oa Modelling Depth Dependency of Rayleigh-wave DAS Amplitudes in Multilayered Media

Depth variations of Rayleigh-wave amplitude in multilayered media are controlled by a complex combination of exponential decay of different modes as well as stiffness of individual layers. The widespread deployment of downhole fibre-optic cables provides an important opportunity for investigating the depth dependence of Rayleigh wave amplitude using distributed acoustic sensing (DAS). However, quantitative understanding of these dependencies is lacking. Here we model the Rayleigh-wave vertical strain amplitude versus depth for frequencies from 0.03 to 0.2 Hz theoretically and numerically, and compare the theoretical and numerical results to observed DAS amplitudes. Modelling results show that both modelled and observed DAS amplitudes display a clear anomaly at the depth of a CO2 saturated reservoir. The numerical amplitudes for a source depth of 5 km, 600 km away from the well location, have a reasonable agreement with the field data at about 0.1 to 0.2 Hz. However, at lower frequencies the agreement is not attainable, which requires further investigation.