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Application of Intensity-Based Coherent Optical Time Domain Reflectometry to Bridge Monitoring
(2022)
Although distributed fiber sensing techniques have been widely used in structural health monitoring, the measurement results of bridge monitoring, particularly under destructive testing, have rarely been reported. To the best of our knowledge, this paper is the first report of distributed vibration measurement results, which we obtained during a three-day destructive test on an abolished bridge. A coherent optical time domain reflectometry (COTDR) was used to acquire the vibration information while the bridge was being sawed. The obtained signal was analyzed in time and frequency domain. Some characteristics of the sawing-induced vibration were retrieved by the short-time Fourier transform; the vibration exhibited several high frequency components within the measured range up to 20 kHz and all the components appeared in the same time slot. Some unexpected signals were also detected. Thorough analysis showed that they are quite different from the sawing-induced vibration and are believed to originate from internal damage to the bridge (probably the occurrence of cracks).
A distributed acoustic sensor (DAS) based on wavelength-scanning coherent optical time domain reflectometer (WS-COTDR) is tested in an underground gas storage based on a multimode fiber for the first time to the best of our knowledge. The WS-COTDR acquires the reflection spectrum by scanning the wavelength of the laser and uses the spectra to calibrate strain. It overcomes the fading problem which is common for all DAS systems, and possesses the advantage of simple configuration and low-cost. The working principle of the system is well explained and the measurement results are presented. Various artificial and natural acoustic events have been successfully detected by the WS-COTDR. Such a sensor demonstrates several advantages over the traditional DAS systems, so it is supposed to be widely used in the oil and gas industry.