Design and Evaluation of a Thermo-TDR Probe for Geothermal Applications

Abstract

A thermo-time domain reflectometry (TDR) probe can function both as a regular probe, for measuring moisture and density, and a dual heat probe, for measuring thermal conductivity, thermal diffusivity, and volumetric heat capacity. This paper describes the design, fabrication, and evaluation of a newly developed thermo-TDR probe. The developed probe was first calibrated for its TDR function in chemicals with known dielectric constants and electrical conductivity. Then the probe was tested in three sands and Kaolin clay at different moisture contents and densities. Available methods for analyses of the thermo-TDR-acquired signals were presented and discussed. The measured thermal properties were also compared against a standard thermal probe KD2 pro. It was found that the thermo-TDR probe has satisfactory accuracy in measurements of soil thermal conductivity, moisture content, and dry density. Based on the test results, recommendations have been provided regarding the use of thermo-TDR probes for geothermal applications.