A saline tracer test monitored via both surface and cross-borehole electrical resistivity tomography: Comparison of time-lapse results

In contrast to traditional field investigation techniques in hydrogeology, geophysical methods are relatively non-invasive, cost effective and can be performed with a higher spatial sampling. The most commonly applied technique in hydrogeophysics is electrical resistivity tomography (ERT), both from the ground surface and in cross-borehole configurations. To infer reliable results from such a hydrogeophysical application, however, the uncertainty related to the data inversion has to be taken into account and specific attention must be paid to the experimental set-up and design, especially when the main target of the study is a quantitative estimation of some relevant hydrological quantity. The sensitivity and resolving power of ERT depend on the type of acquisition methodology; operating from the ground surface only, for example, could lead to severe limitations in terms of resolution, thus limiting the quantitative utilisation from a hydrogeological perspective. In this work, we present the results of a saline tracer test experiment performed in the saturated zone at the water works facility at Valdobbiadene (Treviso, North-East Italy), where an alluvial phreatic aquifer is heavily exploited for irrigation and drinking water supply. The experiment was monitored by time-lapse ERT acquisitions, using both surface and cross-borehole configurations. We compared the results of the two approaches and conclude that, in general, ERT has excellent imaging capabilities for saline tracer tests, however, significant limitations are inherent in the use of surface electrode configurations only. (C) 2011 Elsevier B.V. All rights reserved.