Dynamic strain determination using fibre-optic cables allows imaging of seismological and structural features

Abstract

Natural hazard prediction and efficient crust exploration require dense seismic observations both in time and space. Seismological techniques provide ground-motion data, whose accuracy depends on sensor characteristics and spatial distribution. Here we demonstrate that dynamic strain determination is possible with conventional fibre-optic cables deployed for telecommunication. Extending recently distributed acoustic sensing (DAS) studies, we present high resolution spatially un-aliased broadband strain data. We recorded seismic signals from natural and man-made sources with 4-m spacing along a 15-km-long fibre-optic cable layout on Reykjanes Peninsula, SW-Iceland. We identify with unprecedented resolution structural features such as normal faults and volcanic dykes in the Reykjanes Oblique Rift, allowing us to infer new dynamic fault processes. Conventional seismometer recordings, acquired simultaneously, validate the spectral amplitude DAS response between 0.1 and 100 Hz bandwidth. We suggest that the networks of fibre-optic telecommunication lines worldwide could be used as seismometers opening a new window for Earth hazard assessment and exploration.

On the Broadband Instrument Response of Fiber‐Optic DAS Arrays

“…Here, we restrict our focus to a single commercial instrument, the Silixa iDAS (Version 2), which is a time domain, single‐pulse, phase‐based DAS instrument (Parker et al, ). This particular DAS instrument is among the more widely utilized in the field of earthquake seismology (Ajo‐Franklin et al, ; Jousset et al, ; Lindsey et al, ; Wang et al, ; Yu et al, ). The following discussion of the relationship between ground motion and DAS data is a synthesis of many works including Bakku (), Bóna et al (), Dean et al (), Grattan and Meggitt (), Hartog (), Kreger et al (), Karrenbach et al (), Masoudi and Newson (), Posey et al (), Parker et al (), and Willis et al (), as well as U.S. patents on the technology (Farhadiroushan et al, ).…”

“…Elevated DAS amplitude response at short periods could be caused by the style and degree of rigid coupling through which strain is transferred from the ground to the fiber. Coupling is a necessary condition for any seismometer (Lay & Wallace, 1995), yet many DAS studies have concluded that mechanical coupling of the fiber-optic to the ground had an impact on their results (Becker et al, 2017;Jousset et al, 2018;Lindsey et al, 2017;Wang et al, 2018;Willis et al, 2017;Yu et al, 2019). Coupling is a static feature of a DAS experiment (fiber array; DAS recording parameters) that should be assumed to not change over the duration of a typical geophysical campaign (days to months).…”

“…The broadband nature of DAS has largely been overshadowed by the use of DAS in active‐source experiments at high frequencies ( Hz; s). The long‐period frequency response of the DAS method has been demonstrated in recordings of teleseismic earthquakes (Ajo‐Franklin et al, ; Jousset et al, ) and laboratory and field pump tests (Becker et al, ; Becker & Coleman, ). To date, the longest period earthquake signal observed with DAS used a dark fiber DAS array in the Mojave Desert to capture propagating surface waves down to s (Yu et al, ).…”

“…Generally, DAS studies have focused on seismic wave phase information, which is sufficient to model seismic wavefield velocities, for example, in vertical seismic profiling (Daley et al, ; Mateeva et al, ), ambient noise velocity inversions (Ajo‐Franklin et al, ; Dou et al, ; Zeng et al, ), and earthquake phase identification (Ajo‐Franklin et al, ; Jousset et al, ; Lindsey et al, ; Yu et al, ). However, true ground motion amplitudes are necessary for many other seismological processing tasks, including full‐waveform inversion, AVO analysis, moment tensor inversion, and attenuation analysis, which the DAS community will likely investigate in the near future (Cole et al, ; Paitz et al, ).…”

Vertically Distributed Sensing of Deformation Using Fiber Optic Sensing

“…DFOS with Raman scattering has already been known as distributed temperature sensing and utilized for a host of hydrological and geothermal applications (Briggs et al, 2012;Carlino et al, 2016;Curtis & Kyle, 2011;Selker et al, 2006). Recently, by exploiting changes in Brillouin and Rayleigh scattering induced by external strains, the DFOS technology has been utilized for geohazard sensing including earthquake observations (Dou et al, 2017;Jousset et al, 2018;Lindsey et al, 2017) and landslide detection (Huntley et al, 2014;Lienhart, 2015;Michlmayr et al, 2017;Picarelli et al, 2015;Schenato et al, 2017). While a few studies have explored the feasibility of DFOS for subsidence and strata deformation sensing (Murai et al, 2013;Wu et al, 2015), the understanding of data collected from borehole-embedded FO cables has remained elusive and hence precluded its use in many contexts.…”