Distributed Acoustic Sensing Using Dark Fiber for Near-Surface Characterization and Broadband Seismic Event Detection

Abstract

We present one of the first case studies demonstrating the use of distributed acoustic sensing deployed on regional unlit fiber-optic telecommunication infrastructure (dark fiber) for broadband seismic monitoring of both near-surface soil properties and earthquake seismology. We recorded 7 months of passive seismic data on a 27 km section of dark fiber stretching from West Sacramento, CA to Woodland, CA, densely sampled at 2 m spacing. This dataset was processed to extract surface wave velocity information using ambient noise interferometry techniques; the resulting VS profiles were used to map both shallow structural profiles and groundwater depth, thus demonstrating that basin-scale variations in hydrological state could be resolved using this technique. The same array was utilized for detection of regional and teleseismic earthquakes and evaluated for long period response using records from the M8.1 Chiapas, Mexico 2017, Sep 8th event. The combination of these two sets of observations conclusively demonstrates that regionally extensive fiber-optic networks can effectively be utilized for a host of geoscience observation tasks at a combination of scale and resolution previously inaccessible.

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, ).…”

“…Several authors have observed local noise in DAS data, which we classify here. One type of noise, referred to as common‐mode noise (Ajo‐Franklin et al, ; Bakku, ; Dou et al, ) is characterized by an infinite‐velocity signal (arrives at all channels simultaneously). This is caused by local seismic disturbance near the interrogator, which vibrates the optoelectronic system and leads to an overprinted signal on all channel recordings at the same time.…”

“…One demonstrated fading mitigation strategy employs multiple laser frequencies (Zhou et al, ). Reduced amplitude channel noise can also result from suboptimal fiber‐ground coupling (Ajo‐Franklin et al, ; Becker et al, ; Reinsch et al, ; Willis et al, ). Note that photonic fading noise and sensor coupling noise are difficult to distinguish except at the scale of the array, where sensor coupling is usually identifiable by its systematic pattern or from field installation information.…”

“…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, ).…”