Migration with surface and internal multiples
Shang Huang, Daniel O. Trad
Multiples can provide additional information for subsurface structures compared with primary reflections. In this paper, we consider two different uses of multiples for imaging. First, we will look at the use of the first-order surface multiples for reverse time migration (RTM). Observed primaries are extracted from shot records and injected as virtual sources and surface multiples are used as data and back-propagated in time. Then the cross-correlation between primary wave and the first-order surface multiple is used as image condition. RTM of surface multiples gives a more extensive illumination than RTM of primaries. In addition, least-squares reverse time migration (LSRTM) of surface multiples presents improved vertical resolution compared with RTM. Also, LSRTM of the first-order surface multiple can recover the information from upper-side dipping events as well as some small flanks. The main requirement of these benefits is, however, quite challenging: to achieve multiple separation before migration.
The second use of multiples we examine here is full-wavefield migration (FWM). This method uses an inversion-based approach to update the subsurface image. Reflection co-efficient updates are obtained from scattering effects, including reflections and differential transmissions. A horizontal-layered model is used for proving the benefits of using FWM. Forward modeling by phase shift plus interpolation derives stable downgoing and upgoing wavefields separately, which can predict primary, surface multiples and internal multiples in a full-wavefield response. FWM of total wavefields can provide more details in the image compared with FWM of primary only. Adding energy from multiples into migration is not a replacement for migrating primary reflections, but it can be a useful complement to improve the image resolution and illumination for an accurate geological interpretation.