Geology Reference
In-Depth Information
various interpolation schemes, removing multiples
without distorting relative amplitude variation across
the offset range. Well synthetics, generated with and
without inclusion of multiples, are a useful tool to
check for the presence of residual multiples or for
damage to primary reflections by over-aggressive
application of demultiple techniques. In order to
model seabed multiples a water layer would need to
be inserted into the model.
as the individual interface reflection coefficients
change with angle, and individual peaks and troughs
will not necessarily line up exactly horizontally. Class
IIp AVO effects, with a polarity flip at some inter-
mediate offset, will similarly not give rise to a neat
alignment across the offset range. In these cases, the
apparent residual moveout should be the same as that
seen in a well synthetic. In practice, the synthetic gives
a general idea of whether apparent residual moveout
is an issue, but does not give much assistance in
calculating a detailed residual moveout correction
(
Fig. 6.5
).
6.2.4 Migration
In all but the simplest subsurface geometry (i.e. hori-
zontal reflectors at all levels), pre-stack migration is
needed for detailed AVO work. Without migration,
the traces of a common midpoint (CMP) gather will
contain data reflected from different subsurface loca-
tions and these will not in general be positioned
directly below the common midpoint. The purpose
of migration is to process the signal collected from
many surface locations so that the gathers contain
traces with reflection points vertically below the
source
6.2.5.2 Higher order moveout corrections
In some cases, trying to flatten reflections across the
gather using a hyperbolic equation may distort the
moveout correction, particularly at mid and far
offsets. In such cases the first refinement to be con-
sidered is a more accurate allowance for vertical vel-
ocity heterogeneity by including higher order terms
in the moveout equation. The simple hyperbolic
moveout approximation is
receiver midpoint, focusing energy and
increasing lateral resolution. There are many different
algorithms in use for pre-stack migration, and the
choice of method depends on the complexity of the
structure of both target and overburden. Jones (
2010
)
provides a useful review and discussion. For AVO, an
issue is the amplitude fidelity in the migrated gather.
This is partly determined by the type of algorithm
used, but also by its detailed implementation; where
possible, this needs to be discussed with the seismic
processor in the context of the intended use of the
gathers.
-
T
0
+
X
2
T
X, n
¼
,
ð
6
1
Þ
:
V
rms
where T
0
is the travel time at zero offset and V
rms
is
the RMS average velocity between the surface and the
base of the nth layer. This is derived from a horizon-
tally layered earth model, in which the travel time for
a reflection from the base of the nth layer at a source
-
receiver offset x is given by (Taner and Koehler,
1969
):
T
X
,
n
¼
c
1
+c
2
X
2
+c
3
X
4
+c
4
X
6
+
,
ð
6
:
2
Þ
6.2.5 Moveout correction
Given the importance for AVO analysis of time
registration of reflectors across the gather, accurate
moveout correction is necessary. For an isolated
interface, this means that the reflector should be flat
across the gather enabling, for example, the calcula-
tion of a reasonably accurate AVO gradient at the
same TWT on every trace.
where the coefficients c depend on the layer thick-
nesses and the velocities in each layer.
Thus, a more accurate approximation is obtained
in most cases by retaining the term in X
4
.
6.2.5.3 Anisotropy
Another complication for gather flattening is
anisotropy. When fine scale layering (i.e. transverse
isotropy, see
Chapter 5
) becomes important it is not
possible to flatten gathers beyond an incidence angle
of about 30° with standard moveout corrections.
Alkhalifa (
1997
) addressed the effect of transverse
isotropy (VTI) on the seismic travel time. For a hori-
zontal interface in a homogeneous VTI medium then
the travel time can be described by:
6.2.5.1 Problems in residual moveout identification
Whilst flat gathers are generally a requirement for
AVO, evaluating the
of gathers may not
always be straightforward. For example, if there are
multiple interfaces in a stack of thin beds, then inter-
ference effects (tuning) will change across the gather
'
flatness
'
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