Bridge Displacement Estimation Using Distributed Strain from Pre-Existing Telecommunication Fiber

Monitoring the condition and performance of bridges is critical for enhancing the resilience and safety of urban infrastructure. In particular, displacement is a critical measurement for many bridge health monitoring tasks such as load rating and serviceability assessment. Thus, many sensing modalities and sensing fusion methods have been developed for displacement estimation; however, such methods often require fixed reference points, are sensitive to occlusion, or costly to install and maintain, and thus not scalable at a city level. Our previous works have demonstrated that we can convert pre-existing telecommunication fiber optic cables into a dense array of vibration sensors using distributed acoustic sensing (DAS). This enables scalable, high spatial resolution (meter-level) bridge vibration sensing, as a single interrogator can turn more than 100 km of telecommunication fiber into distributed strain sensors. Thus far, our group has used this to measure the dynamic component (1-10 Hz for short to medium-span bridges) of bridge vibrations and identify modal parameters. However, in order to accurately estimate displacement, we need to extract the scaled, quasi-static (0.1 – 1 Hz) component of the bridge vibration as well. To this end, we develop a novel displacement estimation framework for distributed strain measured from non-dedicated telecommunication fiber optic cables. The key challenge is the coupling between the fiber and the bridge that distorts the bridge response measurements. The pre-existing telecommunication fiber optic cables are often laid inside conduits, which are attached to bridges. As the bridge vibration passes through these attachments, they introduce noise as well as frequency-dependent (for different bridge modes) amplitude distortion. Using double integration amplifies such noise, especially in the lower quasi-static band, resulting in low accuracy. To address this, we introduce a structure-aware FIR filter to reconstruct bridge displacement from telecommunication fiber. We first address the higher noise in non-dedicated fiber with a finite impulse response (FIR) filter based on the regularized distributed strain-displacement relation. This suppresses the low spatial frequency drift and enables reliable estimation of the quasi-static components of bridge vibration. A transfer function parameterized for each mode band is then calculated during the calibration step to address the frequency-dependent distortion from the coupling structure. This framework requires no prior knowledge or assumptions about modal shapes, and only a temporary sensor needs to be deployed at calibration time to identify the transfer function. Based on this framework, we estimate the transfer function between the distributed strain measurements from the telecommunication fiber and actual bridge response, using a temporarily placed reference accelerometer sensor for calibration. Our evaluation results from two real-world in-service highway bridges (Bakdal 2 and Miho River bridges in Korea) demonstrate that DAS can serve as a displacement sensor with sub-millimeter accuracy.