Self-potential response to drainage–imbibition cycles

Abstract

An experimental study of the self-potential (SP) response to a periodic succession of drainage and imbibition cycles was carried out in a decimetric, vertical column filled with Fontainebleau sand. The SP difference between two electrodes placed at two different heights, the pressure heads between two tensiometers located vis-à-vis the electrodes, and the cumulative outflow extracted from the sand column have been measured for three different periods (130, 260 and 380 min). The SP difference presents a hysteretic behaviour with respect to the pressure head difference and to the extracted volume, whose features depend on the flow conditions (i.e. drainage or imbibition phase) and on the flow dynamics. The singular spectrum analysis (SSA) has been applied to the signals to extract their oscillatory content. It evidences that the SP and the pressure head differences are phase-shifted in quadrature, and that the SP difference and the cumulative outflow are linearly related and in phase. The data can be well reproduced only if a semi-empirical component, which depends on the cumulative flow, is added to the classical electrokinetic equation. This suggests that the air–water interface that develops for unsaturated conditions polarizes and therefore generates an electrical response.