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The Leading Edge; October 2004; v. 23; no. 10; p. 1038-1041; DOI: 10.1190/1.1813358
© 2004 Society of Exploration Geophysicists
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CMP fold

A meaningless number?

R. Malcolm Lansley

PGS Onshore, Houston, Texas, U.S.

Corresponding author: malcolm.lansley@pgs.com

The first 20% of the full text of this article appears below.

How many times do we hear "It's a very high-fold survey" or "It's an xx-fold survey"? Most people who work in seismic data acquisition or processing are used to discussing "fold" as though this is a universal parameter that defines the quality of any given survey, with higher fold automatically implying better data quality. Is this really true?

Figure 1 shows a portion of the same line extracted from two different 3D volumes. Data quality is very different ... yet all the same recorded field data is included in each data set, the velocity functions used for stacking and migration are the same, and the fold of the two surveys is identical. So what is causing the difference?


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  		Figure 1. (a) High-density 3D data with 48 fold. (b) Simulated conventional 3D data with 48 fold.

 
The difference results from the improved wavefield sampling of a high-density volume (Figure 1a) compared with the simulated conventional recording in Figure 1b. This paper will address the issue of wavefield sampling, and show that a better method of quantifying how well the recorded wavefield is sampled is by "trace density" rather than "CMP fold." In fact, unless the bin size is specified, the term "fold" has no real meaning.


   Definition of trace density
 
In 1987, Theodor Krey described a method for calculating the fold required for a 3D survey based upon analysis of 2D data in the same area that has adequate signal-to-random-noise ratio. His method is based on the theory that reflected signals within the Fresnel zone (Gijs Vermeer, in his various books and papers on survey design and resolution, would say that we should use the "zone of influence," not the Fresnel zone, but the effect is the same) are imaged in the migration process whilst random noises are attenuated using . . . [Full Text of this Article]






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