Distributed fiber-optic temperature sensor validations using field deployments in the flooded Orphan Boy mine shaft in Butte, MT

The process of sensor validation through experimentation with the Omnisens Distributed Temperature and Strain (DITEST) Brillouin Optical Time Domain Analyzer (BOTDA) proved to be a challenging project. The project encompassed sensor calibrations, system error minimization, sensor network design and deployment, and the characterization of temperatures in the Orphan Boy Mine shaft. Fiber-optic cable sensor calibrations yielded linear relationship coefficients 0.6-1.0MHZ/°F, indicating a strong positive correlation between Brillouin Frequency Shifts and temperature. Calibrated sensors demonstrated accuracies near ±0.8°F using the corrected error bounds from residual analyses as the benchmark. Fiber-optic measurement accuracy and repeatability were controlled by user-selected signal interrogator settings and design limitations within the system. Temperatures monitored during the February-July 2016 period showed little variation except when the Geothermal Heat Exchange System was in operation. A test of the geothermal system (used to heat the Natural Resources Building on the Montana Tech campus) was documented by the fiber-optic sensor cluster deployed in this project and separately by a temperature transducer from the Montana Bureau of Mines and Geology. Temperature was an auxiliary sensing function of the DITEST; temperature profiles recorded in time and depth demonstrated the capability of the Brillouin-based signal interrogator when used primarily as a temperature sensing system

Strain Distribution and Crack Detection in Concrete Overlays with Pulse Pre-Pump Brillouin Optical Time Domain Analysis

This report is focused on the measurement of strain distributions and crack detection in unbonded and bonded pavement overlays. The main objectives of this study are: (a) to characterize the strain sensing properties of distributed fiber optic sensors with recently developed pulse pre-pump Brillouin optical time domain analysis (PPP-BOTDA), (b) to develop an installation method for real world applications, (c) to document the performance of the PPP-BOTDA technology in unbonded/bonded pavement applications, and (d) to develop a numerical model to facilitate the analysis of mechanical behavior of unbonded pavement overlay under vehicle wheel loads. A thin concrete layer can be cast on top of a severely deteriorated pavement layer with a fabric sheet in between to rapidly and cost effectively improve the driving condition of existing roadways. Once cured, the concrete layer is divided into many panels and often referred to as the unbonded Portland cement concrete (PCC) overlay. The service life of PCC overlays can be appreciably extended by appropriate rehabilitation strategies at early stages of deterioration based on the information provided by health monitoring. The strain distribution and crack detection are of interest to engineers in this application. Minor or moderately deteriorated existing concrete pavements can also be resurfaced with a thin concrete layer to improve their driving condition. In this case, potential cracks in the existing pavement may easily penetrate through the new concrete layer. The way the potential slip at their interface develops over time is an interesting question to answer. This study reports an application of a commercial single mode optical fiber to measure strain distributions in full-scale fiber reinforced unbonded overlays. Prefabricated cementitious mortar grid instrumented with distributed fiber optic sensors, namely smart grid, was developed and proposed to address the logistics of handling delicate optical fibers, and thus facilitate the in-situ construction. The smart grids can be laid on top of the fabric sheet and embedded in concrete overlay. With the proposed method, the pavement overlays instrumented with distributed sensors were successfully constructed in Minnesota\u27s Cold Weather Road Research Facility (MnROAD). The optical fibers were characterized on a precision load frame at room temperature. A Neubrescope was used to measure strain distributions based on the pulse pre-pump Brillouin optical time domain analysis (PPPBOTDA). The overlays were subjected to repeated truck loads and eventually cracked. Strain distributions were obtained from the distributed fiber optic sensor. Cracks were identified and localized by mapping the strain distribution in which the sharp peaks represent the cracks. The strain distribution was further investigated using a three-dimensional finite element model incorporating nonlinear boundary conditions. Opening between substrate and overlay concrete was demonstrated, and strain distributions in overlay and substrate concrete were determined with the numerical model. For the bonded concrete overlays on existing pavement, a delamination detection method was developed and implemented using the distributed fiber optic sensors. Delamination can be identified as sharp peaks in the measured strain distributions

Optical fiber sensing cables for brillouin-based distributed measurements

Brillouin distributed optical fiber sensing (Brillouin D-FOS) is a powerful technology for real-time in situ monitoring of various physical quantities, such as strain, temperature, and pressure. Compared to local or multi-point fiber optic sensing techniques, in Brillouin-based sensing, the optical fiber is interrogated along its complete length with a resolution down to decimeters and with a frequency encoding of the measure information that is not affected by changes in the optical attenuation. The fiber sensing cable plays a significant role since it must ensure a low optical loss and optimal transfer of the measured parameters for a long time and in harsh conditions, e.g., the presence of moisture, corrosion, and relevant mechanical or thermal stresses. In this paper, research and application regarding optical fiber cables for Brillouin distributed sensing are reviewed, connected, and extended. It is shown how appropriate cable design can give a significant contribution toward the successful exploitation of the Brillouin D-FOS technique

