INJECTION ELECTRODE POLARIZATION IN RESISTIVITY AND INDUCED POLARIZATION

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1 INJECTION ELECTRODE POLARIZATION IN RESISTIVITY AND INDUCED POLARIZATION J.B. Merriam University of Saskatchewan Department of Geological Sciences 4 Science Pl Saskatoon, SK S7N 5E jim.merriam@usask.ca Abstract Polarization of injection electrodes in resistivity and induced polarization may reach several volts and persist for several seconds or more after current injection has ceased. It has been suggested elsewhere that this may present a problem for programmable switching resistivity meters because of the possibility of an electrode pair being used as a (polarized receiving pair immediately following duty as a transmitting pair. Field trials presented here show that modern instruments are very efficient at detecting and removing fluctuating background potentials. For such instruments, injection electrode polarization will only be a problem if the received signal is very much smaller than the polarization. In induced polarization, the receiving electrodes are never used as injection electrodes, so the concern is somewhat different. Polarization of the injection electrodes implies an impedance change at these electrodes. In extreme cases, the impedance can vary so widely and so rapidly that the injected current never approaches a constant value. This is routinely detected in the field if the impedance changes are large and is remedied by use of a smaller current. More problematical is the intrinsically different polarization of the anode and cathode, which may lead to asymmetric received pulses and consequently Cole-Cole parameters that are different for positive and negative pulses. This is particularly a problem when one electrode is very much larger than the other, as is commonly the case with a pole-dipole survey. Introduction It is well known that injection electrodes become polarized by the passage of current. Such polarizations may reach several volts and persist for seconds, or in some cases much longer. Dahlin() has warned that in multi-electrode automatic switching resistivity systems, injection electrodes used as an injection pair may subsequently be used as a receiving pair before the induced polarization has decayed. As a precaution, he advises that the measurement sequence be programmed so that no electrode is used as a receiving electrode immediately following its use as an injection electrode. The results presented here confirm the presence of large (-3 Volt) polarization voltages, but it is shown that the resistivity meter is very effective at removing the effect of the polarization potential on the apparent resistivity. It is likely that all modern instruments would be just as effective. The study is broadened to examine the polarization at anode and cathode, which may be very different, and to include induced polarization, where considerably larger currents are injected into the ground.

2 Figure shows the decay of the polarization overvoltage on the AB electrodes following an injection of a sequence of four 5 V pulses. The current density through the electrodes is about 4 A/m. The initial decay of polarization is rapid, but the polarization persists for more than a minute. When such an electrode pair is subsequently used as a receiving pair, the decay of the induced polarization becomes part of the spontaneous background potential at that pair and must be compensated by the measurement software of the instrument. INJECTION ELECTRODE POLARIZATION 3.5 A B POTENTIAL (V) TIME FROM CURRENT OFF (s) Figure. Polarization of the injection electrodes after four second, ± 5 V, pulses with a current density at the electrode of 4 A/m. Time is measured from the end of the last pulse. An injection sequence generated by a SYSCAL Jr with programmable switching and received on a polarized pair of electrodes is shown in figure. In this case, the injection sequence is dipole-dipole with nodes one and two used initially as an injection pair and subsequently as a receiving pair. The end of the initial series of injection pulses is at t=, and the received pulses from the second injection series begins three seconds later. The three second delay is important, as this is the interval during which polarization is decaying rapidly and thus presents the greatest difficulty to effective correction. Between each nominally positive and negative pulse the current is off for nearly ten ms. This delay is also important as it presents an opportunity to monitor the decay of the background throughout the injection sequence. Note that the delay between pulses is only effective if the measuring electrodes themselves do not become polarized. This could happen, for example, when large injection voltages are used with small electrode separation.

