Groundwater flow after heavy rain in landslide-slope area from 2-D inversion of resistivity monitoring data

In the interest of monitoring the flow of groundwater from heavy rains, we conducted surface resistivity tomography through the vadose zone over several days. The measured data were modeled by 2-D inversion. A laboratory experiment was carried out to estimate the difference in geological characteristics. We measured the effects of groundwater fluctuation using two tank models: one using sand and one using bricks, which represent unconsolidated sediments and jointed rocks, respectively. In the sand model, we succeeded in monitoring water-level fluctuation as the change in resistivity over time. In the brick model, the water level did not appear as a distinct boundary. Field experiments were carried out in an area in which the near-surface conditions consisted of heavily weathered rocks with an extensive shear zone several tens of meters thick. Measurements were performed over 42 days, during which we observed changes in resistivity associated with heavy rainfall. It became clear that resistivity decreases of up to 8% were present in highly porous zones in heavily weathered layers. This result suggests a connection between a decrease in resistivity and the infiltration of rainwater. In particular, a pronounced zone of minimum resistivity may indicate infiltration of rainwater into the vadose zone. The relationship between resistivity changes and rainfall was inferred to depend on the permeability of geological structure.