Recent progress in calibrated optical fibre distributed sensing for full-bandwidth seismology

Distributed acoustic sensing (DAS) along optical fibres allows for fast and accurate measurements of variations occurring in the fibre. However, the intrinsic shot-to-shot small measurable ranges require for slow processes to be measured by successively integrating variations over time thus introducing a $1 / \mathrm{f}$ noise. The DAS strain sensitivities over hour-daily long measurements are therefore generally expected to be orders of magnitude lower than those recorded at frequencies $\gt 10$’s Hz, which limits the range of processes that can be mapped by DAS. In this work we review the recent developments for “calibrated” DAS measurements, allowing for arbitrarily slow processes to be measured with high sensitivity without sacrificing the optimal DAS characteristics at high frequencies. The developments are aimed to be readily applied in seismology– an area where DAS has emerged as a powerful tool, as it allows for the turnkey deployment of thousands of strain sensor arrays along already available fibre networks. The real-time distributed mapping of slow-varying low-amplitude processes (previously inaccessible to traditional DAS), could provide unique advantages and high impact measurements, ranging from earthquake wave propagation, to the measurement of infragravity waves, earth and ocean tides, and, ultimately, being compatible with the mapping of day-to-year-long tectonic plaque movements.