Geoscience Reference
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well control, this process is straightforward; where there are few wells, there is obvi-
ously a danger of imposing an incorrect low-frequency trend on the final result. This is
particularly true if the wells are drilled in updip locations and the target for the inversion
is downdip, with a greater depth of burial causing additional compaction and perhaps
containing additional stratigraphic intervals not sampled at all by the well control. It may
be possible to allow for compaction effects by putting in trends of increasing impedance
with depth, but thick intervals with no well control are bound to cause problems. A
last resort if there is no well control is to create the low-frequency model from seismic
velocities (stacking or migration). The snag with this, apart from the general issues of
the unreliability of seismic velocities as a measure of real rock properties discussed in
chapter 3 , is that density has to be inferred from velocity in order to calculate impedance;
to do this, it is necessary to know the lithology, which may be just the point at issue
in an undrilled section.
A section resulting from this process is shown in fig. 6.5 . It is created from the
same reflectivity data as were used to create fig. 6.2 . The effect of the low-frequency
information is apparent in the progression from low impedance (purple) at the top of the
section to high impedance (yellow/white) at the base. The high-impedance precursor
(green/red) to the target horizon (i.e. above the yellow marker) visible in fig. 6.2 is not
a feature of fig. 6.5 ; probably it was an artefact produced by minor departure of the
wavelet from the zero-phase assumed in the SAIL processing. However, the definition
of the pay sand is not much different from that in fig. 6.2 ; in this particular case the
simple SAIL approach is adequate, partly because the reflectivity data are close to
zero-phase and partly because the target layer is the right thickness to have a response
within the seismic bandwidth.
Fig. 6.5
Full-bandwidth inversion over same section as fig. 6.2 .
 
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