Rock conductivity depends on salinity history

The researchers measured the electrical properties of 16 diatomite and chalk samples while changing pore-water salinity in both directions. At the lowest salinities, conductivity was up to 120% higher when salinity was decreasing than when it was increasing. Above a bulk water conductivity of around 3 S/m, the difference was negligible.

The researchers attribute the hysteresis primarily to processes at the mineral–water interface. Their interpretation is that sodium ions are retained in the structured water layer at the mineral surface during decreasing salinity, increasing surface conductivity. Repeated measurements confirmed the effect is not due to kinetics, and that each sample had reached a stable equilibrium. Independent measurements showed no significant changes in pore structure following the salinity cycling.

The study also shows that electrical conductivity and induced polarisation do not respond in the same way to salinity history. While conductivity showed pronounced hysteresis at low salinities, the Cole–Cole relaxation time and distribution exponent showed no discernible hysteresis, and normalised chargeability was substantially less sensitive to the direction of salinity change. This indicates a decoupling between conduction and polarisation.