- 1.
- Migrate common shot gathers to offset-domain CIGs.
- 2.
- Transform offset-domain gathers into angle-domain gathers Sava and Fomel (2000).
- 3.
- Determine the polarity flip angle, with equation (9).
- 4.
- Flip negative polarities in the angle domain.
- 5.
- Stack over angle to produce a final structural image.

The first step is to transform the data to the temporal frequency
domain; subsequently, we downward continue the shot and receiver
wavefields with *v*_{p} and *v*_{s} respectively, and image the data at
zero time.
In this way, we obtain the offset-domain CIGs, and if we use the
correct velocity models, the energy is focused close to zero offset.

The common angle gathers are evaluated in the Fourier-domain
by equation (10). The method
involves a radial-trace mapping after migration
in the *k*_{h}-*k*_{z} domain Sava and Fomel (2000).
Rickett and Sava (2001) describe how we prepare the
offset-domain CIGs with a shot-profile migration algorithm.

(10) |

To determine the structural dip, we apply plane-wave destructors Fomel (2000), which characterize the seismic images as a superposition of local plane waves. With the dip field and the two migration velocity models, we calculate the polarity flip angle using equation (9). Flip the polarity is an easy process after we have the polarity flip angle: we just apply a mask to the common angle gathers that follows the polarity flip angles and multiply the data by -1.

4/29/2001