1. Fiber optics-based CO2 sensing
Soga’s Research Group collaborate with Redondo Optics Inc to develop a multi-channel (4-sensing channels) fiber optic intensity radiometric interrogation system (FIRIS™-M400) which is suitable for the real-time monitoring of the light transmission status of ROI’s absorption and/or fluorescence-based distributed fiber optic chemical sensors (DCS).
A 2-week-long calibration test was conducted in order to obtain the relationship between the 3 variables and the real CO2 concentration. Linear regression and higher-order regression would show a relationship if they were correlated. However, the result did not show good repeatability. Besides, the residual of the regression does not converge to zero. These issues might be due to the nonlinear relationship. The long reaction period may also cause the delay in the response, which requires a deconvolution method. In the future, we seek the possibility to test this system in a large soil tank in order to validify the idea of measuring CO2 concentrations in soils using fiber optics. Notice that now the system can only do pointwise gas sensing. Similar to DSS and DTS technology, an OFDR-BOTDR analyzer will be developed to make this gas sensing technique distributed. Then the next task of the project is conducting a laboratory test with this system to verify the feasibility of distributed gas sensing.
Inorganic carbon can occur in different forms in soil, including dissolved species, and solid mineral phases. The dissolved species in soil are controlled by acid-base equilibria: , (Bargrizan et al., 2020). The reaction of CO₂ with water leads to hydronium ions that affect the pH and can be sensed by the color change of the dye. We are generating a type of sol-gel, which adheres well to the sensor and can preserve the dye for sensing. We used plastic optical fiber (POF) as the gas sensor. Our POF has a core of 400 micrometers OD. The cladding and the coating of the fiber need to be removed so that the core can be directly exposed to the sol-gel. We selected methyl red as the dye and the sol-gel is doped with methyl red and other chemicals, which either serves as the solution or the precursor of the reaction (Wysokiński et al., 2015).
Schematic diagram of FO gas sensor
Response of the CO2 cable in the 2 weeks test
2. Intermediate-scale testing of a spatially distributed sensing technology for monitoring greenhouse gas migration through vadose zone to the atmosphere
A combined laboratory and field study is in progress to develop and deploy a spatially distributed gas sensing technology with an embedded fiber-optics component for use in agricultural and natural environments (as shown in the pictures below). The approach involved the testing of the sensor technologies in intermediate-scale test systems before field deployment. The primary goal of this research is to understand how soil heterogeneity affects the development of the gas migration pathways and subsequent spatial distribution of gas
concentrations across the land surface.In this experiment, Optical Distributed Sensor Interrogator (ODISI-6100, from LUNA.inc) is applied to acquire and analyze the fiber-optics signal in the fiber-optic cable (NZS-DSS-C07, from NanZee Sensing). Such DSS system can provide a strain distribution profile up to 40m offset, with 0.65mm gauge pitch and more than 10 Hz measurement rate. 4 MPPTs are placed in different depths of the soil. These MPPTs serve as tensiometers, such that the matrix suction of the soil can be related to the water content measured by EC5 moisture sensor.
The objective is to try to find the feasibility of DFOS to detect the sign of crack generation in soil. The propagation of the crack in solid usually brings a localized strain concentration on the crack tip. With the help of the high-resolution OFDR strain sensing, the crack generation can be captured based on the strain concentration shown by the DFOS measurements. In addition, soil cracking is governed by the change in effective stress during the tensile process. To analyze the mechanism of soil cracking, the measurement of negative pore pressure during soil tensile progress is vital to evaluate the effective stress. In this test, to measure the pore pressure at the location in front of soil fracture, we utilize the miniature fiber-optic-based pore pressure transducers. These pore pressure transducers need to be small enough to capture the very localized tensile stress concentration at the crack tip.
MPPTs and EC5 Locations (Plan View)
MPPTs and EC5 Locations (Section View)
After 2 days of heating, the soil generates multiple cracks. All cracks that have obvious apertures have been indexed carefully. A big strain concentration is recorded near the crack location, which indicates the generation of the crack. In this report, all 3 sections for both top and bottom layers are plotted. A 20cm section near the notch and a 20cm section near the crack 5 location are plotted for the interest of the investigation.
DFOS result of different sections and different layers, with local stress profiles near the crack
Collaborator: Redondo Optics Inc
Researchers: Yaobin Yang, Kenichi Soga (UC Berkeley)
Domains: Smart City Tech, Energy Systems
Capabilities: Distributed Fiber Optic Sensing (DFOS)