Classification of Multiples

There are many types of multiples, some of which are illustrated in Figure 1. For the purpose of this thesis, however, multiples will be classified in two main categories: specular multiples and diffracted multiples. Specular multiples are those that reflect as light rays, following Snell's law at the reflection points. Diffracted multiples, in contrast, are scattered in all directions at the diffractor location. I will further classify the diffracted first-order multiples into receiver-side and source-side multiples depending on which side the diffractor lies as shown in panels (a) and (b) of Figure 2 respectively. The dashed lines represent the arbitrary trajectories that the multiple may take from the diffractor to the receiver (for a receiver-side diffracted multiple) or from the source to the diffractor (for a source-side diffracted multiple). The travelpath from the diffractor to the receiver is independent of the source location for a receiver-side multiple and likewise the travelpath from the source to the diffractor is independent of the receiver location for a source-side multiple. This behavior makes the kinematics of specular and diffracted multiples very different in Common-midpoint (CMP) gathers. Specular multiples have a moveout curve that is symmetric around their apex at zero offset, since reciprocity requires the same traveltime for rays from the source location to the receiver location and from the receiver location to the source location. Diffracted multiples, on the other hand, do not have their apex at zero offset (, ) and are therefore not symmetric around zero offset as shown in Figure 3. The travelpath of the receiver side multiple, for example, is not the same if the source and receiver locations are interchanged. Reciprocity is not violated, however. Receiver-side multiples just become source-side multiples and vice-versa as illustrated in Figure 2. A similar splitting of the source- and receiver-side multiple happen with peg-leg multiples from a dipping reflector (, ).

rays
Figure 1. Examples of 2D specular multiples.

diffracted-muls-rays
Figure 2. Schematics of receiver-side (a) and source-side (b) diffracted multiples. The asterisk represents the source and the triangle represents a receiver. The dashed lines indicate possible trajectories of the diffracted rays from the diffractor to the receiver or from the source to the diffractor. The diffractor itself is indicated by the empty circle at about 1600 m.

moveouts3 Figure 3. Comparison of the moveout curves of a primary, a specular multiple and a diffracted multiple in a CMP gather. Notice that the apex of the diffracted multiple is not at zero offset.