Geophysical characterization using optical fiber measurements: PREMISE2 experiment in LSBB, first results on the effect of fiber coupling conditions

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 cable and over tens of kilometers of cable. However, there is a critical need to better understand how the measurements are influenced by the nature of the fiber optic cable, its coupling to the ground. To assess this issue, I will present results from the active seismic experiment PREMISE2 which was carried out in southeastern France in 2020, in an underground laboratory, the LSBB tunnel (Laboratoire Sous-terrain Bas Bruit, https://lsbb.cnrs.fr/). Multiple active shots were recorded both with a 4 km long underground optical fiber and with traditional seismic sensors. Different optical fiber cables and different types of coupling were tested along this 4 km optical fiber, besides the use of a pre-installed vertical fiber. This experiment brings a unique opportunity to examine in detail the possible variations in the strain signals recovered from DAS data in diverse deployment conditions (sealed, sandbag weighted, freely posed), with the utmost aim to understand the potential implications of these conditions for the geophysical characterization of sites of interest in the next stages of this research.