Geoscience Reference
In-Depth Information
A
B
A
B
Delay generator
Stacking
velocity
Unperturbed velocity
CMP location
Fig. 3.26
Sketch of effect of near-surface static-type delays on stacking velocity (after Al-Chalabi,
1979)
.
across all the data will be steeper than the hyperbolae followed by the inner or outer
trace arrivals separately, corresponding to a lower velocity. When the CMP location is
at B, most of the rays see a double delay, once on the shot side and once on the receiver
side; the outer traces experience only a single delay. The best-fit hyperbola across all the
traces is then flatter (higher-velocity) than would be found for the unperturbed traces. As
shown at the bottom of the figure, the result is an approximately antisymmetric variation
of stacking velocity, with a wavelength equal to the spread length. The effect can be
large, with oscillations up to 15% of the average velocity to the reflector. If the velocity
anomaly is at depth, rather than at the surface, both the width of the stacking velocity
response and its amplitude are reduced. In general, lateral variations in the stacking
velocity field with a wavelength less than the spread length (maximum source to receiver
distance) are not to be trusted; if a sufficiently large number of stacking velocity data
are available, the spurious effects can be largely removed by smoothing the data.
The effect of structure arises in several ways. Dip can have a significant effect on
velocity estimates. Often dip-independent velocities will be available if DMO has been
applied during processing; if not, a correction can be made (Levin,
1971)
. However,
to use the seismic velocities for depth conversion we usually want to calculate interval
velocities in each individual layer. This is done by means of the Dix formula (Dix,
1955)
:
V
b
T
b
−
V
a
T
a
(
T
b
−
T
a
)
1
/
2
V
int
=
,
