Marine Examples and Applications
Because there is no viable S-wave source that can be deployed on the seafloor, multicomponent seismic data acquired in marine environments are constrained to data recorded by seafloor sensors (typically four-component [4C] sensors) and generated by air-gun arrays towed at the sea surface. Thus, SH-SH data are not available for marine applications. An SV-SV mode can be used in rare instances in which the seafloor is sufficiently hard for a downgoing P-wave to generate a robust P-to-SV mode conversion directly at the water-seafloor interface (Tatham and Goolsbee, 1984). The seafloor then becomes a secondary source from which a downgoing SV mode illuminates subseafloor strata.
For those reasons, only two wave modes are emphasized in marine multicomponent seismic data — the P-P mode and the P-SV mode. The applications that are illustrated in this chapter apply equally well to onshore prospects. Those examples are collected into this chapter only because the data were acquired in a marine environment, not because there is some uniqueness to marine geology or to marine seismic data. One exception to this generalization is the use of 4C data to image near-seafloor strata in deep water. This application is unique to the marine environment because there is a large elevation difference between the surface source and the seafloor receiver that allows P-P and P-SV data to be processed like walkaway vertical-seismic-profile (VSP) data. This extension of VSP data-processing principles to marine 4C data allows near-seafloor strata immediately below a receiver station to be imaged with high
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.