Development and Testing of Displacement Inversion to Track Electrode Movements on 3-D Electrical Resistivity Tomography Monitoring Grids

n. napoliwa;

Electrodes installed on active landslides and vulnerable earthworks to monitor changes in resistivity associated with moisture dynamics can be subject to movement. This affects the geoelectrical data and leads to errors in the resulting electrical resistivity tomography (ERT) images. This paper demonstrates the selection of appropriate ERT measurements to provide sensitivity to electrode displacements in both directions on a surface grid. Combinations of linear and equatorial dipole–dipole measurements are considered, which permit use on rectangular grids of any aspect ratio. A Gauss–Newton inversion scheme, initially based on simple homogeneous resistivity model calculations, is developed that allows for the incorporation of constraints based on the magnitude and direction of movement. The effects of the constraints are demonstrated with synthetic data, which are also used to show that displacement inversion can track electro despositions during movement as a function of time. The conclusions of these simulations are subsequently confirmed by analogous experiments in a laboratory tank. The results show that tracking the positions of the electrodes is possible with sufficient accuracy, even in the presence of realistic subsurface resistivity structures, to correct the majority of distortions and resistivity anomalies caused by using the wrong electrode locations in ERT inversion. By incorporating estimates of the resistivity structure into the forward response modelling, the accuracy of the recovered displacements is improved. This also enables an iterative displacement and resistivity inversion to be developed that, for the first time, demonstrates the principle of using 3-D ERT data to monitor both sub surface geoelectrical properties and surface movements simultaneously.