Geoscience Reference
In-Depth Information
record not the true formation values, but rather those in the zone around the borehole where the drilling
fluid has
invaded
the formation, replacing the original fluid. Particularly where oil and gas reservoirs
have been drilled with a water-based mud, these invasion effects may be significant and will need to
be corrected before using the well data as a basis for seismic modelling.
Lacustrine
A lacustrine system is one where sediment deposition occurs in a lake. At the present day, only about
1% of the Earth's land surface is covered by lakes, though inland seas such as the Black Sea have
been large lakes at times of lowered sea-level.
Levee
Levees are ridges built on either side of a river channel. They are typically formed from coalesc-
ing crevasse splays, and consist of fine-grained sands and silts. Similar features are seen along the
distributary channels of a delta plain, and along deep-water channels of a submarine fan.
Loop
A seismic loop is a single wiggle of a seismic trace, from one zero-crossing to the next.
Migration
Suppose we make a set of seismic records across an area by keeping the source and receiver together
and moving the combined source-receiver point around on a regular grid. We could then plot the
recorded seismic traces vertically downwards at the proper position on a map of the grid, creating a
3-D volume of seismic traces. This would not give us a correct picture of subsurface reflector geometry,
because the reflection points are not in reality vertically below the source-receiver point. If we traced
rays from a source position, propagating in all directions into the subsurface, we could find the one
that hits a given reflector at right angles. This ray will be reflected back, exactly retracing its path,
until it arrives at the receiver. We could also determine the time that the ray would take along this path.
Now, what we would like to do is to rearrange the traces in our 3-D volume so that the reflected signal
at this travel-time, on the as-recorded trace plotted below the receiver location, is moved laterally and
vertically to the real location in space of the reflection point. Migration is this process of moving the
as-recorded data to the correct location in space. In reality, of course, seismic data are recorded with
a range of source-receiver separations. Ideally, each one should be migrated separately, although it is
common to cut down on the computation effort by
stacking
data before migration, which transforms
them to the travel-time that they would have for zero separation between source and receiver and
then sums them. There are further complications in practice because there may be several rays from
one surface point that hit a given reflecting surface at right angles, perhaps in widely separated
locations.
Migration aperture
In order to get a satisfactory subsurface image from the migration process, it is necessary to have avail-
able a volume of traces surrounding the point at which we want the image. One way of implementing
migration is to use the Kirchhoff approach described in
section 1.2
;
for every point in the output
image, we want to sum data along a hyperboloid surface in the unmigrated data set. The aperture is
the lateral extent of the traces that we take into this summation. Clearly, a very small aperture (a few
traces) will do little to reposition data; on the other hand, a large aperture will include very distant
traces that will have little influence on the result because the signal at long travel-times will be small.
A rule of thumb is that the aperture needs to be twice the lateral distance over which reflections will
be moved; a larger aperture is therefore needed for steeper dips, particularly on deep reflectors. When
planning a 3-D survey, it is important to acquire the data around the edge of the area of interest that
