View source for Structure-independent model estimation

{{SDA publication 
 | title   = Seismic Data Analysis
 | image   = [[File:Seismic-data-analysis.jpg|frameless|180px]]
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 | series  = Investigations in Geophysics
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 | author  = [[Öz Yilmaz]]
 | doi     = 10.1190/1.9781560801580
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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 gathers|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.

<gallery>
file:ch10_fig2-4.png|{{figure number|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.
file:ch10_fig2-10.png|{{figure number|10.2-10}} The Gulf of Mexico line: stacking velocity field.
file:ch10_fig2-11.png|{{figure number|10.2-11}} The Gulf of Mexico line: rms velocity field derived from the stacking velocity field in Figure 10.2-10.
file:ch10_fig2-12.png|{{figure number|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.
file:ch10_fig2-13a.png|{{figure number|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.
file:ch10_fig2-13b.png|{{figure number|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.
file:ch10_fig2-13c.png|{{figure number|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.
file:ch10_fig2-14.png|{{figure number|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.
file:ch10_fig2-15.png|{{figure number|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.
file:ch10_fig2-16.png|{{figure number|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.
file:ch10_fig2-17.png|{{figure number|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.
file:ch10_fig2-18.png|{{figure number|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.
file:ch10_fig2-19.png|{{figure number|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.
file:ch10_fig2-20.png|{{figure number|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.
file:ch10_fig2-21.png|{{figure number|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.
file:ch10_fig2-22.png|{{figure number|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.
file:ch10_fig2-23.png|{{figure number|10.2-23}} The Gulf of Mexico line: final velocity-depth model.
file:ch10_fig2-24.png|{{figure number|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.
file:ch10_fig2-25a.png|{{figure number|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.
file:ch10_fig2-25b.png|{{figure number|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.
file:ch10_fig2-25c.png|{{figure number|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.
file:ch10_fig2-26.png|{{figure number|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.)
</gallery>

==See also==
* [[Subsalt imaging in the Gulf of Mexico]]
* [[Layered earth model estimation]]

==External links==
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[[Category: Subsalt imaging in the Gulf of Mexico]]