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
The correct depth model results in better volumetric estimates
Time vs. Depth Migration
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
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
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, time
consuming and 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 similar geometry as 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
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.
Seismic for Cross-Section Balancing, Reservoir Modeling
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 were 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
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