Understanding fiber-optic sensitivity to a wavefield: A framework to separate site amplification from orientation effects

Olivier Fontaine, Andreas Fichtner, Thomas Samuel Hudson , Thomas Lecocq, Corentin Caudron

When analyzing signals from Distributed Acoustic Sensing (DAS), the recorded amplitude across the array can be difficult to interpret, as it is influenced by many parameters.
In this work, we explore the theoretical foundations of fiber sensing amplitude transfer functions. We begin with linear fiber segments and progressively extend to more complex geometries to create polarization filters. To build a filter from the gauge length we explore horizontal or vertical loops to average the signal over all azimuths or dips respectively. These geometries could cancel the component of the shear waves polarization contained within the loop plain.
From these transfer functions, we explore the wavefield as seen by the DAS through a forward model based on ray theory. This model predicts the distribution of amplitude across a DAS array from a point source at low computational cost. The difference between our model and the measured wavefield relates to local site amplification, from which we derive an amplitude correction factor.
The results, along with the foundational components of our approach, can be adapted for a broad range of applications, enabling novel sensing strategies. These include optimization of fiber deployment geometry, the creation of synthetic data, and the acceleration and refinement of existing location methods through amplitude and phase correction that account for the distinct sensitivity of DAS.