Structure-independent model estimation

**Seismic Data Analysis**
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 shall now implement the procedure outlined at the beginning of this section.

Start with the stacking velocity field (Figure 10.2-10). Note that already a pattern is inferred about the velocity-depth model — an upper region where the velocity field shows a consistent increase with depth and moderate lateral variations, and a lower region where anomalous zones exist. Actually, the upper region coincides with the suprasalt region and the lower region coincides with the salt sills and the subsalt region.
Apply smoothing to the stacking velocity field in Figure 10.2-10 to derive what may be considered an rms velocity field (Figure 10.2-11) for Dix conversion.
Perform Dix conversion to derive an initial velocity-depth model in the form of an interval velocity field (Figure 10.2-12). The upper half of this model appears geologically plausable — a velocity field typical of a sedimentary sequence without any distinct layer boundaries with significant velocity contrast. The lower half exhibits anomalous behavior that geologically is not meaningful. It is this region where Dix conversion would fail because of the presence of salt sills with rugose boundary that causes severe raypath distortions.
Using the initial velocity-depth model from step (c) (Figure 10.2-12), perform prestack depth migration and generate image gathers (Figure 10.2-13). Flatness of events indicates that the initial velocity-depth model (Figure 10.2-12) based on Dix conversion of rms velocities is remarkably accurate within the suprasalt region (above event A which corresponds to the top-salt boundary).
Nevertheless, small adjustments may be required to the velocity-depth model within the suprasalt region before moving down to the salt and subsalt regions. To update the model in the suprasalt region, we shall perform residual moveout analysis of image gathers. Specifically, a variation of the procedure described in model updating will be followed here. First, convert the image gathers at selected locations along the line from depth (Figure 10.2-13) to time domain (Figure 10.2-14) using the initial velocity-depth model (Figure 10.2-12). The image gathers now are equivalent to moveout-corrected CMP gathers, except that they are in their migrated positions.
The next step is the application of inverse moveout correction to the image gathers from step (e) as shown in Figure 10.2-14. The velocity field used for inverse moveout correction is the rms velocity field shown in Figure 10.2-11, which is consistent with the interval velocity field (Figure 10.2-12) used to create the image gathers themselves.
Now perform conventional velocity analysis using the selected image gathers in time (Figure 10.2-14) and create vertical rms velocity functions at all analysis locations along the line.
Create a new rms velocity field from the vertical functions as shown in Figure 10.2-15. Compare with the initial rms velocity field in Figure 10.2-11 and note the details introduced to the lateral variations in the upper region.
Using the updated rms velocity field from step (h), perform Dix conversion and create an updated velocity-depth model (Figure 10.2-16). Note that the upper region now closely resembles the depositional sequence in the suprasalt region as seen in the time-migrated stacked section (Figure 10.2-2). Also note that the top-salt boundary is now more evident in this model. The lower region still has to be considered as geologically implausable.
Using the updated velocity-depth model (Figure 10.2-16), perform prestack depth migration. Selected image gathers are shown in Figure 10.2-17 and the depth image derived from stacking of the image gathers is shown in Figure 10.2-18. From the flatness of events on image gathers, we may convince ourselves that the model in Figure 10.2-16 can now be considered as final for the suprasalt region. Any further iteration of steps (e) through (j) would only yield insignificant refinement to the model. The model updating described in steps (e) through (j) is only valid if the velocity errors in the initial model are fairly small (model updating).
Interpret the top-salt boundary from the depth image (Figure 10.2-18) and insert it as a layer boundary into the velocity-depth model (Figure 10.2-19). Often, it is adequate to use the depth image from poststack depth migration, rather than prestack depth migration as in this case, to interpret the top-salt boundary.
Assign the salt velocity into the region below the top-salt boundary (Figure 10.2-20). Assume that the velocity within the salt bodies is constant (4450 m/s). This assumption in some cases may not be valid — shale intrusions into halite crystalline rock can alter the velocity within the salt sills, substantially.
Using the new velocity-depth model (Figure 10.2-20), perform prestack depth migration to get a new depth image (Figure 10.2-21).
Interpret the base-salt boundary from the depth image (Figure 10.2-21) and insert it as a layer boundary into the velocity-depth model (Figure 10.2-22).
Finally, introduce the background velocity field into the subsalt region in the form of a vertically varying velocity function (Figure 10.2-23). As for the layered earth model in Figure 10.2-4, the velocity field in the subsalt region is referenced to the water bottom, since the salt bodies have very little influence on the vertical variations in the background velocity field.
Using the final velocity-depth model (Figure 10.2-23), perform prestack depth migration.
Convert the image-gather stack from step (p) from depth to time using the final velocity-depth model, apply poststack spiking deconvolution, band-pass filtering, and AGC scaling, then convert back to depth. Figures 10.2-24 and 10.2-25 show the depth image and selected image gathers, respectively.

