Click
to article in PDF format.
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
|
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 elastic wavefield is traveling vertically through a horizontally layered medium. The P-wave particle displacement vector associated with that wavefield then senses the fabric of the medium in a direction normal to the layering, and the S-wave particle displacement vector senses the fabric in a direction parallel to the layering. The elastic constants of the medium (i.e., the fabric of the medium) differ in these two directions. For example, forces of different magnitudes have to be applied to flex a deck of playing cards or the sheets of a notepad when those forces are directed normal to and parallel to layering (Figure 1). In this simple test, the medium is the same at the common point where forces are applied, but the fabric (or strength) of the material is not the same in the two force directions.
Thus, P-wave 2. The reflectivity of each mode of an elastic wavefield at an interface differs from the reflectivities of its companion modes. 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 elastic wavefield
An example of
elastic wavefield There is an obvious
facies change in the P-SV image that segregates the interval above unit
C into two distinct 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
Reference
Payton, C.E. (ed.), 1977,
Acknowledgment
The U.S. Department of Energy provided funding that allowed the
Exploration Geophysics Laboratory to initiate the elastic wavefield
|