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GC
Seismic
Depth
Interpretation
in Thrustbelts*
By
Nancy House1
Search and Discovery Article #40131 (2004)
*Adapted from the Geophysical Corner column in AAPG Explorer, May, 2004, entitled “Depth Reckoning speaks Volumes” and prepared by the author. Appreciation is expressed to the author, to Alistar R. Brown, editor of Geophysical Corner, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1Geophysicist, EnCana Oil & Gas USA, Denver CO ([email protected])
A
big challenge for modern seismic
is the ability to image complicated structures.
Fold and thrustbelts are characterized by rapid velocity variations due to
juxtaposed rock types.
Generally, if you
can see a structural image on seismic
, the next step is to determine where that
structure is actually located in depth. Once the
interpretation
is correctly
depth positioned, cross-section balancing can be used to help create a
geologically viable three-dimensional model. The correct depth model results in
better volumetric estimates of reserves.
uGeneral statementuFigure captionsuMigrationuThrustbelt exampleuCross-section balancing, reservoir modelinguConclusions
uGeneral statementuFigure captionsuMigrationuThrustbelt exampleuCross-section balancing, reservoir modelinguConclusions
uGeneral statementuFigure captionsuMigrationuThrustbelt exampleuCross-section balancing, reservoir modelinguConclusions
uGeneral statementuFigure captionsuMigrationuThrustbelt exampleuCross-section balancing, reservoir modelinguConclusions |
Time vs. Depth Migration
One of the
first lessons geophysicists learn about
Generally
an
For cases
where beds are dipping, the energy is refracted at high contrast
interfaces, similar to the effect on the image of a straight pole
inserted at an angle into a smooth pool of water; the pole appears bent
at the air-water interface. In severe cases there may be no Both "pre" and "post" stack depth migration were developed to address ray bending in areas of high velocity contrasts and dipping interfaces. However, pre-stack depth migration is expensive and time-consuming, and it requires a detailed prior understanding of the velocity depth model to achieve a solution.
Because
time and money are always limited, where there is an adequate image to
start with, a simplified depth migration technique can be used. Image
rays are the theoretical ray paths taken by time-migrated
Figure 2
illustrates a depth-migrated The time migration (Figure 1) adequately corrects for the shallowest interface, but it incorrectly positions the deeper events. The depth-migrated model (Figure 2) correctly positions the steepened flanks of the anticline with the horizontal position also changed along the dipping flanks compared to the inaccurate time-migrated structure. Thrustbelt Example
An example
from South America (Figure 3) is used to
illustrate typical thrustbelt
Image ray
depth migrating the
The
time-migrated
|