3-D Geological Velocity Model Construction For 3-D Depth Imaging
Authors: Kunz T.J. , Jones T.K. 
Published: October 2008

Description
A fundamental problem in structural imaging, in addition to the need for fast iterative migration, is that of parametrizing velocity models (Johnson, 1992). Different approaches to parametrizing 3-D velocity models have been previously described in Pereyra (1989), Brat (1992), and Mallet (1989). We have used the Gocad triangular mesh surface representation described in Mallet (1989) to construct 3-D surface models. A simplified version of our approach has been used by Ratcliff(1991) and Ratcliff et al. (1991) to construct salt models in the Gulf Coast.
Using parallel 2-D velocity models to construct 3-D velocity models may be adequate in areas of simple structure, however more complex areas require additional control in dip, strike, and depth directions. This is particularly true in areas having complex faulting, overthrusts, salt holes, spires, or complex velocity variations. We will show example models that exhibit these complexities, and illustrate how powerful 3-D surface generation can be coupled with 3-D seismic displays to produce better 3-D velocity models.

Application
In this talk we will illustrate tools that allow the interpreter to generate a smooth iso-velocity surface while viewing seismic data on a perspective cube. The interpreter can use any combination of dip, strike or depth sections to create iso-veIocity surfaces. In addition, after the surface is generated, the 3-D surface model can be cut by dip strike and depth section planes that can then be posted on the seismic display to verify the fit between the surface model and seismic data. In addition, the user can include the results of velocity analysis or velocity inversion within his model, and visualize velocity variation within layers. We will show examples of how this technique can be used in complex geological regions to produce improved 3-D velocity models, The techniques we will describe do not require hardware graphic accelerators, however when hardware graphic accelerators are available, the software can make use of them.

Complex Faults and Overthrust Model
This model is from a complex faulted and overthrust geological region. The 3-D iso-velocity surface model was generated from 13 key parallel lines extracted from 250 lines. Since the faults and overthrust were predominantly in the dip direction, 3-D tools in Gocad were used to construct surfaces having nonorthogonal overthrust and pitchouts.
The resulting velocity model was compared with the migrated seismic data in both dip and strike and depth sections to verify the accuracy of the model with the seismic data (Figure 1). Volume rendering of the velocity model and migrated seismic was used to visualize the depth migrated seismic along the iso-velocity surfaces across the entire 3-D survey.

Complex Salt Body with Hole and Spire
This model demonstrates a closed salt body that not only contains a hole or window in the salt, but a salt spire that turns on itself. Construction of this model is very difficult since both the top of salt and bottom of salt are each multivalued functions. This model is further complicated due to the fact a salt window occurs very near a thick vertical salt section.