The seismic wavefield as seen by distributed acoustic sensing arrays: local, regional and teleseismic sources

Abstract

Distributed acoustic sensing (DAS) exploiting fibre optic cables provides high-density sampling of the seismic wavefield. Scattered returns from multiple laser pulses provide local averages of strain rate over a finite gauge length. The nature of the signal depends on the orientation of the cable with respect to the passing seismic waves. For local events, the dominant part of the strain rate can be extracted from the difference of ground velocity resolved along the fibre at the ends of the gauge interval. For more distant events the response at seismic frequencies can be represented as the acceleration along the fibre modulated by the wave slowness resolved in the same direction, which means there is a strong dependence on cable orientation. Slowness–frequency representations of the wavefield provide insight, via modelling, into the character of the DAS wavefield in a range of situations from a local jump source, through a regional earthquake to teleseismic recording. The slowness-domain representation of the DAS signal allows analysis of the array response of cable configurations indicating a bias due to the slowness weighting associated with the effect of gauge length. Unlike seismometer arrays the response is not described by a single generic stacking function.

Assessment of Distributed Acoustic Sensing (DAS) performance for geotechnical applications

“…13A). This is consistent with the fact that the measurements recorded by the fiber optics are a combination (i.e., a weighted average) of the vertical and horizontal components of the signal propagating from the source (e.g., Kennett, 2022). Considering again Fig.…”

High resolution seafloor thermometry for internal wave and upwelling monitoring using Distributed Acoustic Sensing

“…The detection of mechanical strains at tidal frequencies has only been demonstrated for DAS under laboratory conditions, although only for controlled deformations that were several orders of magnitude larger than actual tides 94 . Furthermore, DAS is also known to have a highly directional sensitivity pattern 20 , 95 , as the deformation response of optical fibers to tangential (broadside) stresses is generally expected to be much lower than longitudinal (axial) ones 96 , meaning that vertical pressure waves induced by tidal oscillations inciding a fiber in a gently sloping, flat bottom might go undetected. Furthermore, it is known that the response of DAS is generally inversely proportional to the apparent wavelengths generated by such broadside incidence angles 97 – 100 .…”

Potential of Earthquake Strong Motion Observation Utilizing a Linear Estimation Method for Phase Cycle Skipping in Distributed Acoustic Sensing

“…In DAS recordings obtained through cables on structures, the site characteristics differ from seismic instruments placed on the ground. Consequently, the differential phase of DAS measurement (∆Φ ni ( t )) while using the existing fiber cables along the railway with the seismic event n at channel # i is expressed as follows: $$\begin{align*}{{\increment}{\Phi }}_{ni}(f)\equiv {S}_{n}(f)\cdot {P}_{ni}(f)\cdot {G}_{i}(f)\cdot {C}_{i}(f)\cdot {R}_{i}(f)\cdot {O}_{n}(f,\theta ),\end{align*}$$ here, S n ( f ) is the source, P ni ( f ) is the propagation path, G i ( f ) is the site amplification on the ground, C i ( f ) is the coupling between the fiber cable and installation surface, R i ( f ) is the effect of amplification of the seismic wave by the railway structure, and O n ( f , θ ) is the orientation factor (Kennett, 2022; Kuvshinov, 2016; Martin et al., 2018), which represents the effect of polarization and arrival direction on the seismic waveform.…”