Chapter 2 of this thesis more thoroughly explains the problems encountered when attempting to image areas with strong velocity contrasts using migration. The migration algorithms it describes are designed to produce an image with angle-domain common image gathers (ADCIGs). The creation of ADCIGs helps to reduce artifacts caused by multipathing, a problem that is also explained in Chapter 2 and which is common in areas with strong velocity contrasts.
I will first explain how multipathing occurs in conventional shot- and offset-domain type migration. Then I will review a Kirchhoff migration that produces ADCIGs (). As with most Kirchhoff methods, this does not necessarily perform well in areas with strong velocity contrasts. Therefore, I will introduce a wave-equation based migration that produces ADCIGs.
The migration algorithm used in this thesis is called downward-continuation migration (). It uses the Double Square Root (DSR) equation to downward continue the wavefield recorded at the surface, then at each depth step performs slant stacking () to create the ADCIGs. The slant stacking step creates an offset ray parameter axis in the resulting image, which is equivalent to reflection angle. The DSR equation allows me to use all of the information recorded at the surface rather than making the type of high frequency assumption a Kirchhoff method requires. By slant stacking at each depth step, the algorithm can handle lateral velocity variations, unlike the method described by ().
Although downward-continuation migration reduces artifacts seen in traditional shot- and offset-domain migration and Kirchhoff methods that create ADCIGs, it still has difficulty with poor illumination. In areas with strong velocity contrasts, the resulting image will have shadow zones. Migration is not capable of properly imaging these areas.