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^{GC}3D Design Philosophy – Part 3: Is Stacking Fold Acceptable?*
Bob Hardage^{1}
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 Fold 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
^{1}Bureau of Economic Geology, The University of Texas at Austin ([email protected])
This article is the third of a fourarticle series – this topic considers Part 3 and Part 4 labeled on the Figure 1 flow chart of 3D seismic design methodology.
Stacking fold 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 fold inside the bin varies with depth. Referring to Figure 2, when a stacking bin is centered about a deep reflection point B, the stacking fold 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 sourcereceiver 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 sourcereceiver pairs that contribute to a seismic image at any subsurface point, the stacking fold at depth Z2 would be N2 – because N2 unique sourcereceiver 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 fold decreases to a smaller number N1 – because only N1 sourcereceiver pairs generate field traces that reflect from A and still satisfy the geometrical constraint that the sourcereceiver pairs are offset a distance DE (or EF) or less that does not exceed depth Z1.
In a 3D context, stacking fold is the product of inline stacking fold (the fold in the direction that receiver cables are deployed) and crossline stacking fold (the fold perpendicular to the direction that receiver cables are positioned). Defining F as 3D stacking fold, FIL as inline fold and FXL as crossline fold, this principle leads to the design equation:
(1) F = FIL x FXL.
To build a highquality 3D image, it is critical to not only create a proper stacking fold across the image space but also to ensure the traces involved in that fold have a wide range of offset distances and azimuths. Equation 1 provides no information about the distribution of sourcetoreceiver offset distances or azimuths that are involved in a stacking fold. If it is critical to know the magnitudes and azimuth orientations of sourcereceiver offsets, then commercial 3D 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 3D seismic design. All discussions of 3D stacking fold will be based totally on equation 1. It is the simplicity of this equation that makes it appealing to use to explain to nongeophysicists how stacking fold and 3D recording geometry link together.
Stacking Fold

2D vs 3D Stacking Fold Considerations
In 2D and 3D acquisition geometry, inline stacking fold FIL is a function of two geometrical properties:
● The number of active receiver channels.
● The ratio of the sourcestation interval and the receiverstation interval.
Specifically, inline stacking fold is given by the equation:
(2) FIL = (1/2) (Number of receiver channels) X [(receiverstation interval)/ (sourcestation interval)].
In 2D seismic profiling, the sourcestation interval is usually the same as the receiverstation interval, making the ratio term in the square brackets equal to unity. However, in 3D profiling, the sourcestation spacing along a receiver line is the same as the sourceline spacing, which is several times larger than the receiverstation spacing. For example, if the receiverstation 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 inline fold for 3D data acquisition is thus considerably less than it is for 2D recording geometries. In this hypothetical example, it is 12 times less. Crossline stacking fold FXL – created by a 3D acquisition geometry – is controlled by the number of receiver lines that are incorporated into the 3D recording swath and is given by:
(3) FXL = (1/2) (Number of receiver lines in recording swath).
The last step in the 3D design procedure (Part 4 of Figure 1) is to compare the designed stacking fold with the predefined stacking fold that is desired. A key question at this stage is, “How do you preselect a stacking fold that is appropriate for comparison?”
There are several ways to answer this question. The ideal situation is to have access to 3D seismic data previously recorded near the prospect area. If those data have good signaltonoise character, then one should simply define the stacking fold that was used in recording these older 3D data as being the stackingfold objective for the new 3D data acquisition program.
If the signaltonoise character of these preexisting 3D data is not acceptable, a higher stacking fold should be considered. If only 2D seismic data are available in the area of interest – and these 2D data adequately image the subsurface geology – a popular design guideline is:
(4) 3D stacking fold = (1/2) (2D stacking fold)
This is a statement of a commonly observed condition that 3D stacking fold often needs to be only onehalf the value of 2D stacking fold to cause 3D data to have equivalent signal quality. If neither 2D nor 3D 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 fold is significantly different from the intended value of stacking fold, then the design procedure must be repeated. In this second iteration, one or more of the critical geometrical parameters (source/receiverstation spacings, source/ receiverline spacings or recording swath size) must be adjusted to cause the stacking fold 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
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