Geology Reference
In-Depth Information
Offset
Velocity
D
1
D
2
D
3
D
4
D
5
D
6
x
V
1
V
2
V
3
Fig. 4.17
A set of reflection events in a
CMP gather is corrected for NMO using
a range of velocity values.The stacking
velocity is that which produces peak
cross-power from the stacked events; that
is, the velocity that most successfully
removes the NMO. In the case illustrated,
V
2
represents the stacking velocity. (After
Taner & Koehler 1969.)
t
0
t
0
V
3
Peak power
defines correct
stacking velocity
V
2
V
1
Fig. 4.18
The velocity spectrum is used to
determine the stacking velocity as a function of
reflection time.The cross-power function
(semblance) is calculated over a large number of
narrow time windows down the seismic trace, and
for a range of possible velocities for each time
window.The velocity spectrum is typically
displayed alongside the relevant CMP gather as
shown. Peaks in the contoured semblance values
correspond to appropriate velocities for that travel
time, where a reflection phase occurs in the CMP
gather.
stacked wavelet. A velocity function defining the in-
crease of velocity with depth for that CMP is derived by
picking the location of the peaks on the velocity spec-
trum plot.
Velocity functions are derived at regular intervals
along a CMP profile to provide stacking velocity values
for use in the dynamic correction of each individual
trace.
4.8 Filtering of seismic data
Several digital data processing techniques are available
for the enhancement of seismic sections. In general, the
aim of reflection data processing is to increase further the
SNR and improve the vertical resolution of the individ-
ual seismic traces. As a broad generalization, these dual
objectives have to be pursued independently. The two
