Second Generation of Lead-lead Chloride Electrodes for Geophysical Applications

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-PbCl2 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 PbCl2 and KCl, or PbCl2 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 (tD time of dissaturation), and to know how to increase it. Two solutions are possible. The first one is to increase the electrode length, because tD 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, tD 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.