SEISMAC WAVES SERIES

Focal mechanisms may be based on more sophisticated optimal waveform fitting between real and synthesized records ( Song and Toksöz, 2011), on considering azimuthal variations in amplitude ratios between P and S waves ( Julian and Foulger, 1996 Hardebeck and Shearer, 2003), or on simple “classical” P-wave first-motion polarity readings only. Focal mechanisms use the first arrival patterns of radiated seismic waves between seismic stations at different directions from an earthquake. As such, they can be used to obtain the fault kinematics. Seismic waves radiated from an earthquake reflect the geometry of the fault and the motion it experiences during rupture. Qin, in Unconventional Oil and Gas Resources Handbook, 2016 9.2.3.7 Focal Mechanisms The wave equation for seismic vibrations is discussed in the next section. The mathematical equation for describing the motion of the rope has the same form as the equation for describing the motion of a vibration propagating through the earth.

SEISMAC WAVES SERIES

The ensuing series of waves is called a wavetrain. We can make many of these waves by moving the rope up and down rhythmically. The resulting pulse should look something like Figure 5-2c. All of these motions should be made smoothly and continuously. To make a whole wave, move the rope up, back to its original position, down, and then up to its original position. This pulse is one half of a wave and it has one half a wavelength.

When the pulse strikes the door it will be reflected back towards us as shown in Figure 5-2b. A pulse like that shown in Figure 5-2a should proceed toward the door. Hold the rope tight and move it up and then back to its original position. Suppose we tie a rope to the doorknob of a closed door. The concept of a wave is easily illustrated. The display of the output is called the seismogram or seismic section. The graph representing the motion of a single seismometer, such as a geophone or hydrophone, is called the trace of the seismometer. The output from the seismometer is transmitted to a recording station where the signals are recorded by a seismograph. Seismic waves can be detected by a seismometer, which is often referred to as a geophone on land or a hydrophone in a marine environment. P waves are sometimes called compressional, primary or pressure waves, while S waves are sometimes called shear or secondary waves. Other names exist for P waves and S waves. S waves do not travel as fast as P waves and S waves do not propagate through fluids. The velocity of S waves depends on the shear modulus and density of the medium. The velocity of P waves depends on the elastic properties and density of the medium. Transverse waves are a class of waves in which the particles of the disturbed medium are displaced in a direction that is perpendicular to the direction of propagation of the wave. Longitudinal waves are a class of waves in which the particles of the disturbed medium are displaced in a direction that is parallel to the direction of propagation of the wave. P waves are longitudinal waves while S waves are transverse waves. The seismic waves that propagate through the earth are called body waves and are either P waves or S waves. The energy released by the disturbance propagates away from the source of the disturbance as seismic waves. Seismic waves are vibrations, or oscillating displacements from an undisturbed position, that propagate from a source, such as an explosion or mechanical vibrator, through the earth. Fanchi, in Shared Earth Modeling, 2002 5.1 Physics of Waves A seismic reflection occurs at the interface between two regions with different acoustic impedances.

When the seismic wave encounters a reflecting surface, it is partially transmitted and partially reflected.

These waves have many attributes, such as frequency content and transit time, but the focus in this chapter is on reflection amplitude. The displacement can be caused by a mechanical source. Seismic waves are displacements that propagate through the earth.

The output from the seismometer is transmitted to a recording station. Fanchi, in Integrated Reservoir Asset Management, 2010 Publisher Summary

Seismac waves