Geoscience Reference
In-Depth Information
vertical section through a time-migrated volume of 3D
seismic data where the Prairie Evaporite is in the subsur-
face. The prominent vertical zone where there is a loss of
coherent re ections is caused by the intense disruption of
the stratigraphy associated with a collapse chimney (1).
The low velocities associated with the collapsed area lead
to an apparent increase in depth of the underlying hori-
zons (2), an example of velocity
Position of diffractor
a)
Location ( X )
V 1
V 1
V 2
V 2
Apex
'
'
. The signifi- -
cant lateral change in velocity prevents time migration
from working well in the vicinity of the collapse structure,
which is a major shortcoming since accurately locating its
margins is essential for mine safety. The depth-migrated
data ( Fig. 6.27b ) provide greater clarity of the subsurface
with the extent of the collapse chimney well defined. Fur-
thermore, since the section is now presented in terms of
depth, there is no spurious structure in the stratigraphy
near the collapse. Note that the apparent thicknesses of the
various units have also changed in the conversion from
two-way time to depth, their thickness now being their
'
push-down
V 2 =
V 1
V 2 > V 1
Position of diffractor
b)
Location ( X )
V 1
V 1
Apex
V 2
V 2
Hyperbola from a)
true
'
thickness rather than their
'
time
'
thickness.
V 2 =
V 1
V 2 > V 1
Migration artefacts
Migration of 2D seismic data can only reposition features
within the plane of the seismic section. Arrivals caused by
features in the subsurface outside the vertical plane con-
taining the source and line of detectors comprise an effect
known as sideswipe. For example, when a re ector has a
component of dip across the survey profile, reflection will
occur on the up-dip side. When dips are shallow the effects
are not severe, but steeply dipping reflectors can cause
significant problems.
The width of the diffraction hyperbola in diffraction-
summation migration, i.e. how many traces are involved in
the summation process, is known as the migration aper-
ture. If it is too narrow, the full bene
c)
Location ( X )
V 1
V 1
V 2
V 2
Gap
Reflection from
horizontal interface
Pull-up
Phantom diffraction
hyperbolae
V 2 =
V 1
V 2 > V 1
Figure 6.26 The effects of lateral variations in velocity on zero-offset
seismic data. (a) Diffraction with no lateral velocity variation, (b)
diffraction with an overlying lateral velocity change, and (c)
distortion of a continuous re
it of summing the
hyperbola is not achieved. For example, the steeper the
dip of an event the more displaced is its response in
the unmigrated data, so it is more likely to lie outside the
aperture and not be corrected repositioned. Also, the
edges of the seismic data present a problem, since the full
extent of the hyperbola will not have been recorded. This
causes the quality of the migrated data to deteriorate rap-
idly towards the edges where it may appear that re ectors
have been truncated by a fault.
Seismic data are usually sampled more intensively in
time than spatially, as represented by the wider trace inter-
val along the section compared with the closer time sam-
pling interval of the traces. Sometimes steeply dipping
ector in the presence of an overlying
lateral change in velocity.
of a seismic survey designed to image the Prairie Evaporite
Formation, located in Saskatchewan, Canada (Nemeth
et al., 2002 ). The Prairie Evaporite is a major source of
potash and is mined using longwall methods. Dissolution
of the evaporite horizon causes voids into which the over-
lying strata falls creating salt collapse chimneys. These are
a major hazard to mining, but can be detected using
seismic reflection surveys. The geological environment
comprises sub-horizontal sedimentary units and is, there-
fore, well suited to seismic surveying. Figure 6.27a shows a
 
Search WWH ::




Custom Search