3 MN POTENTIAL DISTURBED BY POLARIZATION DECAY MN POTENTIAL (mv) TIME SINCE END OF PREVIOUS INJECTION (s) Figure. The MN potential measured on an electrode pair previously used as an injection pair. The decay of the polarization produced by the previous injection on this pair is the quasi exponential function that decays from t=. Six positive and seven negative pulses received from the next injection are also shown. An expanded view of the on time of the sequence is shown in Figure 3, together with a drift curve (dashed line) interpolated from the off times. The absolute value of the corrected pulses are also shown in the upper part of Figure 3. Other than a slight stagger between pulses of different polarity, there is no relic of the initial polarization in the corrected MN potential. After removing a sixty Hz background, the standard deviation of the corrected MN voltage is three percent. The resistivity meter has a programmed procedure to remove the background potential and the sixty Hz line voltage and also takes advantage of a substantial delay after every polarity reversal to allow impedance changes at the injection electrodes to stabilize. The standard deviation on the apparent resistivity reported by the resistivity meter for this trial is less than one percent. The importance of the three second delay between node switches is apparent; it allows time for the polarization to decay so that the rate of decay is almost linear in any on interval. In this example, the linear decay is about mv during the first on pulse, and much less than this for every succeeding pulse. The non-linear decay, which would be much more difficult to compensate, is less than a mv for all pulses. The instantaneous rate of decay is most influenced by the elapsed time from the end of the injection, but also by the identity of the dominant ions responsible for the polarization as well as the properties of the metal electrode. Thus, it is difficult to guarantee that the three second delay would be sufficient in all environments. Further tests were conducted with a SYSCAL Jr in a dipole-dipole configuration from 3

4 N= to N=6. The injection sequence was programmed so that an injection pair was immediately used as a receiving pair. At small offsets, N=-3, the apparent resistivities measured on a polarized electrode pair and on an unpolarized electrode pair are essentially the same, despite a Volt initial polarization. Only at the far offset dipoles, where the received pulses are less than mv in amplitude, does the difference between a measurement on a polarized pair and on an unpolarized pair exceed a few percent. With received signals this small, the measured apparent resistivity is probably already compromised by background noise. Thus, the advice in Dahlin() is correct, but perhaps overstated, in that only far offset dipoles with weak signals present a problem. MN POTENTIAL DISTURBED BY POLARIZATION DECAY 3 MN POTENTIAL (mv) TIME (s) Figure 3. The data of figure corrected for the decay of the original polarization by a high order polynomial. In induced polarization, the injection electrodes are never used as a receiving pair, so the concern is somewhat different. Here, impedance changes at the injection electrodes force the transmitter to adjust the injection voltage to maintain a constant current. The primary may never reach a stable plateau and the constant primary used in the definition of the chargeability is then uncertain and may vary from pulse to pulse. Secondly, an asymmetric response at the cathode and anode may mean that nominally positive pulses have a different primary from nominally negative pulses. In extreme situations, where the injection current waveform is polarity dependent, the positive secondary may exhibit different decay characteristics from the negative secondary. For careful work, where Cole-Cole parameters are to be inverted from the measured decays, more attention to the evolution of the primary may be required. 4

5 Impedance changes at the injection electrodes are caused by a variety of different phenomena; activation overvoltage, concentration overvoltage, ohmic overvoltage, and electroosmosis. Activation overvoltage is arguably the most important. Activation overvoltage is a consequence of the transition from electronic to electrolytic conduction and is affected by the hydration of cations and anions in the pore fluid. A polarity dependence is implicit because of the different degree of hydration of anions and cations. Hydrated ions cannot approach the electrode closer than a nanometer to discharge because of a possible layer of adsorbed water on the electrode and the ions own hydration shell. Discharge can thus only occur by an electron tunneling across the potential barrier implied by the physical separation of ions and electrode. When current is flowing, the polarization grows exponentially with time towards a peak value that is a function of the current density across the electrode-water interface. Activation impedance, R a, increases as R a = η/v R o where V is the injection voltage, R o is the grounding resistance (the total resistance in the absence of overvoltage) and η is the overvoltage. Thus, activation impedance is greatest when the overvoltage is similar to the injection voltage. With an injection voltage of say 3V, a grounding resistance of 3 Ohm and a maximum overvoltage of 3V, the activation impedance is then 3 Ohm, or one percent of the grounding resistance. In extreme cases, where the injection voltage and grounding resistance are small, activation impedance can grow to become several orders of magnitude larger than the grounding resistance. Figure 4 shows an example of what is probably activation impedance. In this case the impedance at the cathode grows from about 3 Ohm to more than 5 Ohm. Concentration impedance is the result of induced concentration gradients near the electrodes either in the current carrying ions, their discharged products, or both. In this case, the impedance increases as the square root of time. Concentration impedance is more likely to be an issue at the larger current densities used in induced polarization. The different diffusion rates of anions and cations imply a slight polarity dependence for concentration impedance. Ohmic impedance is a chemical change at the surface of the electrode ie oxidation or reduction of the electrode. Ohmic impedance is intrinsically polarity dependent. Electro-osmosis moves pore water away from the anode and towards the cathode (if the zeta potential of the soil is negative). Thus, the contact resistance at the anode will increase dramatically, perhaps by several orders of magnitude, and the contact resistance at the cathode will decrease moderately. The electro-osmotic velocity is a function of current density and the zeta potential of the solid matrix, so this phenomenon will be observed in soils with a high cationic exchange capacity when a large current density is applied for some time. Many induced polarization injections meet these criteria. The polarity dependence of these phenomena is not be an issue if the electrodes are identical. In such a case, the evolution of total impedance is the same regardless of the nominal polarity of the pulse. However, when one electrode is made very large compared to the other, the evolution of impedance depends on which electrode is the anode and which is the cathode. In a pole-dipole or pole-pole survey the fixed electrode is often constructed to have a very large surface area, and the roving electrode is made with a much smaller surface area. In such a situation, the change in the net impedance in the circuit will be dominantly the change in impedance at the roving electrode and therefore the impedance will evolve according to whether this electrode is an anode or cathode on any pulse. The use of asymmetric electrodes is common practice in pole-pole and pole-dipole surveys to increase the total current injected into the ground. The results presented here suggest that while the use of a larger electrode may initially decrease the impedance and allow a larger current to flow, the affect of this larger current at the smaller electrode is to increase its impedance, often by a much greater amount than the initial decrease. These results also suggest that some care needs to be taken to ensure that asymmetric impedance is small so that asymmetric pulses are avoided. 5

