Development and Assessment of a Quick Subsurface Thermal Characterization Method to Map Soil Thermal and Physical Properties

The subsurface composition including carbon, minerals, and water are essential attributes of terrestrial ecosystems which are potentially subject to change under a warming climate. Mapping their distribution and heterogeneity is critical to understanding physical and biological processes modulating these changes, yet this task remains a significant challenge using traditional measurement techniques. Recent development of low-cost Distributed Temperature Profiling (DTP) systems provides improved spatial resolution for measuring in-situ temperature time series. Such spatial refinement holds promise for characterizing the heterogeneity of soil thermal and physical properties, leading to a better understanding of subsurface stores. In order to minimize the amount of time needed to estimate thermal parameters from temperature time-series, here we developed a novel combination of instrumentation and software tools to quickly measure soil thermal properties with high spatial resolution. The system was used at about 300 locations in the upper East River watershed in Colorado, USA and in a discontinuous permafrost environment near Nome, Alaska. For this study, DTP probes were specially designed to be quickly deployed and recovered. Each location required between 1 and 5 minutes of active work, enabling up to a few hundred measurement locations per day. For each location, temperature time series were recorded for at least 30 minutes along a 75 cm vertical profile with 5 cm spacing. Along with recording temperature, the first 5 minutes of temperature equilibration data were compared with results from a forward finite volume model, allowing for inversion of thermal parameters. Our approach was assessed with independent measurements of the soil thermal parameters. Combined with measured petrophysical relationships, these inversion results provided insight into the distribution of and heterogeneity of soil composition and moisture content. The results underline the potential of this novel approach, as well as the remaining challenges, such as unknown contact resistance and variable petrophysical relationships.