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One 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.
An example
from South America (Figure 3) is used to
illustrate typical thrustbelt interpretation challenges. This seismic
cross-section has a geometry similar to the models with a younger
formation above the main detachment fault. It has a strong compaction
gradient in the velocity field combined with steeply dipping beds. This
geometry causes the apparent location of the points below this interface
to be affected by the gradual bending of the rays through the velocity
gradient and refraction at the interfaces.
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 section.
Balancing
geologic cross-sections is an important geologic tool for working in
thrustbelts. By using a grid of 2-D seismic profiles in which each
profile is image ray depth-migrated prior to cross-section balancing,
the interpreter can produce a 3-D structurally balanced interpretation
based on 2-D seismic. This in turn produces less error in drilling
prognosis and tying wells in structurally complex areas -- and it also
improves the ultimate volume calculations of trapped hydrocarbons.
In the
complex overthrust model example here, the output of the image ray
depth-migrated interpretations was used as input to a balanced geologic
cross-section. The resulting depth-migrated interpretation required
little or no correction of the basic shape of the formations or the
faults to produce a geologically feasible balanced cross-section (Figure
5).
With a
more accurate depth representation of the structural geometry of a
reservoir, the resulting volume calculations are more accurate. This is
commonly the largest variable in the reserve calculations.
Three-D
visualization, attribute analysis and interpretation with accurate well
ties, and reservoir model building for simulation are significantly
improved by creating more accurate depth representation of surfaces and
faults (Figure 6).
Today's
seismic processing produces not only zero offset data (un-migrated) and
time-migrated data sets, but with the increase in computer capabilities,
depth-migrated volumes are becoming readily available to the
interpreter.
In complex
areas, accurate well ties are important to help define a proper velocity
field for creating a depth-migrated image. In these cases, it is also
important to understand the raypaths and to use the best estimate of
travel time velocity fields before proceeding with well design, depth
prognosis, and volumetric estimates of the reserves.
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