Numerical study of long-electrode electric resistivity tomography — Accuracy, sensitivity, and resolution

Abstract

Electric resistivity tomography (ERT) is a widespread technique used for geologic and hydrological subsurface characterization. The 3D application is limited to small scales due to the enormous effort required for field surveys. Long-electrode (LE)-ERT using steel-cased boreholes can overcome small-scale limitations and has recently gained significant attention. However, no systematic investigation concerning the performance of long electrodes has been performed. We have conducted synthetic studies of LE-ERT to understand its advantages and limitations. We have compared three different approaches of modeling long electrodes, placing particular emphasis on the complete electrode model, to determine which reflects the physical reality best and thus can be used as a benchmark. The conductive cell model led to comparable results but was numerically more expensive. An interesting alternative approach was the shunt electrode model, which does not require discretizing the lateral extension of the casing. Systematic sensitivity studies revealed that model resolution could be enhanced in the borehole depth range through the use of electrodes of different lengths or surface electrodes. Varying contact impedances along different sections of the borehole can change the electric field visibly and influence four-point measurements. Even if large electrode segments show reduced coupling, the influence on voltage measurements was below 4% for realistic contact impedances. We have proven the applicability of LE-ERT for imaging lateral saltwater movement through the simulation of simple scenarios in the context of saltwater intrusion. Our synthetic examples determined the advantage of long electrodes compared with simulations using surface electrodes. The 3D inversion of synthetic data sets revealed that the modeled anomalies could be imaged in most cases. Different models revealed limitations due to poor vertical resolution and supported the usage of casings with different lengths or in combination with surface electrodes to improve resolution.