Investigating temperature and moisture profiles of seasonally frozen soil under different land covers using actively heated fiber Bragg grating sensors

Abstract

In this study, the actively heated fiber Bragg grating (AH-FBG) technique is developed to measure temperature and moisture profiles of partially frozen soil. The working principle of the AH-FBG sensing system is introduced, which employs an aluminum oxide tube sensor (AOTS). The unfrozen water contents and ice contents are estimated by combining AH-FBG with the frequency-domain reflection (FDR) technique. The feasibility of AH-FBG for soil moisture measurement is evaluated by laboratory and field tests under four different land covers, i.e., bare soil, concrete slab, plastic mulch (PM), and grass. It is found that AH-FBG can be used to capture unfrozen water contents and ice contents with high accuracy. The relationships between the thermal conductivity measured by AOTSs and the total water contents of frozen and unfrozen soils can be well fitted by the Côté and Konrad model. Both PM and concrete slab have a warming effect on the ground soil, which makes their depths of soil freezing shallower than that of bare soil, while the grassy land has a deeper freezing depth due to the inherent plant root-to-soil structure. The field monitoring results indicate that the PM can effectively block water migration channels at the ground surface and prevent water vapor exchange between the atmosphere and the soil, resulting in a “pot effect” that commonly exists under an impermeable layer.