Heat tracing in the variably saturated shallow subsurface

Abstract

Temperature and soil moisture are two of the most important parameters that affect a wide variety of ecological, geochemical, and hydrological processes in the shallow subsurface. By understanding and exploiting the coupled physics of heat transport and water content, temperature can be used as a proxy for soil moisture measurement in variably saturated conditions. This dissertation develops and refines novel temperature-based hydrogeophysical methods while expanding the applicability of heat-tracing to unsaturated conditions. To this end, a comprehensive review of existing research into heat-tracing in the vadose zone is presented. Thermo-hydraulic physics of vadose zone porous media, field-based and numerical methods, thermal regime studies, and both active and passive heat tracing methods are reviewed. Several knowledge gaps and opportunities that exist in its application to unsaturated conditions are identified.

Two novel methodologies related to heat-tracing are developed. The first, a matrix method, delineates contrasting subsurface conditions and compares existing saturated zone velocity heat-tracing methods. The method is applied to both coiled fibre-optic distributed temperature sensing (FO-DTS) measurements and finite element (FE) model output. These are used to illustrate the thermal signature of common subsurface conditions as well as to assess the reciprocal relationship between sensor spacing and noise in water flux estimates. Next, a semi-analytical vadose zone heat tracing model and associated methodology, based on the full advection-conduction-dispersion equation coupled with an established empirical thermal conductivity-saturation model, are derived for the calculation of soil moisture profiles. The method exploits measurements of ambient temperature profiles in the subsurface and is shown to predict soil moisture profiles accurately under certain limits on percolation rate or saturation level.

Finally, the subsurface thermal regime of an estuarine inter-tidal zone is investigated. In this highly transient and variably saturated zone, analytical heat-tracing methods are not applicable and common electromagnetic probes can fail due to high salinity. Measured FO-DTS temperature profiles and a fully coupled thermo-hydraulic FE model are used to evaluate the relative propagation depths of tidal and diurnal signals and to quantify the importance of variably unsaturated conditions for heat transport.