[First Hit]

Datapages, Inc.Print this page

 Click to view article in PDF format.

 

 

GC3-D Design Philosophy – Part 3: Is Stacking Previous HitFoldNext Hit Acceptable?*

 

Bob Hardage1

 

Search and Discovery Article #40663 (2010)

Posted December 17, 2010

 

*Adapted from the Geophysical Corner column, prepared by the author, in AAPG Explorer, November, 2010, and entitled “Next Step: Is Stacking Previous HitFoldNext Hit Acceptable?”. Editor of Geophysical Corner is Bob A. Hardage ([email protected]). Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.   Click for remainder of series:  Part 1 Part 2 Part 4

 

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

 

 

Stacking Previous HitFoldNext Hit

 

This article is the third of a four-article series – this topic considers Part 3 and Part 4 labeled on the Figure 1 flow chart of 3-D seismic design methodology.

 

Stacking Previous HitfoldNext Hit is the number of field traces that are summed during data processing to create a single image trace positioned at the center of each bin. At any stacking bin coordinate, the stacking Previous HitfoldNext Hit inside the bin varies with depth. Referring to Figure 2, when a stacking bin is centered about a deep reflection point B, the stacking Previous HitfoldNext Hit is a maximum at depth B because the largest number of source and receiver pairs can be utilized to produce individual reflection field traces inside the bin.

 

The number of source-receiver pairs that can contribute to the image at B is typically confined to those source and receiver stations that are offset horizontally from B a distance that is no larger than depth Z2 to reflection point B. Thus, distances CE and EG shown on Figure 2 are each equal to Z2.

 

Using this offset criterion to determine the number of source-receiver pairs that contribute to a seismic image at any subsurface point, the stacking Previous HitfoldNext Hit at depth Z2 would be N2 – because N2 unique source-receiver pairs can be found that produce distinct field traces reflecting from point B.

 

When the stacking bin moves to a shallower depth Z1, the stacking Previous HitfoldNext Hit decreases to a smaller number N1 – because only N1 source-receiver pairs generate field traces that reflect from A and still satisfy the geometrical constraint that the source-receiver pairs are offset a distance DE (or EF) or less that does not exceed depth Z1.

 

In a 3-D context, stacking Previous HitfoldNext Hit is the product of in-line stacking Previous HitfoldNext Hit (the Previous HitfoldNext Hit in the direction that receiver cables are deployed) and cross-line stacking Previous HitfoldNext Hit (the Previous HitfoldNext Hit perpendicular to the direction that receiver cables are positioned). Defining F as 3-D stacking Previous HitfoldNext Hit, FIL as in-line Previous HitfoldNext Hit and FXL as cross-line Previous HitfoldNext Hit, this principle leads to the design equation:

 

 (1) F = FIL x FXL.

 

To build a high-quality 3-D image, it is critical to not only create a proper stacking Previous HitfoldNext Hit across the image space but also to ensure the traces involved in that Previous HitfoldNext Hit have a wide range of offset distances and azimuths. Equation 1 provides no information about the distribution of source-to-receiver offset distances or azimuths that are involved in a stacking Previous HitfoldNext Hit. If it is critical to know the magnitudes and azimuth orientations of source-receiver offsets, then commercial 3-D design software must be used.

 

Offset analysis is a topic that goes beyond the scope of this discussion, which is structured to provide simple explanations of the basic principles of 3-D seismic design. All discussions of 3-D stacking Previous HitfoldNext Hit will be based totally on equation 1. It is the simplicity of this equation that makes it appealing to use to explain to non-geophysicists how stacking Previous HitfoldNext Hit and 3-D recording geometry link together.

 

Figures

 

Stacking Previous HitFoldNext Hit
Figures
2-D vs 3-D
Final Step



















Stacking Previous HitFoldNext Hit
Figures
2-D vs 3-D
Final Step

















Stacking Previous HitFoldNext Hit
Figures
2-D vs 3-D
Final Step

















Stacking Previous HitFoldNext Hit
Figures
2-D vs 3-D
Final Step

















Stacking Previous HitFoldNext Hit
Figures
2-D vs 3-D
Final Step

















Stacking Previous HitFoldNext Hit
Figures
2-D vs 3-D
Final Step


















 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

fig01

Figure 1. Planning steps that can be followed to design a 3-D seismic acquisition geometry. This article discusses the topics identified by the areas labeled Part 3 and Part 4.

