SWLGS Luncheon Topics
Updated August 05, 2003
AVO Crossplot Evaluation Using Visualization Techniques
Abstract Summary: AVO
crossplotting is a widely employed AVO analysis technique that has gained
acceptance in the geophysical community over the past decade. Crossplotting AVO
attributes such as the AVO intercept and gradient has proven useful in
hydrocarbon diagnostics in unconsolidated clastic basins worldwide.
Improved understanding and interpretation of the crossplot results can be
obtained through 3D visualization of the AVO crossplot.
Modeled and observed 3D seismic crossplots, through visualization
techniques, demonstrate the improved recognition of background trends and
associated AVO anomalies, which in turn improves interpretation and analysis.
the past decade, AVO analysis has shifted from single (combined) attribute
analysis to a multi-attribute approach, and AVO (attribute) crossplotting is one
such technique that is widely practiced. In
addition to computing combined AVO attribute sections using the intercept,
gradient, near angle stacks and far angle stacks, or their equivalents, both
data sets can be plotted against one another through crossplotting techniques.
Under ideal situations, common lithologies and fluid types generally
cluster together in AVO crossplot space permitting a straightforward
interpretation. Together with AVO modeling and the AVO attribute section, the
crossplot is used to evaluate the potential for discriminating fluid-type and or
lithology of the objectives. AVO
crossplotting, which typically uses the AVO intercept (A) and gradient (B),
assists standard AVO analysis techniques by identifying background trends and
anomalous responses (off trend aggregations) that may or may not be associated
with hydrocarbons (Castagna and Swan, among others, have presented work
regarding these concepts). Using
the A and B data volumes and small target specific windows where Vp/Vs are
nearly invariant, a background trend can often be determined which defines the
wet-sand/shale interfaces and other similar lithologies.
AVO (A-B) pairs lying off the trend are considered anomalous. Through
interactive testing, with a priori information through modeling, and with
geological integration of the basin geology, the crossplot can be used to assess
hydrocarbon-bearing strata or key lithologies. In light of the aforementioned, crossplotting has evolved to
be a crucial component in AVO analysis.
even with current computer speeds and inexpensive disk drive costs, interactive
crossplotting of seismic data is computationally intensive.
A 23 square kilometer area with 25 meter square bins and 6 s records at a
4ms sample rate results in nearly 3.6x10 8
pairs, and for a 100 sq. km area using a 1 s window, 4.0x10 7
pairs. Because of the large
quantities of crossplot pairs, much of the seismic crossplotting performed to
date on projects has taken the form of 2D profiles or small subsets of seismic
volumes, resulting in a composite-summary crossplot of the A-B pairs.
These composite-summary crossplots have worked fairly well for the
classic AVO anomalies, but can be confusing for the not-so-obvious objectives.
The dominant reasons for the shortcomings of the composite-summary crossplot
are: the immense amount of points associated with the background responses which
clutters the displays (even for reduced subsets of the entire volume); and
second, the three dimensional nature of the subsurface being analyzed. These two
detractors to the composite-summary crossplot are linked, and through standard
seismic visualization techniques can be overcome.
Biographical Sketch: Mark
Sparlin works with Hampson-Russell in the capacity of Senior Research
Geoscientist where he is currently involved with consulting services in AVO
analysis and neural-network /geostatistical seismic attribute evaluation in
applications to 3D seismic/borehole reservoir characterization projects. Mark
has twenty years of experience working in the oil industry for major oil
companies as an exploration geophysicist and Schlumberger Technology Corporation
as Manager of Geophysical product development. He has also been employed with
The University of Texas Research Campus involved in geophysical/geological
evaluations in international reservoir characterization studies.
Mark received a B.S.
in geology from the University of Akron with honors and an M.S. in Geophysics
from Purdue University. Email: