Initial migration

The initial velocity model was created using the S.M.A.R.T method (, ). Early migration tests showed that a better migration result could be obtained by smoothing the model () . As a result the S.M.A.R.T. model was smoothed, preserving the sharp salt boundary (two slices through the cube are shown in Figure 3).

Using the velocity in Figure 3, the data was migrated with six reference velocities and frequency range of 5 to 60 Hz (, ). Figures 4 and 5 show two different slices through the initial migration cube. Note how in Figure 4 the chalk boundary reflection (`B') varies in amplitude. Further, the reflectors under the salt edge die out (`A'). In addition the syncline and fault structure to the right of the salt (`C' and `D') lack crispness and some reflector continuity. In the second view, Figure 5, we again see the problems with the chalk boundary (`B'). As reflectors approach the salt edge they seem to loose coherency (`A' and `D'). The valley to the right also has problems. Reflectors seem to blur together in both the inline view (`C') and the depth slice (`E'). If we look at the gathers we still see significant moveout errors (Figure 6) that the tomography can attempt to correct.

 

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Figure 3 Initial ELF velocity model.

 

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Figure 4 Slice through the migrated cube using the velocity of Figure 3.

 

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Figure 5 A second slice through the migrated cube using the velocity of Figure 3.

 

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Figure 6 Every 10th gather from a crossline (Y=1900).