Changes

The second type of inversion, which is more recent than the band-limited method, involves producing a “blocky” output rather than a band-limited output. There are several methods that produce this type of output, and they are sometimes referred to as ''sparse-spike'' or ''model-based methods''. These methods work by producing a forward model that best reproduces the seismic data when converted to synthetic form (that is, when the reflection coefficients are convolved with the wavelet). This method involves starting with a simple “guess” of this model and changing this guess iteratively until the error between the model and the observed seismic data is minimized (see [[Forward modeling of seismic data]]). The results of doing such a model-based inversion are shown in [[:file:seismic-inversion_fig3.png|Figure 3]] for the same traces shown in [[:file:seismic-inversion_fig2.png|Figure 2]]. Notice that the carbonate reef is visible, and looks like the blocked version of the log.

The second type of inversion, which is more recent than the band-limited method, involves producing a “blocky” output rather than a band-limited output. There are several methods that produce this type of output, and they are sometimes referred to as ''sparse-spike'' or ''model-based methods''. These methods work by producing a forward model that best reproduces the seismic data when converted to synthetic form (that is, when the reflection coefficients are convolved with the wavelet). This method involves starting with a simple “guess” of this model and changing this guess iteratively until the error between the model and the observed seismic data is minimized (see [[Forward modeling of seismic data]]). The results of doing such a model-based inversion are shown in [[:file:seismic-inversion_fig3.png|Figure 3]] for the same traces shown in [[:file:seismic-inversion_fig2.png|Figure 2]]. Notice that the carbonate reef is visible, and looks like the blocked version of the log.

==Three-dimensional case study==

==Three-dimensional case study==

 

[[file:seismic-inversion_fig5.png|left|thumb|{{figure number|5}}The model-based inversion of the line through the river channel system of Figure 4. Copyright: Western Geophysical.]]

seismic-inversion_fig4.png|{{figure number|4}}A schematic diagram of a river channel zone. Copyright: the Taber area of southern Alberta. The 3-D survey was done in the area outlined by the rectangle in the center of the map. Courtesy of Western Geophysical.

seismic-inversion_fig5.png|{{figure number|5}}The model-based inversion of the line through the river channel system of Figure 4. Copyright: Western Geophysical.

As an example of inversion applied to real data, let's consider a 3-D case study that was done in the Taber area of southern Alberta, Canada, by Western Geophysical. [[:file:seismic-inversion_fig4.png|Figure 4]] shows a schematic interpretation of the geology of a river channel zone. The area of interest is the Glauconitic Formation of the lower Cretaceous upper Mannville Group, which is characterized by rapidly changing lithological facies. The reservoirs are sandstones with porosities of 15% surrounded by impermeable siltstone.

As an example of inversion applied to real data, let's consider a 3-D case study that was done in the Taber area of southern Alberta, Canada, by Western Geophysical. [[:file:seismic-inversion_fig4.png|Figure 4]] shows a schematic interpretation of the geology of a river channel zone. The area of interest is the Glauconitic Formation of the lower Cretaceous upper Mannville Group, which is characterized by rapidly changing lithological facies. The reservoirs are sandstones with porosities of 15% surrounded by impermeable siltstone.

A 3-D survey measuring 2.2 by [[length::1.7 km]] was acquired, as indicated by the rectangle in [[:file:seismic-inversion_fig4.png|Figure 4]]. These data were then processed through to final migrated stack, and the result was inverted using the SLIM inversion method of Western Geophysical. This approach is similar to the model-based algorithm discussed earlier. The results of inverting one particular line from the 3-D volume are shown in [[:file:seismic-inversion_fig5.png|Figure 5]]. Two producing wells and one dry hole are shown in this line. The producing interval is between 640 and 660 msec on the two producers, whereas the low velocity zone on the dry hole comes from a sand that is porous but not prospective. [[:file:seismic-inversion_fig6.png|Figure 6]] shows a series of time slices (that is, horizontal cuts through the 3-D data volume at constant time intervals) over the complete dataset (see [[Seismic data - mapping with two-dimensional data]]). Borehole B from [[:file:seismic-inversion_fig5.png|Figure 5]], which was the dry hole, is indicated by the faint white crosshairs at the centers of the graphs in [[:file:seismic-inversion_fig6.png|Figure 6]]. The lightly shaded areas show areas of low velocity material at the zone of interest. Notice that this well is obviously mispositioned with respect to the low velocity reservoir material.

A 3-D survey measuring 2.2 by [[length::1.7 km]] was acquired, as indicated by the rectangle in [[:file:seismic-inversion_fig4.png|Figure 4]]. These data were then processed through to final migrated stack, and the result was inverted using the SLIM inversion method of Western Geophysical. This approach is similar to the model-based algorithm discussed earlier. The results of inverting one particular line from the 3-D volume are shown in [[:file:seismic-inversion_fig5.png|Figure 5]]. Two producing wells and one dry hole are shown in this line. The producing interval is between 640 and 660 msec on the two producers, whereas the low velocity zone on the dry hole comes from a sand that is porous but not prospective. [[:file:seismic-inversion_fig6.png|Figure 6]] shows a series of time slices (that is, horizontal cuts through the 3-D data volume at constant time intervals) over the complete dataset (see [[Seismic data - mapping with two-dimensional data]]). Borehole B from [[:file:seismic-inversion_fig5.png|Figure 5]], which was the dry hole, is indicated by the faint white crosshairs at the centers of the graphs in [[:file:seismic-inversion_fig6.png|Figure 6]]. The lightly shaded areas show areas of low velocity material at the zone of interest. Notice that this well is obviously mispositioned with respect to the low velocity reservoir material.