COBURN, GARY W., Bell Geospace Inc., Houston, TX
Abstract: Base of Salt Determination
Utilizing Full Tensor Gradient Data
The determination of the base of a salt
body in some areas of the Gulf of Mexico is extremely difficult utilizing
seismic
data
alone. This is due primarily to subsalt imaging problems.
Full Tensor Gradient
data
was combined with the
seismic
data
in an effort
to determine the edges and base of salt in the Green Canyon area. A 2-D
TGS regional line that runs from Green Canyon southeast to Walker Ridge
was used in this study. A section (approximately 30 mi) of this line runs
diagonally across a grid of 3-D gradient
data
acquired by Bell Geospace,
Inc. in 1995. The top of salt was identified and a reasonable
interpretation
made as to the bases and edges of the salt bodies. These horizons were
then imported into a gravity gradient-modeling program. The response of
the
data
measured by Bell Geospace Inc., versus that of the original model
indicated significant differences relating to the size, shape, and thickness
of the salt bodies within the area covered by the gradient
data
. The bases
and edges of the salt bodies in the model were then altered to match the
various gradients and gravity
data
as closely as possible. The gradient
data
indicated that various horizons continue through the
seismic
data
"wipe out" zones under the salt bodies. The salt at one point is less than
2,500 ft thick, creating the potential for subsalt objectives.
A comparison of the original salt model, the gradient
salt model and a model using only conventional gravity data
was made. The
gravity
data
was unable to determine the thickness of the salt as it is
composed primarily of long wavelengths which inhibit its ability to image
shallow mass density changes. The differences between the three models
show that the gradient
data
is much more sensitive to the salt masses than
conventional gravity
data
. A determination of the base of salt can be accurately
made by incorporating Full Tensor Gradient
data
in the
interpretation
process.
AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana