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GCThin-Bed Interpretation Using Reference Surfaces*

 

Bob Hardage1

 

Search and Discovery Article #40535 (2010)

Posted May 31, 2010

 

*Adapted from the Geophysical Corner column, prepared by the author, in AAPG Explorer, May, 2010, and entitled “Looking High and Low for References”. Editor of Geophysical Corner is Bob A. Hardage (mailto:[email protected]). Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.

 

1Bureau of Economic Geology, The University of Texas at Austin ([email protected])

 

 

General Statement

 

The fundamental criteria required of a seismic Previous HitreflectionNext Hit event that is to be used as a reference surface for interpreting thin-bed geology are that the seismic Previous HitreflectionNext Hit should:

 

1) Extend across the entire seismic image space and have a good signal-to-noise character.

 

2) Be reasonably close (vertically) to the geology that is to be interpreted.

 

3) Be conformable to the strata that need to be analyzed.

 

Criterion 3 is probably the most important requirement on this list.

 

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General statement
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Example
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General statement
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Example
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General statement
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Example
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General statement
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Example
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fig01

Figure 1. Vertical slices through a 3-D seismic volume showing a targeted channel system (a), and horizon slices A, B, C and D which cut through the channel window (b). These four horizon surfaces are approximations to stratal surfaces because they are conformable to the indicated reference surface, which is a chronostratigraphic Previous HitreflectionNext Hit event. This profile is labeled 200 on Figure 2.

fig02

Figure 2. (a) Previous HitReflectionNext Hit amplitude across seismic image space on horizon B (Figure 1b); (b) Previous HitReflectionNext Hit amplitude across seismic image space on a horizon 26 meters below reference surface 2 (Figure 3, below).

fig03

Figure 3. Seismic profile 222 shown on Figure 2. Reference surface 1 is the surface defined on profile 200 (Figure 1) below the channel target. Reference surface 2 is a second choice for a reference surface located above the channel that can be used to construct horizon slices that cut through the targeted channel complex.

 

Example

 

Figure 1 shows a data window from a vertical slice through a 3-D seismic volume  centered on a channel system that is to be interpreted. The seismic Previous HitreflectionNext Hit event labeled “reference surface” was selected as an appropriate conformable Previous HitreflectionNext Hit for interpreting the thin-bed channel system identified on Figure 1a. The reference surface in this case follows the peak of the seismic Previous HitreflectionNext Hit event on which it is positioned.

 

Four horizon surfaces labeled A, B, C and D, each conformable to the reference surface, pass through the targeted channel system on Figure 1b. Each of these horizon surfaces can tentatively be assumed to be a reasonable approximation of a stratal surface that intersects the channel system because each horizon is conformable to the selected reference Previous HitreflectionNext Hit event, and a fundamental thesis of seismic stratigraphy is that seismic Previous HitreflectionNext Hit events are chronostratigraphic by definition.

 

Figure 2a shows Previous HitreflectionNext Hit amplitude behavior on horizon surface B. This horizon surface does a reasonable job of defining the targeted channel system (channel 1) across the lower right quadrant of the display and also depicts a second channel system (channel 2) at the top of the image display. The image on Figure 2a is a horizon-based image, meaning that the seismic attribute that is displayed is limited to a data window that vertically spans only one data point.

 

In challenging interpretation problems, it is important to try to define two seismic reference surfaces that bracket the geological interval that is to be interpreted – one reference surface being below the geological target and the other being above the target. An interpreter can then extend conformable surfaces across a targeted interval from two directions (from above and from below). Sometimes one set of conformable surfaces will be more valuable as stratal surfaces than the other at the level of a targeted thin bed.

 

To illustrate the advantage of this opposite-direction convergence of seismic horizon surfaces, a second reference surface was interpreted above the targeted channel system and was placed closer to the target interval. This second reference surface followed the apex of the Previous HitreflectionNext Hit troughs immediately above the channel system. The two bracketing reference surfaces are shown on Figure 3.

 

The Previous HitreflectionTop amplitude response on a horizon surface conformable to reference surface 2 and positioned 26 milliseconds below that reference surface is displayed on Figure 2b. This image is again a one-point-thick attribute display (i.e. a horizon-based attribute). The channel systems are a bit crisper in appearance and their geometries are more definitive on this second imaging attempt than they were on the first effort (Figure 2a).

 

Conclusion

 

This dual-direction approach to constructing horizon surfaces that traverse thin-bed targets is a concept that often will provide valuable results. An even better approach would be to calculate stratal slices through a bracketed data window – a concept discussed and illustrated in the Geophysical Corner article published June 2006 (http://www.searchanddiscovery.net/documents/2006/06036zeng_gc/index.htm).

 

Unfortunately, not all interpretation software provides a stratal slicing option. In those cases, a dual-direction-approach strategy such as described here can be valuable for constructing horizon slices that approximate stratal slices.

 

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