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