Evaluating seismic beamforming capabilities of distributed acoustic sensing arrays

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.

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

“…As a result, Rayleigh waves are even more pronounced on broad-band DAS records of distant events than on comparable seismograms [ 22 ], and the surface wave coda is extended. The amplification of locally scattered surface waves can also have the effect of reducing the coherence of DAS recording of body waves [ 20 ].…”

“…Van den Ende & Ampuero [ 20 ] have made an empirical analysis of array beamforming for the March 2016 ML 4.3 Hawthorne earthquake recorded on both dense nodal seismometer and a complex geometry DAS array at the Brady Hot Springs site in Nevada. The event was about 150 km south of the site.…”

Enhancing fibre-optic distributed acoustic sensing capabilities with blind near-field array signal processing

“…In that early work, the localisation of human footsteps at 10 m distance from the sensing fibre was performed using small linear fibre sections, whilst no further details on the processing method and approach are provided. On the other hand, some works in the field of DAS-based seismology have reported the use of seismic array processing assuming a plane mechanical wave propagation 26 – 29 , limiting processing methods to detect only the direction of arrival of seismic waves using far-field narrowband approaches. Some of these techniques make use of acoustic wave information 28 or hybrid systems based on a combination of DAS and traditional seismometers 29 .…”

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

“…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.…”