Examine the image gathers in Figure 10.2-25 associated with the final velocity-depth model (Figure 10.2-23) and observe that there still are some events that do not meet the flatness criterion. Violation of this criterion, in this case, largely is due to 3-D effects that are not accounted for by 2-D earth modeling and imaging. In the Gulf of Mexico, the salt sills can have complex shapes as shown in Figure 10.2-26. Only by 3-D structural inversion of 3-D data, we can hope to delineate such salt bodies and image the subsalt regions.

Figure 10.2-4 The Gulf of Mexico line: layered earth model in depth estimated by a combination of coherency inversion to estimate layer velocities and poststack depth migration to delineate reflector geometries within the suprasalt region, and prestack depth migration to delineate the base-salt boundary.
Figure 10.2-10 The Gulf of Mexico line: stacking velocity field.
Figure 10.2-11 The Gulf of Mexico line: rms velocity field derived from the stacking velocity field in Figure 10.2-10.
Figure 10.2-12 The Gulf of Mexico line: an initial velocity-depth model based on Dix conversion of the rms velocity field in Figure 10.2-11.
Figure 10.2-13 Part 1: The Gulf of Mexico line: selected image gathers from prestack depth migration using the initial velocity-depth model shown in Figure 10.2-12.
Figure 10.2-13 Part 2: The Gulf of Mexico line: selected image gathers from prestack depth migration using the initial velocity-depth model shown in Figure 10.2-12.
Figure 10.2-13 Part 3: The Gulf of Mexico line: selected image gathers from prestack depth migration using the initial velocity-depth model shown in Figure 10.2-12.
Figure 10.2-14 The Gulf of Mexico line: (top) selected image gathers from prestack depth migration after converting from depth (Figure 10.2-13) to time domain using the initial velocity-depth model shown in Figure 10.2-12; (bottom) after applying inverse moveout correction using the rms velocity field shown in Figure 10.2-11.
Figure 10.2-15 The Gulf of Mexico line: updated rms velocity field derived from the velocity analysis of the image gathers in Figure 10.2-14.
Figure 10.2-16 The Gulf of Mexico line: updated velocity-depth model based on the Dix conversion of the updated rms velocity field shown in Figure 10.2-15.
Figure 10.2-17 The Gulf of Mexico line: selected image gathers from prestack depth migration using the updated velocity-depth model shown in Figure 10.2-16.
Figure 10.2-18 The Gulf of Mexico line: depth image from prestack depth migration using the updated velocity-depth model shown in Figure 10.2-16. Image gathers are shown in Figure 10.2-17.
Figure 10.2-19 The Gulf of Mexico line: updated velocity-depth model as in Figure 10.2-16 with the top-salt boundary interpreted from the depth image in Figure 10.2-18.
Figure 10.2-20 The Gulf of Mexico line: new velocity-depth model as in Figure 10.2-19 with the salt velocity introduced to the region below the top-salt boundary.
Figure 10.2-21 The Gulf of Mexico line: depth image from prestack depth migration using the new velocity-depth model shown in Figure 10.2-20.
Figure 10.2-22 The Gulf of Mexico line: new velocity-depth model as in Figure 10.2-20 with the base-salt boundary interpreted from the depth image in Figure 10.2-21.
Figure 10.2-23 The Gulf of Mexico line: final velocity-depth model.
Figure 10.2-24 The Gulf of Mexico line: depth image from prestack depth migration using the final velocity-depth model shown in Figure 10.2-23.
Figure 10.2-25 Part 1: The Gulf of Mexico line: selected image gathers from prestack depth migration using the final velocity-depth model shown in Figure 10.2-23. The depth image is shown in Figure 10.2-24.
Figure 10.2-25 Part 2: The Gulf of Mexico line: selected image gathers from prestack depth migration using the final velocity-depth model shown in Figure 10.2-23. The depth image is shown in Figure 10.2-24.
Figure 10.2-25 Part 3: The Gulf of Mexico line: selected image gathers from prestack depth migration using the final velocity-depth model shown in Figure 10.2-23. The depth image is shown in Figure 10.2-24.
Figure 10.2-26 A 3-D perspective view of detached salt sills from the Gulf of Mexico. The silver surface represents the top-salt boundary and the gold surface represents the base-salt boundary. (Courtesy Schlumberger Geco-Prakla.)