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CIGs and velocity analysis resolution

Not only the zero-offset image resolution is important. Having a better image resolution for a range of offsets and angles can lead to better estimation of the reservoir properties and better velocity analysis. The angle-domain Common Image Gather (CIG) is the domain where AVA analysis is performed, because it has information about the reflectivity variation with angle Shuey (1985). Angle-domain CIGs have also information about how well events are focused at depth, thus providing a natural domain for migration-focusing velocity analysis Biondi and Sava (1999).

Following equation (10), we compute offset-domain CIGs with the 2-D deconvolution imaging condition. Figure [*]a shows the offset-domain CIGs computed with the 1-D crosscorrelation imaging condition. As expected, they show less resolution than the offset-domain CIGs computed with the 2-D deconvolution imaging condition (Figure [*]c).

 
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Figure 7
Comparison between 1-D crosscorrelation and 2-D deconvolution imaging conditions. (a) 1-D crosscorrelation offset-domain CIGs. (b) 1-D crosscorrelation angle-domain CIGs. (c) 2-D deconvolution offset-domain CIGs. (d) 2-D deconvolution angle-domain CIGs.
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We transform offset-domain CIGs into angle-domain CIGs following the method presented by Sava and Fomel (2000). Figure [*]b shows the angle-domain CIGs computed with the 1-D crosscorrelation imaging condition. They also show less resolution than the angle-domain CIGs computed with the 2-D deconvolution imaging condition (Figure [*]d).

We now compare the previous gathers with the gathers resulting from the migration of the data with a $3\%$ lower velocity. Figures [*]a to [*]d display the offset-domain and the angle-domain CIGS computed with the 1-D crosscorrelation imaging condition and the 2-D deconvolution imaging conditon for the low velocity, respectively. Notice that with the 2-D deconvolution the curvature of the events, due to the low migration velocity, appears more clear.

 
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Figure 8
Comparison between 1-D crosscorrelation and 2-D deconvolution imaging conditions for a $3\%$ lower velocity. (a) 1-D crosscorrelation offset-domain CIGs. (b) 1-D crosscorrelation angle-domain CIGs. (c) 2-D deconvolution offset-domain CIGs. (d) 2-D deconvolution angle-domain CIGs.
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Finally, we use the angle-domain Radon transform (ART) methodology described in Sava and Guitton (2003) to estimate the curvature of the reflectors in the angle-domain CIGs. Since the curvature of the reflectors in the ART domain is related to velocity errors, a better curvature estimation can lead to a better velocity estimation. A flat reflector in the angle-domain CIG maps at zero q (curvature index) in the ART CIGs. Conversely, a reflector with moveout in the angle-domain CIG should map away from zero q in the ART CIG.

In Figure [*] we compare the ART CIGs with the correct velocity (Figure [*]a and Figure [*]b) and the low velocity (Figure [*]c and Figure [*]d). In both cases we calculated the angle-domain CIGs with 1-D crosscorrelation (Figure [*]a and Figure [*]c) and 2-D deconvolution (Figure [*]b and Figure [*]d).

Notice that 2-D deconvolution imaging condition gives ART CIGs that are easier to interpret. Also notice that due to the lack of resolution, ART CIGs computed with 1-D crosscorrelation and the correct velocity Figure ([*]a) show events that can be misinterpreted as reflectors with the wrong velocity. In addition, ART CIGs computed with 1-D crosscorrelation and the wrong velocity Figure ([*]c) show events that can be misinterpreted as reflectors with the correct velocity. This problem is reduced with the 2-D deconvolution imaging condition (Figure [*]b and Figure [*]d) .

 
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Figure 9
Comparison between the 1-D crosscorrelation and the 2-D deconvolution imaging conditions for the correct and the low velocity. (a) Angle-domain and ART CIGs computed with the 1-D crosscorrelation and the correct velocity. (b) Angle-domain and ART CIGs computed with the 2-D deconvolution and the correct velocity. (c) Angle-domain and ART CIGs computed with the 1-D crosscorrelation and the low velocity. (d) Angle-domain and ART CIGs computed with the 2-D deconvolution and the low velocity.
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next up previous print clean
Next: Conclusions Up: Results with synthetic data Previous: Zero offset image resolution
Stanford Exploration Project
7/8/2003