An Experiment Study On Distributed Fiber Optic Sensing Technology For The Measurement Of Soil Water Content

Water content is a basic physical parameter for soil as well as a fundamental physical index in the evaluation of soil engineering properties. However, at present, a long-distance and large-area field measurement method is still in need, which seriously impeded the researches of ecological environment and engineering geological. Raman Optical Time Reflectometer (ROTDR) has broken through the traditional ways of points measurements. It can attain continuous and real-time information of objects. Furthermore, some other corresponding information can be achieved indirectly. A kind of Distributed Water content of soil System (DWS) based on ROTDR and internal-heated by carbon fiber was proposed in this paper. Its feasibility was demonstrated systematically through theoretical analysis, process design, indoor and field tests. The following results were obtained.1. The main method and technique for measuring water content of soil were reviewed, and their advantages and disadvantages were analyzed. At last, a new system—DWS—was introduced based on ROTDR technology.2. The relationship between temperature variation of heat source in the process of heating and coefficient of thermal conductivity of surrounding media was determined. The main factors that influence soil thermal conductivity were summarized, mainly including the water content, dry density and silt content.3. Based on DWS, a new method for the measurement of water content of soil was put forward, and its principle was introduced as well. A concept named temperature characteristic value (Tt) was proposed, its relationship with water content has been established; its error was reduced by average processing the slope of heating curves.4. Distributed Water content of soil measuring System (DWS) was designed, which contain four subsystems—heating subsystem, sensing subsystem, demodulating subsystem and calculating subsystem.5. The calibration test apparatus was designed to determine the relationship between Tt and water content, dry density and silt content at specific heating power by control variate method.6. Based on DTS technology, a field test was carried out by DWS system. The performance and precision for DWS was measured in a 250 m backfill soil bin. Compared with the results by drying method, the absolute error of DWS can be less than 2.1%.7. The feasibility of DWS for landslide monitoring was carried out by indoor experiment, and verified by applying DWS to locate the slide surface and measure the water content in unsaturated mass. The experiment suggested that the positioning error of DWS is less than 2.5cm, and water content in unsaturated mass could be measured effectively. At the same time, the wiring scheme of DWS was designed for landside monitoring.The results of this paper suggested that water content in-situ soil can be measured by DWS, it also realized long-distance and large-area field measurment. DWS has vast applied prospects.