Interpreting seismic amplitudes - 3-D Seismic Interpretation - page 133

Residual Moveout and AVO

reflection

0.15

y = − 0.0823x + 0 .1159

0.1

0.05

sample

0

y = − 0.4535x + 0.1304

− 0.05

− 0.1

0 0.1 0.2 0.3 0.4 0.5

sin 2 θ

Fig. 5.11 Traces from a CDP gather (offset increasing to the left) and measured amplitudes.

sensitivity can similarly favour the vertical arrival. This is not necessarily a problem
if we are interested in looking for lateral change in AVO response on a particular
reflector, but it will confuse comparison of the measured response with models based
on well data. It may be possible to correct the error by applying a scaling factor if we
understand the cause well enough. Alternatively, we can normalise the amplitude of a
target reflector against that of another reflector (or group of reflectors) of known AVO
response. One way to do this is to compare the seismic trace data with well synthetics
over a range of incidence angles. It is possible to calculate a well synthetic for any
angle of incidence by using the Zoeppritz equations to work out the reflection and
transmission coefficients at every interface, so long as we have a shear sonic log (or can
predict one using the methods of section 5.5.5 ) . One of the benefits of elastic inversion,
described in chapter 6 , is that it forces a careful study of such well ties to be made, to
determine the wavelet amplitude at different offsets. Where there is no well control, it
will be necessary to make some assumption about how amplitudes should on average
behave across the offset range. This will depend on whether we expect to have an even
balance of class I and class III responses, or a majority of one or the other. If we expect
an even balance, then the average amplitude over a series of reflectors in a long gate
can be used to scale the amplitude of the target event. In general, amplitude scaling is
a major source of uncertainty. It is much easier to use AVO qualitatively, as a tool to
look for lateral variation in reflector properties (e.g. to recognise pay zones), than to
use it to make quantitative predictions, e.g. of reservoir porosity.
Noise on the trace data tends to have quite a large effect on the gradient calculation.
A more robust approach to poor data is to use partial stacks. The simplest method is to
divide the traces into two sets, nears and fars, with equal numbers of traces in each. The