Calibration, Installation Techniques, And Equilibration Considerations For Vertical Resistivity Probes Used In Hydrogeologic Investigations

Vertical Resistivity Probes (VRPs) are being increasingly used in a variety of applications<br>where detailed vertical resistivity information in both the vadose and saturated zones is needed.<br>We have twenty installations at five sites in Michigan of probes varying between 15 and 30 feet in<br>length, with stainless steel screw-head electrodes every two inches. They are being used to<br>monitor resistivity in residual LNAPL plumes, dissolved LNAPL plumes, and vadose zone water<br>content / infiltration fronts in an agricultural problem area. All the different possible array types<br>and spacings must be calibrated for the effect of the 2” (outer diameter) insulating PVC cylinder<br>on which the electrodes are mounted. Apparent resistivities must be corrected by calibration<br>factors ranging between 0.72 for the 2” Wenner array, to nearly 1 for the 6” pole-pole array.<br>Varying the installation parameters greatly influences the measured apparent resistivity because of<br>the disturbed annulus and the composition of the backfill materials. Depending upon the purpose,<br>installations are made with or without heavy bentonite slurry that also contains silica flour or very<br>tine sand. Where the saturated zone is of primary interest, the installation can be with a hollowstem<br>auger, using natural collapse or an introduced sand to fill the annulus around the PVC pipe.<br>In this application, the electrodes are always in good, wetted contact with the formation. For<br>sites where vadose zone monitoring is desired, installations are usually into a pilot hole, which is<br>kept open by bentonite slurry. After installation of the probe, this material then serves to provide<br>electrical contact with the unsaturated zone. Otherwise, particularly for the case of dry sand<br>formations, extremely high contact resistances will preclude making reliable resistivity<br>measurements. However, it is known that the bentonite will be initially quite conductive, but with<br>the passage of time loses ions to the formation by diffision or leaching. The equilibration time for<br>this process is thus important to document, if repeat readings of resistivity are to be interpreted<br>properly. The effect of the bentonite slurry is also a function of the initial volume required,<br>whether it was mixed with distilled water, and whether it was uniformly distributed vertically<br>along the annular space. Properly calibrating for geometry, and correcting for temporal changes<br>in the bentonite-based annular space filling, allow for very useful resistivity/conductivity<br>information to be extracted from these probes.