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GCSeismic 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 example
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Time vs. Depth MigrationOne of the first lessons geophysicists learn about seismic data interpretation is that the seismic image is not located where it appears. It gets "migrated" to compensate for reflections not emanating from directly beneath the surface recording point, or zero offset trace location. Traditional time migration methods using smoothed stacking velocities are considered good when diffractions are collapsed to a point and the image appears focused -- but this may not correctly position the images in depth. Time migration appropriately locates most events for simple cases where there is not a significant lateral velocity contrast across layers or steep dip in the overlying velocity boundaries. Generally an interpretation is done using time-migrated data that is converted to depth by vertically stretching the observed travel times. Known depths from well ties are used to adjust the final map to fit the structure depths. Depth converting by vertically stretching the interpretation in Figure 1 would result in the same structural shape, with each layer scaled in depth based on the velocities used for the migration. 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 seismic image below high contrast boundaries. 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 seismic events. The time-migrated data can be depth-migrated by image ray migrating the interpreted interfaces. Figure 2 illustrates a depth-migrated interpretation of the same model shown in Figure 1, accounting for the refraction and ray bending at the interfaces. The model exhibits a compaction velocity in the shallowest layer and constant, highly contrasted velocities in the two deeper layers. 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 interpretation challenges. This seismic
Image ray depth migrating the interpretation results in the image produced in Figure 4, where the depth-migrated result is based on the interpreted velocity field. Deeper events that appear chaotic in this figure indicate areas where the interpreted events are not resolved by the velocity model.
The
time-migrated interpretation and velocity model can be iteratively
modified until the resulting depth-migrated model is geologically
reasonable. Iterating the model interactively -- so one can see the
changes -- allows the interpreter to gain insight into the raypaths that
produced the images on the time-migrated seismic
Seismic for
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