Oral Presentation

Fibre-optic Distributed Acoustic Sensing (DAS) is an emerging technology that enables the recording of ground motions with fibre-optic (telecommunication) cables. Since these cables can be deployed in environments that are traditionally challenging for seismometers (underwater, in cities, glaciers, deep boreholes, etc.), we can use DAS to make measurements of seismic waves in sparsely instrumented locations. Moreover, the high spatial- and temporal resolution of DAS data and the low cost of optic fibre makes fibre-optic cables an attractive, low-cost alternative to expensive seismometer arrays. But since DAS is a relatively new technique in seismology, we do not yet know the extent to which DAS is a valid substitution to seismometers.

We evaluate the performance of DAS by comparing the seismic beamforming capabilities of a DAS array with those of a co-located seismometer array at the Brady Hot Springs geothermal field. From this comparison we conclude that the DAS array exhibits poor waveform coherence and consequently produces inadequate beamforming results, which are dominated by the signatures of shallow scattered waves. 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. We propose a simple strategy of converting the DAS strain rate data into particle motions (velocity), which mitigates issues of scattering and waveform incoherence, bringing the DAS beamforming capabilities on par with those of conventional seismometers. This solution motivates new types of “hybrid” arrays that combine dense DAS arrays with sparse seismometer arrays to achieve the best performance.