Exploring limitations in the induced polarization versus surface conductivity relationship in the case of wetland soils

Abstract

Recent induced polarization studies suggest that the real part of surface conductivity ( σ ′ surf ) scales linearly with the imaginary conductivity ( σ ″ = σ ″ surf ) or normalized chargeability (Mn) for a range of soil types. The coefficients of this relationship l and l_Mn ( l = σ ″ / σ ′ surf or l_Mn = Mn/ σ ′ surf ) allow the separation of the surface and electrolytic conductivities from the bulk conductivity. However, the dependence of these constants on varying soil physicochemical properties, including under unsaturated conditions, is yet to be assessed. Using estimates of σ ′ surf from 18 undisturbed soil samples from a restored wetland and σ ″ measured over a frequency range of 0.01 Hz to 10 kHz, we compare the σ ′ surf and σ ″ with the laboratory measurements of soil properties. We calculate l and l_Mn for each soil sample and regress them against the soil properties. We find an apparent dependence of l on soil texture, bulk density, organic matter, and moisture contents, with coefficients of determination ( R 2 ) ranging from 0.5 to 0.65 at low frequencies (e.g., 1 Hz) but not at high frequencies (e.g., 936 Hz). This dependence of l on soil texture results from the insensitivity of σ ″ at low frequency to σ ′ surf and, by implication, to the soil properties controlling σ ′ surf . In contrast, l_Mn indicates no correlation with the soil properties because Mn is linearly correlated with σ ′ surf and correlated with the soil properties controlling σ ′ surf . Our results call for caution on the application of σ ″ at a single frequency as a proxy of σ ′ surf because σ ″ is not necessarily correlated with σ ′ surf across all soil types. Although using l_Mn derived from multifrequency measurements overcome this limitation, field acquisition of spectral information (e.g., up to 1000 Hz) remains a challenge.