Seismic imaging : a review of the techniques, their by Etienne Robein

By Etienne Robein

Subject Contents: Introduction-What is mirrored image seismic picture precisely? Wave prpagation: a few reminders ideas of Ray-based and Kirchhoff summation migrations. Pre-stack time migrations: rules and pace research. Ray-based Kirchhoff Migrations anf Tomography. Ray-based Beam Migrations. Wavefield Extrapolation Migrations. Wavefield Extrapolation Migration speed research and Inversion-based thoughts. Examples, dialogue and end Acknowledgements. References. Index

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Velocity values are highlighted at the picked markers at location VA The set of pairs of {Vnmo_eff; tvert} defines the continuous velocity function Vnmo_eff, (tvert), for which we want to compute the migration operators at each vertical time tvert. Fig. 08 schematically represents the ray-based technique used to compute the operator at that location. In the left-hand part of the figure is a plot of the Vnmo_eff (tvert) function, with the markers picked on the section. On the cartoon to its right, we show the interval velocity computed using inverse Dix’s formula (Dix, 1955; see also fig.

In theory, this diffraction curve is not limited in X, although the amplitude of the reflected wave decreases with travel paths according to spherical divergence. Hence, it is not necessary to sum from minus to plus infinity along the X axis (the whole surface in 3D). The maximum distance of summation actually used is called the ‘migration 32 aperture’; this is an important parameter in imaging. Kirchhoff summation concept In the imaging techniques based on Kirchhoff summation, we actually assume that each point in the subsurface is a diffractor similar to the one in the cartoon in fig.

The final image is the juxtaposition of the respective images of all points D. In this final migrated image, reflectors simply appear automatically due to the fact that there exists effectively a non-zero reflection coefficient along them. Note an interesting benefit of imaging by Kirchhoff summation: imaging can be performed for a limited area or selected sections only, which can be a serious time saver in the many iterations required by velocity model building. The velocity model, however, needs to be defined everywhere from the surface.

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