Geoscience Reference
In-Depth Information
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Fig. 1.2
Sketch of rays reflected from a point scatterer and resulting time section.
The snag with such a procedure is that it repositions data only within the seismic
section. If data were acquired along a seismic line in the dip direction, this should
work fairly well; if, however, we acquire data along a line in the strike direction, it
will not give correct results. If we have a 2.5-D structure, i.e. a 3-D structure in which
the dip section is the same at all points along the structure, then on the strike section
all reflectors will be horizontal, and the migration process will not reposition them at
all. After migration, dip and strike sections will therefore not tie at their intersection
(
fig. 1.3(a)
)
. This makes interpretation of a close grid of 2-D lines over a complex
structure very difficult to carry out, especially since in the real world the local dip and
strike directions will change across the structure.
In general, some of the reflections on any seismic line will come from subsurface
points that do not lie directly below the line, and migrating reflections as though they
do belong in the vertical plane below the line will give misleading results. For example,
fig. 1.3(b)
shows a sketch map of a seismic line shot obliquely across a slope. The
reflection points are located offline by an amount that varies with the local dip, but is
typically 250 m. If we see some feature on this line that is important to precise placing
of an exploration well (for example a small fault or an amplitude anomaly), we have to
bear in mind that the feature is in reality some 250 m away from the seismic line that
shows it. Of course, in such a simple case it would be fairly easy to allow for these
shifts by interpreting a grid of 2-D lines. If, however, the structure is complex, perhaps
with many small fault blocks each with a different dip on the target level, it becomes
almost impossible to map the structure from such a grid.
Migration of a 3-D survey, on the other hand, gathers together energy in 3-D;
Kirchhoff summation is across the surface of a hyperboloid rather than along a hy-
perbola
(fig. 1.4)
. Migration of a trace in a 3-D survey gathers together all the reflected
energy that belongs to it, from all other traces recorded over the whole (
x
,
y
) plane. This
