Figure
Captions
The Problem
We will approach the problem from a
simple point of view. Imagine that you are trying to find your car keys
in a dark room with a hand-held spotlight. If you think the keys are
underneath the chair, it may do no good to illuminate the chair with the
spotlight pointed down from a high position. Instead, you have to lower
the spotlight (the illuminating source) and your eyes (the backscatter
receivers) so that the hidden area beneath the chair is first
illuminated, and then is imaged.
A geological equivalent to this “find the
hidden keys” problem is illustrated in Figure 1, where a reservoir is
hidden beneath a geological interval that does not allow it to be imaged
with surface-positioned sources and receivers.
If a well is
near the hidden target, a method now being developed and applied by some
companies involves the acquisition of vertical seismic profile (VSP)
data with downhole receivers. Such a VSP data-acquisition concept is
shown in Figure 1.
For the geology illustrated here, surface-positioned sources
and receivers cannot image the reservoir target. The VSP source-receiver
geometry shown in Figure 1 helps in that the receivers are moved to deep
positions where they can better image the hidden target; however, the
sources are still on the Earth surface and are not illuminating beneath
the geology that distorts the raypaths.
To illuminate
the reservoir, the source also has to be lowered – just as was the light
source that was used to illuminate beneath the chair to find the car
keys.
This
source repositioning is done mathematically, not by lowering a real
source down a well (which is one option). The concept that is
implemented is described in Figure 2. In the left diagram, we have a
source-receiver geometry similar to the VSP geometry shown in
Figure 1;
in the right diagram, VSP surface-source (S) is repositioned to one of
the downhole geophone stations by wavefield extrapolation.
The lower
position of this new virtual source (VS) now allows a different
imaging strategy to be implemented. Although it requires numerous
mathematical equations for most geophysicists to explain the procedure,
the process of repositioning the source from the Earth surface to a deep
borehole location can be illustrated with the graphic shown in
Figure 3.
In this figure, data traces X1 through X5 are the VSP data acquired with
the surface-positioned source.
In
Figures 1 and
2, we positioned our virtual source at the third receiver station, but
here we arbitrarily select VSP receiver Station 1 as the location
for the downhole virtual source. The original VSP data are next
converted to data that would be produced as if the source was at
downhole Station 1 by:
1) Time
reversing the data trace recorded at Station 1.
2) Convolving
this time-reversed trace with all of the other data traces.
If you do not
know what the mathematical process of “convolving” means, it is not
important because your friends in the seismic data-processing community
do. For this discussion, we will indicate convolution at receiver
Station 3 by using an arrow to show the mathematical movement of
time-reversed trace X1 across data trace X3. This mathematical process
also is called cross-correlation, which may be a more familiar term.
Once this
cross-correlation is done at all receiver stations, we have created data
that are equivalent to data acquired with a deep illuminating source at
Station 1 and deep imaging sensors at all other stations. Initial
results by those who practice this imaging technique confirm you can
often see into places that are hidden from surface-based sources and
receivers.
Shell has a
powerful patent on this virtual-source concept that some companies are
interested in licensing. No doubt
intellectual property owned by
others will come into play also. Some contractors are working to develop
equivalents to what has been described here.
My purpose is not to promote anyone’s business or intellectual property,
but to alert you to an emerging and valuable seismic technology that
will allow you, in some cases, to image geology that is difficult to see
with other imaging strategies. Check with your favorite borehole seismic
contractors about their ability to “look into hidden places” and inquire
what they know about “virtual source” imaging and “seismic
interferometry,” which are terminologies used by some when referring to
this technique.
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