Operators are feeling the pressure to acquire
and understand new data to gain insight into the deep structures
they're targeting in the Gulf of Mexico.
The western Gulf with its geologically-complex subsurface
is a prime example of the obstacles they must surmount. It's an
area of sutured salt canopies, hidden salt roots and evacuated/welded
basins — enough geological intrigue to challenge most any geoscience
But there's documented progress being made.
For instance, a project undertaken by a team of geoscientists
in Houston has made significant strides in unraveling some of the
mystery of the area using long offset seismic and marine gravity
data. The data combo enabled them to create a 3-D earth model of
the area to use in assessing 3-D shapes — especially salt.
Details of the project, which is already drawing
attention among operators, will be presented in a paper, "From 2-D
to 3-D: Modeling the Western Gulf Using Recently Acquired Long-Offset
Seismic and Gravity Data," at the Gulf Coast Association of Geological
Societies' convention in San Antonio in October.
The project began as a 2-D
modeling endeavor along a recently acquired long offset seismic
line in the Western Gulf, according to Marianne Parsons, senior
geophysicist at Fugro Robertson. Gravity data were acquired concurrently
with the seismic data, with the idea that gravity modeling can aid
seismic interpretation and pre-stack depth migration (PSDM) efforts.
According to Parsons, the gravity modeling was used
to test and constrain the shapes and extents of the salt bodies
— and, therefore, better resolve the velocity model prior to the
"As more seismic lines in the program were completed,
it was possible to create a grid of the bathymetry surface, the
top salt surface and parts of the deeper sub-surface," Parsons said.
"With these surfaces, a three-dimensional
earth model was built," she continued, "and input from wells
and seismic velocities was used to establish the sediment properties.
Three-dimensional gravity modeling of the area occurred simultaneously
to when the seismic interpreter was defining salt body extents through
various steps of the seismic processing."
This is the first time in the Gulf of Mexico that
a commercial seismic program has attached to it a depth migration
constrained by gravity data, according to Brian Anderson, Fugro's
vice president of marketing, "meaning you can use the gravity to
model the salt and create a density model tied to the seismic velocities.
"That's part of a feedback loop between the gravity
and the seismic," he said, "such that we can help determine what's
salt and what's a multiple on the seismic data. Based on the gravity,
we can help to determine in areas where you have no illumination
from the seismic, what's the most likely shape you're trying to
"This is an approach that's been done a lot in terms
of prospect level stuff," he added, "but as part of a regional program,
it's never been done."
Anderson summed up the advantages of integrating
the gravity throughout, from seismic processing for the de-multiple
of seismic right through the PSDM:
- Better velocity model for the seismic.
- Better de-multiple for the seismic.
- Provides regional starting point for
gravity modeling — for a company to do a prospect 3-D gravity
model of the salt for a well in the deepwater Gulf it would only
be necessary to add whatever proprietary well data there are,
along with the proprietary seismic interpretation, and turn the
Two of the significant findings from the modeling
exercise the geophysicists engaged in are:
- Higher degree of confidence in the shape
of the salt and a refinement in sub-salt seismic imaging as a
- Parameters of the data being recorded
are such that coherent reflectors in the 7-10 second range can
"The modeling is a story in and of itself, and the
model they did was a real arm wrestling match," Anderson said. "It's
a real issue to a lot of people, especially in southern Alaminos
Canyon, where there's a well known major salt/gravity feature that's
the biggest gravity anomaly in the Gulf of Mexico.
"It's either a salt wall that runs down to the Moho
darn near," he said, "or some density anomaly there that's not yet
fully explained. One of the ways we kind of explain it is, in the
model you see there's a very thick autochthonous salt layer down
deep that had to be added to make up for all the gravity anomaly.
"People try to add salt shallow, add salt deep,"
Anderson said. "If you don't do that, you can still gain a seismic
image, but it's difficult.
"But the gravity is not lying, and there's got to
be something extremely low density and big there — maybe it's salt,
maybe it's gas. Who knows?
"The model crosses it and defines one of several