3-D coherency inversion combined with 3-D poststack depth migration

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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


Examination of the earth image volume in time (Figure 10.7-2) and the velocity field (Figure 10.7-4) leads to the following characterization of the earth model in depth:

  1. The complex geometry of the second anhydrite-dolomite unit within Zechstein may influence the geometry of the target level — base Zechstein, and therefore, it may have to be included in the earth model.
  2. Lateral velocity variations within the overburden layers are, in some parts of the survey area, require imaging in depth.
  3. Velocity contrast at layer boundaries causes significant ray bending.
  4. Vertical velocity gradients within the overburden layers are significant and vary spatially. For instance, the gradient within the Cretaceous chalk layer can be as much as 1.5 m/s/m.

Based on these characteristics, an appropriate procedure for estimating an earth model in depth involves a layer-by-layer application of 3-D coherency inversion to estimate layer velocities and 3-D poststack depth migration to delineate reflector geometries. Assume that we already have estimated the velocity-depth model for the first n − 1 layers, and that we want to estimate the layer velocity for the nth layer and delineate the reflector geometry associated with the base of that layer. Hence, we may consider the subsurface velocity-depth model made up of two parts — the known part that includes the first n − 1 layers, and the unknown part that includes the nth layer and the layers beneath. The following sequence is conducted for each of the layers in the unknown part of the model.

  1. Create a 3-D velocity-depth model represented in the form of a tessellated volume.
  2. Perform 3-D coherency inversion to estimate velocity nodes for the layer under consideration. Coherency inversion requires time horizons picked from a volume of 3-D unmigrated CMP-stacked data (Figure 10.7-3) and CMP gathers at analysis locations. For each layer, estimate layer velocities using 3-D coherency inversion at grid locations 1 km apart in the inline and crossline directions. Figure 10.7-5 shows results of coherency inversion at one specific analysis location. The bottom frames are copies of the CMP gather at the analysis location, while the top frames are copies of the semblance curves derived from coherency inversion (in blue) and stacking velocity inversion (in red). The 3-D coherency inversion semblance curve is a measure of the discrepancy between the modeled and actual CMP traveltimes associated with the reflection event from the layer boundary under consideration. The stacking velocity inversion semblance curve is a measure of the discrepancy between the modeled and actual stacking velocities associated with the reflection event from the layer boundary under consideration. The vertical bars in red are aligned with the selected trial velocities that correspond to the center data windows in the panels shown in Figure 10.7-6. The modeled traveltime trajectories associated with the three selected velocities are plotted on the CMP gather displays in Figure 10.7-5. Note from Figure 10.7-6a, the center data window exhibits residual moveout which suggests erroneously too low velocity, corresponding to the selection made in Figure 10.7-5a. Similarly, the center data window in Figure 10.7-6c exhibits residual moveout which suggests erroneously too high velocity, corresponding to the selection made in Figure 10.7-5c. The center data window in Figure 10.7-6b, however, exhibits a flat-event character, which suggests that the selected velocity in Figure 10.7-5b is optimum. This velocity coincides with the peak value of the 3-D coherency inversion semblance curve and the minimum of the stacking velocity inversion semblance curve.
  3. Use the results of 3-D coherency inversion — the semblance spectrum and the modeled CMP traveltimes (Figure 10.7-5), and the residual moveout panels (Figure 10.7-6) to pick velocities at analysis locations.
  4. Assign the velocity nodes to the half space defined by the unknown part of the model which includes the layer to be delineated. Then, by spatial interpolation between the nodes, a velocity field is created for the half-space.
  5. Next, create a gridded velocity-depth model that includes the known and unknown part of the model.
  6. By using the gridded velocity field, perform 3-D poststack depth migration down to a depth just below the current layer under consideration.
  7. The next step involves interpretation of the base of the layer under consideration to delineate the reflector geometry in depth. Interpretation of the depth horizon is done on cross-sections from the 3-D volume of poststack depth-migrated volume of data at an interval that accounts for the complexity of the reflector geometry. By using the interpretation results from the cross-sections, we perform a surface fit to obtain the 3-D representation of the reflector geometry associated with the base of the layer.

Figure 10.7-7 shows selected inlines as in Figure 10.7-1 from the volume of 3-D poststack depth-migrated data following the layer-by-layer application of the procedure described above. Compare with the results of 3-D poststack time migration (Figure 10.7-2) and note the subtle differences in the geometry of the base-Zechstein boundary.

The second anhydrite-dolomite unit of Zechstein was included in the earth modeling as a layer of constant thickness (100 m). Note from Figure 10.7-8 the complex geometry of this unit. Although it has a complex geometry and a significant velocity contrast, this unit does not have much effect on the geometry of the base-Zechstein boundary. This can be explained by noting that the thickness of the unit is sufficiently small making the traveltime within the unit insignificant despite its very fast velocity (5900 m/s).

Figure 10.7-9 shows the results of earth modeling using the procedure described above. Starting from the top, we see layer velocities and reflector geometries for Upper and Lower Tertiary, base Cretaceous chalk, base Upper and Lower Triassic, and base Zechstein. To obtain an accurate depth structure map for the base-Zechstein boundary that is of interest to the explorationist, the layer velocities and reflector geometries of the layers above are estimated one layer at a time starting from the surface. As a result, the accumulation of errors in the earth model parameters at the zone of interest (base Zechstein) is minimized.

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