A laboratory experience to assess the 3D effects on 2D ERT monitoring of river levees

In this study, a small-scale flume was prepared in laboratory to reproduce the earthen levee of a real test site (Mantua, northern Italy), where a permanent geo-electrical monitoring system has been operating since 2015. In the lab, time lapse geo-electrical measurements were performed using the Wenner configuration with miniaturized electrodes and cables with the aim of monitoring soil saturation variations due to both changes in the water level in the channel and rainfalls. The measured apparent resistivity values and preliminary inverted sections showed no reasonable correlation with the actual levee conditions. This was mainly caused by 3D effects due to the geometry of the levee as well as to the resistive sides and bottom of the flume. Therefore, a forward modelling procedure was followed to estimate distortions of the 2D data. The effect of buried electrodes was also taken into account. Data were corrected according to the outcomes of the forward modelling tests, and the results showed that, although the best layout to effectively investigate levees in monitoring projects is to perform measurements along the levee, collected 2D data suffer from 3D effects that must be corrected before any interpretation is carried out.