Phantom diffractor

A second example of lateral velocity variation is Figure 22, also taken from Kjartansson's dissertation. The physical model shown on the inset in Figure 22 is three constant velocity wedges separated by broken line segments representing reflectors.

 

westdepthmig
Figure 22 The model in the upper panel was taken from Western Geophysical's Depth Migration brochure. The model is not physical because of the segmenting of the interface; however, the segments make it a good case for the study of lateral shifts. The synthetic data is in the lower panel (from Kjartansson). The phantom diffraction is on the latest arrival just below the tip of the wedge.

The bottom edge of the model also represents a reflector. The wavefield in Figure 22 was made using the exploding-reflector calculation, which Kjartansson regarded as a reasonable approximation to a zero-offset section. Notice that under the tip of the 4 km/sec wedge is a small diffraction on the bottom horizontal reflector. Because such a diffraction has nothing to do with the flat reflector on which it is seen, it is termed a ``phantom'' diffraction. Phantom diffractions are not easy to recognize, but they do occur. In reality, the ``bright spots'' in Kjartansson's section were probably phantom diffractions. It has been reported that phantom diffractions provide a means of prospecting for small, high-velocity, carbonate reefs.