Geology Reference
In-Depth Information
Fig. 6.8.
Effect of a 3 : 1 vertical exag-
geration on thickness. a Pro-
file vertically exaggerated 3 : 1.
Bed thickness increases as the
dip decreases. b Unexagger-
ated profile. Bed thickness is
constant
or thickening or by structural growth during deposition. The profile can be easily
corrected when the true horizontal and vertical scales are known. The correction factor
is the inverse of the horizontal or vertical exaggeration. Create an unexaggerated profile
by multiplying the correction factor times the scale of the exaggerated axis. If the
cross section is in digital form, this is a simple operation using a computer drafting
program.
Seismic time sections are commonly displayed with both horizontal and vertical
exaggerations (Stone 1991). Horizontal exaggeration may be applied to obtain a leg-
ible horizontal trace spacing. The amount of horizontal exaggeration is most conve-
niently determined by comparing the distance between shot or vibration points marked
on the profile with the scale between the corresponding points on the location map.
The vertical scale on a time section is in two-way-travel time, and the determination
of the vertical exaggeration requires depth conversion as well as scaling. A few simple
techniques can provide the necessary scaling information without geophysical depth
migration. If the depth to a particular horizon is known independently, as from a
well, then the vertical exaggeration at that well can be determined directly from the
definition (Eq. 6.1). If the true dip is known for a unit or a fault, then the vertical
exaggeration can be found by solving Eq. 6.3, given the exaggerated dip from the profile.
If there is a unit on a seismic time section that can be expected to have constant
depositional thickness and minimal structural thickness changes, then any observed
thickness change in the unit is caused by the exaggeration (Fig. 6.9a,b). The vertical
exaggeration of the time section can be removed by restoring the bed thickness to
constant (Stone 1991). A package of reflectors should be chosen that retains its reflec-
tion character and proportional spacing regardless of dip. The reflectors should be
parallel to one another and not terminate up or down dip. The thickness changes of
such a package are more likely to be caused by exaggeration than by deposition. A
simple procedure for removing the vertical exaggeration is to change the vertical scale
until the unit maintains constant thickness regardless of dip (Fig. 6.9c). This pro-
vides a quick depth migration that applies to the depth interval over which the unit
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