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

Abstract

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