Summary
We perform seismic diffraction imaging and velocity analysis by separating diffractions from specular reflections and decomposing them into slope components. We image slope components using extrapolation in migration velocity in timespace- slope coordinates. The extrapolation is described by a convection-type partial differential equation and implemented efficiently in the Fourier domain. Synthetic and field data experiments show that the proposed algorithm is able to detect accurate time-migration velocities by automatically measuring the flatness of events in dip-angle gathers.
Introduction
The idea of separating diffractions from specular reflections and using diffraction focusing as a tool for velocity analysis goes back to the work of Harlan et al. (1984). Sava et al. (2005) adopted it for wave-equation migration velocity analysis in depth migration. Fomel et al. (2007) developed a constructive procedure for diffraction separation based on planewave destruction and diffraction focusing analysis based on velocity continuation and local kurtosis. The procedure was extended to 3-D azimuthally-anisotropic velocity analysis by Burnett and Fomel (2011). However, local kurtosis may not be an optimal measure for diffraction focusing, because it requires smoothing or windowing in space, reducing resolution. A particularly convenient domain for separating diffractions and reflections and for analyzing migration velocities is dipangle gathers (Landa et al., 2008; Reshef and Landa, 2009; Klokov and Fomel, 2012). In the dip-angle domain, specular reflections appear as hyperbolic events centered at the reflector dip, and diffractions appear flat when imaged at the location of the diffractor with the correct velocity. Measuring flatness of diffraction events in dip-angle gathers, as opposed to flatness of reflection and diffraction events in reflection-angle gathers, provides an alternative constraint on seismic velocity. Traditionally, dip-angle gathers are constructed with Kirchhoff migration (Fomel and Prucha, 1999; Xu et al., 2001; Brandsberg- Dahl et al., 2003; Cheng et al., 2011; Koren and Ravve, 2011; Bashkardin et al., 2012; Klokov and Fomel, 2013).
