Seismic anisotropy
Encyclopedia
Seismic anisotropy
is a term used in seismology
to describe the directional dependence of seismic wavespeed in a medium (rock
) within the Earth
.
Both seismic P-waves and S-waves may exhibit anisotropy. For both, the anisotropy may appear as a (continuous) dependence of velocity upon the direction of propagation. For S-waves, it may also appear as a (discrete) dependence of velocity upon the direction of polarization. For a given direction of propagation in any homogeneous medium, only two polarization directions are allowed, with other polarizations decomposing trigonometrically into these two. Hence, shear waves naturally "split" into separate arrivals with these two polarizations; in optics this is called birefringence
.
Crustal anisotropy is very important in the production of oil reservoirs, as the seismically fast directions can indicate preferred directions of fluid flow.
In crustal geophysics, the anisotropy is usually weak; this enables a simplification of the expressions for seismic velocities and reflectivities, as functions of propagation (and polarization) direction. In the simplest geophysically plausible case, that of polar anisotropy, the analysis is most conveniently done in terms of Thomsen Parameters.
. Anisotropy has long been used to argue whether plate tectonics is driven from below by mantle convection or from above by the plates, i.e. slab pull and ridge push.
Anisotropy
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...
is a term used in seismology
Seismology
Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...
to describe the directional dependence of seismic wavespeed in a medium (rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
) within the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
.
Crustal Anisotropy
In the Earth's crust, anisotropy may be caused by preferentially aligned joints or microcracks, by layered bedding in sedimentary formations, or by highly foliated metamorphic rocks. Crustal anisotropy resulting from aligned cracks can be used to determine the state of stress in the crust, since in many cases, cracks are preferentially aligned with their flat faces oriented in the direction of minimum compressive stress. In active tectonic areas, such as near faults and volcanoes, anisotropy can be used to look for changes in preferred orientation of cracks that may indicate a rotation of the stress field.Both seismic P-waves and S-waves may exhibit anisotropy. For both, the anisotropy may appear as a (continuous) dependence of velocity upon the direction of propagation. For S-waves, it may also appear as a (discrete) dependence of velocity upon the direction of polarization. For a given direction of propagation in any homogeneous medium, only two polarization directions are allowed, with other polarizations decomposing trigonometrically into these two. Hence, shear waves naturally "split" into separate arrivals with these two polarizations; in optics this is called birefringence
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
.
Crustal anisotropy is very important in the production of oil reservoirs, as the seismically fast directions can indicate preferred directions of fluid flow.
In crustal geophysics, the anisotropy is usually weak; this enables a simplification of the expressions for seismic velocities and reflectivities, as functions of propagation (and polarization) direction. In the simplest geophysically plausible case, that of polar anisotropy, the analysis is most conveniently done in terms of Thomsen Parameters.
Mantle anisotropy
In the mantle, anisotropy is normally associated with crystals aligned with the mantle flow direction. Due to their elongate crystalline structure, olivine crystals tend to align with the flow due to mantle convectionMantle convection
Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, which rides atop the asthenosphere , is divided into a number of plates that are continuously being...
. Anisotropy has long been used to argue whether plate tectonics is driven from below by mantle convection or from above by the plates, i.e. slab pull and ridge push.
External links
- http://www1.gly.bris.ac.uk/~wookey/MMA/index.htm
- http://geophysics.asu.edu/anisotropy