# Iteration with CMP-stacked data

Series | Investigations in Geophysics |
---|---|

Author | Öz Yilmaz |

DOI | http://dx.doi.org/10.1190/1.9781560801580 |

ISBN | ISBN 978-1-56080-094-1 |

Store | SEG Online Store |

Depth migration of the CMP-stacked section in Figure 8.2-1 using Model A required two iterations to achieve convergence (Figure 8.2-21). The resulting image and the model inferred from it have some inaccuracies. It appears that starting with the true model (Model A), iterative poststack depth migration does not exactly reproduce the true model. For the zero-offset section, starting with the true model, convergence was achieved after one iteration and the resulting model was the same as the input model (Figure 8.2-11). Again, the reason for the less-than-ideal performance of poststack depth migration is that the CMP stack is only an approximation to the zero-offset wavefield. The more the stacked section departs from the zero-offset wavefield, the more the final model will depart from the true model.

Now consider Model D in Figure 8.2-22 in which the salt velocity and the base-salt boundary are incorrectly specified. After the third iteration with the stacked section, we find that convergence is achieved. Nevertheless, the final solution has converged to a velocity-depth model that is different from the true model.

If the initial model contains errors in reflector geometries only, and layer velocities are specified correctly, then convergence using the stacked section to the true velocity-depth model is *nearly* achievable (Figure 8.2-23).

**Figure 8.2-21**Iterative depth migration using the CMP-stacked section as in Figure 8.2-1 and Model A (same as the true velocity-depth model as in Figure 8.2-1).**Figure 8.2-22**Iterative depth migration using the CMP-stacked section as in Figure 8.2-1 and Model D that contains errors in salt velocity and base-salt reflector geometry. For comparison, the true velocity-depth model (Model A in Figure 8.2-11) is superimposed on the final velocity-depth model (bottom of left column).**Figure 8.2-23**Iterative depth migration using the CMP-stacked section as in Figure 8.2-1 and Model F that contains errors in top-salt and base-salt reflector geometries. For comparison, the true velocity-depth model (Model A in Figure 8.2-11) is superimposed on the final velocity-depth model (bottom of left column).**Figure 8.2-1**(Top) An earth model in depth with a salt diapir; (middle) CMP-stacked section derived from the modeled prestack data in Figure 8.2-3; (bottom) the modeled zero-offset section. The stacked and zero-offset sections are appropriately aligned in the lateral direction with respect to the earth model above. Trace spacings in the stacked and zero-offset sections are 25 m and 50 m, respectively. No amplitude scaling has been applied to the sections. The aspect ratio of the horizontal and vertical axes in the velocity-depth model is 1.**Figure 8.2-11**Iterative depth migration using the zero-offset section as in Figure 8.2-1 and Model A (same as the true velocity-depth model as in Figure 8.2-1).

## See also

- 2-D poststack depth migration
- Image rays and lateral velocity variations
- Time versus depth migration
- Iterative depth migration
- Iteration with zero-offset data
- Iteration with prestack data
- Iteration in practice