic measurement array. The cross-well slug interference tests were evaluated using type curve analysis, which provided detailed information concerning the vertical changes of hydraulic conductivity and specific storage. To assess hydraulic strata connectivity a travel time based tomographic inversion approach was utilized. The potential of the inversion approach to determine lateral changes could be successfully demonstrated by the reconstruction of the pinch out of a high diffusivity layer close to the bottom of the aquifer. The results demonstrate that the combined evaluation of cross-well slug interference tests based on type curve analysis and travel time inversion allows for the development of a detailed model about subsurface hydraulic heterogeneity.