We introduce a new design for direct-push electrical resistivity probes, as well as document an inversion procedure for processing the data. We demonstrate the effectiveness of these probes at an artificial recharge site in Watsonville, CA. The data from these probes lends new insight into processes governing infiltration at the site.
We have developed a direct-push probe that can be permanently installed and used to continually monitor changes in the electrical conductivity of the vadose zone. We present an inversion processing workflow for producing images of electrical conductivity as a function of time from the measured resistances. We monitored time-lapse changes in the sediments directly beneath an artificial recharge pond to obtain information about the controls on the decreasing infiltration rates and associated clogging in the recharge pond. Three probes, placed at a depth of 2 m below the bottom of the pond, were monitored for 120 d, with data being acquired every 18 min. Through the use of one- and two-dimensional inversions, we identified what we believe to be a disturbed zone immediately adjacent to the probes. Based on an assessment of the error in the one- and two-dimensional models, however, we chose to work with the one-dimensional inversion results, converting them from electrical conductivity estimates to estimates of saturation as a function of time. The saturation maps show clear evidence of clogging. In particular, at late time, we could see that a thin layer (<10 cm), possibly associated with biological activity, controlled infiltration. Our ability to obtain high-resolution spatial and temporal sampling of the subsurface with electrical conductivity probes offers a new approach to acquiring data about properties and processes in the vadose zone. The permanent installation of such probes could provide valuable information about natural processes governing infiltration and recharge and aid in the operation of engineered sites.