Geoscience Reference
In-Depth Information
marks the top of a particular stratigraphic interval. The SAIL section is displayed with a
colour bar such that hard intervals are red and soft intervals are violet-purple; the same
convention will be used for other impedance sections in this chapter. Patches of soft
colours below the yellow marker show the location of soft oil-filled sands. These are ac-
tually channels crossing the plane of section, as would be clear from a map view; note the
small bumps in the yellow horizon above the soft patches, which are formed because the
sandy channel-fill compacts less than the shales on either side as it is buried (
differential
compaction
, which often results in sandy intervals being marked by mounded topog-
raphy). Such a display can be used in a qualitative way, to map out the extent of a
hydrocarbon accumulation. It would be less straightforward to try to use the results to
make a quantitative prediction of, for example, reservoir porosity, because the traces
show only relative impedance. Making a plot of well impedance data, with bandwidth
limited to the seismic spectrum, is a useful aid to understanding how the SAIL results
relate to the real earth. They often turn out to be surprisingly useful for such a sim-
ple technique, because in many cases reservoirs are not thick enough for the missing
low-frequency component to be critical.
Arefinement of this approach is to try to match the spectrum of the SAIL data to
the spectrum of impedance in the real earth. If we compute reflectivity from well log
data, we usually find that the earth reflectivity spectrum is not flat over the seismic
bandwidth, as is commonly assumed in seismic processing; instead, it has more energy
at higher frequencies. By analogy with the optical light spectrum, we can say that
the earth reflectivity is not white (i.e. flat) but blue (i.e. higher amplitude at higher
frequencies). A first approximation is often that the well log reflectivity has a slope
of 3 dB per octave across the seismic spectrum. This can be allowed for by high-cut
filtering the 90
◦
phase-rotated trace at 3 dB per octave instead of 6 dB. A more exact
approach is to determine the actual reflectivity spectrum from a well log (Lancaster &
6.2.2
Extending the bandwidth
To get more information into a seismic section than is actually present in the seismic
traces, extra data have to be obtained from elsewhere; different algorithms differ in
detail, but they all have the following general features.
(1) Begin with a model that describes the subsurface; explicitly or implicitly, this will
contain a number of layers of different acoustic impedance.
(2) Calculate the seismic response from this model, using the wavelet present in the
seismic dataset.
(3) Compare the calculated seismic response with the real data.
(4) Modify the model so as to reduce the misfit between the calculated and real seismic,
perhaps iteratively and perhaps incorporating constraints on the impedance values
that may be assigned to particular layers, on the complexity of the model, and on
