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

Ding-Feng Cao ab, Bin Shi a, Guang-Qing Wei c, Shen-En Chen d, Hong-Hu Zhu a

School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

Department of Civil and Environmental Engineering Department, Engineering College, University of Wisconsin-Madison, Madison, WI,53706, USA

Suzhou NanZee Sensing Technology Co. Ltd, Suzhou 215123, China

Department of Civil and Environmental Engineering, University of North Carolina at Charlotte, NC 28223, USA

Measurement, 2018.

DOI: 10.1016/j.measurement.2018.03.052

Abstract

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 screw 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.

Keywords: soil moisture profile, distributed temperature sensing (DTS), carbon fiber, fiber optic sensing


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