Interpreting seismic amplitudes - 3-D Seismic Interpretation - page 135

Angle Mutes from Processed Gather

Fig. 5.13 Selection of data from a gather for creation of angle stacks.

(4) Pre-stack migration is becoming routine, and is the best approach for serious AVO
study as it provides some guarantee that the traces in a gather all belong to the same
subsurface reflection point. Where this has not been done, migration of the near
and far sub-stacks is a cheap way of positioning the stacked amplitude data in the
right place in space.
A possible problem with near and far offset stacks is that the sub-stacks represent
different incidence angle ranges at different two-way times. This can make it more
difficult than it need be to compare observed data with a modelled response for a
particular incidence angle. The solution is to produce angle stacks ; these are made
by stacking data within an angle range calculated by ray-tracing or the formula given
above. Figure 5.13 illustrates the principle.
The presence of different fluid fills (brine, oil or gas) affects the R 0 and G values of
areflector. In general, the progression from brine to oil to gas will move R 0 and G for
the reflector at the top of the reservoir towards more negative values. Some schematic
examples are shown in fig. 5.14 . Sand A has small positive R 0 and negative G for the
brine and oil cases and negative R 0 for gas. The response will be class II for brine or
oil, and class III for gas. The expected stacked reflection from top sand is also shown
(red = impedance increase downwards = positive reflection coefficient). For brine