Understanding DAS records and their response with full-waveform modelling

Distributed Acoustic Sensing (DAS) can provide unprecedented spatial resolution and sensitivity to a wide frequency band. The instrument response of the interrogated optical fibre cables, however, is largely unknown and difficult to separate from source, path, and directivity effects on seismic records. This prevents us from using DAS in many staple seismological techniques that require either absolute amplitude values or a complete understanding of the full waveform (e.g., earthquake magnitude estimation, waveform tomography). Here we present a full-waveform simulation scheme developed to model the DAS instrument response using a particle-based Elastic Lattice Model (ELM-DAS). The scheme allows us to simulate a virtual cable embedded in the medium and made of a string of connected particles. By measuring the strain along these particles, we are able to replicate the axial strain natively measured by DAS as well as the effects of irregular cable geometries. Analysing synthetic DAS data allows us to focus on the main factors that are believed to determine the instrument response: cable-ground coupling and local site effects. The particle-based numerical scheme allows us to easily simulate complex properties of the material around the cable (e.g., unconsolidated sediments, nonlinear materials) as well as different degrees of cable-ground coupling. By simulating DAS cables in 2D, we observe that at the meter scale, realistic DAS materials, cable-ground coupling, and the presence of unconsolidated trench materials around it dramatically affect wave propagation, each change affecting the synthetic DAS record, with differences exceeding at times the magnitude of the recorded signal. By expanding the scheme to 3D, we can accurately include the effects of realistic, complexand at times sub-wavelengthcable geometries and how they influence DAS records. Our observations show that cable coupling and local site effects have to be considered both when designing a DAS deployment and analysing its data when either true or along-cable relative amplitudes are considered.