Sensitivity kernels for transmission fibre optics

Fibre-optic sensing based on transmission offer an alternative to scattering-based distributed
acoustic sensing (DAS). The ability to interrogate fibres that are thousands of kilometres
long opens opportunities for studies of remote regions, including ocean basins. However, by
averaging deformation along the fibre, transmission systems produce integrated instead of
distributed measurements. They defy traditional interpretations in terms of simple seismic
phases, thereby inherently requiring a full-waveform approach. For this, we develop a formal-
ism to calculate sensitivity kernels of transmitted optical phase changes with respect to (Earth)
structure using optical phase delay measurements. We demonstrate that transmission-based
sensing can effectively provide distributed measurements when optical phase delays are anal-
ysed in different time windows. The extent to which a potentially useful sensitivity coverage
can be achieved depends on the fibre geometry, and specifically on its local curvature. This
work establishes a theoretical foundation for tomographic inversions and experimental design
using transmission-based optical sensing.