Solid Earth Geophysics-Geop503

Ali Onceloncel@kfupm.edu.

saDepartment of Earth Sciences, KFUPM

Anisotropy and Attenuation

Reading: Fowler Chapter 8- Section 8.1

Summary: Previous Lecture 6

Seismic Instruments and Networks3D Mantle Tomography Seismic Velocity Contrast

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

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.

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:

klijklij c 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 propagation The polarization of compressional and shear waves may not be perpendicular or parallel to the wavefront, resp.

Reference: Stein, 2003, pp. 177

AnisotropyShape-preferred orientation anisotropy

Stein & Wysession, 2003Stein & Wysession, 2003

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

shear-wave splitting

• Indicator of order in a medium.• 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

VP a

nom

aly

(km

s-1)

CentralPacific

Morris et al., (1969) JGR

.

ANISOTROPY FROM RELATIVE PLATE MOTION

Stein 2003, pp. 180

Overview: Seismic anisotropy

wave speeds are heterogeneous in an isotropic and anisotropic sense

boundary layers of the mantle show radial (VSH > VSV) and azimuthal anisotropy

anisotropy in mantle is related to lattice preferred orientation of intrinsically anisotropic olivine crystals

can relate flow with shear with strain with anisotropy

Attenuation of Seismic Waves

Q= Quality Factor=2π x elasticity energy stored in the wave

Energy lost in one cycle or wavelength

From: Panning and Romanowicz, 2004

Radial Anisotropy

Depth = 250 km

Under Pacific: transverse isotropy versus Q

Anisotropy versus attenuation

Central Pacific

Anisotropy

See: pp. 346-348 of Fowler’s book

Gung et al., 2003Gung et al., 2003

Image courtesy of Ed Garnero (ASU)Image courtesy of Ed Garnero (ASU)

Garnero, Ann. Rev. 2000

“Scenario for CMB”

From: Sean Ford, 2004

From: Christine Thomas, 2004

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B11, 2274, 2002

http://www.liv.ac.uk/earth/Staff/academic/TineThomas.htm

For more information:

From: Sean Ford, 2004

Cause of Discontinuity in D”?

• 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

Highly attenuative region in the Earth

Low Q-region or High Q-1 -region

Evidence for super plume beneath south Pacific Very difficult to resolve small scale structures like individual plumes

increasedecrease

Q= Quality Factor=2π x elasticity energy stored in the wave

Energy lost in one cycle or wavelength

Eq. 8.21 Fowler