JFM 608E SPECIAL TOPICS IN SEISMOLOGY Seismic Anisotropy

1
JFM 608E SPECIAL TOPICS IN SEISMOLOGY(Seismic
Anisotropy)

• Semester 2005-Spring
• Time 14-17 p.m.
• Place MDB-Yüksek Lisans Odasi
• Date Monday

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Course Outline

• Weeks Lectures
• 7 February 2005 Introduction
• 14 February 2005 Elastic anisotropy of rock
  forming minerals polycrystalline rocks
• 21 February 2005 Seismic wave propagation in
  anisotropic medium
• 28 February 2005 Interpretation of seismic data
  in terms of anisotropy
• 7 March 2005 Case Study-1
• 14 March 2005 Case Study-2
• 21 March 2005 Case Study-3
• 28 March 2005 Case Study-4
• 4 April 2005 Midterm Exam
• 11 April 2005 Projects
• 18 April 2005 Projects
• 25 April 2005 Projects
• 2 May 2005 Presentation of Projects
• 9 May 2005 Review

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Grading System

• Final Exam (1) 40 points
• Midterm Exam (1) 20 points
• Seminar (1) 10 points (Oral Written
  Presentations)
• Project (1) 30 points (Calculation
  Interpretation20 points Oral Written
  Presentations10 points)

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LECTURE - 1

• 1. Elastic anisotropy of rock forming minerals
  polycrystalline
• rocks
• 1.1 Definition of seismic anisotropy
• 1.2 Methods of measuring of elastic parameters of
  rocks and minerals
• 1.3 Velocity anisotropy of rock forming minerals
• 1.4 Elastic anisotropy of polycrystalline
  aggregates

5
1.1 Definition of seismic anisotropy

• What is the seismic anisotropy ?
• THE SEISMIC ANISOTROPY is the dependence of
  SEISMIC VELOCITY
• upon ANGLE.
• Is the seismic anisotropy a new concept for the
  seismologists and
• exploration geophysicsts ?
• The minerals and many sedimentary and crytalline
  rocks are anisotropic.
• The seismologists knew that their isotropic
  models were not good but they
• had no other choice since the computers were not
  large enough to integrate
• the anisotropy parameter.
• Presently, they can deal with the anisotropic
  models since they have
• the better data (3-D,4-D data acquisition,
  broader band width, new survey
• methodologies), better ideas (better algorithms
  for imaging) and
• better tools (powerful computers for number
  crunching and interpretation).

6
How many elastic coefficients are necessary to
define the seismic anisotropy of minerals ?

• sij Cijkl ekl Hookes law
• sij Stress tensor components
• ekl Strain tensor components
• Cijkl Components of the fourth-order stiffness
  tensor depending on the local elastic
  properties of the material
• (Babuska and Cara, 1991)

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Number of independent elastic coefficients (n)
for selected symmetry systems and typical
minerals or Earths materials (page 10, Babuska
and Cara, 1991)

• Type of Symmetry n Typical Mineral
• Triclinic 21 Plagioclase
• Monoclinic 13 Hornblende
• Ortorhombic 9 Olivine
• Tetragonal 6 Stishovite
• Trigonal I 7 Ilmenite
• Trigonal II 6 Quartz
• Hexagonal 5 Ice
• Cubic 3 Garnet
• Isotropic Solid 2 Volcanic glass

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(Page 11, Babuska and Cara, 1991)
9
Isometric (or Cubic)SystemThree axes of
symmetry, all at right angles to one another and
all of equal length (Chesterman, 1987).
10
Tetragonal SystemThree axes of symmetry two
axes, of equal length, lie in a plane at 90o the
third is longer or shorter and is at right angles
to the others (Chesterman, 1987).
11
Hexagonal (or Trigonal) SystemFour axes of
symmetry three, of equal length, lie in a plane
at 120o the fourth axis is longer or shorter
and is at right angles to the others (Chesterman,
1987).
12
Orthorhombic SystemThree unequal axes, all at
right angles to one another (Chesterman, 1987).
13
MonoclinicThree unequal axes two axes, at right
angles to each other, lie in a plane the third
axis is inclined to the plane of the other two.
There is one twofold axis (Chesterman, 1987).
14
Triclinic SystemThree axes, all of different
length and none perpendicular to the others
(Chesterman, 1987).
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Symmetry (Chesterman, 1987)

• Plane Symmetry a mirror image
• Axial Symmetry symmetry about an axis
• Symmetry about a Center symmetry about a center
  (each face has a corresponding face parallel to
  it on the opposite side)
• Isometric Highest degree of symmery
• Tetragonal, Hexagonal and Orthorhombic Less
  symmetry
• Monoclinic and Tricilinic No symmetry

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What are the possible causes of the observed
seismic anisotropy ?

