Quantitative Imaging Of 3D Solute Transport Using 2D Time-Lapse Ert: A Synthetic Feasibility Study

Time-lapse electrical resistance tomography (ERT) has proven significant potential to monitor<br>solute plumes in the subsurface. However, the ultimate value of ERT for quantitative imaging of solute<br>transport, for example in heterogeneous aquifers, is still under dispute. Here, difficulties may be<br>expected to arise particularly from the fact that ERT data acquisition and interpretation is often limited<br>to 2D image planes, while aquifers are generally characterized by a 3D structure involving considerable<br>variability of flow and transport properties. The potential of time-lapse ERT in such a situation is<br>investigated by means of a synthetic tracer experiment. For this purpose, 3D solute transport in a<br>heterogeneous hydraulic conductivity field, characterized by an exponential covariance function, is<br>simulated. Assuming that solute concentration is linearly related to electrical conductivity, the spatiotemporal<br>evolution of the tracer plume is imaged in a transect spanned by a set of fictive boreholes using<br>2D time-lapse ERT. Although the 3D process is imaged using a 2D inversion approach, the recovered<br>electrical conductivity distributions coincide well with the input distributions. The obtained images are<br>interpreted as concentration maps and then analyzed in terms of transport properties. By adopting a<br>stream-tube model, an equivalent advection velocity and longitudinal dispersivity can be quantified for<br>each pixel in the ERT image plane. The recovered equivalent advection velocities exhibit fair agreement<br>with those obtained from the original model. The results of the synthetic study demonstrate that<br>quantitative imaging of 3D solute transport by means of time-lapse ERT is feasible. Importantly,<br>systematic errors associated with the 2D representation of a 3D model are found to play an insignificant<br>role concerning the quantification of transport properties, justifying the use of simple 2D imaging, for<br>instance if equipment, time, and/or budget is limited.