Distributed Temperature and Strain Discrimination with Stimulated Brillouin Scattering and Rayleigh Backscatter in an Optical Fiber

A distributed optical fiber sensor with the capability of simultaneously measuring temperature and strain is proposed using a large effective area non-zero dispersion shifted fiber (LEAF) with sub-meter spatial resolution. The Brillouin frequency shift is measured using Brillouin optical time-domain analysis (BOTDA) with differential pulse-width pair technique, while the spectrum shift of the Rayleigh backscatter is measured using optical frequency-domain reflectometry (OFDR). These shifts are the functions of both temperature and strain, and can be used as two independent parameters for the discrimination of temperature and strain. A 92 m measurable range with the spatial resolution of 50 cm is demonstrated experimentally, and accuracies of ±1.2 °C in temperature and ±15 με in strain could be achieved

Unbonded Portland Cement Concrete Overlay/Pavement Monitoring with Integrated Grating and Scattering Optical Fiber Sensors

This report summarizes the findings and results from a laboratory and field study on the strain distribution and crack development in 3 thick concrete panels cast on top of existing concrete pavements as a rapid rehabilitation strategy for roadways. Both fiber Bragg gratings (FBG) and Brillouin Optical Time Domain Reflectometry/Analysis (BOTDR/A) were applied and tested for their feasibility and effectiveness in distributed strain measurement and crack detection. For laboratory tests, six 6\u27×6 panels were cast similar to their corresponding field construction. Each was tested under both truck loads and under threepoint loads. The performance of distributed BOTDR/A strain measurements was compared with that of FBG sensors. In field study, the performance of FBG sensors was compared with that from strain gauges when the ambient temperature was measured with thermocouples. Overall, hairline to major cracks can be successfully detected with the distributed BOTDA measurements. The strain distributions measured from the FBG and BOTDR/A sensors are consistent. The FBG readings are in good agreement with those of strain gauges. Both FBG and BOTDR/A technologies are promising for pavement monitoring

AN EVALUATION OF THE DEPLOYMENT OF A DISTRIBUTED STRAIN AND TEMPERATURE (DST) FIBER OPTIC SENSING SYSTEM IN AN UNDERGROUND FACILITY

In an effort to further mine safety and utilize the recent advances in fiber optic sensors’ distributed sensing capabilities, a research project at Montana Tech has deployed fiber optic sensors in an underground environment. The method of Brillouin scattering that has documented success in a mining environment will be used because it shows the greatest potential for detecting both temperature and strain in the deployed environment, as well as the ability to differentiate between the two. The primary objective of the Montana Tech research is to create a scenario where the distributed sensing technology can be evaluated for its sensing capabilities in a rugged underground environment, at the same time creating a protocol for this particular type of sensor deployment. A Ominsens DITEST signal interrogator, along with two strain sensing and two temperature sensing cables will constitute the fiber optic sensor component, and an array of documented point sensing instrumentation consisting of both strain and temperature sensors will validate the FOS sensor data. Calibrations of the FOS cables were extensively conducted in a Montana Tech laboratory to accuracy quantify the response of the FOS cables to strain and temperature. All four FOS cables and traditional sensors were deployed around and through boreholes in a structural pillar in the Underground Mine Education Center (UMEC). During the 3-month long data collection campaign multiple events such as the construction of the protective trench and the direct FOS cable to rock attachment allowed for indepth response analysis because the nature of the deployment area was static. A comparison of the FOS data and the traditional sensory data showed that the strain detection of the FOS technology was comparable to the traditional technologies in the microstrain range, however, the FOS temperature data was not as accurate without a relative baseline. Comparing the two strain cables showed similarities, with the Mil-Tec OCC cable being less expensive and easier to repair. Of the two temperature sensing cables, only the Brugg T-85 temperature cable lasted longer than a week and provided the only reliable temperature data. The lessons learned throughout the UMEC deployment and data collection campaign were compiled into a FOS deployment protocol in an effort to pass on the knowledge gained from these experiences. From all of the data collected and comparisons made, the FOS cables proved their durability and ability to detect environmental changes within +/-100 microstrain of traditional sensors. Of the FOS cables deployed, the cable that demonstrated the overall best value was the Mil-Tac OCC cable. Future research that could benefit FOS sensing the most would be additional calibration research and deployment of the FOS cables to a site with less static ground conditions