Mediate relation between electrical and thermal conductivity of soil

Hans Schwarz,David Bertermann

doi:10.1007/s40948-020-00173-x

Abstract

Thermal conductivity is a key parameter for many soil applications, especially for dimensioning shallow and very shallow geothermal systems based on the possible heat extraction rate and for modelling heat transfer processes around high voltage underground cables. Due to the limited purview of direct thermal conductivity measurements, for an investigation of extensive areas, usually other geophysical methods like electrical resistivity tomography measurements are applied. To derive thermal conductivity of soil from geoelectrical measurements a relation between electrical and thermal conductivity is needed. Until now only few approaches worked on a direct correlation between both conductivities. Due to the difficulties of a direct relation, within this study a modular approach of a mediate correlation between electrical and thermal conductivity was investigated. Therefore, a direct relationship between a corrected electrical conductivity and water content as well as the standard and simple thermal conductivity model of Kersten (Bull of the Univ Minnesota 28:1–227, 1949) was used. To develop this concept soil types of sand, silt loam and clay were investigated where different saturation steps and pressure loads were applied. For each configuration electrical and thermal conductivity as well as water content and bulk density was determined. To refine the results of the calculated water content a corrective factor was applied. Furthermore, bulk density as an inlet parameter of the Kersten equation was also derived based on electrical conductivity. The suggested proceeding enables the determination of thermal conductivity solely based on electrical conductivity without prior soil property information.

Highlights

Thermal conductivity (TC), which is a valuable soil parameter especially within the subjects of shallow and very shallow geothermal applications (Bertermann et al 2014; Saez Blazquez et al 2017; Vieira et al 2017; Di Sipio and Bertermann 2018) and high voltage underground cable surroundings, is tricky to determine accurately in the field
electrical conductivity (EC) as well as TC are depending on the same soil properties, a direct correlation between both conductivities is not possible
This suggested approach may be one way to bypass the complication of a direct correlation between EC and TC

Summary

Introduction

Thermal conductivity (TC), which is a valuable soil parameter especially within the subjects of shallow and very shallow geothermal applications (Bertermann et al 2014; Saez Blazquez et al 2017; Vieira et al 2017; Di Sipio and Bertermann 2018) and high voltage underground cable surroundings (de Lieto et al 2014; Salata et al 2016; Chatzipanagiotou et al 2017; Drefke et al 2017; Rerak and Oclon 2017), is tricky to determine accurately in the field. Thermal conductivity itself can only be determined by sensors with direct contact at selective points of interest, indirect proceedings concerning other geophysical applications like electrical resistivity. A proper processing of measured data is key to produce valid results and enables an adequate and not time-consuming assessment. Expert knowledge and a proper assessment of geophysical measurements as well as good and practicable concepts and models to derive applicable soil parameters like TC are essential. Results based on examined derivations are just as meaningful as quality of the implemented concept allows. The focus of this study is the determination of TC based on geoelectric measurements which is a standard application for underground hydraulic assessments (Dahlin and Loke 2018; Ghalamkari et al 2019)

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Conclusion