The most effective field technique is to deploy 10, 12, 16, or more geophones at a uniform spacing at each receiver station so that the distance from the first geophone to the last geophone is the same as the dominant wavelength of the ground-roll event.
Much of the field effort in onshore seismic programs concentrates on designing and deploying receiver arrays that can attenuate horizontally traveling surface waves (ground-roll noise) and, at the same time, amplify upward-traveling reflection signals. 3 – Particle motions produced by the two principal seismic surface-wave noise modes: the Love wave and the Rayleigh wave. Love waves create particle displacements in the horizontal plane Rayleigh wave displacements are in the vertical plane ( Fig. The more common surface wave is the Rayleigh wave, which combines P and SV motions and is referred to as ground roll on P-wave seismic field records. Love waves are a serious noise mode only when the objective is to record reflected SH wavefields. Love waves are an SH-mode surface wave and do not affect conventional P-wave seismic data. There are two principal surface waves: Love waves and Rayleigh waves ( Fig. An exception in the marine case is sometimes encountered when data are recorded with ocean-bottom sensors (OBS) because interface waves can propagate along the water/sediment boundary and become a type of surface-wave noise that degrades OBS marine seismic data. Surface waves do not affect towed-cable marine data because they require some shear-wave component to propagate, and shear waves cannot propagate along the air/water interface. Surface waves can be a serious problem in onshore seismic surveys. Surface waves are noise modes that overlay the desired body-wave reflections. Surface waves travel along the Earth/air interface and do not illuminate geologic targets in the interior of the Earth. Reflected (or scattered) body waves are the fundamental signals sought in seismic data-acquisition programs. These waves generate the reflected P, SH, and SV signals that are needed to evaluate prospects and to characterize reservoirs. Body waves propagate in the interior (body) of the Earth and illuminate deep geologic targets.
Seismic wavefields propagate through the Earth in two ways: body waves and surface waves. To date, most exploration seismic data have been recorded with single-component sensors that emphasize P-wave modes and do not capture SH or SV wave modes. The fundamental requirement of multicomponent seismic imaging is that reflection wavefields must be recorded with orthogonal 3C sensors that allow these P, SH, and SV particle motions to be recognized. The P, SH, and SV particle displacements shown in Fig. An equally powerful technique for separating a seismic wavefield into its component parts is to use data-processing techniques that concentrate on the distinctions in the particle displacements associated with the P, SH, and SV modes ( Fig. This velocity difference aids in separating interfering P and S wave modes during data processing. In siliciclastics, V p/ V s varies from approximately 1.6 in hard sandstones to approximately 3 in some shales. In carbonates, the velocity ratio ( V p/ V s) tends to be approximately 1.7 or 1.8. Likewise, an SV image must have no interfering P and SH modes, and an SH image must be devoid of P and SV contamination.Ī P wave travels at velocity V p in consolidated rocks, which is approximately two times faster than velocity V s of either the SH or SV wave. To create optimal images of subsurface targets, a seismic wavefield must be segregated into its P, SV, and SH component parts so that a P-wave image can be made that has minimal contamination from interfering SV and SH modes. 2 – Distinction between the three components of an elastic wavefield. In a flat-layered isotropic Earth, the SH displacement vector is parallel to stratal bedding, and SV displacement is in the plane that is perpendicular to bedding.įig. A shear-wave particle-displacement vector is thus tangent to its associated wavefront. In contrast, SV and SH waves cause rock particles to oscillate perpendicular to the direction that the wavefront is moving, with the SH and SV displacement vectors orthogonal to each other. In other words, a P-wave particle displacement vector is perpendicular to its associated P-wave wavefront.
A compressional wave causes rock particles to oscillate in the direction that the wavefront is propagating. 2 illustrates the relationships between propagation direction and particle-displacement direction for these three wave modes. A principal difference among P, SV, and SH wavefields is the manner in which they cause rock particles to oscillate.
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