Geoscience Reference
In-Depth Information
using a wedge model, a single layer whose thickness is systematically varied from zero
to the desired maximum. A further step toward realism is to construct a model using
all the different layers within some interval, as recognised from wireline well logs.
This is important for relating well data to real seismic response, but the interference
effects between a whole stack of interfaces can be hard to understand unless the main
individual layers are modelled separately first.
For small incidence angles, a further simplification of the Zoeppritz equations is
possible; the term in the above formula involving C can be neglected. This is certainly
the case out to incidence angles of 30 or so, and is often a reasonable approximation out
to 40-45 , beyond which the data are in any case often muted out from gathers because
of NMO stretch and the presence of direct arrivals. Then we can write the reflection
coefficient in terms of the normal incidence reflectivity R 0 and the AVO gradient , G :
R ( θ ) = R 0 + G sin 2
θ.
5.3
Interpreting amplitudes
Sometimes we can interpret fluid fill from amplitudes on seismic data. Before we can
do so, we need to have reasonable confidence in the validity of the amplitudes in the
seismic dataset. As explained in chapter 2 , modern processing will try to avoid any
steps that cause amplitude artefacts. Ideally, we would like to have seismic data where
amplitudes are everywhere proportional to reflectivity. This is not achievable, but what
can be done is to make sure that local lateral variation of amplitude (over a distance of,
say, a kilometre) on a particular group of reflectors is proportional to reflectivity change.
We can often assume that the average absolute reflectivity over a long time window
varies little, so a long-gate AGC can be applied to the data. It is essential, though, that
the gate is long enough (1000 ms or more) to avoid destroying the lateral variations
we are looking for; the gate should include many reflectors, so that the target event
makes very little contribution to the average amplitude in the window. Calibration of
amplitude to reflectivity is possible from a well tie, but the calibration is valid only over
a limited interval vertically. In any case, it is a good idea to inspect the entire section
from surface to the target event and below; if amplitude anomalies at target level are
seen to be correlated with overlying or underlying changes (high or low amplitudes
due to lithology or gas effects, or overburden faulting, for example), then they should
be treated with suspicion. Such a correlation might have a genuine geological cause,
but careful thought is needed to establish that the effect is not an artefact. Following the
amplitude anomaly through the seismic processing sequence from the raw gathers may
be helpful; this may reveal an artefact being introduced in a particular processing step.
To recognise hydrocarbon effects (Direct Hydrocarbon Indicators, DHIs) for what
they are, we need to know what to expect. The sketch in fig. 5.5 shows what to look for
 
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