GCRandomness
in 
3-D
Seismic Survey Design*
By
Engin Alkan1 and Bob Hardage2
Search and Discovery Article #40265 (2007)
Posted November 20, 2007
*Adapted from the Geophysical
Corner column, prepared by the authors, in AAPG Explorer, October, 2007, and
entitled “Was That Survey Crew Sober?”. Editor of Geophysical Corner is Bob A. Hardage. Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is
Communications Director.
1Graduate student at the Jackson School of Geosciences.
2Bureau of Economic Geology, The University of Texas at
Austin ([email protected])
Considerable effort can
be expended in onshore 3-D seismic data acquisition in surveying the coordinates
where source-station and receiver-station flags are placed, because these flags
will later instruct field personnel exactly where to plant geophones and
vibrator drivers exactly where to position their vehicles. Sometimes there is a
long delay (perhaps weeks or months) between the deployment of these station
flags and the arrival of the seismic crew. In such instances, a
station-surveying crew may visit the prospect a second time and invest
additional time and expense to reset station flags that have disappeared for any
reason.
The justification for this emphasis on precise, pre-survey station-flag
positioning is partly tradition that holds over from days when GPS technology
was not available and there was no other way to define the X, Y and Z
coordinates of each source and receiver station. But the justification also is
partly based on seismic data-processing requirements. Numerous data-processing
algorithms require seismic data to be sampled at regularly spaced intervals in
X, Y space. To ensure correct data processing, some explorationists exert a
serious effort to positioning source-station and receiver-station flags at
precise, regularly spaced intervals before any data-acquisition activity is
initiated.
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An issue to consider is, “Is it necessary
to position station flags accurately before a seismic survey begins, or
is it only necessary to know station coordinates accurately after they
have been occupied?” Almost every vehicle and every person on a modern
seismic crew has a GPS system, and their positions are known at all
times. The GPS systems in vibrator trucks define precisely where the
source is positioned; GPS units carried by the geophone-deployment crew
define precisely where they planted the geophones.
Regarding the issue of regularity of data
sampling, powerful algorithms exist to convert irregularly
sampled data to regularly sampled data. Thus, if post-survey station
coordinates are know with high accuracy, there is less need to expend
cost in pre-survey work to position station flags at precisely known
coordinates.
If vibrator drivers and
geophone-deployment crews are allowed to position their stations
according to their best judgment during the course of a seismic survey
(rather than following trails of pre-survey positioned flags), the
stations will be positioned with some amount of randomness and will not
be at precise, regular intervals. This randomness in the positions of
source and receiver stations can be beneficial.
For example, consider the two seismic
data-acquisition concepts illustrated as Figures 1 and
2. Great care and
expense were taken to make the acquisition geometry in
Figure 1 have
source/receiver stations at precise, regular intervals. In contrast, the
erratic positioning of the stations in Figure 2 suggests one question:
“Was the station-surveying crew ill or inebriated?”
Now look at the
plots of stacking fold, source-to-receiver offsets and
source-to-receiver azimuths that accompany each acquisition geometry.
The random-station geometry increases stacking fold in many bins by 10
to 20 percent (panel b in each figure), which is good. Randomness in
station positions does introduce minor, erratic, bin-to-bin variations
in fold, but these variations are not a serious problem in this example.
More importantly, randomness creates more uniform distributions of
offset and azimuth than does the regular-station
geometry
(compare panels c and d in Figure 1 with their equivalents in
Figure 2).
The offset and azimuth behaviors created by random-station geometry
(Figure 2) are preferred for amplitude-vs.-offset and attribute-vs.-azimuth
studies.
There are
situations where source and receiver station coordinates must be
known with precision before any seismic data-acquisition commences: In
archeologically sensitive areas, regulatory agencies have to inspect
each station to determine whether archeological damage will occur if
vehicles or people occupy the station coordinates. Some farmers want to
know exactly where vehicles will travel across croplands before they
will allow a seismic crew to enter their property. Some pipeline
companies insist on knowing exactly where each source station is
relative to each of their underground lines. The list goes on and on.
We
are not advocating that seismic source and receiver stations be
positioned willy-nilly across a prospect; we intend only to show that in
some seismic surveys, accurate and costly pre-survey positioning of
source and receiver stations is not necessary – and that the randomness
introduced into an acquisition geometry by casual positioning of
source/receiver stations can be advantageous if the amount of randomness
is kept within reason.
A good analogy to the introduction of randomness into station
positioning is the use of salt on food. A modest amount does
considerable good; too much can be a disaster. Perhaps it is time to
think about relaxing some of the time demands and expense we invest in
precise pre-survey station positioning in those numerous situations
where it is really not necessary to do such pre-survey effort.
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