Geoscience Reference
In-Depth Information
M ultip le
Energy
Fig. 2.24 CMP gather at 100 and 50 m trace spacing. Comparison of CMP gather at original trace
spacing and after shot record interpolation (see fig. 2.25 ) to interpolate traces so that the new gather
has half the trace spacing of the original. The high-energy multiple packet is badly aliased in the
original gather, making removal of these multiples very difficult. After shot record interpolation the
multiples are better sampled and there is little aliased energy.
contributes to data on different subsurface lines (fig. 2.6 ) . The time interval between
successive shots is unchanged from the single source case, because a source cannot be
fired until the listening period for the previous shot has ended. When the dual sources
are fired alternately, there are therefore half as many shots contributing to every CMP.
This may lead to data aliasing of any steeply dipping multiples within a CMP gather.
For example, a common geometry is to have 25 m receiver group intervals within a
cable and two sources fired alternately, with a shot every 25 m. This means that each
source is fired only every 50 m. The fold in a CMP gather will be only one-quarter
of the number of traces in a shot gather, and the offset increment in a CMP gather
will be four times that in a shot gather. Figure 2.24 shows an example CMP gather at
50 and 100 m trace spacing. To reduce the possibility of aliasing, additional shots are
created by interpolation between the actual shots. Generally, frequency-space methods
are employed since these use the lower unaliased frequencies to generate estimates of
the higher frequencies. The procedure is explained by the stacking diagram in fig. 2.25 .
The shot recorded data are sorted into receiver gathers; each receiver gather is then
interpolated to add one new trace between each existing trace. These traces are then used
together with the original data to form better-sampled CMPs. Usually shot interpolation
is required for multiple suppression and when this has been performed the additional
(invented) traces are discarded.
 
Search WWH ::




Custom Search