Monitoring Distributed, Dynamic Strain in Civil Infrastructure using Phase-Sensitive Optical Time-Domain Reflectometry
The micro-scale mechanical behavior of civil infrastructure and the earth immediately sur-rounding it is indicative of material properties, condition and use level. In addition, the geometry of infrastructure systems can be complex and spanning large extents. Phase- sensitive optical time domain reflectometry (φ-OTDR), also known as distributed acoustic sensing (DAS) or distributed vibration sensing (DVS), is a revolutionary technology that uses interferometry within optical fibers to make distributed measurements of strain at higher precision than ever before possible. This technology can measure extremely small strains (nanostrain level) at high spatial resolutions (several meters) and over long distances (10’s of kilometers). Since φ-OTDR can measure such small perturbations, it has gained widespread attention within the earth and planetary science community for measuring seismic wave propagation. These measurements have been predominantly focused on capturing the phase behavior of seismic waves at a crustal or exploration scale. However, for application to mea- suring dynamic strain in civil infrastructure the measurements must be understood for their strain magnitude as well as phase behavior. This dissertation serves to demonstrate the use of φ-OTDR for monitoring civil infrastructure systems through the presentation of several practical verification cases. Optical fibers were coupled to a soil site, a pavement system, and a superstructure to demonstrate how φ-OTDR can be used to quantitatively measure dynamic strain in these elements. The φ-OTDR measurements were verified through the use of velocity transducers (geophones), strain gauges, and another distributed fiber optic sens- ing technique called optical frequency-domain reflectometry (OFDR). Numerical modeling of φ-OTDR measurements is demonstrated for describing the measurements in a pseudo- static case as well as during stress-wave propagation. It is shown that φ-OTDR provides capabilities for monitoring civil infrastructure that currently-used techniques cannot due to its superior measurement precision and high acquisition rates. φ-OTDR is demonstrated as a new tool for monitoring civil infrastructure.