Structural inversion

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Seismic Data Analysis
Seismic-data-analysis.jpg
Series Investigations in Geophysics
Author Öz Yilmaz
DOI http://dx.doi.org/10.1190/1.9781560801580
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store


We want to construct a structurally consistent 3-D velocity-depth model using the deliverables from phase 3 and update it to obtain a final 3-D velocity-depth model. We then want to use this earth model in depth to create an earth image in depth from 3-D prestack depth migration.

  1. Interpret a set of depth horizons from the image volume derived from 3-D poststack depth migration (Figure 10.9-20a). These depth horizons correspond to layer boundaries with significant velocity contrast.
  2. To preserve the vertical and lateral velocity variations in the 3-D interval velocity field derived from Dix conversion (Figure 10.9-19), create a set of phantom depth horizons by subdividing each of the layers bounded by the depth horizons (Figure 10.9-20a) interpreted from the 3-D poststack depth-migrated volume of data into a set of 10 sublayers. The phantom horizons can be conformed either to the top-boundary or the base-boundary, or, as in the present case, to both the top- and base-boundary of the layer under consideration. Figure 10.9-20b shows the principal depth horizons as in Figure 10.9-20a interleaved with the phantom horizons in green.
  3. Intersect the 3-D interval velocity volume (Figure 10.9-19) from step (c) of phase 3 with the principal and phantom depth horizons from step (b) of the present phase and extract horizon-consistent interval velocity surfaces. Then, combine these layer velocities with the reflector geometries represented by the depth horizons to create an initial structurally consistent 3-D velocity-depth model. Figure 10.9-21 shows selected inline cross-sections from the initial 3-D velocity-depth model.
  4. Perform 3-D prestack depth migration and generate a set of image gathers along every tenth inline, and compute the residual moveout semblance spectra for model updating (model updating). Figure 10.9-22 shows selected panels of residual-moveout analysis. Each panel comprises the image gather at the analysis location and the residual moveout semblance spectrum computed from it. While many of the events in the image gathers show very small or no residual moveout (panel I-51), note that some events exhibit significant residual moveout errors (panel I-41). Perform residual moveout corrections and update the initial velocity-depth model (model updating).
  5. Repeat the residual-moveout analysis a few times until the residual-moveout errors have been reduced significantly. Figures 10.9-23 and 10.9-24 show inline and crossline cross-sections, respectively, from the final 3-D velocity-depth model. The residual-moveout analysis from the last iteration of model updating shown in Figure 10.9-25 indicates significant reduction of the residual-moveout errors (compare, for instance, panel I-41 in Figure 10.9-25 with that in Figure 10.9-22).
  6. Perform 3-D prestack depth migration using the final 3-D velocity-depth model from step (e). The input to 3-D prestack depth migration is the prestack data set with signal processing applied as in Figure 10.9-5b. Figures 10.9-26 and 10.9-27 show selected inlines and crosslines, respectively, from the image volume in depth derived from 3-D prestack depth migration.

The deliverables from phase 4 — structural inversion, include a volume of structurally consistent 3-D interval velocity field and an image volume derived from 3-D prestack depth migration.

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Structural inversion
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