Geology Reference
In-Depth Information
a)
b)
a)
Offset
Offset (ft)
Offset (ft)
3400
440
10100
440
10100
1.4
3500
1.6
3600
1.8
2.0
3700
2.2
3800
2.4
b)
2
2.6
0
Figure 6.15
An example of gathers and supergathers (after Allen
and Peddy,
1993
); (a) processed gather, (b) supergather generated
by stacking offsets across an odd number of cdps. Noise levels are
significantly reduced throughout the traces in the supergather, but
lateral resolution is compromised to some extent.
-2
L
-4
the case as the seismic trend is not simply a function
of geology but a combination of the geology and noise
(e.g. Hendrickson,
1999
; Simm et al.,
2000
). In fact,
most of the crossplot data in
Fig. 6.14
is heavily
contaminated with noise. In such a case the calibra-
tion should be done on the reflectivity associated with
the target sand (shown as the red point on the plots).
Effectively, the calibration is done by deriving differ-
ent scalers for near and far angles, then re-calculating
the intercept and gradient. Given the noise contamin-
ation, the background trend on the calibrated cross-
plot (
Fig. 6.14c
) in this case is no different from the
original seismic.
R
-6
0
0.05
0.1
0.15
0.2
sin
2
θ
Figure 6.16
Comparison of AVO intercept and gradient calculation
by least squares (L) and by robust fit (R) (after Walden,
1991
).
of lateral resolution is traded for improved signal-to-
noise ratio. An example is shown in
Fig. 6.15
.
A practical issue is the amount of fold (or
number of traces) available to perform the super-
gather operation. In 3D datasets the fold varies across
the offset range; typically, fold is low at near and
far offsets, and traces with intermediate offsets dom-
inate the overall fold. Xu and Chopra (
2007
) have
described how using an adaptive approach, in which
traces are borrowed from adjacent CMP locations to
create an even fold distribution with offset, provides
6.3.4 Supergathers
A way of enhancing signal-to-noise ratio is by
forming a supergather. This is done by collecting
traces from adjacent CMPs and stacking traces with
similar offsets or angles. Of course, the lateral stacking
element of supergathers means that a certain amount
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