Geoscience Reference
In-Depth Information
it might not be possible to replicate them. This is typically the case for a marine
survey, where streamer locations are affected by surface currents; recent technology
allows steering of the individual streamers, which will reduce, but not eliminate, the
problem. Secondly, there will always be inaccuracies in position-fixing of shots and
receivers. Considerable improvement in positional accuracy of marine surveys has
been achieved from about 1990 onwards, owing to satellite positioning, the tracking
of streamer tailbuoys, and the use of acoustic transponder networks. If the baseline
survey is old enough to have been shot before these improvements, then there may
be quite large uncertainties on the exact locations of sources and receivers. One
simulation showed that errors of 80 m in the location of the tail of a 4000 m marine
streamer could give rise to difference sections with an rms amplitude 20-30% of
the baseline data. Study of VSP data by Landro
(1999)
showed that shifting a shot
location by 10 m resulted in an rms change in the record (after static alignment) of
20%, and a 20 m shift resulted in a 30% change. The records were dominated by the
downgoing signal, so these changes are caused by transmission response variations
between neighbouring ray-paths. One way of reducing acquisition differences is
to leave receivers permanently in place on the seabed, as has been tried in the
BP/Shell Foinaven Field. Even so, difference sections between surveys showed
(f) Processing parameters. There are many steps in the processing sequence that can
introduce differences similar to the time-lapse signal we are looking for. These in-
clude statics correction, mute design, pre-stack deconvolution, stack and migration
velocity derivation, and amplitude balancing. It is often necessary to reprocess the
baseline survey together with the repeat survey, as discussed in the next section.
8.4
Seismic processing
Specialised processing can alleviate these problems of repeatability. Where the baseline
survey is a 'legacy' dataset, not originally acquired with time-lapse in mind, it will be
beneficial to reprocess it in parallel with the monitor survey. This ensures that the same
algorithms are used at each stage, with so far as possible the same choice of parameters.
At each step, the surveys can be compared and action taken to match them if necessary.
This might include time- and space-variant amplitude and spectral trace matching
between the two datasets. A detailed discussion is given by Ross
et al.
(
1996)
. The idea
is to match the two surveys over the intervals outside the reservoir, where no time-lapse
changes are expected. The quality of the match can be seen from the difference volume
between the baseline and monitor surveys, which should ideally be zero except where
the production-related effects are present. The matching process can include several
elements. Time corrections are needed where there are systematic shifts, for example
as a result of salinity variations in the water column in the marine case, or changes
