Multicomponent Data Processing
Multicomponent seismic data processing is a complex subject that would require a separate book to cover all aspects o the topic in a thorough manner. This chapter summarizes only basic principles and is not intended to be a complete treatise on multicomponent data-processing concepts and strategies.
When nine-component (9C) data are acquired, processing S-wave data propagating in isotropic media is in concept no different than processing conventional single-component P-wave data because SH-SH and SV-SV modes satisfy the constraints of common-midpoint (CMP) data processing just as P data do. The fundamental requirement for CMP processing is that the velocity of the downgoing mode must be the same as the velocity of the upgoing mode. That assumption is valid for SH-SH and SV-SV data just as it is for P-P data. Because CMP data-processing software and expertise are widespread, processing 9C data to make SH-SH and SV-SV images is not a great challenge to a data processor skilled in processing conventional P-P data.
Processing three-component (3C) and four-component (4C) data is a different matter. For those data, the velocity of the downgoing wavefield (P-wave) is not the same as the velocity of the upgoing wavefield (SV-wave), and CMP principles no longer apply. A different data-processing strategy based on common-conversion-point (CCP) principles has to be implemented. Some of the better CCP processing software is proprietary to seismic contractors and to a few research groups and service providers. The use of CCP software is beginning to be reasonably widespread, and CCP data-processing skills are expanding annually.
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.