Summary

Understanding the influence of pore space characteristics on the hydraulic conductivity and spectral induced polarization (SIP) response is critical for establishing relationships between the electrical and hydrological properties of surficial sedimentary deposits. Here, we present the results of laboratory SIP measurements on saturated quartz samples with granulometric characteristics ranging from fine sand to fine gravel. We alter the pore characteristics using three principal methods: (i) variation of the grain sizes, (ii) changing the degree of compaction, and (iii) changing the level of sorting. We then examine how these changes affect both the SIP response and the hydraulic conductivity. In general, the results indicate a clear connection between the applied changes in pore characteristics and the SIP response. In particular, we observe a systematic correlation between the hydraulic conductivity and the relaxation time of the Cole-Cole model describing the observed SIP effect for the whole range of considered grain sizes.

Introduction

Geoelectrical methods offer much promise in hydrology for resolving parameters directly relevant to groundwater flow and contaminant transport. In particular, induced polarization (IP) methods show significant potential for providing first-order estimates of the spatial structure of the hydraulic conductivity. The reason for this is the high sensitivity of these methods to hydrologically relevant characteristics of the pore space. Measuring the IP response essentially captures the capacity of a probed subsurface region to store electrical energy in terms of local charge accumulations. In the absence of metallic conductors, the IP response of fluid-saturated porous media is largely affected by current flow in the vicinity of grain surfaces. Consequently, there exists the potential to link such measurements, especially if performed in a spectral manner, to the grain diameter and/or specific surface of the pore space, both of which show correlation with hydraulic conductivity.

An excellent description of the underlying electrical theory of the IP method is provided in Slater and Lesmes (2002). In the absence of metallic conductors, such as ore minerals or graphite, the low-frequency IP effect is commonly associated with membrane polarization effects related to a polarized electrical double layer (EDL). The EDL schematically describes the organization of ionic charges at the interface between solid and fluid and was first introduced by Helmholtz in the 19th century. The inner layer is given by the negatively charged mineral surface attracting positively charged ions which form the firmly attached layer known as the Stern layer. Beyond the Stern layer, positively charged ions remain attracted but are at the same time repelled by each other and the Stern layer. The resulting dynamic equilibrium is referred to as the diffuse layer and represents the transition zone towards the outer limit of the EDL where ions are in equilibrium with the solution. The EDL provides the conceptual background for the electrochemical processes responsible for much of the observed IP response as documented by the recent study of Leroy et al. (2008).

Titov et al. (2004) provide us with a visualization of the two basic conceptual views on the origin of the IP effect in porous media, which link the effect to either the grain-size distribution or the pore-size distribution of the material.

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