A Practical Approach to One-Pass 3-D Depth Migration

Accurate processing of 3-D seismic data sets is essential to meet the objectives of today’s explorationist. Migration is a process that is particularly important in 3-D seismic processing. Originally, 3-D migration was performed in two separate 2-D passes but this suffered from the inaccuracies of a constant velocity assumption. The splitting algorithm is now a popular one-pass approach to 3-D migration. One-pass migration algorithms avoid the assumption of constant velocities and more correctly position the seismic energy. A more flexible approach to one-pass 3-D migration is known as Ky partitioning. This algorithm allows for a choice of currently available 2-D migrations to be used to perform an accurate one-pass 3-D migration. The Ky partitioning approach is also more efficient.

When lateral variations in velocity become significant, a depth migration algorithm will more accurately image the data than the standard time migration algorithm. The splitting algorithm is converted into a depth migration by including the thin lens term. For the Ky partitioning algorithm this is achieved by the use of dilation. Dilation consists of small dynamic time shifts applied to the 3-D data before migration to compensate for the existence of lateral velocity variations.

Application of both splitting and Ky-dilation one-pass 3-D migration algorithms to a synthetic data set show that the proposed dilation factors can produce sharp migrated images in the presence of lateral velocity variations. In this case the Ky-dilation results exhibit superior clarity compared with the image obtained from the splitting approach. The extra flexibility of the Ky-dilation approach allowed the use of a high-dip time migration algorithm which avoided the frequency dispersion effects that degrade the image of the finite-difference splitting migration. An improved image has been obtained for less computational effort.