Geology Reference
In-Depth Information
AI Syn
Extracted
wavelet
-100
-50
0
50
150
Amplitude
Figure 4.21
A well tie where the synthetic apparently needs to be
stretched (seismic timing lines have 100 ms separation).
for there to be residual timing differences between
synthetic and seismic related to the way that arrival
times are picked. If the onset of the wavelet (the first
break) is picked, then the effect of absorption is to
make the travel time less than it would be if picking is
based on the first peak of the waveform (
Fig. 4.23
).
This effect is usually not large, amounting to only a
few milliseconds.
What certainly has to be avoided is a loop-by-loop
stretch and squeeze, which can be made to produce an
excellent but spurious tie. At this level of detail, there
are many reasons why the tie will not be perfect,
including issues with both well log and seismic data.
If available, a VSP may help to identify the underlying
problems, but in many cases the exact cause of a
mistie will remain unknown; its presence is a warning
against trying to over-interpret the data.
Figure 4.22
Stretch or residual migration effect? This is
Fig. 4.21
with the synthetic rotated to achieve the tie (seismic timing lines
have 100 ms separation).
wavelet extracted is peak-dominated and the visual tie
is shown. The character match is quite good, though
relative amplitudes of events are not well matched.
However, other wells in the area are characterised by
trough-dominated wavelets. The synthetic tie with the
trough-dominated wavelet (extracted from other wells
in the area and consistent with polarity of major
sequence boundaries) generally looks better in terms
of matching peak and trough amplitudes (
Fig. 4.25
),
except for the highlighted high-amplitude (red)
trough at a major unconformity. It is probable that
this has biased the wavelet estimation in
Fig. 4.24
. The
reasons for this are not clear but it may be that a
short-period interbed multiple has modified the
appearance of the reflection at the unconformity.
4.6.3 Sense checking and phase perception
The interpreter should not necessarily believe all the
results of well matching or indeed any other well tie
approach. Geological sense gained from the interpret-
ation should be brought to bear on the choices made
in well ties.
Figure 4.24
shows an example where the
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