Time and Frequency analysis of DAS in cemented fibers: Insights from the Black Forest Observatory test-bed

Distributed Acoustic Sensing (DAS) has established itself as a robust technology for detecting thermal and mechanical variations along fiber-optic cables. Historically, DAS applications have predominantly targeted dynamic processes above 1 Hz, such as anthropogenic noise (e.g., traffic, railway monitoring, or structural health engineering) and high-frequency seismic waves from earthquakes. Conversely, many critical geophysical phenomena that characteristically manifest at frequencies well below 1 Hz —including volcanic magma migration, tidal fluctuations, ocean infra-gravity waves, and slow crustal deformation— can also be measured by DAS. Consequently, there is a growing scientific interest in exploring this ultra-low frequency spectrum, an area that remains largely understudied in distributed sensing frameworks.

Commercial DAS interrogators utilize diverse optical principles to quantify strain. Within phase-sensitive optical time-domain reflectometry (φ-OTDR), some systems extract the raw optical phase to calculate deformation, while alternative approaches, such as chirped-pulse DAS (CP-DAS), derive strain by tracking local spectral shifts in the fiber's backscattered response. Recent literature highlights the distinct metrological advantages of CP-DAS, particularly its wider dynamic range, uniform longitudinal sensitivity, and localized measurement principle. These features minimize the need for frequent reference updates and significantly enhance instrument stability at lower frequencies.

To evaluate these capabilities under optimal low-frequency conditions, a specialized experimental setup was deployed at the Black Forest Observatory (BFO). Here, eight single-mode patch fibers were rigidly coupled to the surrounding medium by cementing them into a 250-meter-long groove excavated directly into the gallery's concrete floor. The utilized cables comprise a standard 9 um core, 125 mm cladding, 250 mm coating, and a 900 mm tight buffer, resulting in a total outer diameter of 0.9 mm. This high-rigidity installation ensures near-ideal strain transfer from the host rock to the fiber core. Operating as a controlled geophysical test-bed, this layout allows for a cross-comparison between the DAS channels and BFO’s permanent Invar-wire strainmeter array, facilitating a precise evaluation of signal fidelity.

In this work, we present a comprehensive time- and frequency-domain intercomparison between a commercial CP DAS interrogator platform and the reference strainmeter array during periods of microseismic activity and teleseismic wave arrivals. Our experimental results demonstrate highly equivalent performance across both domains, validating the quantitative reliability of the distributed optical approach for broadband geophysical monitoring.