Prestack Migration Processing

Numerous investigations have shown the advantages of prestack migration before AVO analysis over the conventional AVO analysis on unmigrated CMP gathers Beydoun et al. (1994); Mosher et al. (1996); Resnick et al. (1986). Prestack migration is required in areas of strong structural dips, but also improves the data quality when the sediment structure is relatively flat: diffractions that can interfere with the primary reflections are collapsed and the lateral resolution of the data is increased as migration shrinks the size of the Fresnel zone. Furthermore, migration provides a better estimate of the subsurface reflectivity, which is proportional to the relative P-impedance contrasts in velocity and density. Therefore, I perform a Kirchhoff prestack migration/inversion on the data which is based on the method developed by Lumley 1993a. This method estimates the reflection coefficients and the reflection angles directly from the prestack data and compensates internally for spherical divergence. In order to avoid spatial aliasing, I interpolate the data to a CMP spacing of 12.5 m and use a smoothed 2-D RMS velocity field as the migration velocity.

Since it is essential for any subsequent AVO analysis that the reflector moveout is very flat, I apply an additional residual moveout correction (RNMO) to the data. In order to improve the coherency of the events prior to this correction, I also apply the first part of the amplitude calibration (normalization of the amplitudes) which will be described in the next section. An example of a prestack migration/inversion reflectivity (left) after RNMO and the reflection angles (right) is shown in Figure . The angle contours start at 5 degrees at the near offset and increase in 5 degree increments. The maximum angle coverage in this gather is approximately 25 degrees.

 

mig-ann
Figure 13 Migrated gather after RNMO and reflection angles. Angle contours are displayed in 5 degree intervals. The first contour represents an angle of 5 degree.