American Oil and Gas Reporter - May 2015 - 81

SpecialReport: Study In Geophysics

Reprocessing older seismic data can provide significant new in- shot in the late 1970s and early 1980s. At right is the same image
sight by adding greater detail and more accurate placement of sub- after reprocessing by ION Geophysical.
surface features. At left is an image from a Gulf of Mexico dataset

shot data are not always available. Poststack data can be used in their place, but
Gresko says they will not gain as much
improvement and may cause striping.
There also may be data gaps because
some portions of the line were not shot,
he adds. To fill these gaps, data must
sometimes be interpolated.

Regional GOM Insight
The Yucatan survey is an example of
an older Gulf of Mexico dataset that is
garnering new attention, according to
Gresko. Shot in the late 1970s and early
1980s by University of Texas students,
the survey data extend from the Mexican
shoreline south to shallow Mexican waters,
east to and around the Yucatan Peninsula,
and over to Florida.
The Yucatan survey was conducted
over 11 seasons and covers 9,300 miles.
It was shot with 20 or more seismic
lines of 100-500 miles each, most of
them in water depths from 300 to 3,000
meters. Gresko says these 2-D lines
were gathered using towed streamers
that were three kilometers long and
had maximum offsets of 2,380-3,500
meters. Record length was 10-14 seconds. He says some of the lines extended
into the actively explored area of the
northern Gulf of Mexico, although they
were not continued if they overlapped
with newer data.
The Yucatan data are of interest for
two key reasons, says Gresko. First, Mexico-side data can provide insight into the
U.S. side, he reasons. "Geology does not
stop at borders," he quips.
Second, when Mexico opens its waters
to foreign companies, operators want to
be ready. Gresko says virtually all data
on the Mexican side of the Gulf have
been proprietary to PetrĂ³leos Mexicanos.
He says the team was able to recover
70-75 percent of the shot records. The

remainder were available only as initial
processed stack data. While the team
could improve the post-stack data with
newer techniques, the potential was limited, compared with the original shot
data, notes Gresko.
Because some original field tapes were
lost, data gaps exist in several of the reprocessed lines, he observes. Some of
the gaps are several kilometers wide.
Gresko says reprocessed data from shot
gathers were blended with the reprocessed
data from stacked data to form a single,
relatively seamless composite line.
Global positioning satellites were not
available when the survey was completed,
so Gresko says the precise positioning of
the lines was not known. "We do know
water depths, so we used that as our
quality control," he reports.
Both bathymetry lines and modern
data help in verifying or correcting the
locational data. Student notes also contribute, but must be considered skeptically,
he adds.
Gresko says the reprocessed data provide a deeper view of the subsurface,
bringing out more detail in the Wilcox,
for example. He says a key question has
been how much the Mexican river system
contributed to Wilcox sediments. A recent
paper addresses this question using Yucatan study data showing significant thickness into Mexican waters.
"The Yucatan survey also can help
determine what the Perdido Fold Belt
looks like as it extends south," Gresko
ventures, pointing out that 25 percent of
the Yucatan survey lines address that
area. He says reprocessing should improve
resolution and structural accuracy.
Carbonate plays in the southern part
of Mexican waters also are covered by
the survey. Gresko says reprocessed data
should shed light on the extent of these
carbonates.

Integrating Geophysical Data
One of the immutable "laws" of geophysical interpretation over the past 50
years is that over time, there are always
more data to be evaluated in less time,
posits Geoffrey Dorn, senior adviser for
seismic interpretation technology at CGG
GeoSoftware. As a result, he says, the
"user experience" has become a more
critical element of interpretation software,
and directly impacts successful interpretation. "A poor user experience in the
design of the interpretation system may
limit what interpreters can do and how
fast they can do it," he remarks.
"Interpreters can be much more effective when the software application requires fewer windows, parameter settings
and button clicks, and the design allows
them to keep their focus on the data and
the geology," Dorn goes on. "Data relationship and errors are far more apparent,
and changes are made faster when multiple
datasets can be visualized and edited in a
single 3-D space."
He says that enabling this integrated
workspace for increasingly diverse data
is an ongoing challenge for software developers.
He suggests that another key to efficiency is how users interact with data as
they visualize them. He says interfaces
should be as simple as possible, providing
intuitive options for interaction and interpretation. Video gaming has changed
expectations significantly in this area,
Dorn points out, and has introduced new
technologies that can be leveraged for
faster interpretations.
He says improvements in the user experience help free interpreters to focus
on providing greater insights into geology,
identifying and delineating prospects, and
optimizing well paths. "With an integrated
approach to geophysical and borehole
data, we can understand the combined
MAY 2015 81



American Oil and Gas Reporter - May 2015

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Contents
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American Oil and Gas Reporter - May 2015 - Contents
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