Maximum dip to migrate
During migration, we can specify the maximum dip we want migrated in the section. This may be useful when we need to suppress the steeply dipping coherent noise. Figure 4.2-8 shows migrations of the dipping events with four different maximum allowable dips. For a 4 ms/trace dip limit, events with dips greater than this value are suppressed. Similarly, for an 8 ms/trace dip value, events with dips greater than this value are suppressed. When the dip value is 12 ms/trace, no suppression occurs, since all events in the input section have dips less than this value. Limiting the dip parameter is a way to reduce computational cost, since it is related to aperture width (equation 1), which determines the cost.
From Figure 4.2-1, note that the Kirchhoff migration impulse response can be limited to various maximum dips. The smaller the maximum allowable dip, the smaller the aperture. This combination of maximum aperture width and maximum dip limit determines the actual effective aperture width used in migration. In particular, diffraction hyperbolas along which summation is done are truncated beyond the specified maximum dip limit.
A field data example of testing the maximum dip parameter is shown in Figure 4.2-9. Some steep dips are lost on the section that corresponds to the 2 ms/trace maximum allowable dip. The 8 ms/trace dip appears to be optimum. The maximum dip parameter must be chosen carefully so that the steep dips of interest in the input section are preserved. Finally, dip value can be changed spatially and in time; however, practical implementation can be cumbersome.