A soil moisture estimation method using actively heated fiber Bragg grating sensors

Although many methods of soil moisture measurement exist, there remains a lack of soil moisture sensor with small volume, high precision, excellent real-time performance and distributed monitoring capability. In this paper, a fiber Bragg grating (FBG)-based carbon fiber heated sensor (CFHS) that can overcome the deficiencies in soil moisture movement is proposed. The CFHS consists of a carbon fiber rod with a diameter of 5 mm, quasi-distributed FBG sensors on an optical fiber, a coating, two binding posts, and two electric cables. Laboratory calibration tests have been conducted to establish an empirical piece-wise relationship between the temperature characteristic value and soil moisture for sand, silt and clay. A laboratory validation test under normal gravity and a centrifuge test under overweight conditions, have been performed to verify the feasibility of this technique for soil moisture monitoring. The test results show that the CFHS can (i) accurately record soil moisture under normal gravitational and overweight conditions and (ii) track the evolution of capillary zones, and the generation and migration of moisture transition zones under centrifuge acceleration. During a response period of 5 min, the accuracy is 0.033 m3/m3 under 1 g condition, and 0.047 m3/m3 under overweight condition. With the advantages of small sensor size, low disturbance of soil mass and wide adaptability in physical model tests, the proposed method can achieve distributed soil moisture measurement at different scales and with high efficiency.