3.1 Contact Resistance

Contact resistance refers to the resistance to current flow between the current electrodes and the surrounding medium (soil, water, rock). Recorded contact resistances are the sum of the resistances of the contacts between each current electrode and the ground and the resistance of the formation between the current electrodes. Resistance checks should be run on the electrodes prior to data collection to assure that contact resistances are not too large. This test is the key quality control in the field. Cutoffs on the order of tens of kiloohm (kΩ) may help determine where limited current will be injected for an electrode pair. It is difficult to come up with absolute numbers for this metric because contact resistance measurements vary with geology. For example, crystalline rock may have high contact resistance measurements due to the formation resistivity, but quality data are still possible to collect. Contact resistances should be recorded manually if not automatically recorded by the instrument software. Contact resistance measurements commonly can be made automatically with modern ER instrumentation, which applies a small voltage to the current injection electrodes, measures the resulting current with the ER instrumentation, and computes the resistance by dividing the applied voltage by the injected current. Alternatively, manual contact resistance measurements for a given electrode pair can be made with a voltmeter by measuring the resistance between corresponding pins in the head of the electrode cable. Contact resistance values can provide a basis for editing data associated with particular electrodes that exhibit poor contact with the formation, allowing corrections to be made prior to a survey. Low-contact resistances are critically important for the collection of reliable IP datasets because the signal-to-noise ratio of IP measurements is typically 2.5 to 3 orders of magnitude smaller than resistance measurements. In fact, contact resistance is often the limiting factor preventing acquisition of meaningful IP data (Zarif et al., 2017), as is also true for ER. For more advanced reading on contact resistances, we point readers to the 2013 paper by Hördt and others.

In surface arrays, it is possible to add a small amount of saltwater around electrodes to improve contact resistance, but this generally is not possible for cross-well arrays. Note that the introduction of saltwater would be a poor idea if one were interested in monitoring salinity or saturation changes, in which case a metallic anti-seize paste or ultrasound gel may also work to increase the electrical contact of the electrodes with the ground around them. If the soil drains too quickly to add water, these materials might also be helpful, or electrodes can be placed in bentonite or saturated sponges. Below the water table, borehole electrodes are generally in good contact with the formation as a function of the presence of water. In the vadose zone, electrode surface areas may need to be larger to provide good coupling with the subsurface. Contact resistance generally decreases notably as the size of the electrode increases. However, care must be taken not to violate the point–source approximation made by most processing codes—where electrodes are assumed to be infinitesimally small points in numerical modeling codes. The actual sizes and shapes of large electrodes may require explicit representation in the numerical model used by the inversion software. A common rule is that the size of the electrode should not exceed 10 percent of the distance between electrodes for the point-source assumption to be approximately valid (e.g., Rücker and Günther, 2011).