Another possible imaging condition, advocated in this paper,
involves shifting of the source and receiver
wavefields in time, as opposed to space,
followed by image extraction at zero time:
(6)
(7)
Here, is a scalar describing the time-shift
between the source and receiver wavefields prior to imaging.
This imaging condition can be implemented in the
Fourier domain using the expression
(8)
which simply involves a phase-shift applied
to the wavefields prior to summation
over frequency for imaging at zero time.