Geology Reference
In-Depth Information
Outside
corridor
Inside
corridor
Outside
corridor
Inside
corridor
Surface
seismic
Inside
corridor
Filtered upgoing wavefield
Interbed multiple
Event
termination
Figure 4.7
Example of a VSP corridor stack (after Campbell et al.,
2005
). The up-going wavefield is shown on the left with shading to indicate
the zones stacked to make the outside corridor stack (blue) and inside corridor stack (red).
stacking the data in a time window immediately
following the first arrival. This should be free
from multiples and is therefore the ideal reference
trace to compare with both the well synthetic and
the actual surface seismic.
constrained to be nearly vertical. An image is pro-
duced of the subsurface directly below the borehole,
which can be compared with the surface seismic along
the well trajectory. An example is shown in
Fig. 4.8
.
The benefits of the VSP are numerous (Campbell
et al.,
2005
).
Figure 4.7
compares a multiple-free
'
outside
'
corridor
High-density sampling gives good control on the
time
stack with an
corridor stack which is contam-
inated by interbed multiples. At the horizontal red
line there is a strong event on the inside corridor stack
that is absent from the outside stack, and is thus
inferred to be an intra-bed multiple. The event is
absent from the surface seismic, implying that the
processor has been successful
'
inside
'
-
depth relationship.
Control on phase: trace by trace deconvolution of
the upwave using the down-going wavefield
results in a zero phase upwave corridor stack.
The upwave corridor stack is largely multiple free;
comparison with seismic can highlight potential
multiple problems.
in removing the
multiple.
The VSP can be particularly useful in highly devi-
ated wells. It is common for well synthetics to tie
surface seismic poorly for such wells, perhaps because
of anisotropic effects (see
Section 8.4.5
). In a walk-
above acquisition, the surface source is positioned
vertically above the geophone at a series of levels in
the deviated borehole. This makes acquisition more
time-consuming and costly, as the source has to be
moved to an accurately determined location for each
level. However, the benefit is that the raypaths are
In favourable situations the absorption parameter
Q can be estimated from wavelet shape changes in
the downgoing wavefield (e.g. Tonn,
1991
; Harris
et al.,
1997
).
It may be possible to apply inverse Q filtering to
optimise resolution as well as using the VSP to
determine zero phasing operators for surface
seismic.
However, there are several reasons why VSPs may not
tie exactly to seismic.
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