Discussion

Our examples considered microseismic sources as impulsive compressional sources similar to subsurface scatteres that generate diffractions. As noted by Anikiev et al. (2013) and Staněk et al. (2015), microseismic sources are mostly shear sources that have different radiation patterns. For example, the stacking value along moveout curve of a double-couple source event is ideally zero. Therefore, a radiation pattern correction for the source mechanism should be taken into account in diffraction imaging applied to passive data.

Conversion from time coordinates to depth coordinates after time-domain imaging is necessary and requires time-domain velocity analysis. Velocity analysis can be performed along with the migration process, by using the double path-integral formulation (Schleicher and Costa, 2009; Merzlikin and Fomel, 2015). Recent advances in time-domain seismic imaging bring other possibilities, including velocity-independent imaging using local slopes (Cooke et al., 2009; Fomel, 2007) and time-to-depth conversion in the presence of lateral-velocity variations (Cameron et al., 2008; Li and Fomel, 2015).