Multicomponent Data Acquisition
When acquiring multicomponent seismic data, careful attention must be given to the vector motions associated with P and S seismic displacements. For example, when acquiring onshore data with a vertical-displacement source, it is not necessary to be concerned about the azimuth orientation of the source at a source station. In contrast, when a horizontal-displacement source is used to generate S-wave data, it is essential to know the azimuth orientation of the source baseplate at each source station and the direction of first motion of that baseplate and to create consistent baseplate azimuth orientations at all source stations across a survey area.
Likewise, it is mandatory to know the positive-polarity ends of the two horizontal sensor elements in a three-component (3C) receiver and to orient the horizontal sensors so that the positive-polarity ends point in consistent azimuths at all receiver stations. Such caution is not required when deploying vertical sensors used to acquire one-component (1C) P-wave data. If it is not possible to orient horizontal sensors in a consistent vector azimuth, as can be the case when four-component (4C) receiver nodes are deployed in deep water, a data-processing procedure must be implemented to determine sensor orientations at every receiver station.
Analysis of the vector motion induced in seafloor sensors by first-arrival wavelets traveling from a large number of surface source-station coordinates is a common method used to determine 4C sensor orientation. This orientation information then can be used to mathematically transform data to a new coordinate system that describes data that would be
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.