The Effect of Electrode Contact Resistance On Electric Field Measurements

A simple equivalent circuit model and field measurements show that dipolar electric field measurements can be changed by up to 50% due to the effects of electrode contact resistance (RC). The equivalent circuit model shows that a high RC enhances the effective wire-to-ground capacitive coupling, leading to a complex dependence of received voltage on frequency, electrode contact resistance, wire length, and wire capacitance. The model shows that measured electric field voltages will fall between a perfectly grounded asymptote (RC -> 0) and an ungrounded asymptote (RC -> infinity). Field tests were made of this model using the controlled source audio-frequency magnetotelluric (CSAMT) technique. By varying the effective RC and the signal frequency, the behavior predicted by the model was confirmed. The tests indicate that electrode contact resistance or ECR effects cannot be ignored in CSAMT data, and that they may influence complex resistivity measurements in certain conditions.

A simple, workable solution to the ECR problem was devised by inserting a high-impedance amplifier in series with the electrodes and by shielding the lead wires, grounding the shield to a common-mode reference pot. Measurements using this configuration show that ECR effects virtually can be eliminated even at high RC values.