Changes

==Seismic inversion methods==

==Seismic inversion methods==

As just mentioned, seismic data give us a “fuzzy” picture of the subsurface over a large area, whereas wells give us detailed geological information at a few points. This suggests a general approach to the seismic inversion method, as shown in Figure 1. On the left side of this flowchart, the input is our best reflectivity estimate from the seismic data. On the right side, we introduce geological constraints taken from a sonic log or seismic velocity information and produce a forward model. We then combine the information from the seismic data with controlling information from the seismic model and produce a final inversion result.

[[file:seismic-inversion_fig1.png|left|thumb|{{figure number|1}}A flowchart showing the general concept behind seismic Inversion.]]

[[file:seismic-inversion_fig1.png|thumb|{{figure number|1}}A flowchart showing the general concept behind seismic Inversion.]]

[[file:seismic-inversion_fig2.png|thumb|{{figure number|2}}Band-limited inversion of several traces around a reef. The integrated sonic log from the reef well is shown as the dark trace in the central part of the figure.]]

There are two main types of inversion currently being used. The first is ''band-limited inversion'', which involves directly integrating the seismic trace. Since the seismic trace lacks a low frequency velocity trend because of the band-limited wavelet, the inverted trace lacks the “excursions” seen on the original sonic log. We must therefore add in the low frequency component from the geological model. The result is a frequency band-limited version of the original sonic logs. This is shown in Figure 2 for a carbonate reef example. The heavy trace in the center is a sonic log from the producing well. The zone of interest is at 1150 msec.

As just mentioned, seismic data give us a “fuzzy” picture of the subsurface over a large area, whereas wells give us detailed geological information at a few points. This suggests a general approach to the seismic inversion method, as shown in [[:file:seismic-inversion_fig1.png|Figure 1]]. On the left side of this flowchart, the input is our best reflectivity estimate from the seismic data. On the right side, we introduce geological constraints taken from a sonic log or seismic velocity information and produce a forward model. We then combine the information from the seismic data with controlling information from the seismic model and produce a final inversion result.

[[file:seismic-inversion_fig2.png|thumb|{{figure number|2}}Band-limited inversion of several traces around a reef. The integrated sonic log from the reef well is shown as the dark trace in the central part of the figure.]]

There are two main types of inversion currently being used. The first is ''band-limited inversion'', which involves directly integrating the seismic trace. Since the seismic trace lacks a low frequency velocity trend because of the band-limited wavelet, the inverted trace lacks the “excursions” seen on the original sonic log. We must therefore add in the low frequency component from the geological model. The result is a frequency band-limited version of the original sonic logs. This is shown in [[:file:seismic-inversion_fig2.png|Figure 2]] for a carbonate reef example. The heavy trace in the center is a sonic log from the producing well. The zone of interest is at 1150 msec.

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 Figure 3 for the same traces shown in Figure 2. Notice that the carbonate reef is visible, and looks like the blocked version of the log.

[[file:seismic-inversion_fig3.png|left|thumb|{{figure number|3}}Model-based inversion of several traces from around a reef. The integrated sonic log from the reef well is shown as the dark trace in the central part of the figure.]]

[[file:seismic-inversion_fig3.png|thumb|{{figure number|3}}Model-based inversion of several traces from around a reef. The integrated sonic log from the reef well is shown as the dark trace in the central part of the figure.]]

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==