Modelling of fibre-optic DAS response to microseismic arrivals in anisotropic media

Fibre-optic Distributed Acoustic Sensing (DAS) cables are now used to monitor microseismicity during hydraulic fracture stimulations of unconventional gas reservoirs. Unlike geophone arrays, DAS systems are sensitive to uniaxial strain along the fibre direction and thus provide a single-component recording, which makes identifying the directionality of incoming waves difficult to infer. Using synthetic examples, we show some fundamental characteristics of microseismic recordings on DAS systems for purposes of hydraulic fracture monitoring in a horizontal well in anisotropic (VTI) shales. We demonstrate that SH arrivals dominate the recorded signals since their polarization is aligned along the horizontal cable at near offset. The amplitude of the SH phase along the cable exhibits a characteristic pattern with bimodal peaks, the width of which relates to the distance of the event from the cable. Furthermore, we find that shear-wave splitting recorded on DAS systems can be used to infer the inclination of the incoming waves, overcoming a current limitation of event locations which have constrained events to lie in a horizontal plane. Low amplitude qSV arrivals suggest an event depth similar to that of the DAS cable. Conversely, steep arrivals produce higher amplitude qSV waves, with shear-wave splitting increasing with offset along the cable.