6 x ANODIC AND CATHODIC IMPEDANCE AT LOW CURRENT DENSITY CATHODIC IMPEDANCE IMPEDANCE (Ohm).5 ANODIC IMPEDANCE TIME (s) Figure 4. The evolution of impedance at the two injection electrodes during an injection of a low current density (.5A/m ). The anode and cathode have radically different behavior at low current density and at high current density. Figure 4 shows the evolution of impedance at the anode and cathode during a six second injection of a relatively small current density(.5a/m )attheelectrode. The impedance at both electrodes rises with time, although the evolution at the anode and cathode is very different. At such a low current density, the impedance is most likely due to activation overvoltage, although some contribution from ohmic and concentration overvoltage may also be present. Figure 5 shows the evolution of impedance at the anode and cathode during injection of a much larger current density (3A/m ). In this case, the impedance at the anode increases and the impedance at the cathode decreases. The polarity and the relative magnitude suggest that this is dominantly electro-osmotic impedance. In both cases, low current density and high current density, the impedance evolves differently depending on whether the small electrode is an anode or a cathode. The greater the difference in impedance at the two electrodes, the greater the difference that will be seen in the primary in a positive pulse versus a negative pulse. In cases like the latter, where there is a large difference in the evolution of the impedance, the recovery from a positive pulse will have a different shape from the recovery from a negative pulse, and hence the Cole-Cole parameters inverted from a negative pulse will be different from those of a positive pulse. 6

7 6 ANODIC AND CATHODIC IMPEDANCE AT HIGH CURRENT DENSITY 4 ANODIC IMPEDANCE IMPEDANCE (Ohm) CATHODIC IMPEDANCE TIME (s) Figure 5. The evolution of impedance at the injection electrodes during injection of a high current density (3A/m ) Summary Large polarization voltages, -3 Volts, may be induced on injection electrodes during a resistivity injection. In automatic switching resistivity meters, these electrodes may subsequently be used as a receiving pair which is now polarized with a rapidly decaying background potential. It has been shown here that modern equipment is very effective at subtracting this bias potential and delivering a reliable apparent resistivity, except in cases where the received signal is very much smaller than the polarization. I have examined the causes of electrode polarization and found that there is a considerable polarity bias, that is the anode will polarize differently from the cathode. This is especially important when the two injection electrodes are not identical in contact area and environment, as would be the case in a pole-pole or pole-dipole injection in resistivity or induced polarization. It is suggested that the use of a remote pole with large surface area may not be advisable. There are two reasons for this. The use of asymmetrical electrodes introduces the possibility of asymmetric polarization and ultimately of asymmetric pulses. Secondly, a large surface area remote electrode is used to lower the grounding resistance in the circuit, so that a larger current can be injected. This may be counter-productive since the effective impedance at the smaller roving electrode may be increased more as the result of the larger current than the impedance at the remote electrode is decreased by increasing its area. References Dahlin, Torleif. Short note on electrode charge-up effects in DC resistivity data acquisition using multi-electrode arrays. Geophys. Prospecting, 48,