Geology 493M
*3D Horizon/Fault Interpretation Workshop
Day 3 - Horizon Interpretation and Correlation
*This workshop is based in part on procedural steps developed by Mike Enomoto of Seismic MicroTechnology Inc. Houston, TX.
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42. Creation of a depth map is a two-stage effort that begins
with construction of an average velocity map. To create a
velocity map, select Tools from the main Menu Bar and then
Depth from the drop down list. Under Depth there are several
selections. Click on Compute Average Velocity Map. For Type,
select Horizon. The program computes the average velocity at
each well using one of three options (Apparent, Time Grid or
Formation Top) (Figure 27).
Figure 27: Method used to compute the Average Velocity Map
of a selected horizon is selected in this menu.
1) The Apparent method used to construct the average
velocity map uses the horizon time and formation top depth.
You must provide a velocity map name (Figure 27).
Gridding parameters can be tailored to individual needs
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(Figure 28). Time and depth pairs are then combined to form
an average velocity grid.
Figure 28: Gridding parameters selections menu.
Average velocity in this (Apparent) approach is computed by
dividing formation top depth by half the horizon time. Whether
you Extrapolate (Figures 27 and 29) or not (Figure 30) will
yield two different results. Extrapolation will project the
resulting velocity grid outside the area of well control.
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Figure 29: Velocity map formed by extrapolation.
Figure 30: Velocity grid of C38 horizon without extrapolation.
Velocities were derived using the Apparent method.
2) The Time Grid method of deriving the velocity map uses
the horizon time picks, converts them to depth using the
well time/depth function and then generates a velocity grid.
This velocity map (Figure 31) is considerably different from
that obtained using the apparent method.
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Figure 31: Velocity grid from Time Gridding (no extrapolation).
Figure 32: Velocity grid from time gridding with extrapolation.
Comparison of Figures 29 and 32 reveal notable but minor
differences in this example.
3) The Formation Top method of deriving a velocity map
starts with the formation top depth, converts it to time using
the well time/depth function and then generates a velocity
grid. If the horizon and formation top do not tie, three
different velocities can be generated. Use the default grid
parameters as a first pass for each velocity map. Depths
obtained from this approach (Figure 33 and 34) reveal subtle
differences.
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Figure 33: Velocity grid obtained from Formation Top method
with extrapolation to the borders of the survey.
Figure 34: Formation Top conversion without extrapolation
yields this velocity grid, which has been extended to incorporate
well #10 along one of the 2D lines external to the 3D survey.
43. Once the velocity map has been generated, you are ready to
convert times to depth. From the top menu bar click on Tools
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then Depth, then Depth Map by Average Velocity Map. This
will open up the menu shown below (Figure 35).
Figure 35: Depth Map by Average Velocity Map menu.
Supply the appropriate information in the above menu (Figure
35) and then click OK. The Grid Parameters window (not
shown) will then appear. Parameters can be tailored to your
specific project. Click OK to generate the Depth Map (Figure
36).
Figure 36: Depth map derived from the average velocity map.
Edge effects may be noticeable on your depth map. These areas
can easily be removed using the Grids, Polygon Erase utility.
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Polygon erase will provide you with a "rubber band" that you
can use to define an enclosed region to delete. The operation is
identical to that used to digitize fault polygons. Digitize the
points around the region you wish to delete and double click to
complete the operation.
44. Contour the depth map and display the amplitudes
under the contours. Remember that you can contour your
maps using the Map Select Contour Overlay options.
Contours of velocity for the C38 horizon are shown below
(Figure 37).
Figure 37: Depths obtained from time-gridding (Figure 32) have
been contoured for the C38 horizon.
Amplitudes of the C38 reflection event can then be followed
along the structure by dragging the amplitude horizon from the
project menu onto the map (see montage Figure 38). Note the
association of amplitude anomalies with the faults in this
example.
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Click on Amplitudes
and drag to map.
Figure 38: Reflection event amplitudes combined with a depth
contour overlay for the C38 Horizon.
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