Characterization of aquifer heterogeneity by tomographic slug test responses considering wellbore effects

Abstract

Slug tests provide an efficient way to obtain information about the spatial heterogeneity of an investigated site. Combining with the imaging method of hydraulic tomography, they can present broad application prospects in aquifer reconstruction. However, unlike pumping tests, the water level responses of slug tests may be significantly affected by wellbore effects, especially for underdamped cases. This study proposes a new characterization framework for characterization of aquifer heterogeneity based on water level responses recorded during cross well slug tests with a tomographical arrangement. The peak head of each water level response and its corresponding time, i.e. the hydraulic travel time, are utilized in the inversion process, rather than the whole water level time series. To eliminate the influence of wellbore effects, two correction methods with respect to the hydraulic travel time and peak head amplitude are developed. Based on corrected data, the distributions of hydraulic diffusivity (D) and specific storage (Ss) are obtained through travel time-based and attenuation-based inversions, respectively. The new framework is verified by a series of simulated tomographic slug tests through a synthetic example based on an aquifer outcrop analog, where the hydraulic parameter distribution is known. The verification results indicate that the distributions of D and Ss are well estimated, and their mean values are much improved compared to the old framework. With the same method, a field application is employed at a fractured rock experimental site located in Göttingen, Germany. Three potential transmissive fractures are revealed with hydraulic conductivities varying from 6.9 × 10–5 to 3.4 × 10–4 m/s. The uncertainty of the inversion results is discussed for future studies and related applications.