Quantifying the spatio-temporal variability of total water content in seasonally frozen soil using actively heated fiber Bragg grating sensing

Abstract

Actively heated fiber-optic (AHFO) method has become a research focus for soil water content measurement in recent years due to its advantages such as small size, distributed measurement and good durability. However, the AHFO method is presently coupled to distributed temperature sensing (AHFO-DTS) which is inevitably limited by low temperature measurement accuracy and spatial resolution. Furthermore, the existing studies about AHFO method do not consider the existence of frozen soil caused by temperature changes, which further limits its application in the field. Here a new method for total water content measurement in frozen soil using actively heated fiber Bragg grating (AHFO-FBG) sensing is proposed for the first time, which directly determines the calibration formulas of frozen soil under different soil temperatures from the calibration formula of unfrozen soil. Moreover, the feasibility and reliability of AHFO-FBG technology are proved through laboratory calibration tests and in-situ monitoring data of Chinese loess, and the temporal and spatial distribution of total water content in shallow seasonally frozen loess is also revealed. It is suggested that the AHFO-FBG technology can complement with the traditional techniques to achieve quasi-distributed and in-situ total water content monitoring in seasonally frozen soil.