THERMOPHYSICAL CHARACTERIZATION OF THE HETEROGENEOUS SUBSURFACE (Invited Presentation)

Presently, our primary understanding of the terrestrial subsurface thermal regime is elucidated from studies of energy resources and agricultural productivity, but often the subsurface is not considered as heterogeneous and anisotropic in fine scale with groundwater flow. Emerging research from the Critical Zone community suggests the thermal regime to be much more complex, controlling microbial activity, the fate and transport of metals, and cycling of nutrients. There appears to be a dynamic coupling between the thermal regime and the geochemical, microbiological, and physical processes.

In this study, we have developed a methodology to further characterize this complexity and heterogeneity in the part of the subsurface spanning the shallow and deep critical zones down to 100 m depth in sub-meter scale. By integrating geologic mapping, geophysical logging, thermal property analyses, and fiber-optic distributed temperature sensing (FO-DTS) at two research sites: the subsurface temperature profiling site at Rantoul, IL as part of Intensively Managed Landscapes Critical Zone Observatory, and the Geothermal Research Station at the University of Illinois Energy Farm, a better understanding of the vertical thermal regime was obtained. Variations in the physical and thermophysical properties of the different geologic units (bulk density, water saturation, mineralogy, and hardness) are strongly correlated with changes in the thermal gradient.

We identified temperature profiles distinct from that predicted by a typical temperature diffusion equation. Besides the influence of surface temperatures in the shallow subsurface and the Earth's geothermal heat flux from below, groundwater flow could be responsible for the conduction of heat. The thermal conductivity of saturated materials was 2 to 3 times higher than dry formations. Saturated units with higher permeability and porosity also had higher conductivities. Based on these findings, a new heat conduction equation might be needed for better embracing scenarios with different hydrostratigraphic characteristics, varying soil moisture, and dynamic groundwater flow.