Interpreting seismic amplitudes - 3-D Seismic Interpretation - page 138

AVO Mapping -
Weighted Stack or Equivalent Angle Maps

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Fig. 5.16 Amplitude maps at different angles over the Auger Field in the Gulf of Mexico. The
optimum coincidence of structure and amplitude anomaly is at an angle of 18 ◦ , which is therefore
the optimum fluid stack. Reproduced with permission from Hendrickson (1999) .

shale content in the sand, would give much lower values, in the range 3.

The slope of the trend observed in the real seismic trace data is strongly affected by
noise. The calculated AVO gradient is very sensitive to random noise on the seismic
traces, whereas the intercept is fairly insensitive. Where signal to noise ratio is low
(i.e. weak reflectors), the slope on the crossplot will be much higher than the well-
based expectation. A complementary approach to this crossplot analysis is to make a
series of amplitude maps for different angle ranges and compare the results with the
expected behaviour for fluid effects, e.g. amplitude change conforming to structural
contours (Hendrickson, 1999 ; see also fig. 5.16 ) .
A different way of visualising the intercept and gradient data is to create a trace
volume of R 0 ∗ G values. This is useful where pay sand has a class III response but
brine sand is class I. Pay sand will then have a positive R 0 ∗ G value and brine sand
a negative one. By using a colour bar that accentuates the positive values, pay sand is
easily identified in the R 0 ∗ G volume (fig. 5.17) .
The presence of AVO effects can be a problem for well-to-seismic ties if synthetic
seismograms are constructed for the zero-offset case. In principle, well synthetics should

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