Geoscience Reference
In-Depth Information
Appendix 3
Recent developments
In this section we briefly discuss several areas that have seen significant development since the topic
was originally written.
Seismic acquisition: multi-azimuth and wide azimuth
A3.1
There has been a growing realisation that the reason for poor imaging with standard seismic surveys
over regions of rapid velocity change is not just the fact that the image is often highly distorted and
needs special migration schemes to image it correctly. In many instances, it is caused by the fact that
energy from the target horizons has never made it from the source down to the target and back to
the receivers. Often no data have been collected that can be used as the basis for imaging. In such
circumstances, no matter how hard we try in the processing, we can never deliver an image of the target
reflectors. In addition, strong multiples can interfere with the signal, degrading what illumination we
do have.
A good analogy to this situation is an obstacle in the way of your line of sight. If we are trying
to look at something and there is an object in the way, the obvious solution is to move to a different
position where the view is unimpeded. If the obstacle is small then we might not have to move very
far to get a good view, but if the obstacle is large we may need to move some distance before we can
see past it. If the obstacle is both large and irregular (say with a few holes or windows through it)
then we may not know exactly where we need to stand to get a good view. In such a situation we may
just try a number of locations, some of which may give us a complete or partial view of the object
beyond. If we take a photo from each location where we get a partial view of our target then it might
be possible to combine all of these pictures together to get a more complete image.
Recently there have been a number of new marine acquisition geometries based on the above
principle. Essentially they aim to increase the diversity of ray-paths from source to reflector and back
again in order to ensure that at least some energy illuminates the target and is recorded. They do
this by increasing the azimuthal range in the data. There are basically three schemes that have been
used.
Multi-Azimuth surveys (MAZ). In this method, several conventional towed streamer surveys are
collected over the same area but are shot with the boat sailing in different directions.
Wide Azimuth Towed Streamer (WATS). Additional shooting boats are configured at the side of
the main streamer boat to increase the cross-line separation between source and receiver.
Deepwater nodes: receivers are placed on the sea bottom and the source boat sails around the area
shooting into a large patch of receivers (somewhat similar to some land acquisition geometries)
Figures A3.1
and
A3.2
illustrate these survey geometries.
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