Spectral induced polarization responses of carbonate precipitation in gel-saturated media with and without electron conductors

Abstract

SUMMARY Interest in the use of carbonate precipitation for the purposes of soil stabilization, carbon capture and environmental remediation has created a need for geophysical techniques capable of characterizing the 3-D extent of precipitation in the subsurface. The use of spectral induced polarization, a geophysical technique sensitive to mineral precipitation and dissolution processes, has been investigated in previous studies; nevertheless, debate still exists regarding measured induced polarization (IP) signals relating to the formation of calcite. In this study, calcite was precipitated within gel-saturated media through highly controlled double-diffusion methods. Three experiments were conducted to study both the inherent IP response of calcite in isolation, as well as the IP response when precipitated within a low-polarizability background medium (quartz sand) and a high-polarizability medium (a sand–magnetite mixture). The results support that in isolation, the polarization of the electrical double layer surrounding individual calcite grains is not a significant contributor to IP signatures from calcite precipitation. However, under certain circumstances the precipitation of calcite, in the quartz-sand and sand–magnetite mixtures, can produce a strong IP response when the precipitation blocks a current conduction pathway. If metallic mineral grains are present then they may act as conduits for current if they become embedded in an insulating calcite wall, creating a significant electrode polarization which dominates the IP signal. Equivalent circuit models indicate that the precipitation of calcite as a thin planar front normal to the current path is analogous to a simple lossy capacitor blocking most DC current but being thin enough to measurably polarize. Circuit models also suggest that the total impedance seen from the sand–magnetite sample is simply the sum of the impedance of the disseminated grains and the grains encased in the calcite front. The encased grains also have a higher relaxation time which may indicate a different polarization mechanism than from disseminated magnetite.