Unveiling type of fiber and coupling conditions effects on geophysical DAS measurements, results from underground experiments

Optical fiber measurements have been demonstrated to be useful in assessing geophysical near-surface parameters and in detecting seismological events in newly accessible regions (e.g. cities, ocean floor, highways) by leveraging the existing fiber-optic infrastructure. In particular, laser interferometry performed with DAS systems (Distributed Acoustic Sensing) allows measuring the cable axial strain related to passing seismo-acoustic waves, at any point along the fiber and over tens of kilometers of cable.

However, compared to traditional seismic sensors the instrumental response of DAS remains unclear, and there is in particular a critical need to better understand how the measurements are influenced by the nature of the fiber optic cable and its coupling to the ground or medium under study. To explore this question, we present results from two active seismic campaigns carried out in the low-noise  underground tunnel LSBB (Laboratoire Souterrain à Bas Bruit), in southeastern France.

We recorded multiple active sources (TNT detonations and hammer shots) by a 10km and 2km long underground optical fiber set-ups and with conventional seismic sensors as well. We tested along both campaigns different optical fiber cable designs and different types of coupling conditions (sealed, sandbags weighted, freely posed) installed in parallel. This experimental setup provides a unique opportunity to examine in detail and quantify the possible variations in the strain signals recovered from DAS data.

Preliminary observations reveal significant discrepancies in the recorded data depending on the coupling conditions. The characteristics of the deployed source result in a signal that is primarily concentrated in the high-frequency range, for which the sealed fiber does not necessarily exhibit a significantly improved response. Additionally, the acoustic wave generated by the hammer-shot echo, propagating through the air, is strongly amplified in all cables covered by sandbags. We propose that the sandbags increase the interaction area between that signal and the cables, thereby enhancing reverberation.

Furthermore, we observe systematic differences in the maximum amplitudes recorded by the different cables tested, with the telecom cable consistently exhibiting lower amplitudes than other specialized cables, suggesting a lower sensitivity. However, this reduction is relatively modest, and when combined with the substantially lower cost of telecom cables, indicates that they remain a cost-efficient alternative for certain experiments. Additional observations and detailed analyses from this study will be presented.

 

Keywords: Coupling, fiber optics, DAS measurements, strain rate, active seismic, LSBB.