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a)
b)
-400
Cretaceous
shales
0.2
Cretaceous
shales
0
2
2
0.4
Devonian
carbonates
1
1
400
Prairie
Evaporite
Devonian
carbonates
0.6
Prairie
Evaporite
800
0.8
1200
0
200
1.0
Metres
Figure 6.27
Vertical slices through a post-stack migrated 3D seismic volume across a collapse chimney above the Prairie Evaporite Formation.
6.6
Variations in seismic properties in the
geological environment
high-frequency arrivals are not adequately sampled spatially,
presenting another problem for migration. If the response is
aliased (see
Section 2.6.1
) dips are misrepresented in the
data and so are incorrectly repositioned by migration.
If the migration velocities are incorrect, the shape of the
diffraction hyperbola used for the summation will also be
incorrect, and obviously so too will be the result. When the
migration velocity is too small, it may cause in concave-
downward arcs to appear, known as migration frowns, and
the section is under-migrated. If the velocities are too high,
concave-upward smiles result and the section is over-
migrated. Noise in the data may also cause smiles, and
these are commonly seen in the deeper parts of the section
where the signal-to-noise ratio tends to be small. A tell-tale
sign on correct migration is re
ectors cross-cutting each
other to create geologically implausible structures.
The seismic response of the subsurface is controlled by
variations in seismic wave velocity and attenuation, and
changes in acoustic impedance. These are collectively
referred to here as seismic properties. An understanding
of the geological controls on seismic properties is funda-
mental to making geologically realistic interpretations of
seismic data, so we describe the subject here in some detail.
Note that only P-wave seismic properties are considered.
Since there are far more measurements available for dens-
ity and velocity than for attenuation, and because the latter
is rarely used in interpretations, it is discussed brie
y and
separately. Also, although geological controls on density
affect acoustic impedance, they are only alluded to here as
they are described in detail in
Section 3.8
.
Various geological factors affect seismic velocity and
acoustic impedance, and there are numerous laboratory
studies focusing on the sedimentary rocks of Mesozoic
and Cainozoic sedimentary basins. Many laboratory meas-
urements have been made on igneous and metamorphic
rocks under the extreme temperature and pressure condi-
tions expected deep within the Earth
Choice of migration type
In many relatively undeformed soft-rock terrains, post-
stack time migration gives acceptable results, whilst in
more geologically complex terrains DMO processing is
required prior to post-stack migration. Where the geology
is most complex, expensive pre-stack depth migration is
required. A review of recent publications on seismic re
ec-
tion surveying in hard-rock mineral provinces revealed
that the most common processing strategy involved the
use of DMO and post-stack time migration.
s interior. Neither
area of investigation is especially relevant to mineral
exploration. The discussion below is based on a database
of published velocity and density data comprising labora-
tory measurements made under conditions expected at
'
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