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

Ding-feng Cao1, 2, Bin Shi1*, Hong-hu Zhu1*, Hilary I. Inyang3, Guang-qing Wei4, Chao-zhe Duan1

1 School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
2 Department of Civil and Environmental Engineering Department, Engineering College, University of Wisconsin-Madison, Madison, WI,53706, USA
3 Global Education and Infrastructure Services LLC, Charlotte, North Carolina, USA 
4 Suzhou NanZee Sensing Technology Co. Ltd, Suzhou 215123, China 

Abstract: Although many methods exist to measure soil moisture, 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 counter 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 gravitational condition and a centrifuge test under overweight condition have been implemented 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) demonstrate the evolution of capillary zones, and the generation and migration of moisture transition zones under centrifuge acceleration. During a response period of 5 minutes, the accuracy is 0.033 m3/m3 under 1g condition, and 0.047 m3/m3 under overweight condition. With the advantages of small sensor size, low disturbance to soil mass and wide adaptability in physical model tests, the proposed method can achieve distributed soil moisture measurement at different scales.

Keywords: Soil moisture, fiber Bragg grating (FBG), carbon fiber heated sensor (CFHS), fiber optic sensing

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