1: Introduction and Historical Review
Until the 1970s, applications of seismology in petroleum exploration were limited, to a great extent, to P-wave energy recorded on single-component vertical geophones or pressure sensitive hydrophones. With the advent of S-wave recording in the 1970s, use of both vertically and horizontally polarized sources and multicomponent receivers has become more common. Thus, applications of S-wave technology have led the exploration geophysicist to multicomponent recording, including use of both multicomponent sources and receivers.
The following review traces the use of S-wave applications in exploration, and the resultant evolution to multicomponent recording. Chapter 1 is followed by a brief discussion of the basic principles of elastic theory, including polarization and anisotropy as they relate to acquisition, processing, and interpretation of multicomponent seismic data. Field examples and case histories in later chapters illustrate the practical applications of the basic principles.
Some of the earliest work in seismology involved the use of seismic S-waves. Observations of body waves made by earthquake seismologists were principally S-waves. This was a result of both earthquake sources generating, in many cases, predominately S-wave energy, and the sensitivity of early instrumentation to horizontal ground motion. The very earliest instruments, such as torsion seismometers, responded primarily to horizontal ground motion. Further, the development of sensitive instruments to detect horizontal ground motion preceded the development of sensitive vertical instruments by a number of years. Even modem earthquake seismology is very dependent upon S-wave observations. In fact, the original definition of earthquake magnitude given in Richter (1935) was based upon displacement observed on a Wood-Anderson torsional seismometer, although the horizontal motion associated with the first-arriving P-wave is what was measured.
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Many prospective basins of the world are, or will soon become, “mature” in the exploration sense. Increasingly we must resort to nonconventional technology and techniques to uncover the remaining hydrocarbon reserves that are often found in complex or subtle traps. Multicomponent seismology-the use of concurrent, combined shear (S)- and compressional (P)-wave seismology-is gaining acceptance in the exploration community as one tool that can provide direct measurements of subsurface rock properties. These measurements can detect new hydrocarbon accumulations, and aid in the efficient and economic development of newly found or existing reservoirs by providing detailed maps of reservoir porosity, lithology, and pore fluid distributions.