Fiber Optic Acoustic Detection for Condition monitering

Safety is the top priority for every transportation system. Although various aspects of transportation infrastructure’s safety have been studied, in-motion monitoring and detection of defect occurrence, cause and severity is still a big concern. Understanding the trend of anomalies, the significant factors affecting these trends, and how to monitor and predict undesired conditions are of high interest in transportation safety, and necessary for development of transportation infrastructure. In this study, the state-of-the-art technology of Distributed Acoustic Sensing (DAS) for in-motion rail condition monitoring is studied and evaluated through experimental testing, theoretical assessment and simulation modeling. DAS uses fiber optic cables along the track to detect any anomaly indicator such as vibration, so it can be represented as the transportation operators’ ear on the track. The highest state of the art in optical sensing is achieved with optical fiber distributed sensors. Such sensors permit the measurement of a desired parameter as a function of length along the fiber. This is clearly of particular advantage for applications such as smart skins, since a sensor can measure the variation or temperature over significant areas of the outer layer of vehicles. There are few main criteria which must be satisfied to achieve a distributed sensor. Firstly, it is necessary to construct (or select) a fiber which will modify the propagation of light in a way which can be relied upon to be dependent on the parameter to be measured. Secondly, one must be able to detect the changes in transmission (or light scattering) arising from the parameter to be measured. Despite any conventional inspection and monitoring Nondestructive Testing (NDT) technique where the coverage or scanning area of the sensors are very limited, DAS technology provides a full, fast and accurate coverage of all transportation section under the test which is a huge advantage for in-line inspection and monitoring. Using DAS technology, a long track can be continuously monitored for several different purposes as is described in this paper. The objective of this research work is to provide critical and detailed assessment of current and emerging anomaly detection and monitoring techniques based on DAS for transportation investigation. This study presents the literature survey results, experimental evaluations, preliminary theoretical assessment and numerical simulations on the current methodologies in transportation monitoring and detection DAS systems. In this study, DAS was used to evaluate the transportation traffic condition in a rural area with connecting an available underground dark fiber to the DAS interrogator and system as well as simulated traffic condition in smaller scale in a parking lot. In addition, COMSOL Multiphysics finite element software was used to model the interaction of ambient vibration due to the traffic with the fiber optic in a DAS setup. Results show that the condition and variations of the transportation can be monitored and detected by DAS with an appropriate accuracy. DAS information can be used for traffic condition monitoring, object tracking and flaw detections in the transportation lines