Geology Reference
In-Depth Information
Tie at Well A
Wavelet 10-50Hz
Reservoir
a)
Section through input volume (bandwidth 10-50Hz)
Tie at Well A
Wavelet 10-75Hz
Reservoir
b)
After FSSOF and whitening (bandwidth 10-75Hz)
Figure 3.14.
Seismic section (a) before and (b) after processing with frequency split structurally oriented filtering and trace whitening
(after Helmore
et al
3.7 Resolution
A key issue in the interpretation of bandlimited seis-
mic data is the fact that there is a lower limit (imposed
by bandwidth and wavelet shape) to the bed thickness
that can be uniquely resolved. This limit defines the
boundary between
shales. With a positive standard polarity zero phase
wavelet, a peak would be expected at the top of the
sand and a trough at the base and in a general way this
is what is seen. In detail, however, the acoustic imped-
ance curve shows variability within the sand. The
effect of this fine layering is to modify the seismic
response. Owing to the fact that the seismic wavelet is
longer than the spacing between impedance contrasts,
the character of the top and base sand reflections is
dependent not only on the contrast at the reflecting
interface but also on the layering close to the bound-
ary. The amount of interference is controlled by the
length of the seismic pulse and the spacing in two way
time of the impedance contrasts (a function of the
interval velocity).
'
thick
'
beds (above the resolution
limit) and
beds (below the resolution limit). The
uncertainty in the interpretation of
'
thin
'
beds that
results from resolution limits is a major issue for
quantitative interpretation and this discussion will
be developed in
Chapters 5
,
9
and
10
. Although verti-
cal resolution is a key concept for the interpreter to
consider, in the context of 3D amplitude and other
seismic attribute maps the idea of resolution also
tends to incorporate the notion of detectability.
'
thin
'
3.7.2 Simple models of interference
A simple way of looking at wavelet interference is via
the
3.7.1 The problem of interference
The modelled seismic trace in
Fig. 3.15
illustrates the
fact that a seismic section is a complex interaction of
the wavelet and reflecting surfaces.
Figure 3.15
shows
a high-impedance sand encased in low-impedance
thought of as a wedge of sand encased in shale (the
sand may be higher or lower impedance than the
'
wedge
'
32
