Geoscience Reference
In-Depth Information
better accuracy. Such time shifts are often the time-lapse effect that is most ro-
bust against differences in acquisition and processing between baseline and repeat
surveys.
at top reservoir) may be diagnostic of its fluid fill. Changes in amplitude between
baseline and repeat survey may therefore be diagnostic of changes in fluid fill. As
we shall see, amplitude changes between the surveys may also be caused by several
factors related to data acquisition and processing, but it can still be possible to
extract useful time-lapse information.
(c) Seismic velocities. Subsurface velocities are routinely estimated as a step in stacking
and migrating seismic data, and in principle they will change with change in fluid fill.
However, the accuracy with which stacking velocities can be estimated is limited;
it will often be hard to get a higher accuracy than 1%. In a realistic case, the
effect of a change in fluid fill on stack or migration velocity to a base-reservoir
reflector will usually be less than this. The problem is that the velocity change
is confined to a fairly thin layer, but stacking velocities are an average velocity
from surface to the reflector concerned, and are dominated by the thick unchanged
acquisition azimuth owing to genuine anisotropy or to dip and curvature of the
reflector; it will also change with azimuth and maximum offset due to near-surface
effects. It is therefore sensitive to acquisition differences, particularly when very
high accuracy is wanted. All this makes velocities hard to use as a time-lapse
diagnostic.
If we are intending to make use of differences in TWT and amplitude between
surveys, there are two possible approaches. Where fluid or pressure effects are large,
and fluid movement follows a simple pattern, it may be sufficient to make maps of TWT
and amplitude for a reservoir-related reflector on both surveys, and compare them. If,
for example, hydrocarbons cause obvious bright spots at top reservoir, then the extent
of the bright spots on amplitude maps of the two surveys can be compared visually.
This does not require that the acquisition or processing should be exactly the same
in the two surveys. It should certainly follow good practice for obtaining reasonably
accurate amplitudes, but what we are looking for is a lateral change in amplitude on
each map independently (marking the fluid contact), followed by a comparison of the
positions of these amplitude changes on the maps of the two surveys. If, on the other
hand, we are looking for small effects, then we may have to subtract one trace volume
from the other in order to reveal subtle differences, such as small amplitude changes
within a reservoir interval caused by interfingering of oil and water in the producing
zone. This is much more difficult to do, because there are many possible reasons for
differences between surveys that are related to acquisition and processing differences.
These differences will appear as unwanted noise in the difference volume, and their
elimination requires careful processing.
