Distributed Acoustic Fiber Sensing for Research Campuses and Large Scientific Infrastructures -- The Hamburg WAVE proto-network

TL;DR

This paper investigates how distributed acoustic sensing (DAS) can monitor environmental vibrations on research campuses and large infrastructures to protect high-precision experiments. It describes a two-week proto-network campaign over 12.132 km of fiber on the Hamburg campus using two DAS interrogators and a sensor suite to map the vibration field. Key contributions include observing P- and A-waves, imaging earthquake wave propagation (notably the magnitude 7.4 Qinghai earthquake) and anthropogenic sources, and characterizing DAS self-noise in a tunnel environment. The results support deploying permanent campus-scale DAS networks to enable real-time source localization, vibration mitigation, and subsurface studies, with strong implications for accelerator physics, gravitational-wave experiments, and urban science campuses.

Abstract

Here, we demonstrate and investigate how Distributed Acoustic Sensing (DAS) can be utilized on research campuses and in large scientific infrastructures to study environmental vibrations and reduce their impact on high-precision experiments. We first discuss the potential of DAS in the context of particle accelerators, gravitational wave detection experiments and research campuses. Next, we present the results of our seismic measurement campaign conducted with our proto-network, which involved the probing of over 12 km of fiber, in May 2021. This campaign was conducted by the Hamburg WAVE initiative in Science City Hamburg Bahrenfeld and included DESY, the European XFEL, PETRA III and the University of Hamburg. Our proto-network confirms the ability to observe natural, anthropogenic, and infrastructural vibrations and how and where these couple into different parts of the heterogeneously set up fiber network. We also present results on a study of noise and motion coupling aspects of DAS probing double-redundant fiber loops in a unique environment, the European XFEL. Our results show that DAS greatly benefits research campuses and large scientific infrastructures and they highlight the opportunities and challenges of implementing and operating such seismic networks.

Figures (6)

• Figure 1: Map of the fiber route during the demonstration study in the Science City Hamburg Bahrenfeld, on the campus of DESY and University of Hamburg, and in the tunnel of the European XFEL. In the north-east the fiber is routed through the experimental hall of the PETRA III accelerator, where the beam lines are situated, and follows the circular shape of the ring. The additional sensors and vibro-truck excitation points are also marked, as is the routing of the fiber either underground in ducts or over ground in cable trays. The upper maps show the position of the campus in the West of Hamburg in the North of Germany and details of the campus itself, with roughly 100 km of distance to both the North and the Baltic sea. The lower sketch illustrates the fiber paths. Multiple fibers within a single fiber optic cable were spliced together to form one loop. Another set of fibers, also spliced together within the same optical cable, was monitored using a second interrogator system.
• Figure 2: Screenshot of the waterfall plot showing geo-referencing of the DAS fiber loop using hammer taps on campus and research infrastructure, such as HF stations in the EuXFEL, where the hammer taps were not visible due to the concrete floor.
• Figure 3: Spatiotemporal propagation of seismic waves through the European XFEL tunnel (a) caused by a strong earthquake (magnitude 7.4) with its epicenter in Qinghai, China. A typical representation for DAS data (waterfall diagram) is shown in (b), which color-codes the time series of strain amplitude (y-axis) for each fiber sensor (channel, x-axis). Here, the dark blue color represents compression and light yellow represents dilatation of a fiber segment. Four different points in time, marked by numbered horizontal lines in the waterfall diagram, correspond to snapshots of strain rate amplitudes of all DAS sensors in (a). These are plotted in the same colormap, projected to their corresponding location above the fiber trajectory in the European XFEL tunnel. Here, the colors and dashes of the horizontal lines and frames of the snapshots match. Although the wavelength exceeds that of the tunnel considerably, the spatial shaping of the wave crests and troughs can be seen at a fixed point in time (animation available at http://www.wave-hamburg.eu/). (c) shows an illustration of SNR enhancement through stacking of 600 adjacent DAS channels, compared to single channel waveform. (d) shows a spectrogram of single DAS channel recording, showing both P- and S-wave arrivals of the earthquake, along with the permanently active ocean microseism band.
• Figure 4: Waterfall plots of the strain rate measured by the DAS network along the 12.132 km-long fiber. Several activities—highlighted in (a) are directly visible without the need for extensive data processing. Additional events are shown in (b)–(d), where low-pass (LP) filters were applied to enhance the visibility of traffic-induced vibrations and temperature-related variations. (a) Overview of noise and signal as waterfall plot of all DAS sensors/channels over the 12.132km fiber. Max. amplitude strain rate of 20s intervals is color coded. (b) Passing cars close to the mirror axis with a speed of $18\pm 5\,$km/h. (c) Passing bus on Flurstraße 10m above the EuXFEL. (d) Longterm (1 minute) averaged strain rate variations along the XTL tunnel showing temperature changes.
• Figure 5: Spectrograms for parts of the DAS network. Frequency dependent amplitudes varying over distance in the fiber are shown in (a) and (c). Variations of the frequency dependent amplitude over time for specific points in the fiber are shown in (b) and (d) to visualize temporal variations of signals. (a) DAS spectrogram along the EuXFEL. (b) DAS spectrogram over time shows the 200 Hz harmonic of the 50 Hz power line signal measured near a tranformer house. The inset shows the transformer house's mains 50 Hz fluctuations measured in the DESY power supply system. (c) 47.5 Hz oscillation caused by Helium compressors in the tunnel. (d) Snapshot of 5.2 Hz oscillation disturbing PETRA III operation.