Hydraulic Tomography and the Impact of A Priori Information: An Alluvial Aquifer Example (Kansas Geological Survey

Hydraulic tomography is a recently developed aquifer characterization technique that involves the performance of a large number of short-term pumping tests, each of which stresses a different vertical interval in a well. Simultaneous analysis of drawdown measurements obtained from the entire set of tests provides the potential to estimate between-well hydraulic conductivity (K) variations at a higher level of detail than is possible with more conventional well tests. Hydraulic tomography is still plagued by the ubiquitous problem of non-uniqueness, so reasonable estimates of the K distribution are difficult to obtain without incorporating independent a priori information on aquifer structure. In this study, we investigate the utility of cross-hole radar surveys as a source of a priori information on the spatial structure of the K field. We compare the results of analyzing a suite of tomographic pumping tests performed in an extensively studied alluvial aquifer using regularly layered zonations and zonations derived from the results of zero-offset cross-hole radar profiles. In all cases, residuals between predicted and observed drawdowns, chi-squared values for the model fit, and K estimates obtained from direct-push slug tests are used as the basis for comparison. Radar-based approaches can potentially provide valuable information about K variations between wells due to the dependence of radar propagation velocity and attenuation on sediment porosity and clay content. This study demons trates that electromagnetic and hydraulic properties at the study site seem to be governed at least to some extent by the same sediment variations, meaning that radar surveys are indeed capable of providing information regarding the K field geometry.