Distributed Acoustic Sensing Using Dark Fiber for Array Detection of Regional Earthquakes

Abstract

The intrinsic array nature of distributed acoustic sensing (DAS) makes it suitable for applying beamforming techniques commonly used in traditional seismometer arrays for enhancing weak and coherent seismic phases from distant seismic events. We test the capacity of a dark-fiber DAS array in the Sacramento basin, northern California, to detect small earthquakes at The Geysers geothermal field, at a distance of ∼100 km from the DAS array, using beamforming. We use a slowness range appropriate for ∼0.5–1.0 Hz surface waves that are well recorded by the DAS array. To take advantage of the large aperture, we divide the ∼20 km DAS cable into eight subarrays of aperture ∼1.5–2.0 km each, and apply beamforming independently to each subarray using phase-weighted stacking. The presence of subarrays of different orientations provides some sensitivity to back azimuth. We apply a short-term average/long-term average detector to the beam at each subarray. Simultaneous detections over multiple subarrays, evaluated using a voting scheme, are inferred to be caused by the same earthquake, whereas false detections caused by anthropogenic noise are expected to be localized to one or two subarrays. Analyzing 45 days of continuous DAS data, we were able to detect all earthquakes with M≥2.4, while missing most of the smaller magnitude earthquakes, with no false detections due to seismic noise. In comparison, a single broadband seismometer co-located with the DAS array was unable to detect any earthquake of M<2.4, many of which were detected successfully by the DAS array. The seismometer also experienced a large number of false detections caused by spatially localized noise. We demonstrate that DAS has significant potential for local and regional detection of small seismic events using beamforming. The ubiquitous presence of dark fiber provides opportunities to extend remote earthquake monitoring to sparsely instrumented and urban areas.

Sensing Shallow Structure and Traffic Noise with Fiber-optic Internet Cables in an Urban Area

“…Compared with seismometer and geophone, DAS greatly improves equipment deployment and recovery efficiency and reduces deployment costs and challenges in the urban areas. A series of achievements have been made based on the existing telecommunication cables in the urban environment, such as earthquake signal identification (Lindsey et al 2017;Nayak & Ajo-Franklin, 2021), large-volume airgun signal detection (Song et al 2021), thunder signal monitoring (Zhu and Stensrud 2019), footstep detection (Jakkampudi et al 2020), near-surface characterization (Spica et al 2020;Fang et al 2020), human and vehicle activity monitoring (Lindsey et al 2020;Wang et al 2020Wang et al , 2021b. These experiments have well demonstrated that DAS with existing urban fiberoptic cables are capable of signal detection and is expected to construct the high-resolution shallow structure.…”

The seismic wavefield as seen by Distributed Acoustic Sensing (DAS) arrays: local, regional and teleseismic sources

“…This means that the effect of a change in the direction of incoming waves by an angle θ is associated with tensorial strain rotation that depends on functions of 2θ rather than just θ for the vector rotation employed for the horizontal components of seismometers (e.g. Martin, 2018;Zhan, 2020). As a result, the orientation of the optical fiber relative to incoming seismic waves plays an important role in determining the relative amplitude of DAS signals.…”

“…The double angle dependence comes from the tensor projection of strain. Martin (2018) provides a detailed discussion of orientation effects for different classes of waves, both for direct measurement and for cross-correlation as in the analysis of ambient noise.…”

“…For Rayleigh waves, the radial orientation term cos 2 ψ is to be applied, with maximum response along the DAS cable (e.g. Martin, 2018, Zhan, 2020. For Love waves, the tangential factor sin ψ cos ψ comes into play and this 4-lobed pattern has nulls for waves travelling along and perpendicular to the DAS cable.…”

“…Earthquake studies have used events at regional ranges Wang et al, 2018;Jousset et al, 2018;Ajo-Franklin et al, 2019;Sladen et al, 2019;van den Ende and Ampuero, 2021) and out to teleseismic distances (Lindsey et al, 2020;Paitz et al, 2020). DAS recording has also been used in the analysis of ambient noise (Dou et al, 2017;Zeng et al, 2017;Martin et al, 2018;Ajo-Franklin et al, 2019;Spica et al, 2020). Some applications of DAS systems have deployed their own fiber optic cables (e.g., , whilst many have exploited unused fiber channels on existing cables, termed 'dark' fibers.…”