Locating microseismic events using multiplicative time reversal imaging based on decoupled wavefields in 2D VTI media: Theoretical and synthetic cases studies

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Estimation of microseismic event location is of great significance in unconventional resources development and exploitation with its advantage of imaging the fractures induced by hydraulic fracturing stimulation. Time reversal imaging technique (TRI), which reversely propagates the recorded microseismic energy to refocus to its real location, has been verified as a satisfying method in noisy environment. However, the location images obtained from TRI techniques are frequently smeared with strong imaging noises and artifacts resulting from the cross talk of coupled wave modes coinciding in space and time with sparse measurements especially in heterogeneous anisotropic media, resulting in erroneous location estimations. To mitigate these disturbances, a multiplicative time reversal imaging method based on decoupled wavefields is proposed (DC-MTRI). Wave-mode separators constructed based on Christoffel equation are first utilized to fully separate quasi P wave (qP) and quasi vertically pozlarized shear wave (qSV) wavefields at each receiver in 2D vertical transverse isotropy (VTI) media. Multiplicative time reversal imaging condition is then applied to the auto and cross-correlations of decoupled wavefields of each receiver to construct the final location image. The synthetic examples show that the wave-mode separators applied in this paper can fully separate qP and qSV potentials in 2D VTI media compared to Helmholtz decomposition. As a consequence, the proposed method substantially improves the quality of the location image with a much sharper focus and less noisy imaging energy at shallow depth of the sophisticated Sigesbee 2A model. In addition, the numerical experiments show that DC-MTRI can give robust source location images with distinct radiation pattern of high resolution for sparse acquisition plan. Furthermore, DC-MTRI has same sensitivity on velocity perturbations compared with conventional TRI techniques. Therefore, the proposed technique can potentially be employed as the constraints to estimate source mechanism and update the velocity structure in the future.