| In the June EXPLORER we discussed
the idea that the best overall 3-D seismic survey is not necessarily
the one with the best quality data. Nor does it have to be the one
with long offset data from all azimuths.
The best survey really depends on balancing a combination
of factors -- in particular, subsurface geology and economic objectives.
For some projects, wide-azimuth data is a necessity;
for others, it can be more of a liability than an asset.
The critical issue is to record seismic data that
is "good enough" to image the geology and still meet the economic
requirements of the user. This is accomplished by recognizing the
important role of survey design in the planning process.
This month we look at offset distribution plots and
offset-limited fold plots from several different wide-azimuth designs.
Then, we will compare these plots to similar plots from a typical
This comparison will reveal some of the adverse effects
that can result from wide-azimuth shooting.
Design Comparison: Offset Distribution
In June we discussed the importance of source-to-detector
offset distribution for each individual cell. We concluded that
for any given fold and bin size, offset distribution is the single
most important design attribute, especially when it comes to processing
and interpreting the final data volume.
One of the best ways to display this offset information
is with a trace offset scatter plot -- also known as a "necklace
plot," which displays source-to-detector offset distances (along
the vertical axis) for every pre-stack trace that belongs within
a particular cell.
Adjacent cells are indicated along the horizontal
axis, so that entire cell-lines can be examined at one time. Gaps
in offset-domain coverage appear as voids in a pattern of overlapping
The larger the void is, the greater the likelihood
of noticeable artifacts in the processed data.
In June we also discussed four different 3-D survey
designs named A, B, C and D. Figures 1A
and 1B are necklace plots that correspond
to designs A and B, respectively.
Recall that design A is the narrow-azimuth survey,
where the cross-line maximum offset is only about 40 percent of
the in-line maximum. Design B, on the other hand, has in-line and
cross-line maximum offsets that are approximately equal to each
Note that even though designs A and B produce the
same fold, the offset distribution for the wide design (figure
1B) is markedly poorer. The same observation also holds true
for wide azimuth design C (figure 1C).
In both cases, near and mid-range offsets have been
sacrificed in order to achieve large cross-line offsets. As a result,
the data volume produced by either design B or design C is likely
to be inferior to the volume produced from A -- the narrow design.
Of the three wide-azimuth designs modeled, only design
D has better offset distribution (figure
1D) than design A. However, the D design also has more than
two and a half times the fold of A, and that extra fold doesn't
The cost of acquiring design D will be substantially
higher than any of the other three designs.
Design Comparison: Shallow Fold
In addition to having poor offset distribution, the
ability of designs B and C to image shallow events is degraded.
We can see this degradation by examining fold plots that have been
offset-limited to source-to-detector distances of 5,000 feet or
less (figures 2A-D). Limiting the offsets
to 5,000 feet or less is representative of the offset mute that
is applied to shallow data by the data processors.
For this example, we will consider geologic depths
of about four to six thousand feet to be "shallow."
Although the nominal fold for wide-azimuth designs
B and C is about the same as narrow-azimuth design A, the offset-restricted
fold is quite different. Figure 2A
shows offset-restricted fold for design A ranges from 10 to 14,
whereas the wide designs B and C (figures
2B and 2C) only have four to eight
traces per cell. This means the ability to accurately map a shallow,
secondary objective, or to use a shallow marker horizon for isochron
mapping, probably will be compromised by using either design B or
Only design D achieves wide-azimuth data and effective
imaging of shallow events (see figure 2D).
Unfortunately, as we mentioned before, design D will
cost more to acquire than any of the other three design options.
The point of this article is not to suggest that
designs B, C or D are necessarily better -- or worse -- than design
Rather, the point is to call attention to the fact
that those extra azimuths are going to cost you in one way or another.
Either the price of your seismic survey will go up, or the offset
distribution and shallow imaging will deteriorate, or both.
Therefore, you must carefully weigh the pluses against
the minuses in the final seismic subsurface image.
What are you getting?
What are you losing?
What will it cost?