A field assessment of high-resolution aquifer characterization based on hydraulic travel time and hydraulic attenuation tomography

In this study the potential of an inversion approach based on hydraulic travel time and hydraulic attenuation tomography was assessed. Both hydraulic travel time and hydraulic attenuation tomography are based on the transformation of the transient groundwater flow equation into the eikonal equation using an asymptotic approach. The eikonal equation allows the calculation of pressure propagation and attenuation along trajectories, which is computationally efficient. The attenuation and travel time-based inversion approaches are naturally complementary: hydraulic travel times are determined by the hydraulic diffusivity, a combination of hydraulic conductivity and specific storage, whereas the attenuation is determined solely by specific storage. The potential of our hydraulic tomographical approach was investigated at a well-characterized sand and gravel aquifer located in the Leine River valley near Gottingen, Germany. The database for the hydraulic inversion consists of 392 cross-well slug interference 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 results have shown that the combination of hydraulic travel time and hydraulic attenuation tomography allows the reconstruction of the diffusivity and storage distribution in two and three dimensions with a resolution and accuracy superior to that possible with type curve analysis.