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GCElastic
Wavefield Seismic Stratigraphy*
By
Bob A. Hardage1 and I.J. Aluka2
Search and Discovery Article #40184 (2006)
Posted January 28, 2006
*Adapted
from the Geophysical Corner column, prepared by the authors and entitled, “Expanding
an Elastic
Definition,” in
AAPG Explorer, January, 2006. Editor of Geophysical Corner is Bob A. Hardage.
Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is
Communications Director.
1Senior research scientist, Bureau of Economic Geology, The University of Texas ([email protected] )
2Professor of physical science, Prairie View A&M University, Prairie View, Texas
General Statement
Seismic stratigraphy has been an important seismic-interpretation science since the 1975 AAPG annual meeting, when its principles were introduced in a series of presentations--and particularly since its documentation two years later as AAPG’s Memoir 26 edited by Payton (1977).
Emerging interest in
multicomponent seismic technology now allows (and demands) the science of
seismic stratigraphy be expanded to include all modes of a multicomponent
seismic wavefield. The term “elastic
wavefield seismic stratigraphy” is now used
when the total
elastic
wavefield, not just the P-wave component, is used in
seismic stratigraphy applications.
In elastic
wavefield
seismic stratigraphy, a seismic sequence is still defined as a succession of
relatively conformable seismic reflections bounded by unconformities or their
correlative conformities, just as Robert M. Mitchum (this year’s AAPG Sidney
Powers Memorial winner), in AAPG Memoir 26, defined the term for P-wave seismic
stratigraphy decades ago--only now the definition is expanded to include
interpretation and utilization of S-wave seismic sequences in addition to P-wave
sequences.
A seismic facies is still defined, using Mitchum’s original definition, as any seismic attribute that distinguishes one succession of reflection events from another. The only difference now is the term is expanded to include interpretation and use of S-wave seismic facies as well as P-wave facies.
uGeneral statementuFigure captionsuSequences & faciesuExampleuReferenceuAcknowledgment
uGeneral statementuFigure captionsuSequences & faciesuExampleuReferenceuAcknowledgment
uGeneral statementuFigure captionsuSequences & faciesuExampleuReferenceuAcknowledgment
uGeneral statementuFigure captionsuSequences & faciesuExampleuReferenceuAcknowledgment
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Sequences and Facies: P-Wave vs. S-Wave Two arguments help explain why P-wave sequences and facies often differ from S-wave sequences and facies:
1. Assume an
The
Thus, P-wave seismic sequences and facies
sometimes differ from S-wave sequences and facies simply because
orthogonal P and S particle-displacement vectors sense and react to
different 2.
The reflectivity of each mode of an The principle is illustrated in Figure 2; the vertical axis Ri,S is the S-wave reflectivity at an interface, the horizontal axis b is the ratio of the velocity ratio VP/VS across that interface (VP = P-wave velocity and VS = S-wave velocity), and the quantity Ri,P labeled on each curve is the P-wave reflectivity at the interface. These curves show there are interfaces that:
Thus, any combination of P and S sequences and facies can be encountered
in
An example of
There is an obvious facies change in the P-SV image that segregates the interval above unit C into two distinct seismic facies A and B. Sequence boundary 1 is defined at the common boundary between these two P-SV seismic facies. An equivalent facies break is not obvious in the P-P image. Boundary 1 drawn across P-P image space and the two P-P units labeled A and B are inferred from the P-SV interpretation. An interpreter would be hard pressed to justify P-P units A and B are different facies only on the basis of the P-wave data. This is only one example whereby expanding seismic stratigraphy beyond the confines of P-wave seismic data provides increased insight into depositional architecture and lithofacies distribution.
ReferencePayton, C.E. (ed.), 1977, Seismic Stratigraphy--Applications to Hydrocarbon Exploration: AAPG Memoir 26, 502 p.
Acknowledgment
The U.S. Department of Energy provided funding that allowed the
Exploration Geophysics Laboratory to initiate the |