Time-Lapse Azimuthal Resistivity Survey In A Highly Industrialized Area: Problems, Diagnosis, and Solutions

A time-lapse azimuthal resistivity survey was proposed to support a fluid flow test pertinent to<br>an upcoming bio-remediation experiment. The site is in a highly industrialized area, and preliminary<br>resistivity measurements showed steady, but substantial, drift in apparent resistivities measured over<br>tens of minutes. The drift was of such magnitude that collection of time-lapse data would be precluded<br>unless a solution could be found. We hypothesized that the problem was either instrumental, was<br>induced by the electrodes, or was site specific. Internal checks of the Sting R1 indicated a properly<br>functioning instrument. We tested three different electrode types and found no particular differences in<br>the rate of drift with any of the three types chosen. A test of the resistivity system in a plastic tub filled<br>with a sodium chloride solution produced steady measurements over a span of about an hour. We<br>concluded that stray currents at the site itself must be producing the drift. We found that by averaging<br>two measurements at a given azimuth, one with electrodes positioned AMNB, the other BNMA, we<br>could obtain steady resistivity results over acceptably long durations, and so we were able to acquire<br>useable data during the flow test.