Onshore Examples and Applications
Applications and examples of multicomponent seismic technology across onshore prospects are assembled in this chapter. Because dipole sources that directly contact earth strata can be used in land-based seismic fieldwork, land-based multicomponent studies involve some S-wave modes that cannot be replicated in marine environments. Specifically, the SH-SH mode is unique to onshore multicomponent applications, and for all practical purposes, so is the SV-SV mode. An SV-SV mode can exist in marine data only when a hard seafloor causes a P-to-SV mode conversion directly at the base of the water column. Even though SH sources cannot be used in offshore areas, most of the multicomponent applications that are illustrated onshore also can be implemented across marine prospects.
Data examples in this chapter span several decades of investigation. Several early S-wave experiments are quite valuable for demonstrating basic principles and are included in the portfolio of rock and fluid applications assembled in this chapter so the work of first-generation proponents of S-wave technology can guide those of us who follow in their steps. The compilations of multicomponent seismic applications prepared by Stewart et al. (2003) and by Hardage (2010) are recommended reading to complement the material presented in this chapter.
Figures & Tables
A principle that is emphasized throughout this book is that the physics of any multicomponent seismic technology cannot be understood unless that technology is viewed in terms of the particle-displacement vectors associated with the various modes of a seismic wavefield. This material therefore begins with a discussion of seismic vector-wavefield behavior to set the stage for subsequent chapters.
Several approaches can be used to explain why each wave mode of nine-component (9C) and three-component (3C) seismic data that propagates through subsurface geology provides a different amount and type of rock/fluid information about the geology that the wave modes illuminate. Some approaches appeal to people who have limited interest in mathematics. Other options need to be structured for people who have an appreciation of the mathematics of wavefield reflectivity. Another argument that can be used focuses on the fundamental differences in P-wave and S-wave radiation patterns and the distinctions in target illuminations associated with 9C and 3C seismic sources. We will consider all of those paths of logic.
A principle that will be stressed is that each mode of a multicomponent seismic wavefield senses a different earth fabric along its propagation path because its particle-displacement vector is oriented in a different direction than are the particle-displacement vectors of its companion modes. Although estimations of earth fabric obtained from various modes of a multicomponent seismic wavefield can differ, each estimate still can be correct because each wave mode deforms a unit volume of rock in a different direction, depending on the orientation of its particle-displacement vector. Those deformations sense a different earth resistance in directions parallel to and normal to various symmetry planes in real-earth media. The logic of that nonmathematical approach appeals to people who are interested in the geologic and petrophysical information that multicomponent seismic data can provide and are less concerned about theory and mathematics.
A second approach that is helpful for distinguishing one-component (1C), 3C, and 9C wavefield behavior focuses on the mathematics of the reflectivity equation associated with each mode of the full-elastic seismic wavefield. The mathematical structure of the reflectivity equation associated with each seismic wave mode describes why and how petrophysical properties of the propagation medium affect different wave modes in different ways. The logic of that analytical approach is appreciated by scientists who are comfortable with mathematics.
All of these concepts lead to the development of a new seismic-interpretation science based on multicomponent seismic data called elastic wavefield seismic stratigraphy.