Overview of whole fiber track:
Waterfall diagram of the records of all DAS sensors along the 12.6 km long fiber. The maximum amplitude of the time series at 20 s intervals is color-coded across the spatially distributed sensors and the 16 hour recording period. The arrows mark events that are described in the listing (click for details), numbered accordingly.

Signals along EuXFEL tunnel:
Spectra of all DAS channels along the accelerator tunnel XTL, during accelerator operation of EuXFEL. Already at first glance, many elements that may cause disturbance or noise can be identified by their characteristic frequencies. Some prominent elements are marked for illustration.

Cars passing the DAS fiber:
Spatiotemporal DAS recording of passing cars is shown at the top. The fiber section between positions 10430 and 10800 is immediately adjacent to a road. A map of the road section is shown at the bottom left. At the bottom right is a recording of the passage of a single car, with horizontal lines framed in appropriate color corresponding to snapshots of the fiber excitation at the bottom center.
Since the fiber passes the road twice in opposite directions, the waterfall diagram is mirrored about the horizontal at a distance of 10645m. The cars are clearly visible as diagonal stripes, and their slope gives immediate information about the speed of the cars (about 18 ± 5km/h). The first car appears at about second 60. Another car travels in the same direction starting at second 125. While the first car crosses a clearly slower road user traveling in the opposite direction towards the end of the road section about 15 seconds later, the second car apparently encounters three other vehicles. One of them leaves its parking place halfway, two others come towards it in close succession.

Animation of car driving along the fiber:
The same section of fiber as shown in the figure on the left. The high amplitudes in the DAS recordings, indicated by yellow color, follows the location of the car.

Earthquake in China recorded along the DAS fiber:
Comparison of seismometer data with DAS data. Shown is an earthquake that occurred on 21.05.2021 in Quinghai (China) with a magnitude of 7.4.
It can be seen that the amplitudes of the sum of 600 data traces in the time window in which the earthquake signal arrives are larger than those of the single trace. The amplitudes of the unwanted noise at the beginning of the data track are reduced. In the lowest plot, many DAS data traces are plotted below each other. The blue line marks the single trace shown in the center plot. The red area covers the 600 traces from which the sum trace was formed.
In the lower graph, in addition to the vertical lines corresponding to the coherent seismic wavefronts, spatial variations of the measured oscillation amplitudes can be seen. These may be related to inhomogeneous subsurface structure and illustrate how DAS measurements provide high temporal and spatial resolution.

Animation of Earthquake waves propagating along EuXFEL tunnel:
Propagation of seismic waves caused by a strong earthquake (magnitude 7.4) with epicenter in China through the EuXFEL tunnel.
On the right, a typical representation for DAS data (waterfall diagram) is shown, 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 elongation of a fiber segment.
On the left, the evolution over time of the strain amplitudes (strain rate) recorded at all DAS sensors is projected onto the fiber track along the EuXFEL tunnel.
Although the length of the waves exceeds that of the tunnel many times over, the spatial shaping of the wave crests and troughs can be clearly recognized as they propagate from East to West.

Vibrotruck of the company Baudynamik Heiland & Mistler GmbH exciting the frequency sweep seen on the right.

Ground motion excited by the Vibrotruck:
Vibration spectra measured in the accelerator tunnel of EuXFEL very clearly show the ground vibrations that a so-called vibrotruck exerts on the subsurface about 100m away.

Spectrogram of the recordings of a single DAS channel over a period of two hours. An irregular signal in the frequency range around 5.2 Hz can be seen, which affects the experimental operation at PETRA.

Frequency components of acoustic signals measured near a transformer station on the DESY site. The lower graph shows the electronically measured variation of the 50 Hz mains frequency. The upper spectrogram shows the variation over time of a one-hour section of the signal from a single position of the optical fiber near the transformer. One can see a vibration at the 5th harmonic of the mains frequency at 300 Hz, whose fluctuations exactly follow the behavior of the mains frequency.