Abstract
Amplitude versus offset concepts can be used to generate weighted
stacking schemes (here called geo-stack) which can be used in an
otherwise standard seismic data processing sequence to display information
about rock properties. The Zoeppritz equations can be simplified
and several different approximations appear in the literature. They
describe the variation of P-wave reflection coefficients with the
angle of incidence of a P-wave as a function of the P-wave velocities,
the S-wave velocities and the densities above and below an interface.
Using a smooth, representative interval velocity model (from boreholes
or velocity analyses) and assuming no dip, the angle of incidence
can be found as a function of time and offset by iterative ray tracing.
In particular, the angle of incidence can be computed for each sample
in a normal moveout corrected CMP gather. The approximated Zoeppritz
equation can then be fitted to the amplitudes of all the traces at
each time sample of the gather, and certain rock properties can be
estimated. The estimation of the rock properties is achieved by the
application of time- and offset-variant weights to the data samples
before stacking. The properties which can be displayed by geo-stack
are: P-wave reflectivity (or true zero-offset reflectivity), S-wave
reflectivity, and the reflectivity of P-wave velocity divided by
S-wave velocity (or pseudo-Poisson's ratio reflectivity). If assumptions
are made about the relation between P-wave velocity and S-wave velocity
for water-bearing clastic silicate rocks, then it is possible to
create a display which highlights the presence of gas.
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