Stein (pp.177): Anisotropy can also occur for homogeneous materials. For example, the crystal structure of the mineral olivine is homogeneous in that it is composed of the same repeating groups of atoms, but acts anisotropically because itss acoustic properties vary in different directions relative to the crystal structure. This situation is called lattice-preferred orientation (LPO) anisotropy.
Sub-crustal oceanic lithosphere shows strong anisotropy. Flow processes associated with plate spreading appear to orient olivine crystals preferentially in the spreading direction Pn head waves that sample the uppermost mantle just below the Moho show a strong azimuthal velocity dependence, with velocity highest in the spreading direction or 180° from it. This anisotropy is &quot;frozen in&quot; as the lithosphere ages Azimuthal variation of velocities in the upper mantle observed under the pacific ocean. What are possible causes for this anisotropy? Aligned crystals Flow processes
Attenuation: For a seismic wave traveling through the Earth, amplitude loss due to several physical processes, including gradual conversion of wave energy into heat absorption (pp.546, Liner).
Solid Earth Geophysics-Geop503 Ali Oncel [email_address] Department of Earth Sciences, KFUPM Anisotropy and Attenuation Reading: Fowler Chapter 8- Section 8.1
Reason to study Anisotropy? http://www.patentstorm.us/patents/5142501-description.html Crampin and Bush (1986) also pointed out that vertical S-wave birefringence might provide a useful tool for reservoir development. The polarization direction of the fast S-wave in simple cases gives the direction of maximum horizontal compressive stress, a quantity much in demand by those who induce fractures in reservoirs by techniques such as hydraulic fracturing Available evidence, (Winterstein, 1990) including offset VSP information supports the notion that the vertical S-wave birefringence is caused by horizontal stresses, and that the polarization direction of the fast S-wave lies in the direction of maximum horizontal compressive stress , even when subsurface structures are steeply dipping.
What is Seismic Anisotropy? Seismic waves are assumed that they propagated through an earth made up of purely isotropic, linearly elastic material. In such material, the stresses are linearly proportional to strains via Hooke’s law:
But, seismic wave propagation in anisotropic media is quite different from isotropic media:
There are in general 21 independent elastic constants, (instead of 2 in the isotropic case)
there is shear wave splitting
waves travel at different speeds depending in the direction of
The polarization of compressional and shear waves may not be
perpendicular or parallel to the wavefront, resp.
Reference: Stein, 2003, pp. 177
Anisotropy Shape-preferred orientation anisotropy A shear wave can be split into two pulses, each with a different polarity and traveling at a different speed Anisotropic materials cause seismic waves traveling through them to travel faster or slower depending on their direction. Shear-wave Splitting
Indicator of style of flow, stress regime or fracturing.
Insights into past and present deformation.
Major source of anisotropy in reservoir rocks is fracturing.
Effect of fractures on anisotropy can be predicted using effective medium theory (e.g. Hudson et al (1996).
Shear-wave splitting Time lag between fast and slow phases, t Polarisation of fast phase,
a axis is fastest direction This is also dominant slip direction, so olivine crystals align in direction of plastic flow OLIVINE IS HOMEGENOUS BUT ANISOTROPIC Lattice-preferred orientation (LPO) anisotropy Babuska and Cara, 1991
East West South PROPAGATION DIRECTION V P anomaly (km s -1 ) Central Pacific Morris et al., (1969) JGR . ANISOTROPY FROM RELATIVE PLATE MOTION Stein 2003, pp. 180
Sidorin et al., 1999 compared scenarios using convection models, and synthetic seismograms
Best fit : phase change with 6 MPa/K Clapeyron slope
No suitable phase change was known at the time
Thermal Gradients Dense Layer Phase Change From: John Hernlund,2005
Global maps of attenuation (1/Q) From: Romanowicz and Gung, 2002
At shallow depths, similar to velocity maps:
beneath ridges: high Q -1
below continents: low Q -1
See: pp.344 of Fowler’s book, Fig.8.8
Evidence for super plume beneath south Pacific
Very difficult to resolve small scale structures like individual plumes
Eq. 8.21 Fowler Highly attenuative region in the Earth Low Q-region or High Q -1 -region increase decrease Q= Quality Factor= 2 π x elasticity energy stored in the wave Energy lost in one cycle or wavelength