Elastic wavefield separation of DAS-VSP data based on non-stationary polarization projection

Distributed acoustic sensing (DAS) generally records the seismic signals by detecting the axial strain or strain rate that is stimulated by the impinging elastic wavefields along the optical fibers. It has become an important seismic observation technology, especially in vertical seismic profiling (VSP) applications, due to its low cost, easy deployment and high-density spatial sampling. Although current DAS-VSP acquisitions typically offer only a single-component observation, they still provide valuable elastic information about the subsurface. Separating P and S waves from the DAS-VSP data and leveraging these wavefield information is very important for the inversion of elastic parameters and seismic imaging. Therefore, a polarization projection method is introduced to deal with the P/S separation of walkaway DAS-VSP data. Firstly, the polarization directions of P and S waves are estimated using the dispersion relation derived from elastodynamic wave equations. Then, the P/S wave separation is achieved through a non-stationary polarization projection that can take into account the effects of spatially varying wave velocities. Finally, a two-step workflow is proposed to successively separate the P and S waves in common-shot and common-receiver gathers. The results of synthetic and real walkaway DAS-VSP data demonstrate that this method can effectively separate P- and S-wave signals from DAS-VSP data, and provide effective data preconditioning for subsequent velocity model building and seismic imaging.