Geology Reference
In-Depth Information
and throughout the data analysis, well ties (
Chapter 4
)
are very important. The steps taken to condition the
data prior to AVO analysis should be checked against
the well synthetics to make sure that they really are
improving the data. It should be recognised that con-
ditioning is not a mechanical process. Careful thought
as to underlying data quality is required to assess the
likelihood of genuine improvement. In some cases no
amount of conditioning will establish the correct gra-
dient. A useful discussion of data conditioning prior
to pre-stack impedance inversion, which is also rele-
vant to AVO analysis, has been published by Single-
ton (
2009
).
between 3D surface streamer and ocean bottom cable
(OBC) data where it is clear that the data quality of
the OBC data is superior. Often, however, data effects
are subtle and require pre-stack data displays for their
evaluation.
Figure 6.2
shows an example of residual
moveout that can only effectively be appreciated in
the pre-stack domain. The presence of this type of
residual moveout can adversely impact the AVO gra-
dient estimation.
6.2.1 Initial amplitude corrections
6.2.1.1 Divergence correction
One of the first processes applied to seismic data,
which is fundamental to the general behaviour of
amplitude with offset, is a scaling step to correct for
the
6.2 General processing issues
The ideal dataset for AVO analysis would have the
following characteristics:
'
'
divergence phenomenon, described in
Chapter 2
. The effect is quite large, and is corrected
for using a simple function of two-way-time (TWT)
and stacking velocity. This correction is based on an
approximation and is unlikely to be entirely accurate,
particularly if there is anisotropy in the overburden.
Anisotropy can cause significant modification of geo-
metric spreading with angle (Stovas and Ursin,
2009
)
and inadequate correction will distort the amplitude
variation with offset. When using AVO for quantita-
tive interpretation this effect has to be corrected prior
to AVO analysis by applying some form of offset-
related scaling (see
Section 6.3.3
).
spherical
zero phase data
clear continuity of reflections across the gather
adequate offset/angle sampling
good multiple and noise attenuation
flat gathers (correct moveout velocities and
algorithm)
correct relative amplitude across the gather
consistent frequency content across the gather
(no moveout stretch)
consistent scaling of each trace (high source and
receiver repeatability)
no critical or post-critical energy
Incidence angle (degrees)
correct pre-stack imaging
6 6 6 6
46
high bandwidth.
3800
Sometimes the evaluation of data quality is straight-
forward simply from inspection of stacked data. For
example,
Fig. 6.1
illustrates a full stack comparison
3900
a)
b)
3D Marine streamer
3D OBS
4000
Residual
NMO
4100
4200
Far
Figure 6.2
Angle gather showing residual moveout (after
Contreras et al.,
2007
).
Near
Mid
Ultra Far
112
Figure 6.1
Comparison of marine surface towed streamer data
with ocean bottom seismic (OBS) data (after Thompson et al.,
2007
).
