Geology Reference
In-Depth Information
diameter). Key factors in lateral resolution for 3D
seismic surveys are bandwidth, accuracy of the migra-
tion velocity model, and adequate sampling of steep-
dip information. Owing to the dense sampling of
modern 3D data lateral resolution (in terms of the
Fresnel zone) is not usually an issue.
3D Model
a)
Depth
3.8 Detectability
Although a gas sand may have a thickness below
tuning thickness it will still potentially have ampli-
tudes higher than background. Thus the sand is
detectable without being fully resolved. The limit
of detectability (i.e. the thickness at which the amp-
litude merges with the background) depends essen-
tially on the acoustic impedance contrast and the
signal-to-noise ratio of the seismic. As a general rule
of thumb, for low impedance hydrocarbon sands
with reasonable data quality the limit of detectabil-
ity can be around
Contact
Inline
Crossline
b)
CDP
Signal only
/30 (Sheriff and Geldart,
1995 ; Sheriff, 2006 ) . Whenever net pay is deter-
mined from seismic ( Chapter 10 )thereisalways
the uncertainty that there may be pay in this
λ
/20 to
λ
'
below
detectability
zone. Whether this hydrocarbon is
producible of course depends on the connectivity
of the reservoir and is a question for the reservoir
engineer.
Much of the power of 3D seismic is in detecting
subtle features below seismic resolution. The notion
of detectability is illustrated in Fig. 3.22 , which con-
trasts the interpretability of subtle effects on ampli-
tude maps and vertical sections. A 3D model is
shown of a formation defined by two bounding
surfaces with a hydrocarbon contact ( Fig. 3.22a ).
The 3D synthetic models were generated to simulate
the seismic before and after production and a differ-
ence dataset was generated ( Fig. 3.22b ). It is evident
that the change in the contact can be seen effectively
as a composite of two interfering reflections (see also
Section 10.4 on time lapse seismic). For realism,
noise was added and the corresponding section is
shown in Fig. 3.22c . In the presence of noise the
time lapse signature is not easily interpretable on the
vertical section. Time slices were also generated at
the level of the original contact ( Figs. 3.22d , e )andit
is clear that even in the noisy data the outline of the
contact change is clearly visible. The effective
dynamic range of amplitude maps (and other seis-
mic attribute maps) is much greater than vertical
sections.
'
c)
CDP
S:N=1.5
d)
e)
Line
Line
Signal only
S:N=1.5
Figure 3.22 Detectability in seismic sections and maps; (a) 3Dmodel
of top and base Oseberg reservoir and a hydrocarbon contact, (b)
vertical difference section after pre-production andmonitor models are
subtracted, (c) vertical difference section with realistic levels of noise
added, (d) time slice through model at level of contact, (e) time slice
throughmodel with added noise. Note how the time lapse signature is
evident in the noise prone map (e) but quite indistinct on the noise
prone section (c) (re-drawn after Archer et al., 1993 ) .
37
 
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