The international campaign on intercomparison between electrodes for geoelectrical measurements

Abstract

In many applications of electrical geophysics, the electrodes used to detect signal seem to be an important source of noise. To identify which electrodes were most stable for electric field measurements; an international campaign to evaluate electrodes for long-term monitoring of the telluric field was held at Garchy, France from April 1995 to April 1996. More than fifty different electrode pairs from researches around the world were tested in the campaign. Most of electrodes were metal-electrolyte designs, but with different construction details. The inter-comparison was carried out through laboratory and field experiments. The results of the experiment showed a surprising variation between the performances of different electrodes. Stability as determined in a brief laboratory experiment proved to be an excellent predictor of long-term field stability. Instabilities of differing qualitative character were observed during the year-long experiment. Linear drift may be caused by gradual changes in ground contact, leaking of electrolyte, or changes in chemical concentration of electrolyte within the electrodes. Sudden onset transient noise, which could be as large as 20 mV, is likely to be an electrochemical process such as reaction charge release and charge accumulation within the electrodes. Other physical causks of drift could be due to mechanical contact problems. Of metal electrodes, stainless steel is by far the best. The conclusions from the Garchy experiment are relevant to exploration geophysics: self-potential surveys are strongly affectedby the long-term stability of electrodes and some electrodes may also have internal noise exceeding 1 mV per .ÖHz in the frequency band around 1Hz, thus potentially affecting measurements of Induced Polarisation data.