Geoscience Reference
In-Depth Information
computationally straightforward enough for many workstations to have the ability
to create instantaneous amplitude and phase displays for any section in not much
more than the time taken to display the data. Essentially, the instantaneous phase
display looks like a seismic display with a very short gate AGC applied; amplitude
information is suppressed, and peaks can be followed across the section with a
constant phase of 0
◦
, troughs with a constant phase of 180
◦
, and zero-crossings
with a phase of
90
◦
. When displayed with a suitable colour-bar (i.e. one that starts
and ends with the same colour so that
±
180
◦
), then
the instantaneous phase section makes it easier for the interpreter to spot angular
relationships (onlap, downlap, etc.) in low-amplitude parts of the seismic section.
phase section shows angularities more clearly than the standard reflectivity data.
(2) Horizontal sections. Time slices can reveal map-view geometry, such as channel
systems. However, if there is structural dip present, the horizontal slice does not
show data referring to a single stratigraphic level.
(3) Horizon slices. By slicing through the data parallel to a particular event it is possible
to see a map of amplitude changes at a single stratigraphic level. This is often the
best way to see channel and fan features, which are recognised by their geometry
in map view. The reference horizon is usually chosen to be the strongest and most
continuous marker within the sequence, as this can be autotracked most easily. This
is a good way to look at the internal geometry of thin layers at or below the limit of
seismic resolution; all the information is encoded in the lateral amplitude variation
of the reflector. In such a case, it may be worthwhile to invert the data by the methods
slightly more information out of horizon slices through the inverted volume. With
thicker layers, it can be more informative to look at amplitudes (e.g. rms average to
avoid mutual cancellation of positive and negative values) within a window whose
thickness is chosen so as to enclose the layer of interest; reconnaissance of the
feature using vertical sections will show what window size to use. A refinement of
this idea is to use 'stratal slices' (Zeng
et al.
,
2001)
. In this method, displays are
produced of a seismic attribute (e.g. amplitude) on a geological time surface. This
surface is created by linear interpolation between picked surfaces that are believed
to be time-parallel reference events, e.g. marine flooding surfaces; as we saw earlier
in the chapter, such sequence boundary reflections are often strong, easily picked
and laterally continuous.
(4) Coherence slices. Use of horizontal slices through the coherence cube to map
map-view information about internal structure of a layer, in exactly the same way
as for reflectivity. Subtle internal discontinuities can be revealed. To understand
features seen in map view, they may need to be compared with their expression
on vertical sections; standard reflectivity sections should be used for this purpose
180
◦
has the same colour as
+
−
