Abstract

Polarization of the injection electrodes in resistivity and induced polarization may in some cases result in an anomalous response at the receiving electrodes. This is referred to here as injection electrode overprinting. Overprinting may occur in resistivity measurements when an electrode is used as a receiving electrode immediately following duty as an injection electrode, such as in multi-electrode systems with automatic switching. The circumstances that produce overprinting are demonstrated here and recommendations are made to limit its occurrence and effect. It is found that the overvoltage on injection electrodes is unlikely to exceed a few volts and that the period of rapid, nonlinear, decay is over in a few seconds. Most modern resistivity equipment can easily accommodate potentials with these characteristics as part of spontaneous potential rejection. A simple procedure is recommended to test if overprinting is occuring. Injection sequences with short and long delays between injector-receiver duty should be executed. Any difference in apparent resistivity indicates a possible problem with overprinting. Situations where one of an injection pair has a large surface area and the other a small surface area, as is often the case in pole-pole or pole-dipole surveys, are more likely to be problematic in that the polarization and circuit impedance may depend on the polarity of the pulse.

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