Geoscience Reference
In-Depth Information
x
T
i
m
e
2-D migration
(sum along hyperbola)
x
y
Time
3-D migration
(sum across hyperboloid)
Fig. 1.4
Kirchhoff migration in 2-D and 3-D.
volume of trace data, from which sections can be chosen for display in any orientation we
want. Furthermore, focussing of the data is also improved. For unmigrated data, the lim-
iting horizontal resolution can be taken as the size of the Fresnel zone, an area surround-
ing any point on the reflector from which reflected energy arrives at the receiver more or
less in phase and thus contributing to the signal at that reflection point. The radius
f
of
this zone is given approximately by
=
λ
h
f
2
2
,
where
is the dominant wavelength of the seismic disturbance and
h
is the depth of
can amount to several hundred metres in a typical case. Migration collapses the Fresnel
zones; 2-D migration collapses the zone only along the line direction, but 3-D migration
collapses it in both inline and crossline directions, to a value approaching
λ/
2, which
may amount to a few tens of metres. This will improve the detail that can be seen in the
seismic image, though various factors will still limit the resolution that can be achieved
in practice (see
section 4.1
).
λ
1.3
Data density
When 3-D seismic first became available, it resulted in an immediate increase in the
accuracy of subsurface structure maps. This was partly because of the improved imaging
