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Fig. 1: Map of the REFIMEVE network (green links) and its connection to European links.In recent years, significant technological progress has demonstrated the feasibility of using the long distance fiber optic links as large scale distributed networks for environmental sensing [1]. Optical fibers are inherently sensitive to external perturbations: their mechanical structure responds to strain, while the light propagating within them undergoes measurable intensity and phase variation when subjected to vibration or seismic waves. A notable example is the French national research infrastructure REFIMEVE [2], which distributes ultrastable time and frequency references across more than 9000 km of fiber links connecting laboratories throughout France and Europe (see Fig. 1). The infrastructure has demonstrated strong potential for geophysical studies [3]. Applications such as earthquake detection, volcano monitoring, and environmental hazard surveillance are attracting increasing interest worldwide, particularly because they can leverage already existing fiber networks. In this context, the European project SENSEI (Smart European Networks for Sensing the Environment and Internet Quality) [4] aims to harness this potential by developing the next generation photonic technologies for detecting both natural phenomena, such as earthquakes, volcano activity, and anthropogenic events including construction activity or vehicular traffic.Within this framework, one of our objectives is to develop a coherent optical frequency domain reflectometry (C-OFDR) [5]. Current systems are limited to approximately 100 km by the coherence length of the laser source. Here, we take benefit from the low frequency noise laser source generated by REFIMEVE frequency reference in order to extend the sensing range. In our setup, the output of a low noise laser is frequency modulated and a fiber under test is studied in a Michelson interferometer configuration. By analyzing the Rayleigh backscattered signal along the fiber, the system enables detailed diagnostics of the fiber under test including the detection of localized fiber deformations, faulty connectors, attenuation variations, and disturbances induced by environmental vibrations. As a first demonstration, we tested a prototype over a long range fiber link made of laboratory spools extending up to 335 km. The system successfully identified the position of the optical amplifier and a PC connector placed at the end of the fiber with km scale spatial resolution. In addition, vibration induced perturbation was observed and is under study, highlighting the potential of this technique for seismic applications. In future work, we plan to deploy the C-OFDR system on the operational REFIMEVE fiber network to evaluate its performance under real field conditions. This approach positions C-OFDR as a powerful tool for telecommunication infrastructure monitoring and distributed geophysical sensing. References :[1] G. Marra et al., Science 361 (2018), https://doi.org/10.1126/science.aat4458[2] REFIMEVE, https://www.refimeve.fr/en/homepage/[3] M. B. K. Tønnes, PhD Thesis (2022), https://hal.science/tel-03984045v1[4] SENSEI, https://senseiproject.eu/[5] C. Liang et al., IEEE Access. 9 (2021), DOI: 10.1109/ACCESS.2021.306125