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Induced Polarization of Unsaturated Sands Determined through Time Domain Measurements

^a Russian Institute of Exploration Geophysics, 20 Fayansovaya St., 193019, St. Petersburg, Russia
^b Presently, St. Petersburg State University, Geophysical Department, 7/9 Universitetskaya nab., 199034, St. Petersburg, Russia
^c Agrosphere Institute (ICG-IV), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
^d State Enterprise, Geologorazvedka, 17 Knipovich St., 193019, St. Petersburg, Russia

Correspondence: ^* Corresponding author (tito{at}kt4096.spb.edu)

Received for publication 31 January 2004. We studied the electrical induced polarization (IP) response of simple multiphase porous systems by conducting time-domain (TD) IP measurements on two different groups of sieved quartz samples: sands containing air in unsaturated pores and sands where the unsaturated pores were filled with kerosene. The analyzed chargeability vs. water content relationship showed an extreme behavior. The resistivity vs. water content relationship exhibited two distinct power law regions characterized by different values of the power-law exponents. Quartz–water–air and quartz–water–kerosene samples showed similar behavior. A conceptual model of polarizing cells is proposed to explain the observed IP phenomena. We consider a sequence of large and narrow passageways for electric current as the elementary polarizing cell. The polarization of the cell is related to the difference in effective radii of the passageways. In the saturated cell, the water-filling intergrain space is considered the large current passageway. Areas of grain contact are considered narrow passageways. In unsaturated cells, the thin water film on the grain surfaces is considered the narrow passageway. Areas of grain contact are viewed as large passageways. The polarization of unsaturated cells in this condition should be larger than that of saturated cells. With further drying, unsaturated cells start releasing water from the grain contact area, which leads to a convergence of the effective radii of large and narrow passageways and, consequently, to a decrease in cell polarization. The conceptual model is capable of explaining the observed dependence of polarization on water content. The chargeability maximum corresponds to the limit between the two distinct regions, each having a different resistivity vs. water content relationship, which we consider as the critical water content. Our model suggests that at the critical water content, pore water becomes predominantly adsorbed on the solid surface.

Abbreviations: EDL, electrical double layer • IP, induced polarization • QWA, quartz–water–air • QWK, quartz–water–kerosene • TD, time domain

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