Geology Reference
In-Depth Information
Fig. 4.16
Major improvement to a
seismic section resulting from residual
static analysis. (a) Field statics only.
(b) After residual static correction.
(Courtesy Prakla Seismos GmbH.)
computer analysis of moveout in the groups of traces
from a common mid-point (
CMP gathers
). Prior to this
velocity analysis
, static corrections must be applied to
the individual traces to remove the effect of the low-
velocity surface layer and to reduce travel times to a
common height datum.The method is exemplified with
reference to Fig. 4.17 which illustrates a set of statically
corrected traces containing a reflection event with a
zero-offset travel time of
t
0
. Dynamic corrections are cal-
culated for a range of velocity values and the dynamically
corrected traces are stacked. The
stacking velocity V
st
is
defined as that velocity value which produces the maxi-
mum amplitude of the reflection event in the stack of
traces.This clearly represents the condition of successful
removal of NMO. Since the stacking velocity is that
which removes NMO, it is given by the equation
As previously noted, the travel-time curve for re-
flected rays in a multilayered ground is not a hyperbola
(see Fig. 4.3(b)). However, if the maximum offset value
x
is small compared with reflector depth, the stacking
velocity closely approximates the root-mean-square
velocity
V
rms
, though it is obviously also affected by any
reflector dip. Values of
V
st
for different reflectors can
therefore be used in a similar way to derive interval
velocities using the Dix formula (see Section 4.2.2). In
practice, NMO corrections are computed for narrow
time windows down the entire trace, and for a range
of velocities, to produce a
velocity spectrum
(Fig. 4.18).
The suitability of each velocity value is assessed by
calculating a form of multitrace correlation, the
semblance
, between the corrected traces of the CMP
gather. This assesses the power of the stacked reflected
wavelet. The semblance values are contoured, such that
contour peaks occur at times corresponding to reflected
wavelets, and at velocities which produce an optimum
2
2
x
V
2
2
(cf. equation (4.4))
t
=+
st
t
0
