A field assessment of high resolution aquifer characterization based on hydraulic tomography with cross-hole slug tests

Slug tests have traditionally been utilized as a means to determine the hydraulic conductivity of an aquifer at a relatively small scale. When performed in a cross-hole mode using a tomographical measurement array, however, slug tests can be used to gain information about the distribution of hydraulic properties between two or more wells. Despite the common perception that a slug test only affects a small volume of the aquifer in the vicinity of the test well, response data with a reasonable signal to noise ratio can be collected at distances of over several hundred times the radius of the screen of the test well. In this work, we use a hydraulic travel time approach to evaluate the cross-hole slug tests. The approach, which is based on the inversion of travel times, is analogous to seismic ray tomography. The key element of this procedure is a travel time integral relating the square root of the peak travel time to the inverse square root of the hydraulic diffusivity for a Dirac point source. The potential of hydraulic tomography using cross-hole slug tests was investigated at a well-characterized sand and gravel aquifer located in the Leine River valley near Goettingen, Germany. The data base for the inversion consists of 200 cross-hole slug tests performed between five wells in which the positions of the sources (injection ports) and the receivers (observation ports), isolated with double packer systems, were varied between tests. The delay in the travel time of the pressure pulse due to wellbore storage and inertial effects was quantified and corrected using semi-analytical solutions based on the assumption of a homogeneous aquifer. The inversion results demonstrate the effectiveness of the proposed approach, as we were able to reconstruct four 2- D diffusivity tomograms with a resolution of 0.15×0.15 m2. The reconstructions were verified using single-well slug tests, grain-size statistics and geophysical well logs. The results of this study show that the combination of tomographical travel time inversion techniques and analytical evaluation approaches is particularly well suited for providing information about the structure and spatial variability of hydraulic properties of an aquifer, which are prerequisites for prediction of contaminant transport.