A large number of geophysical applications need long-term telluric recordings. In order to realise it correctly, the use of very stable electrodes is necessary. The study of the potential variations of Pb-PbCl 2 electrodes as a function of both the ionic composition and the pH of its electrolyte allows one to chose the optimal working criteria. The best stability and the minimum noise for the potential are obtained for a solution saturated in both salts PbCl 2 and KCl, or PbCl 2 and NaCl, in the presence of additional non-dissolved salts, with a pH from 4 to 5. The study of the salts diffusion between the inside of the electrode and the outside medium allows one to compute the time span over which the potential remains stable (t D time of dissaturation), and to know how to increase it. Two solutions are possible. The first one is to increase the electrode length, because t D is proportional to the length squared. The second one is to reduce the exchanges of salts with the external medium, by using an electrode with a narrow channel. In this case, t D is proportional to the quantity of non-dissolved salt in the electrode and to the internal electrical resistance of the electrode. The fabrication of this new electrodes design with a channel is described.

Numerous electrodes, already used in geophysics or just perfected by us, have been compared by measuring the three main characteristics which interest the user: noise spectrum, temperature coefficient and polarization with its stability versus time. Among the most used unpolarizable electrodes, silver‐silver chloride (Ag‐AgCl) are the best ones. But a systematic research of all different possible metal‐salt couples, have led us to use lead‐lead chloride (Pb‐PbCl2) for the following reasons: noise as low as the one of Ag‐AgCl at 1 Hz and even lower for the low frequencies (0.4 μV at 1 Hz and 1.2 μV at 0.01 Hz for peak to peak value and ΔF=F), temperature coefficient about ten times weaker (−40 μV/°C instead of −450 μV/°C) and also better long time stability of the polarization (1 mV/month instead of 2 at 10 mV/month). We have been using these electrodes since 1977 as “tube” electrodes which are very easy to use. They allow us to record correctly the fast variations thanks to their low noise, the very slow variations, their low temperature coefficient and their stability, and this with telluric lines only about 100 m long.

Various types of electrodes designed for the measurement of the electric field in the soil or in sea water at periods larger than one minute have been compared in a one-year experiment in Garchy, France. The experiment included more than fifty electrode pairs with liquid or absorbed electrolytes and Pb/PbCl2, Ag/AgCl, Cu/CuSO4 and Cd/CdC12 metal-ion couples. The electrode parameters were systematically measured in the laboratory and the electrodes were installed in the field to constitute 50-meter long parallel dipoles separated by 2 meters. Pairs of electrodes used for sea measurements were monitored in a salted water vessel. Fourtytwo potential differences were recorded with a sampling interval of 1 minute between May 1995 and April 1996. When electrodes are compared, large differences are observed in the long term stability as well as in the sensitivity to diurnal variations, rainfall and soil saturation. For measurements in soil, the installation method of the electrodes plays an important role. In salted water, the best performing electrode pair has a drift of the order of 0.1 mV per year. In soil, typical drifts for the best sensors are of the order of 0.2 mV per month in dry soil and 0.5 mV per month in soaked soil. Preferred electrode designs and installation methods, depending on the external conditions or the type of geophysical measurement, emerge from this experiment. In addition to the magneto-telluric field, potential variations which are not electrode or installation effects are observed and attributed to electrical sources in the soil.

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