Rayleigh scattering: Distributed Acoustic Sensing (DAS)

Rayleigh scattering is an elastic form of scattering in which photons interact with frozen-in inhomogeneities of the silica: small, static refractive-index variations distributed along the fibre. The back-scattered light carries phase information, of which the two-way traveltime in the fibre reveals the path length to the scatterer. Therefore any deformation of the cable between the interrogator and the scatterer results in a phase change that can be directly related to strain(rate). 

Distributed Acoustic Sensing is based on phase-sensitive optical time-domain reflectometry (Φ-OTDR): finite laser pulses are sent into the fibre, and the phase of the Rayleigh-back-scattered light is recorded as a function of two-way travel time, addressing every location along the cable as a virtual channel. When an incoming wavefield deforms the fibre, compression and extension over a gauge length surrounding each channel change the optical path length, producing a phase difference that is converted to axial strain rate. Because DAS samples strain only along the fibre axis, it does not recover the full strain tensor and its sensitivity depends on the angle of the incoming wavefield, but the measurement is intrinsically distributed, with metre-scale resolution over tens of kilometres and a frequency response that empirically spans from millihertz to kilohertz and beyond. This technology is widely used to repurpose existing telecom networks as dense seismic arrays in environments such as urban areas, glaciers, boreholes and the seafloor. In EGS and CCS, DAS provides high-resolution monitoring of microseismicity, fracture activation and induced events along the injection well and surrounding fibre infrastructure. 

References 

[1] Tuinstra, K. (2024). Enhancing Seismic Monitoring for Geothermal and Carbon Storage Applications using Fibre-Optic Sensing, Doctoral thesis, ETH Zurich. doi: https://doi.org/10.3929/ethz-b-000742567