GNGTS 2013 - Atti del 32° Convegno Nazionale

reservoir itself and the flow path can be mapped (e.g. Bruno et al. , 2000; Garg et al. , 2007). Hermans et al. (2012) demonstrated the ability of ERT to study heat flow and heat storage within a small field experiment in a shallow aquifer. They injected heated water and monitored the electrical values with cross-borehole time-lapse ERT but did not measure the temperature changes. Fragogiannis et al. (2008) also used ERT for monitoring the thermal performance of the ground at the University of Athens with an installed ground-source heat pump system consisting of 12 borehole heat exchangers. Electrical resistivity effecting parameters. The electrical resistivity ρ (or its inverse conductivity σ ) depends on different soil and environmental attributes. Friedman (2005) gave an overview of these parameters and their impact. He stated three categories: 1) parameters describing the bulk soil, such as porosity ( n ), water content ( θ ) and structure, 2) the time- invariable solid particle quantifiers such as particle shape and orientation, particle-size distribution, wettability or cation exchange capacity (CEC) and 3) fast-changing environmental factors, such as ionic strength, cation composition and temperature. However, these factors do not act separately. For unsaturated soils, porosity (n) especially influences the attributes of the second group. So with the main influencing factors of water content and therefore the electrical conductivity of the water/fluid within the soils pores ( σ w ) (Friedman, 2005; Dietrich, 1999), the differentiation of soils as a two-phase system (saturated soils) and as a tri-phase system (unsaturated soils) must be considered. For the electrical resistivity of soils, as a two-phase systems with negligible matrix conductivity of clay-free porous media, the most important attribute is the conductivity of the fluid within the pores. Therefore, the Archie’s empirical law is the most widely used application: (1) whereby ρ a and ρ w are the resistivities of the mixture and of the saturating water respectively (and σ their inverse conductivities), F is a formation factor and m a material-depending exponent (cementation index) of the porosity n (Archie, 1942). Further applications are shown in Friedman (2005) or Sihvola and Kong (1988). For unsaturated soils as part of a three-phase-system, the ρ a is a mixture of the soil properties, especially porosity, saturation degree with the fluid composition (ions) and the air content. In this case, Archie’s law can be rewritten as: (2) here, the saturation degree ( θ ) has to be considered, ρ s is the resistivity of the solid phase and the coefficient x is the saturation coefficient, that takes different values depending on the saturation level (Dietrich, 1999). Under laboratory conditions some of the electrical resistivity-influencing soil parameters can be a-priori known (e.g. medium porosity and composition, saturation degree) so that the temperature is the part which can be analyzed to understand the correlation with electrical resistivity. As these are the same influencing parameters for thermal conductivity (Singh and Konchenapalli, 2000; Singh et al. , 2001), a general relationship between the electrical (ρE) and the thermal resistivities (ρT) can be expected: (3) C R is a multiplier dependent upon the gravel and sand size fraction of the soil (Singh et al. , 2001). Different authors (e.g. Singh et al. , 2001; Sreedeep et al. , 2005; Fragogiannis et al. , 2010) showed this correlation of thermal and electrical conductivities within laboratory measurements for different soils in dependence of the water content. Materials and methods. A common box, sized 1.0 m x 0.4 m x 0.4 m, was prepared for the experimental tests. Within this box, the correlation of the thermal and electrical measurements 124 GNGTS 2013 S essione 3.2