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ADCIGs in the presence of geological structure: a North Sea example

The following marine-data example demonstrates that the application of the robust method for computing ADCIGs presented in this section substantially improves the quality of ADCIGs in the presence of geological structure. Our examples show migration results of a 2-D line extracted from a 3-D data set acquired in the North Sea over a salt body with a vertical edge. The data were imaged using a shot-profile reverse time migration, because the reflections from the salt edge had overturned paths.

As predicted by our theory, in the presence of a wide range of reflector dips (e.g. flat sediments and salt edges), both the HOCIGS and the VOCIGs are affected by artifacts. Figure [*] illustrates this problem. It displays orthogonal sections cut through the HOCIG cube (Figure [*]a), and through the VOCIG cube (Figure [*]b). The front faces show the images at zero offset and are the same in the two cubes. The side face of Figure [*]a shows the HOCIGs taken at the horizontal location corresponding to the vertical salt edge. We immediately notice that, at the depth interval corresponding to the salt edge, the image is smeared along the offset axis, which is consistent with the horizontal-offset stretch described by equation (9). On the contrary, the image of the salt edge is well focused in the VOCIG displayed in the top face of Figure [*]b, which is consistent with the vertical-offset stretch described by equation (10). However, the flattish reflectors are unfocused in the VOCIG cube, whereas they are well focused in the HOCIG cube. The stretching of the offset axes causes useful information to be lost when significant energy is pushed outside the range of offsets actually computed. In this example, the salt edge reflection is clearly truncated in the HOCIG cube displayed in Figure [*]a, notwithstanding that the image was computed for a fairly wide offset-range (800 meters, starting at -375 meters and ending at 425 meters).

The ADCIGs computed from either the HOCIGs or the VOCIGS have similar problems with artifacts caused by the wide range of reflectors dips. Figure [*] shows the ADCIG computed from the offset-domain CIGs shown in Figure [*]. The salt edge is smeared in the ADCIG computed from HOCIG (side face of Figure [*]a), whereas it is fairly well focused in the ADCIG computed from VOCIG (top face of Figure [*]b). Conversely, the flattish reflectors are well focused in the ADCIG computed from HOCIG, whereas they are smeared in the ADCIG computed from VOCIG.

The artifacts mostly disappear when the ADCIG cubes shown in Figure [*] are merged according to the simple scheme discussed above, which uses the weights defined in equations (20). Figure [*] shows the ADCIG cube resulting from the merge. The moveouts for the salt edge and the sediment reflections are now clearly visible in the merged ADCIG cube and could be analyzed for extracting velocity information. To confirm these conclusions we migrated the same data after scaling the slowness function with a constant factor equal to 1.04. Figure [*] shows the ADCIG cubes computed from the HOCIG cube (Figure [*]a), and from the VOCIG cube (Figure [*]b). When comparing Figure [*] with Figure [*], we notice the 175-meter horizontal shift of the salt edge reflection toward the left, caused by the decrease in migration velocity. However, the artifacts related to the salt edge reflection are similar in the two figures, and they similarly obscure the moveout information. On the contrary, the moveout information is ready to be analyzed in the cube displayed in Figure [*], which shows the ADCIG cube resulting from the merge of the ADCIG cubes shown in Figure [*]. In particular, both the flattish event above the salt edge (at about 1,000 meters depth) and the salt edge itself show a typical upward smile in the angle-domain gathers, indicating that the migration velocity was too slow.

 
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Cube-both-v7newsc-overn
Figure 6
Migrated images of North Sea data set. Orthogonal sections cut through offset-domain CIG cubes: a) HOCIG cube, b) VOCIG cube. Notice the artifacts in both cubes.


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Ang-Cube-both-v7newsc-overn
Figure 7
Orthogonal sections cut through ADCIG cubes: a) ADCIG computed from HOCIG cube, b) ADCIG computed from VOCIG cube. Notice the artifacts in both cubes that are related to the artifacts visible in the corresponding offset-domain CIG cubes (Figure [*]).


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Ang-Cube-merge-v7newsc
Figure 8
Orthogonal sections cut through the ADCIG cube that was obtained by merging the cubes displayed in Figure [*] using the proposed method. Notice the lack of artifacts compared with Figure [*].


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Ang-Cube-both-v7new-overn
Ang-Cube-both-v7new-overn
Figure 9
Migrated images of North Sea data set. The migration slowness had been scaled by 1.04 with respect to the migration slowness used for the images shown in Figures [*]-[*]. Orthogonal sections cut through ADCIG cubes: a) ADCIG computed from HOCIG cube, b) ADCIG computed from VOCIG cube. Notice that the artifacts obscure the moveout information in both cubes.


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Ang-Cube-merge-v7new
Ang-Cube-merge-v7new
Figure 10
Orthogonal sections cut through the ADCIG cube that was obtained by merging the cubes displayed in Figure [*] using the proposed method. Notice the typical upward smile in the moveouts from both the salt edge and the flattish event above it.


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next up previous print clean
Next: Illustration of CIGs kinematic Up: Robust computation of ADCIGs Previous: Robust computation of ADCIGs
Stanford Exploration Project
7/8/2003