• The crack-induced anisotropy
• Upper parts of both the oceanic and continental
  crust
• Systems of parallel vertical microcracks or
  fractures A transversely
• isotropic medium with horizontal symmetry axis
• Combination with a subhorizontal layering or a
  schistosity Orthorhombic
• or lower-symmetry system
• Lower crust
• A few large-scale layering Transverse isotropy
  with the vertical symmetry
• axis for large wavelengths
• Mylonite zones Extremely high seismic anisotropy
  gt 20 for P-wave
• velocities Seismic reflectivity of faults

17
(Babuska and Cara, 1991)
18
(Babuska and Cara, 1991)
19
(Babuska and Cara, 1991)
20
Mylonitized Blueschist (Precambrian rocks in
Linville Falls, USA)
21

• A frozen-in alignment of olivine crystals
• Lithosphere
• Crystallographic a-axes oriented horizontally //
• The direction of the upper mantle flow at the
  time when the lithosphere
• was formed by cooling
• Changes in directions of the fast Pn velocity
  (a-axis direction of Olivine)
• The memory of a past mantle flow
• The seismic anisotropy subducting slab and
  shearing strain near
• the contact of the subducting slab with the
  lithosphere above the subduction
• Tectonic processes like rifts and large-scale
  shearing zones

22

• Asthenosphere
• Surface waves, Rayleigh waves
• A present-day convection flow producing a
  preferred orientation of
• crystals, mainly olivine
• Partial melt
• Inner core
• Free osciallations and travel time data
• A hexagonal symmetry with the main axis oriented
  along the axis of
• the Earths rotation
• Alignment of hexagonal close-packed iron

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What is the contribution of the seismic
anisotropy to the Earth Science ?

• Better more realistic seismic models of the
  Earths interior (anisotropic earth models)
• Information on large-scale structural features
  contributing structural geodynamic
  reconstructions (mineral orientation)
• Information on seismogenic zones (crack
  orientation)
• - Information on oil resevoirs (reservoir
  characterization, boreholes VSP in situ
  stress, fractures, pore pressure and
  permeability)

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1.2 Methods of measuring of elastic parameters of
rocks and minerals

• (Babuska and Cara, 1991, p39-70)
• STATIC (bending, twisting, compression of
  oriented crystals)
• DYNAMIC (vibration of crystals)
• At atmospheric pressure ? Results of static
  method ? Results of dynamic method
• At elevated pressure ? Results of static method
  Results of dynamic method
• At high pressure and temperature ? Use the pulse
  transmission the resonance
• method

25
(Christensen, 1985)
26
(page 92, Christensen and Wepfer, 1989)
27
(Christensen, 1971)
28
Dunite (ultramafic igneous rock Olivine gt90,
Pyroxene, chromite, no feldspar)
29
Slate (Metamorphic rock, low grade metamorphism
compressive stress, parent rock mudstone or
shale, foilation mica flakes )
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• Magnitude of elastic anisotropy (k)
• Elastic properties of minerals Crystal
  structure Chemical
• composition

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1.3 Velocity anisotropy of rock forming minerals

• Ortho and ring silicates
• Most abundant rock forming minerals in
  lithosphere
• Olivine
• - (Mg Fe)2SiO4
• - Orthorhombic (9 elastic constants),
• - High anisotropy (25 anisotropy for P-wave
  velocities,
• 22 anisotropy for S-wave velocities)
• - Common in upper mantle
• Garnet
• - Ca(Fe, Mg)2Al2Si3O12
• - Cubic (3 elastic constants)
• - Lowest anisotropy
• - Common in ultramafic rocks

