**AVO Analysis of Long Offset Seismic Data Using 3-D Graphical Analysis**

**F. Zhu ^{1} and R. L. Gibson^{2}**

^{1}Shell International Exploration and Production Inc.^{2}Texas A&M UniversityP-wave AVO intercept (*A*) and gradient (*B*) crossplotting is widely
used for detecting anomalous elastic properties that may be indicative
of hydrocarbons, but long offset seismic data are starting to become
widely available. It is well known that the two-term AVO
equation is only an approximation for angles of incidence less than
30^{o}. However, long-offset seismic data with *P*-wave angle of incidence
much larger than 30^{o} are sometimes used for AVO crossplotting.
We show how inaccurate *A* and *B* values may result if the twoterm
AVO equation is used for prestack seismic data with large
incident angles.

A linear Vp-Vs and a Gardner-like *Vp-Rho* relationship explain the
linear relationship between *A* and *B* for typical brine-saturated sandstones
and shales, and this line passes through the origin. However,
in the Gulf of Mexico (Hilterman, 1990) and Gulf of Thailand (Zhu,
2000), the sand velocity is sometimes larger than the shale velocity,
while sand density is smaller than the shale density. As a result, the *A-B* trend of some brine-saturated sandstones and shales may not
pass through the origin or may not be linear in the *A-B* plane. Nevertheless,
a linear *Vp-Vs* relationship holds and the average *Vp* across
the shale and sand interface does not vary significantly. AVO intercept
(*A*), gradient (*B*) and curvature (*C*), which can be extracted
from long offset prestack seismic data, are expressible in an equation
representing a plane the 3-D *A-B-C* space. Deviations from this plane
suggest abnormal velocities and indicate lithology and fluid
variations.

AAPG Search and Discovery Article #90901©2001 GCAGS, Annual Meeting, Shreveport, Louisiana