Geology Reference
In-Depth Information
Lithology
Im
pedan
ce
Rc
Individual reflections
Synthetic
seismogram
Figure 2.7
Synthetic seismogram using
a causal (i.e. recorded) wavelet with SEG
standard polarity (re-drawn and modified
1234
56
-+
1
3
4
5
6
Causal
(recorded)
wavelet
Symmetrical
(processed)
wavelet
GEOLOGICAL
INTERFACE MODEL
SEG STANDARD
POLARITY
POSITIVE
STANDARD
POLARITY
NEGATIVE
STANDARD
POLARITY
Impedance
(Sheriff & Geldart,1982)
(Thigpen et al,1975)
soft
- +
0
hard
trough
peak
American
Polarity
European
Polarity
(Brown, 2001)
Figure 2.8
Seismic polarity conventions.
starting with a trough and negative reflections (such
as the top of the gas sand) starting with a peak.
One problem with causal wavelets (amongst
others) is that there is a time lag between the position
of the boundary and the energy associated with a
reflection from the boundary, making it difficult to
correlate the geology with the seismic. Thus, there is a
requirement for processing the seismic wavelet to a
symmetrical form which concentrates and correctly
aligns the energy with the position of geological
boundaries.
Figure 2.9
shows the same synthetic but
now with a symmetrical wavelet. It is now much
clearer which loops in the seismic the interpreter
needs to pick for the various geological boundaries.
The polarity conventions in common usage that
apply to symmetrical wavelets have been defined by
positive reflection. If a positive reflection is represented
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