Migration is an attempt to construct a model of the earth's subsurface from data recorded at the earth's surface. The raw data is parameterized by the shot and receiver locations of the survey. This makes it natural for us to migrate the data in terms of shot and receiver locations, common midpoint (CMP) locations, and offset, which are all surface-related coordinates. Unfortunately, these surface-related migration methods often fail in areas where the subsurface is complex for reasons such as reflector ambiguity due to multipathing.
Multipathing is a phenomenon in which a single event in the data recorded at one surface location could be caused by reflectors at two or more subsurface model space locations. This reflector ambiguity will contribute to imaging artifacts when surface-related migration is performed. It can also cause additional problems such as non-flat events in common image gathers that render velocity analysis ambiguous (). If we do not limit ourselves to the surface-related domains for migration and turn to a subsurface-related domain like the reflection angle domain, the problems caused by multipathing can be avoided. In the subsurface-related domain, we deal with specific points in the subsurface and energy that arrives at and departs from these points at specific angles. Using these subsurface-related coordinates means there will not be reflector ambiguity, and thus no multipathing.
The artifacts caused by multipathing energy can cause serious problems for one of the most commonly used migration schemes: Kirchhoff migration using surface-related coordinates. Although much work has been done to try to modify surface-related Kirchhoff migration to better handle areas with complex subsurfaces (, , , ), Kirchhoff methods are fundamentally based on a high frequency assumption, which is known to break down in very complex areas. Fortunately, other migration methods such as downward-continuation migration do not make this assumption. Additionally, downward-continuation migration is well suited to be used in the subsurface-related reflection angle domain.
In this chapter, I will examine the phenomenon of multipathing as it relates to the surface-related shot and offset domains. I will then explain how the artifacts resulting from multipathing can be avoided in the subsurface-related reflection angle domain. I will review how Kirchhoff migration can be performed in the reflection angle domain, then introduce a subsurface-related downward-continuation migration method that is less likely to have imaging artifacts than the Kirchhoff method. Finally, I will show that even the downward-continuation method will not provide a perfect image of a very complex subsurface at least in part because of poor illumination.