Abstract
Abstract. The versatility and cost efficiency of fibre-optic distributed acoustic sensing (DAS) technologies facilitate geophysical monitoring in environments that were previously inaccessible for instrumentation. Moreover, the spatio-temporal data density permitted by DAS naturally appeals to seismic array processing techniques, such as beamforming for source location. However, the measurement principle of DAS is inherently different from that of conventional seismometers, providing measurements of ground strain rather than ground motion, and so the suitability of traditional seismological methods requires in-depth evaluation. In this study, we evaluate the performance of a DAS array in the task of seismic beamforming, in comparison with a co-located nodal seismometer array. We find that, even though the nodal array achieves excellent performance in localising a regional ML 4.3 earthquake, the DAS array exhibits poor waveform coherence and consequently produces inadequate beamforming results that are dominated by the signatures of shallow scattered waves. We demonstrate that this behaviour is likely inherent to the DAS measurement principle, and so new strategies need to be adopted to tailor array processing techniques to this emerging measurement technology. One strategy demonstrated here is to convert the DAS strain rates to particle velocities by spatial integration using the nodal seismometer recordings as a reference, which dramatically improves waveform coherence and beamforming performance and warrants new types of “hybrid” array design that combine dense DAS arrays with sparse seismometer arrays.
Highlights
Dense seismometer arrays play a central role in understanding various geological phenomena, including earthquake rupture behaviour (Kiser and Ishii, 2017; Meng et al, 2011), micro-seismicity (Inbal et al, 2016), fault zone structure (Zigone et al, 2019), and deep crustal and mantle geology (Jiang et al, 2018; Lin et al, 2013)
For a gauge length that is much smaller than the seismic wavelength, the distributed acoustic sensing (DAS) strain rate is proportional to cos2θ for a P wave or SV wave, and sin 2θ for an SH wave, assuming a plane wave with incidence angle θ relative to the fibre (Martin et al, 2018)
On the ground motions generated by the March 2016 ML 4.3 Hawthorne earthquake, as recorded by a DAS array colocated with a dense nodal seismometer array at the Brady Hot Springs geothermal field
Summary
Dense seismometer arrays play a central role in understanding various geological phenomena, including earthquake rupture behaviour (Kiser and Ishii, 2017; Meng et al, 2011), micro-seismicity (Inbal et al, 2016), fault zone structure (Zigone et al, 2019), and deep crustal and mantle geology (Jiang et al, 2018; Lin et al, 2013). The recent emergence of fibre-optic distributed acoustic sensing (DAS; Hartog, 2017; Zhan, 2020) has opened up a plethora of possibilities and applications in seismic and transient deformation monitoring. Fibre-optic cables are relatively inexpensive, require little to no maintenance, and can be deployed in environments that were previously impractical for or inaccessible to traditional seismometers, such as urban environments (Dou et al, 2017; Fang et al, 2020), glaciers and permafrost regions (Ajo-Franklin et al, 2017; Walter et al, 2020), deep boreholes (Cole et al, 2018; Lellouch et al, 2019), and in lakes and submarine environments (Lindsey et al, 2019; Sladen et al, 2019) – see Zhan (2020) for a concise review of applications in geosciences. The measurement principles of DAS are inherently different from those of conventional seismometers (Zhan, 2020), Published by Copernicus Publications on behalf of the European Geosciences Union
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