Prestack Depth Migration for the Sub-salt Structure

Reverse migration and Kirchhoff prestack depth migration was applied to image the sub-salt structure in this study. Both migration methods were based on the one way wave equation. To generate the velocity to input migration process, waveform inversion based on the numerical function analysing method was used. The purpose of this study is to quality control of the developed algorithm and to make the processing flow before applying the field data.

First, synthetic modeling was done for the synthetic structure model. then using the waveform inversion, the successive results of the velocity model and depth migration image was obtained. Although results are applied for the synthetic data, waveform inversion is more powerful method to invert the velocity for the sub-salt imaging than any other conventional velocity modeling methods. Also using the same velocity model, migration types are compared. After comparing the migration results, the reverse time migration algorithm can give better image than that of the Kirchhoff migration algorithm.

Although it was possible to be used as input velocity to migrate the sub-salt structure, since it is very hard to image exactly the sub-salt structure, sub-salt imaging is still focused issue in seismic data processing

So some approaches with variation of the salt velocity are conducted in this study to solve the problem. In case the velocity of the salt is even faster than that of the sedimentary layers(velocity difference in this study is greater than 1500~2000m/s), the exact migration could hardly be done. so firstly, geological case study need to be done to research that case salt velocity is extremely higher than that of the sedimentary layers.

In addition, imaging the sub-salt structure is depend on the size and shape of the salt. sub-salt image distortion is proportional to the size of the salt and salt shape developed in vertical direction.