An improved distributed sensing method for monitoring soil moisture profile using heated carbon fibers

•A carbon fiber heated sensing tube is developed to capture soil moisture profile.•A piecewise function is used to calculate soil moisture from heat pulse.•The parameters of the piecewise functions for sand and clay are obtained.•The performance of this technique is validated through a field dewater...

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Bibliographic Details
Published in:Measurement : journal of the International Measurement Confederation 2018-07, Vol.123, p.175-184
Main Authors: Cao, Ding-Feng, Shi, Bin, Wei, Guang-Qing, Chen, Shen-En, Zhu, Hong-Hu
Format: Article
Language:English
Subjects:
Boreholes
Carbon fiber
Carbon fibers
Detection
distributed temperature sensing (DTS)
Embedded sensors
Fiber optic sensing
Fiber optics
Hydrology
Laboratory tests
Moisture
Monitoring
Optical fibers
Power supplies
Sensors
Soil investigations
Soil mapping
Soil moisture
Soil moisture profile
Soil properties
Soils
Spatial resolution
Thermal response
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Summary:•A carbon fiber heated sensing tube is developed to capture soil moisture profile.•A piecewise function is used to calculate soil moisture from heat pulse.•The parameters of the piecewise functions for sand and clay are obtained.•The performance of this technique is validated through a field dewatering test. Soil moisture variation with respect to depth directly affects the engineering properties of soil and the health state of plants. At the present, there are few techniques that can satisfactorily quantify the vertical moisture profile within the soil medium. In this paper, a fiber optic sensor-based distributed temperature sensing (DTS) technique is introduced for soil moisture profile mapping. In this technique, a carbon fiber heated sensing-tube (CFHST) is integrated into conventional fiber optic sensing cable to improve the sensitivity, accuracy and spatial resolution of the measurement of soil moisture profile. The CFHST consists of three parts: the inner tubing, the carbon fiber heated cable (CFHC) tightly wrapped on inner tubing, and the interface screws installed on both ends of the inner tubing. The length of a unit CFHST is adjustable according to the actual demand of a specified application. The power supply model and installation method in field are introduced. Laboratory tests were conducted to establish the relationship between soil moisture and thermal response of CFHST. A foundation pit dewatering test was also carried out to validate the field performance of this monitoring technique. The test results show that the borehole-embedded sensor monitored and recorded the continuous change of soil moisture profile accurately (RMSE = 0.046 m3/m3). This technique can effectively capture the distribution profile of soil moisture along the depth direction, which provides a new approach to investigate the physical and hydrological properties of soils.
ISSN:0263-2241
1873-412X
DOI:10.1016/j.measurement.2018.03.052