Determining Thermal Properties of Soil Using Fiber Optic Distributed Temperature Sensing Models in COMSOL Multiphysics

Understanding soil water dynamics in agricultural fields is crucial for efficient irrigation management practices. The Dual-Probe Heat Pulse (DPHP) method is commonly used to measure soil thermal heat capacity and soil water content at the point scale. Typically, the heat source and the sensing probe are two short stainless-steel needles 30mm in length and 6mm apart. The thermal properties of soils are informed by the thermal response at the sensing probe. Additionally, advances in Fiber-Optics distributed temperature sensing (FO-DTS) technology provide high-resolution measurements of soil water content and thermal properties of soils at scales ranging from 0.1 to 10,000m in spatial extent using the DPHP approach in fiber optic cables. The heat pulse is applied along the metallic sheath of a fiber optic cable and the thermal response is monitored at a known distance away from the heated cable using a passive FO cable. To account for the influence of the thermal dimensions of the FO cable and non-constant spacing between cables on results are challenges to this new technology. COMSOL Multiphysics is a computational modeling software that can be used to investigate the newest FO-DTS technology in a time- and cost- efficient way. First, we investigated the feasibility of using COMSOL to model FO-DTS DPHP. Then, we used the model to investigate the effect of cable divergence on the results. We found that COMSOL's results and the lab experiments agreed, but there is work left to be done to refine the existing models to further optimize physical experiments.