Conclusion

We image the overturned waves by decomposing the source and receiver wavefields into plane waves. Each plane wave is extrapolated in tilted Cartesian coordinates. Offset-domain CIGs are generated by cross-correlating the source and receiver wavefields, with a shift in the direction normal to the extrapolation direction. The offset-domain CIGs are decomposed into horizontal and vertical CIGs, which are merged into robust, dip-dependent ADCIGs.

We apply our method to a North Sea dataset and compare the image with the ones obtained from downward continuation and reverse-time migration. The image obtained by plane-wave migration in tilted coordinates is better than the image obtained from standard downward continuation. The steeply dipping salt edge is imaged by one-way plane-wave migration in tilted coordinates, but it is missing in the image obtained by downward continuation. The plane-wave migration results are also comparable to those obtained by reverse-time migration and produces similar horizontal and vertical CIGs, while its computational cost is much less than reverse-time migration. The dip-dependent ADCIGs merged from horizontal and vertical CIGs are robust and provide useful moveout information for reflectors with a wide range of dips.

 

elf.hz.cig Figure 6 Vertical CIGs at z=1850m: (a) offset domain CIGs obtained by reverse-time migration; (b) offset domain CIGs obtained by plane-wave migration in tilted coordinates; (c) ADCIGs obtained by reverse-time migration; (d) ADCIGs obtained by plane-wave migration in tilted coordinates.

 

ang.image.elf.dip
Figure 7 Dip-dependent ADCIGs: Notice that both horizontal and vertical reflectors are focused very well.