Abstract
Converted-wave amplitude versus offset (AVO) behavior may be fit with
a cubic relationship between reflection coefficient and ray parameter.
Attributes extracted using this form can be directly related to elastic
parameters with low-contrast or high-contrast approximations to the
Zoeppritz equations. The high-contrast approximation has the advantage
of greater accuracy; the low-contrast approximation is analytically
simpler. The two coefficients of the low-contrast approximation are
a function of the average ratio of compressional-to-shear-wave velocity
(alpha/beta) and the fractional changes in S-wave velocity and density
(Deltabeta/beta and Deltarho/rho). Because of its simplicity, the
low-contrast approximation is subject to errors, particularly for
large positive contrasts in P-wave velocity associated with negative
contrasts in S-wave velocity. However, for incidence angles up to
40 deg and models confined to Deltabeta/beta < 0.25, the errors in
both coefficients are relatively small. Converted-wave AVO crossplotting
of the coefficients of the low-contrast approximation is a useful
interpretation technique. The background trend in this case has a
negative slope and an intercept proportional to the alpha/beta ratio
and the fractional change in S-wave velocity. For constant alpha/beta
ratio, an attribute trace formed by the weighted sum of the coefficients
of the low-contrast approximation provides useful estimates of the
fractional change in S-wave velocity and density. Using synthetic
examples, we investigate the sensitivity of these parameters to random
noise. Integrated P-wave and converted-wave analysis may improve
estimation of rock properties by combining extracted attributes to
yield fractional contrasts in P-wave and S-wave velocities and density.
Together, these parameters may provide improved direct hydrocarbon
indication and can potentially be used to identify anomalies caused
by low gas saturations.
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