fig02

Figure 2. Vertical variation in stacking Previous HitfoldNext Hit. The source-station and receiver-station spacings along this profile have the same value Δx, which results in a stacking bin width of Δx/2. The vertical column shows the coordinate position of one particular stacking bin. For a deep target at depth Z2, the stacking Previous HitfoldNext Hit in this bin is a high number because there is a large number (N2) of source-receiver pairs that produce a raypath that reflects from subsurface point B. Only one of these raypaths, CBG, is shown. For a shallow target at depth Z1, the stacking Previous HitfoldNext Hit is low because there is only a small number (N1) of source-receiver pairs that produce individual raypaths that reflect from point A. One of these shallow raypaths, DAF, is shown. When a 3-D seismic data volume is described as a 20-fold or 30-fold volume, people are usually referring to the maximum stacking Previous HitfoldNext Hit that is created by the 3-D geometry, which is the stacking Previous HitfoldNext Hit at the deepest target.

 

2-D vs 3-D Stacking Previous HitFoldNext Hit Considerations

 

In 2-D and 3-D acquisition geometry, in-line stacking Previous HitfoldNext Hit FIL is a function of two geometrical properties:

 

● The number of active receiver channels.

 

● The ratio of the source-station interval and the receiver-station interval.

 

Specifically, in-line stacking Previous HitfoldNext Hit is given by the equation:

 

(2) FIL = (1/2) (Number of receiver channels) X [(receiver-station interval)/ (source-station interval)].

 

In 2-D seismic profiling, the source-station interval is usually the same as the receiver-station interval, making the ratio term in the square brackets equal to unity. However, in 3-D profiling, the source-station spacing along a receiver line is the same as the source-line spacing, which is several times larger than the receiver-station spacing. For example, if the receiver-station spacing is 110 feet, and the interval between the source lines is 1,320 feet, then there is a source station every 1,320 feet along each receiver line – and the square bracket term in equation 2 has a value of (1/12).

 

The in-line Previous HitfoldNext Hit for 3-D data acquisition is thus considerably less than it is for 2-D recording geometries. In this hypothetical example, it is 12 times less. Cross-line stacking Previous HitfoldNext Hit FXL – created by a 3-D acquisition geometry – is controlled by the number of receiver lines that are incorporated into the 3-D recording swath and is given by:

 

(3) FXL = (1/2) (Number of receiver lines in recording swath).

 

Final Step

 

The last step in the 3-D design procedure (Part 4 of Figure 1) is to compare the designed stacking Previous HitfoldNext Hit with the predefined stacking Previous HitfoldNext Hit that is desired. A key question at this stage is, “How do you preselect a stacking Previous HitfoldNext Hit that is appropriate for comparison?”

 

There are several ways to answer this question. The ideal situation is to have access to 3-D seismic data previously recorded near the prospect area. If those data have good signal-to-noise character, then one should simply define the stacking Previous HitfoldNext Hit that was used in recording these older 3-D data as being the stacking-Previous HitfoldNext Hit objective for the new 3-D data acquisition program.

 

If the signal-to-noise character of these pre-existing 3-D data is not acceptable, a higher stacking Previous HitfoldNext Hit should be considered. If only 2-D seismic data are available in the area of interest – and these 2-D data adequately image the subsurface geology – a popular design guideline is:

 

(4) 3D stacking Previous HitfoldNext Hit = (1/2) (2D stacking Previous HitfoldNext Hit)

 

This is a statement of a commonly observed condition that 3-D stacking Previous HitfoldNext Hit often needs to be only one-half the value of 2-D stacking Previous HitfoldNext Hit to cause 3-D data to have equivalent signal quality. If neither 2-D nor 3-D data are available, the only recourses are to ask advice of people who have recorded data in the area – or to guess.

 

If the calculated stacking Previous HitfoldNext Hit is significantly different from the intended value of stacking Previous HitfoldNext Hit, then the design procedure must be repeated. In this second iteration, one or more of the critical geometrical parameters (source/receiver-station spacings, source/ receiver-line spacings or recording swath size) must be adjusted to cause the stacking Previous HitfoldTop to converge toward the desired value. Because of the simplicity of the method described in this article series, designs can be iterated easily and quickly.

EXPLORER

 

 

Return to top.

 

Copyright © AAPG. Serial rights given by author. For all other rights contact author directly.