Comparing measurement response and inverted results of electrical resistivity tomography instruments

In this investigation, we compare the results of electrical resistivity measurements made by six commercially available instruments on the same line of electrodes to determine if there are differences in the measured data or inverted results. These comparisons are important to determine whether measurements made between different instruments are consistent. We also degraded contact resistance on one quarter of the electrodes to study how each instrument responds to different electrical connection with the ground. We find that each instrument produced statistically similar apparent resistivity results, and that any conservative assessment of the final inverted resistivity models would result in a similar interpretation for each. We also note that inversions, as expected, are affected by measurement error weights. Increased measurement errors were most closely associated with degraded contact resistance in this set of experiments. In a separate test we recorded the full measured waveform for a single four-electrode array to show how poor electrode contact and instrument-specific recording settings can lead to systematic measurement errors. We find that it would be acceptable to use more than one instrument during an investigation with the expectation that the results would be comparable assuming contact resistance remained consistent.