Geology Reference
In-Depth Information
a)
b)
160
0.2
80
0.1
0
0
-80
-0.1
-160
-0.2
-0.2
-0.1
0
0.1
0.2
-160
-80
0
80
160
Intercept
Intercept
c)
0.2
0.1
0
-0.1
-0.2
-0.2
-0.1
0
0.1
0.2
Intercept
Figure 6.14
AVO calibration at a well; (a) AVO crossplot generated from well data (note red dot represents top reservoir), (b) seismic
AVO crossplot determined from the same time segment as the well crossplot, (c) seismic AVO crossplot calibrated by scaling the reflectivities
at top reservoir and re-calculating intercept and gradient. Note how the seismic background trend does not change significantly with
calibration as it is contaminated by noise.
scaling of the seismic has the same relative change as
the synthetic (i.e. the seismic scaling is
It is paramount in offset scaling that the effects of
noise should be avoided.
Figure 6.14
illustrates the
problem in the context of the AVO crossplot, where
the effects of noise are most apparent. The example is
fairly extreme as it is based on an unconsolidated
sand target and data with fairly poor signal-to-noise
ratio. A crossplot derived from the well synthetic
(
Fig. 6.14a
) shows a background trend at an angle of
χ ¼
'
'
) then
the amplitude of the extracted wavelets on nears, mids
and fars should essentially be the same, although the
frequency content and phase may change across the
offset range. Corrections can be derived simply from
the relative scaling of the extracted wavelets. Clearly,
if there are big scaling differences from one well to
another then further analysis is needed. If well ties are
poor then comparison can be made using RMS ampli-
tudes in a window which contains several strong
reflectors to avoid the effects of noise or bias from a
single dominant reflection.
correct
53°. Compared to the well crossplot (
Fig. 6.4a
),
the seismic crossplot (
Fig. 6.14b
) has a much lower
angle of trend (around 11°). It might be supposed that
the AVO calibration step could involve a rotation of
this trend to fit the well data. Unfortunately, this is not
122
