Monitoring The Solute Transport With Active Time Constrained Time-lapse Electrical Resistivity Tomography

Solute transport is a dynamic hydrologic process with the spatial distribution of concentration continuously changing over time. The distribution of resistivity will change in geophysical monitoring. The electrical resistivity tomography (ERT) is a non-invasive method which has been widely applied to monitor the solute transport. However, the commonly used ERT algorithm only introduces the space smooth item for the special continuity, without consideration of the temporal continuity in the solute transport. It is difficult to inverse the true change in this way.The recently proposed active time constrained (ATC) algorithm defines a space-time model, which introduces the regularizations not only in the space but in time as well. This research applies the method in monitoring solute transport. A numerical simulation of tracer experiment was conducted in a synthetic heterogeneous aquifer. Based on the petrophysical model, we established a relationship between concentration and electrical resistivity. Then we deduced the spatial and temporal distribution of electrical resistivity which corresponded to the dynamic process of solute transport and served as the forward model. We calculated the simulated monitoring data and then inverted respectively by ATC, time constrained method and independent method to make the qualitative and quantitative analysis.The spatial and temporal evolution of the injected 4000 mg/L NaCl solute plume was simulated for 50 days in an unconfined aquifer. The dynamic concentration maps were converted to resistivity maps at selected time phases by a postulated petrophysical relationship, then serving as the forward model to calculate the monitoring data. The results of these three algorithm show:the independent algorithm can roughly locate the center of solute. The time constrained algorithm can precisely locate the position of the center of the solution and even a small-scale movement. This algorithm also identify the interface of the aquifer with different permeability. The ATC algorithm inherits the advantages of the time constrained method and identifies the direction more clearly.This research proposes a practical relationship between concentration and resistivity and solves the forward problem from the simulation of solute transport to monitoring with ATC time-lapse ERT. We can also provide valuable information of the hydrogeological structure based on the results, laying a solid foundation to the inverse problem of quantifying the hydrogeological parameters from time-lapse electrical resistivity tomography.