Model updating

From SEG Wiki
Jump to: navigation, search
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


Earth modeling in depth

Limitations in the techniques for velocity estimation and velocity-depth ambiguity inherent to seismic inversion are compelling reasons for the need to update an estimated earth model in depth, however it has been constructed. Unfortunately, model updating tools themselves also have limitations in terms of their ability to resolve lateral velocity variations and refine reflector geometries. Again, the cable length and reflector depth dictate the extent that model updating techniques can resolve the velocity-depth ambiguity. In this section, we shall review residual moveout corrections applied to image gathers as a local method and reflection tomography as a global method for model updating.

3-D structural inversion applied to seismic data from offshore Indonesia

The steps for model updating by reflection tomography are outlined below.

  1. Perform 3-D prestack depth migration using the initial 3-D velocity-depth model (Figure 10.8-6) and generate the image gathers along selected inline traverses. Note from the selected image gathers shown in Figure 10.8-7 that majority of the events are nearly flat. Nevertheless, residual moveout along some of the events calls for a modification to the initial velocity-depth model. Figure 10.8-8 shows three inline sections derived from stacking the image gathers. The same inline sections with reverse polarity for better identification of some of the fault planes are shown in Figure 10.8-9. Compare these image sections from 3-D prestack depth migration with those derived from 3-D poststack depth migration shown in Figures 10.8-3 and 10.8-4, and note that prestack depth migration has improved the image quality within the zone with intensive faulting.
  2. Assume that residual moveout on image gathers can be described by a parabolic moveout equation when analyzed in time, and compute the horizon-consistent residual moveout semblance spectra for events on image gathers that correspond to the layer boundaries included in the velocity-depth model. Shown in Figure 10.8-10 are the semblance spectra for three horizons H2, H3, and H4 as in Figure 10.8-6 along three inline traverses.
  3. For each horizon, pick the residual moveout profiles by tracking the maxima of the semblance spectra for each of the inline traverses and create a residual moveout map as shown in Figure 10.8-11 associated with a reference offset of the image gathers — usually the maximum offset.
  4. Use the residual moveout maps for the reference offset from step (c) and compute the residual move-out for all offsets in the image gathers based on the parabolic moveout assumption. Build the traveltime error vector Δt of equation (9-7) using the residual moveout times.
  5. By using the interval velocity and depth-horizon maps associated with the initial velocity-depth model (Figure 10.8-6), construct the coefficient matrix L in equation (J-88) as described in Section J.6.
  6. Estimate the change in parameters vector Δp by way of the GLI solution given by equation (9-7). The tomography matrix equation (9-7) can be set up such that the residual moveout times given by the traveltime error vector Δt are used to perturb interval velocities and reflector geometries for not necessarily all of the horizons, but any combination of them. In fact, if judged to be appropriate, only one set of the parameters — either layer velocities or reflector geometries may be perturbed.
  7. Update the parameter vector p + Δp. Figure 10.8-12 shows the updated interval velocity and depth-horizon maps. Compare with the interval velocity and depth horizon maps associated with the initial model (Figure 10.8-6) and note that tomographic update was actually applied to the interval velocities while the reflector geometries represented by the depth horizon maps were not altered.
  8. Now combine the new set of interval velocity and depth horizon maps after the tomographic update (Figure 10.8-12) to construct a new velocity-depth model.
  9. Perform 3-D poststack depth migration to create the image volume using the updated velocity-depth model. Shown in Figure 10.8-13 are three inline sections from the image volume. Again, the same inline sections with reverse polarity for better identification of some of the fault planes are shown in Figure 10.8-14. Compare these image sections with those derived from 3-D poststack depth migration before the tomographic update shown in Figures 10.8-3 and 10.8-4, and note that the tomographic update of the model has improved the image quality within the zone with intensive faulting. Selected depth slices from the image volume are shown in Figure 10.8-15.
  10. Re-interpret the depth horizons using the image volume from step (i) and examine discrepancies with the previous interpretation. If required, repeat steps (a) through (i) until residual moveouts for events associated with the layer boundaries included in the model have been reduced to negligible magnitudes.

See also

External links

find literature about
Model updating
SEG button search.png Datapages button.png GeoScienceWorld button.png OnePetro button.png Schlumberger button.png Google button.png AGI button.png