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Igneous Rocks (http//csmres.jmu.edu/geollab/Ficht
er/IgnRx/IgnRx.pdf)
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(Ringwood, 1979)
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Olivine, (Mg Fe)2SiO4
35
Olivine
36
Garnet, Ca(Fe, Mg)2Al2Si3O12
37
(page 218, Meissner, 1986 )
38
(page 219, Meissner, 1986)
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• Chain silicates
• Pyroxenes (Mg Fe)SiO8
• Amphibole (Ca2Mg5)Si8O22(OH)2
• Orthopyroxene (Mg,Fe)SiO3
• Orthorhombic (9 elastic constants)
• 16 anisotropy for P and S wave velocities
• Olivine-Orthopyroxene aggregates
• Vmax in Orthopyroxene (a-axes) // Vmin in
  Olivine (b-axes) ? Anisotropy?
• Anisotropy in Amphibolites ? High anisotropy of
  Hornblende
• Anisotropy of Micas (Sheet Silicates) Perfect
  Basal Cleavage

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• In highly deformed rocks such as schist and
  mylonites, perfect
• cleavage ? accuracy of determining elastic
  parameters ? ? scatter of
• data
• Framework Silicates
• Feldspar NaAlSi3O8
• Triclinic (21 constants)
• Quartz SiO2
• Trigonal (7 constants)
• Potassium feldspar kS-wave gtgt kP-wave
• Anisotropy of qtz lt Anisotropy of feldspar

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Plagioclase Feldspar, Albite (NaAlSi3O8)
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Quartz (SiO2)
43

• High pressure minerals - Deeper mantle
• Olivine (a) ? ß-phase
• 400 km, 13GPa, 1400oC
• ? 9 ?
• Vp 8.81km/s ? 9.47 km/s
• Vs 4.98km/s ? 5.74 km/s
• Vs-aniso does not change.
• Vp-aniso 24 ? 17
• ß-phase ? ?-phase
• ?, Vp, Vs - insignificant change
• Anisotropy ? 4-8

44
(page 45, Babuska and Cara, 1991)
45

• Ilmenite MgSiO3
• High pressure form of enstatite
• Stable at the base of the transition region
  and the top of the lower mantle
• Anisotropy(P-waves) 21
• Anisotropy (S-waves) 37
• Significant contribution to seismic anisotropy
  in the mantle
• Appears in deeply subducted slabs
• Contributes to high velocities in the vicinity
  of deep focus earthquakes
• Stishovite High pressure phase of SiO2
• Tetragonal
• High velocities and anisotropy
• Common in continent-continent collision zone
  like in Alps, earth transition zone and
  lower mantle
• Perovskite (Mg,Fe) SiO3 together with
  Magnetwustit (Mg,Fe)O
• Abundant in lower mantle
• Orthorhombic
• Anisotropy (P-waves) 9.3
• Anisotropy (S-waves) 7.2

46
Ilmenite, FeTiO3
47
1.4 Elastic anisotropy of polycrystalline
aggregates

• Causes of the anisotropy
• Preferred mineral orientation
• Mineral layering
• Anisotropic cracks
• At Uppermost crust Cracks Faults
• At Deeper parts of the Earth The plastic flow
  induced preferred orientation
• of minerals

48

• Effects of cracks and grain boundaries
• Crack porosity ? Elastic wave velocities ?
• Vp Vs ? normal to the plane of oriented cracks
• Fluid filled open cracks ? Vp ? Vp-aniso ?
• Vp Vs anisotropy ? water saturation
• Dry rocks ? Vp anisotropy gt Vs anisotropy
• Pressure effects are much more important than
  temperature effects on
• anisotropy.
• Oriented cracks are important in seismological
  and tectonic investigations
• of the upper crust.

49
Effects of preferred mineral orientation

• At high confining pressure, the effects of
  microcracks on seismic velocity ?
• Velocity-anisotropy orientation of minerals
• Minerals k ()
• Mica 59
• Alkali feldspar 46
• Calcite 33
• Plagioclase 31
• Hornblende 27
• Quartz 26
• Olivine 25
• Pyroxene 16

50
Muscovite, KAl3Si3O10(OH)2
51
Serpentine, (Mg, Fe)3Si2O5(OH)4
52
Biotite, K(Mg, Fe)3(Al, Fe)Si3O10(OH,F)2
53
Orthoclase (KAlSi3O8)
54
Calcite, CaCO3
55
Plagioclase Feldspar, Anorthite, CaAl2Si2O8
56
Hornblende, (Ca,Na, K)2-3(Mg,Fe2,Fe3,Al)5(SiAl)8
O22(OH)2
57

