Two Alternative Seismic Fault Interpretation Techniques—Part Two
by Mike Cline, Consulting Geophysicist, T/X Resources
This is Part Two of an expanded article discussing two alternative seismic fault interpretation techniques that were summarized in my “Rocks-2-Digits” web log some time ago. However, I thought that they might also be of interest to those who may not have Internet access or otherwise have seen them already. Part One, which was published last month, illustrates the Horizontal Fault Interpretation technique.
Technique #2—Shaded Relief Maps
What is a shaded display? It is a map, or 3D surface, that has been artificially illuminated to cause shadows to be cast by minor deflections in the surface. The shadowed deflections face away from the illumination source, exactly like real shadows. Shaded displays are an excellent way to highlight smaller faults or other features that may not have been noticed on the vertical seismic displays.
Figure 1 is an example of a shaded 3D surface map which illustrates some of the shadows created by the small faults in the general direction of the viewer. The illumination point is the red “sun” in the northwestern corner of the surface. Note the annotated small fault that has been partially obscured by this viewing angle—a limitation of 3D perspective maps that can only be resolved by rotating the surface for a better viewing angle for this particular fault.
Figure 1. The shaded 3D surface map in this display illustrates how an artificial point of illumination (shown as the red sun) will cause shadows to be cast by surface deflections sloping away from the light source.
Figure 2 is a shaded relief map (fully illuminated), which was rectified and imported into 2d/3dPAK as a geotiff file. The small black crosses on the map are the manual fault interpretations (at 500-ft increments—or, every fifth inline and crossline in the 3D seismic survey). The small fault, annotated by the arrow pointing to the shadow that intersects Inline-125, is better seen on this display than in Figure 1. As you may be able to see, this fault will probably be more easily interpreted by following the shadows that “connect the dots” of the interpreted faults nearby. Many of the other subtle shadow features seem to better connect, and extend other isolated fault interpretations scattered throughout the rest of the 3D seismic survey area. I use Golden Software’s Surfer program to generate the shaded relief maps, which are then geo-referenced in Global Mapper before importing them as geotiff files into 2d/3dPAK
Figure 2. The darker gray, linear features in this shaded relief map display are the result of shadows created from locally-steepening dips within the fault/fracture zones deforming this auto-picked seismic surface.
In Figure 3, the same small fault mentioned first in Figure 1 is highlighted on the vertical seismic display of 3D Inline-125. There is a small deflection (about 2 milliseconds of throw, which is about 11 ft in this area) on the interpreted red horizon, but it was not initially interpreted on this line.
Figure 3. The vertical seismic display of 3D Inline-125 illustrates the small fault interpreted on the shaded relief map of the red horizon, shown in Figures 1 and 2.
Shaded relief maps, while not always able to image faults with throws as small as 11 ft (may be constrained by geology and/or seismic data) as seen in this example, can highlight many additional faults too small to be noticed on vertical seismic displays. Depending on the local geological circumstances, these small faults could lead to additional drilling opportunities such as undrilled fault blocks in compartmentalized reservoirs. Or, they could help to explain production volumes that cannot be rectified with previous structural mapping. In addition to mapping small faults, shaded relief maps are also an excellent way to visualize stratigraphic changes such as channels, etc.
The summarized blog posting of 12/28/07, along with animated images of the shaded relief map technique, can be found on the Internet at http://txresources.com/rocks2digits/, which is also more specifically located at http://txresources.com/rocks2digits/2007/12/28/shaded-relief-maps-for-subtle-fault-detection/.
For a larger montage of the images in the shaded relief map technique, you can go to the posting of 12/12/07 at http://txresources.com/rocks2digits/ (http://txresources.com/rocks2digits/2007/12/12/easily-create-presentatio...).
For more information on this see June 2008 Bulletin article: Effective (and Not-So-Effective) Channel Maps