Saturation Dependence of the streaming potential coefficient

Abstract

Observations of streaming potential for unsaturated conditions do not always show the same trend depending on the hydrodynamic conditions and because of a lake of coherency between the data processing procedures. We combine the data from three studies published in the literature, acquired during non-steady-state drainage experiments, and apply the same processing steps. We model the hydrodynamic behavior of these experiments to confirm that they experienced different flow dynamics. We argue that the raw self-potential (SP) data should not be corrected unless a clear drift of the electrodes stability is observed. The combined hydrodynamic behavior and the streaming potential response show that (a) the observations of one of the experiment (exp #1) are associated to a limited range of water saturation (0.85–1.00). The corresponding signals could therefore be fairly modeled assuming no saturation dependence of the streaming potential coefficient (SPC) whatsoever; (b) the observations of exp #3 led to a SPC that can be larger than its value at saturation; (c) the observations of the exp #2 show a nonmonotonous behavior of the SPC as saturation decreases. The underlying physics of a nonmonotonous SPC is related to water/air interfaces as shown by the results of the lattice Boltzmann numerical simulations. The main contribution to the SPC behavior comes from the charged water/air interfaces and depends on the dynamic state of moving or entrapped bubbles. We finally describe the consequences of such a behavior on the seismoelectric conversions for unsaturated conditions.