Summary
We present an approach for increasing imaging resolution of reverse-time migration (RTM) by taking advantage of time-shift gathers. The method has two steps: migrating seismic data with the extended imaging condition to get time-shift gathers and accumulating the time-shift gathers along time-shift axis after they are transformed to zero-lag time-shift by a post-stack depth migration on a finer grid. The final image is generated on a grid which is denser than that of the original time-shift images. The proposed method is based on the observation that non-zero-lag time-shift images recorded by the regular computing grid contain the information of zero-lag time-shift image of a denser grid, and such information can be continued to zero-lag time-shift and refocused at the correct locations on the denser grid. The extra computational amount of the method relative to RTM has the computational cost of zero-offset migration, which is almost negligible when compared to prestack shot-record RTM. Numerical tests on synthetic models demonstrate that the method can effectively improve RTM resolution. It can also improve the efficiency of RTM if the source and receiver wavefield extrapolations are performed on a coarse grid.
Introduction
Thanks to its ability of dealing with extreme velocity complexity and imposing no dip limitations on the images, reversetime migration or RTM (Baysal et al., 1983; McMechan, 1983; Whitmore, 1983) has become the standard method for depth imaging in complex areas (Zhang et al., 2007; Etgen et al., 2009; Zhang et al., 2011; Zhu et al., 2014; Casasanta et al., 2015). The image of RTM is traditionally constructed by taking the zero-lag crosscorrelation of the extrapolated source and receiver wavefields, which can provide correct kinematics and is simple to implement (Claerbout, 1985). More imaging information can be obtained by applying extended imaging conditions, such as space-shift or time-shift imaging condition (Sava and Fomel, 2006; Sava and Vasconcelos, 2011). Space-shift or time-shift common-image gathers can be transformed to anglegathers for migration velocity analysis (Sava and Fomel, 2003; Biondi and Symes, 2004; Fomel, 2011; Xu et al., 2011; Tang et al., 2013).
Time-shift gathers were first related to depth focusing analysis (Faye and Jeannot, 1986; MacKay and Abma, 1992; Nemeth, 1996). Recently they have been used to remove RTM artifacts. Kaelin and Carvajal (2011) devised a fan filter based on timeshift imaging condition to eliminate low frequency artifacts and explained its advantages to other noise attenuation methods. Khalil et al. (2013) proposed a new way to transform time-shift gathers to zero-lag time-shift and to attenuate RTM noise by stacking the transformed time-shift gathers. Xu et al. (2014) formulated the transformation of time-shift gathers as a secondpass migration and removed coherent noise in TTI migration by using the original migration velocity along the symmetry axis but with different anisotropic parameters in the second pass migration.
