Estimating high hydraulic conductivity locations through a 3D simulation of water flow in soil and a resistivity survey

In this study, we propose a method to estimate high hydraulic conductivity locations that uses 3D simulation of soil water flow and in-line resistivity survey data acquired during a groundwater recharge experiment, and we apply this method to numerical and field experiments. The high hydraulic conductivity locations are estimated from a combination of field-observed and simulated apparent resistivities using the following simple steps. (1) Assuming that high hydraulic conductivity zones exist in the first layer, simulations of saturated-unsaturated seepage are conducted for several possible water-flow models that have high hydraulic conductivity zones in different locations. (2) The simulated volumetric water contents are converted into bulk resistivities, which are used to produce apparent resistivity data through simulation of a resistivity survey. (3) The differences between the simulated apparent resistivities and the field-observed data are examined, and the best-fit hydraulic conductivity model is identified by minimising the above differences. In the numerical experiment, 3D inversion of the simulated resistivity survey provides an image of the preferential flow, although the infiltration locations are unclear. Comparing the field model with the possible models, the high hydraulic conductivity location in the field model corresponds to the high hydraulic conductivity location in the possible model with the minimum errors. In the field, an in-line resistivity survey was conducted during a groundwater recharge experiment on a pyroclastic plateau. The 3D inversion of the in-line resistivity survey data provides an image of the preferential flow. Comparing the field apparent resistivity data with the simulated apparent resistivity data, the high hydraulic conductivity location of the possible model that provides the minimum error corresponds to the recharge water range, whereas the hydraulic conductivity location of the possible model that gives the maximum errors corresponds to ranges with no recharge water. These results indicate that it is possible to estimate high hydraulic conductivity locations using 3D simulations of the soil water flow and a resistivity survey.