Induced polarization response of porous media with metallic particles — Part 9: Influence of permafrost

Abstract

We consider a mixture made of dispersed metallic particles immersed into a background material saturated by an electrolyte. Below the freezing temperature (typically 0°C to -4°C), a fraction of the liquid water in the pore space is transformed in ice while the salt remains segregated into the liquid pore water solution. Our goal is to understand how freezing affects the complex conductivity (induced polarization) of such mixtures. Complex conductivity measurements (96 spectra) are performed in a temperature-controlled bath with a high-precision impedance meter. We cover the temperature range +20°C down to -15 to -18°C and the frequency range 10-2 Hz to 45 kHz. The spectra are fitted with a double Cole Cole complex conductivity model. A finite element model is used to further analyze the mechanisms of polarization by considering an intra-grain polarization mechanism for the metallic particles and a change of the conductivity of the background material modeled with an exponential freezing curve. This curve is used to connect the liquid water content to the temperature. In the context of freezing, we test all the aspects of the intra-grain polarization model developed in the previous papers of this series, at least for a weakly polarizable background material. The Cole Cole exponent and the chargeability are observed to be essentially independent of temperature including in freezing conditions. This means that all the relaxation times of the system follow the same temperature dependence and that the chargeability is controlled by the volume fraction of metal. The instantaneous conductivity (highfrequency conductivity) and the relaxation times depend on temperature in a predictable way and their product can be considered to be essentially temperature independent. Both the analytical and numerical models can reproduce the inverse relationship between the relaxation time and the instantaneous conductivity. Page 2 of 71 GEOPHYSICS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 This paper presented here as accepted for publication in Geophysics prior to copyediting and composition. © 2019 Society of Exploration Geophysicists. D ow nl oa de d 07 /2 8/ 19 to 5 9. 72 .9 7. 86 . R ed is tr ib ut io n su bj ec t t o SE G li ce ns e or c op yr ig ht ; s ee T er m s of U se a t h ttp :// lib ra ry .s eg .o rg /