Dc-Resistivity Array To Monitor Fluid Flow At The Inel Infiltration Test

During 1994 a semi-permanent DC-resistivity array of nearly 2600 electrodes was installed to monitor vadose zone fluid flow as part of a Large Scale Infiltration Test carried out at the Idaho National Engineering Laboratory (INEL) to determine contaminant transport properties for the highly heterogeneous subsurface at the INEL. The infiltration test consisted of an integration of hydrologic, geochemical, and geophysical techniques used to monitor the migration of tracer-spiked water through the subsurface. Most of the sensors used were down-hole monitoring sensors which have limited spatial coverage. To augment the borehole monitoring, a surface DC-resistivity array was deployed to obtain additional information in the regions between the boreholes. The DC-resistivity array was located in the area external to the infiltration basin :o monitor lateral fluid flow along barriers to vertical flow. The array of electrodes was installed in a pattern of four radial arms and three concentric circles about the infiltration basin. Groups of 120 to 200 electrodes were connected to a set of 22 seismic cables and brought to 8 common points for recording. A PC field computer and a multiplexing system was used to record the array. The array was installed on a semipermanent basis, and remained in place throughout the duration of the Large Scale Infiltration Test, so that small changes in the subsurface resistivity over time could be monitored and related to the vadose zone fluid flow. Data was acquired using multiple current sources and was interpreted to obtain an overall geoelectric structure of the infiltration test site. The DC resistivity data was used lo delineate preferred flow paths in the vadose zone. These zones of preferred fluid flow are seen in features interpreted from the multi-source resistivity data and also from well log data obtained from the monitoring wells situated around the monitoring basin.