Resolving Fine-Scale Hydrological Features In Electrical Resistivity Tomography Images

The resolution of electrical resistivity tomography surveys is limited by the data quantity, data quality, and the survey configuration (electrode count, spacing, borehole separation). Inevitably, we are forced to interpret data from regions containing substantial variability at a scale much finer than the resolution of the method. To understand the effects of these fine-scale structures on interpretation, data were created by converting hydrological parameters from high-resolution flow simulations to electrical conductivity. The data were then forward modeled using fine, high-resolution meshes and inverted using the same coarse meshes that were used for normal inversion of field data. The modeling simulated earlier experiments from the Sandia-Tech Vadose Zone Facility. In those experiments, geophysical imaging techniques were employed to monitor the unsaturated flow of potable water and transport of a salt tracer through fluvial sediments.<br>Comparing images to the original hydrological models shows that the images are smoother and much of the fine detail is lost but major features such as the center of a zone of infiltrating water are correctly represented. However, comparing images of synthetic data with images from previous field studies shows that the hydrological models themselves poorly represent the original field conditions. The synthetic models show narrower zones of infiltration with much higher saturation than the field case.