Case Histories: Seismic Inversion
These case histories show inversions based on acoustic impedance or acoustic velocity. Many inversions can be made based on other attributes, including elastic properties, such as Poisson's Ratio, frequency content, and others.


Devonian Crossfield Strat Trap
In this example, the lightest colors in the Crossfield member at 1.4 seconds (-5200 ft) show where the porosity is highest. The darker blue near shot point 39 corresponds to tighter rock, which bounds this stratigraphic trap. Note that the correlation lines do not always follow color boundaries. The horizon at the top of the Devonian, for example, contacts variable color below the line, corresponding to changes in Devonian facies at the contact.
 


Seismic Inversion on Mississippian strat trap

Devonian Reef Trap
The example shows a conventional seismic section with a portion of a Seislog section across a Devonian reef. The Cretaceous Devonian unconformity is at 1.1 seconds and the reef top at 1.2 seconds near shotpoint 40. Drape over the reef is evident, as well as a porosity halo around the reef, caused by secondary dolomitization. Production is from the dolomite porosity.


Seismic inversion on a Devonian Reef


Reservoir Modeling
This example shows a schematic presentation of a seismic reservoir model based on raw logs, processed seismic data, and a transform of seismic amplitude to lithology and porosity. Such models are really seismic inversions and are discussed more fully later in this Chapter.


Lithology-porosity model in sand shale sequence

By combining sonic log velocity and reservoir contours based on 3-D migration of seismic, the porosity distribution of the pool can be better defined. This permits nonlinear interpolation between well control.


Velocity mapping to find porosity

The penultimate example contrasts three modeling techniques over a porous reef. At top is a seismic inversion which created synthetic sonic logs from seismic traces. The colors represent acoustic impedance, and hence lithology or porosity variations. Sparse spike inversion, in the middle illustration, more closely resembles a blocked sonic log, making it less noisy and easier to interpret than normal inversion. Some fine detail may be lost.


Inversion controlled by sonic log modeling

The bottom image shows a multi trace forward model derived from interpolated sonic logs. Such models are often used to control inversion processing and interpretation. The model can be adjusted until a good fit to the inversion is found, or some inversion parameters can be adjusted until the inversion becomes more realistic. Both models can be altered under user control and viewed on a workstation.

Potash Mining
The final example illustrates synthetics made from sonic logs over a potash-halite-anhydrite sequence in Saskatchewan. Because the density variations between these minerals is so extreme, the synthetics would have been much more realistic if this data had been included. (Density sylvite = 1.86 gm/cc, carnallite = 1.57 gm/cc, halite = 2.03 gm/cc, anhydrite = 2.97 gm/cc, dolomite 2.87 gm/cc, limestone 2.71 gm/cc). The density contrast is larger than the velocity contrast and is an important factor in matching to real seismic.


Synthetic seismograms in potash beds


Synthetic seismograms in potash beds with mine entry edited into sonic log


Synthetic seismograms in potash beds compared to real seismic

This example courtesy of Boyd Geosearch, Calgary.
 

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