Distributed acoustic fibre sensing for large scientific infrastructures: ocean microseism at the European XFEL

The WAVE seismic network is a dense, multi-instrument monitoring system deployed on a scientific campus in Hamburg, Germany. It combines seismometers, geophones, and a 19 km distributed acoustic sensing fiber loop installed in existing telecommunication infrastructure. The network covers large-scale research facilities including the European X-ray Free-Electron Laser (EuXFEL) and particle accelerators at DESY. Its primary goal is to characterise natural and anthropogenic ground vibrations and to quantify how these signals couple into ultra-precise measurement infrastructures that are limited by environmental noise. Beyond local applications, WAVE serves as a testbed for fibre-optic sensing concepts relevant to fundamental physics, including seismic and strain monitoring for gravitational wave detection.

The EuXFEL is a femtosecond X-ray light source designed for ultrafast imaging and spectroscopy. Its performance depends critically on precise timing and synchronisation of the electron bunches along the linear accelerator. Measurements of bunch arrival times reveal significant noise contributions in the 0.05–0.5 Hz frequency band, with peak-to-peak timing jitter of up to 25 femtoseconds. Using distributed acoustic sensing data, we demonstrate that this jitter is largely explained by secondary ocean-generated microseism, which is identified as a significant limiting factor for stable, high-precision XFEL operation in the sub-Hz regime. 

To assess the potential for prediction and mitigation, we investigate whether ocean wave activity in the North Atlantic can be used to anticipate microseismic signals observed at the EuXFEL site. Output from the WAVEWATCH III ocean wave model is used to generate synthetic Rayleigh wave spectrograms with the WMSAN framework. These are compared to seismic observations at the EuXFEL injector. By subdividing the North Atlantic into source regions, we evaluate their relative contributions to the observed seismic wavefield. While absolute amplitude prediction remains challenging, the modelling reproduces key spectral characteristics and temporal variability.

Our results demonstrate that combining dense fibre-optic sensing with physics-based ocean wave modelling provides a framework to characterise microseismic noise and assess its limiting impact on high-precision experiments. This approach supports noise mitigation efforts at high-precision accelerator facilities and is directly relevant to future ground-based gravitational wave detectors.