• Upper mantle rocks-Olivine rich rocks Most
  anisotropic rocks Dunite
• Peridotite include Garnet (16), Olivine (25)
  and Pyroxene (1).
• What are the main factors which determine the
  degree of elastic
• anisotropy of crytalline rocks ?
• Values of anisotropy coefficient of constitutent
  minerals and their volume in the aggregate.
• Degree of preferred orientation of minerals
  other fabrics (oriented micro discontinuties or
  alternating layers)
• Orientation of active slip directions like in
  Olivine 100 wrt the symmetry of single-crystal
  elastic anisotropy (beside garnet)
• Main mechanisms Plastic Viscous flow

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The Grand Canyon, USA (Slide 1-4, page 1-2,
Thomsen, 2002)
59
Slide 1-5, page 1-2 (Thomsen, 2002)
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Heterogeneity

• The heterogeneity is the dependence of physical
  properties upon
• positions.
• Shale sections in the Grand Canyon are
  homogeneous from the distance.
• Microscopic vertical section of the shale is
  heterogeneous.
• Grains are not round but have flat shapes. They
  are aligned in a preferred
• direction.
• What is the cause of the preferred orientation ?
• Physics of sedimentation (Individual particles
  will tend to land on their flat
• sides and survive the lithologic processes)
• Mud ? Shale
• Why have the particles flat sides ?
• Shapes of their crystal cells
• The grains are anisotropic because of their
  crystal structures.

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Side 1-6, page 1-4 (Thomsen,2002)
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Slide 1-7, page 1-4 (Thomsen, 2002)
63

• Fabric gravity forces (preferred orientation in
  the vertical direction)
• The layered sedimentary hand sample
• The tree-trunk
• The outcrop
• Fabric The gradient of temperature in the
  cooling magma
• The jade hand-sample
• Fabric The atomic arrangement
• The crystal
• No fabric The heterogeneity grains (randomly
  oriented) fractures(stress)
• the sandstone core
• Fabric Stress
• The outcrop (rectangular prisms created by two
  vertical orthogonal
• fracure sets)
• HETEROGENEITY ON SMALL SCALE
• ANISOTROPY ON LARGE SCALE
• WAVE LENGTH SCALE OF ORDERED
  HETEROGENEITIES
• ANISOTOPIC WAVE PROPAGATION

64
Wilson Greek Rock, USA (Basement
rock)QuartzFeldsparStrong Foilation
(Muscovite) ? Extremely anisotropicV? 4 km/s,
V// 8 km/s
65
North Carolina, USA (Precambrian)Quartz Large
Feldspars Mica (foilation)
66
Mudstone Slate Phyllite(Parent
rock) (low grade comp stress) (Medium grade
comp. stress)
67
Gneiss
68
Obsidian
69
(Babuska and Cara, 1991, p 55)
70
Shear wave splitting
71
(Babuska and Cara, 1991, page 63)
72
(Babuska and Cara, 1991, page 64)
73
(Babuska and Cara, 1991, page 70)
74
References

• Babuska, V. and M. Cara, 1991, Seismic Anisotropy
  in the Earth, Kluwer Academic
• Pub., 39-63.
• Chesterman, C.W., 1987, The Audubon Society,
  Field Guide to North American Rocks and
• Minerals.
• Christensen, N.I., 1971, Shear wave propagation
  in rocks, Nature, v229, p549-550.
• Christensen, N.I., 1985, Measurements of dynamic
  properties of rocks at elevated
• pressures and temperatures in Pincus, H.J. And
  Hoskins, R.R. eds.,Measurements
• of rock properties at elevated pressures and
  temperatures Philadelphia,
• Penssylvania American Society for Testing and
  Materials ASTM STP869, p93-107.
• Christensen, N.I. and Wepfer, W.W., 1989,
  Laboratory techniques for determining seismic
• velocities and attenuations with applications to
  the continental lithosphere, in Pakiser, L.C.
• and Mooney, W.D., Geophysical framework of the
  continental United States, Boulder,
• Colorado, Geological Society of America Memoir,
  172.
• Thomsen, L., 2002, Understanding Seismic
  Anisotropy in Exploration

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Metamorphic rocks (Smith, 1981)
76
Pressure and Temperature Fields (Smith, 1981)
77
Major metamorphic facies (Smith, 1981)
78
Mineral Assemblages developed in clay-rich rocks
(Smith, 1981)
79
Sedimentary Rocks (Smith, 1981)