PHYSICAL PROPERTIES of the EARTHS INTERIOR: ELECTRICAL CONDUCTIVITY

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PHYSICAL PROPERTIES of the EARTHS INTERIOR
ELECTRICAL CONDUCTIVITY

• Electrical properties of rocks and minerals
• Origin and characteristics of magnetotelluric
  fields and
• telluric currents
• Conductivity of the crust and upper mantle
• Regional and local conductivity anomalies
• Radial distribution of electrical conductivity

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Electrical properties of minerals and rocks

• Physical properties of rocks and minerals
• Parameters Range
• Densities gtgtgt small
• Eastic wave velocities gtgtgt small
• Radioactive content gtgtgt small
• Magnetic susceptibility gtgtgt large 105
• Electrical resistivity gtgtgt greatest
• 10-5 ?m (dry coarse grained rocks) - 107 ?m
  (gabbro)

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• Electrical conduction
• R Resistance, ohm (?) V Volt
• I Amper
• ? Resistivity (?m) ? Conductivity (1/ ?m
  or mho/m)
• A m2 Siemens/mS/m
• l m

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• Materials Conductivity (?)
• Conductor ? gt 105 Sm-1
• Large number of free electrons
• Metals and graphite
• Semiconductor 105 gt ? gt 10-8 Sm-1
• Fewer mobile electrons
• Insulator ? lt 10-8 Sm-1
• Ionic bounding

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• Classification of materials based on the
  conductivity parameter (Schön,
• 1998)
• Metallic conductors
• 2) Nonconductors
• Insulators
• Semiconductors
• Electrolytes

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• Metallic conductors
• Electrons are not tightly bounded or associated
  with any particular atom.
• Electron conductivity ? Temperature ?
• Native Copper, Silver, Gold
• 10-8 ?m
• Non conductors (Insulators Semiconductors
  Electrolytes)
• Electrons are tightly trapped near atoms due to
  large energy barriers
• between atoms.
• Electron conductivity ? Temperature ?
• Most rocks and Minerals

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• Insulators
• Large energy barriers between atoms
• Most silicates
• 109-1017 ?m
• Semiconductors
• Energy barriers are slightly higher than the
  available energy from thermal activation at room
  temperature.
• Most sulfides and oxides 10-6-104 ?m
• Electrolytes
• T (? Tcritical) ? Electrical conductivity ?
• T (gt Tcritical) ? Electrical conductivity ?
• Water 5x10-6 (pure) mho/m
• Saturated water with salt in solution 102 mho/m
• Mixture of these materials ? 10-8 1017 ?m

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• Electrical conduction in rocks and minerals
• Metallic conduction (native metals copper, gold
  and graphite)
• Electronic semiconduction (ilmenite, magnetite,
  pyrite and galena)
• Electrolytic conduction
• Solid electrolytes (ionic crystals) motions of
  ions
• Most rock forming minerals act as solid
  electrolytes with the transfer of electric
  current being by motion of ions through a crystal
  lattice.
• Electrolyte water solutions conductivity of
  pore water
• In water-bearing rocks, the electrolyte
  conductivity of the pore water has dominant
  influence upon the rock conductivity.
• The different kinds of electrical conduction have
  different temperature
• dependencies.
• Metallic conduction ? Temperature ?
  Conductivity ?
• Semiconduction ? Temperature ? Conductivity ?

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• Resistivity of Minerals
• Most rock-forming minerals (silicates and
  carbonates) have very high
• specific resistivities ( gt 109 ?m). They are
  classified as insulators.
• Conductive minerals (sulfides, some oxides and
  native elements) are
• comparatively rare in the crust
• Examples Resistivity (?m)
• Native metals and their natural 10-8-10-5
• paragenetic sequences
• (graphite)
• Native non metals 5x107-2.7x1016
• (sulphur, selenium, diamond)
• Anumber of sulfides, some oxides 10-6-1011

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• Resistivity of rocks
• (Schön, 1998)

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• Electrical properties of rocks
• Dense, pore and fracture free rocks or absolutely
  dry rocks
• Porous or fractured water bearing rocks
• Porosity and fracturing ? ? Resistivity ?
• Geological processes Their effects on
  resistivity (?)
• Clay alteration ?
• Dissolution ?
• Faulting ?
• Salt water intrusion ?
• Shearing ?
• Weathering ?
• Carbonate perticipitation ?
• Silification ?
• Metamorphism ?
• ?

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• (Schön, 1998)

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• (Schön, 1998)

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• Induced Polarization
• Electrode polarization
• Polarization is attributed to the presence of
  interfaces between zones
• of electronic and ionic conduction.
• Pyrite
• Membrane polarization
• Polarization is attributed to the presence of
  interfaces between zones of
• unequal ionic transport properties.
• Clay
• A few percent clay surface of silicate minerals

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• (Schön, 1998)

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Origin and characteristics of magnetotelluric
field and telluric currents

• Terrestrial Currents
• T10 - 40 s
• The current intensity is much larger on the
  daylight side of Earth as well as in aurora
  latitudes.
• Electrical currents range 10 mV/km
• Magnetic fields range milligamma
• Random Fluctuations
• Intensity electrical disturbances in the
  ionosphere f100 kHz
• Electric storms (f ?) used in AFMAG methods
  (thunder storm energy)
• Amplitude peaks at 8, 14 and 760 Hz

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• Audio and subaudio telluric and MT signals
• Diurnal amplitude and azimuthal variations of
  telluric signals at
• 1, 8, 145 and 3000 Hz
• Amp. ? ? sunset
• Amp. ? ? sunrise

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• (Telford et al., 1990)

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• Source I Periodic transient
  fluctuations
• diurnal variations in the
  Earths
• Magnetic field
• Large scale (low f)
• Source II Activities in Source I
• influence the currents in
• the ionosphere.
• Magnetotelluric Fields (MT) penetrate
• the Earths surface to produce
• the telluric currents.
• The telluric currents are induced
  in
• the Earth by ionospheric
  currents.
• (Telford et al., 1990)

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• Conductivity of the crust and upper mantle
• Conductivity of rock unit
• 1) Saline water in pores and cracks
• 2) Conductive solid minerals
• 3) High temperature
• 4) Partial melts
• or combination of these four factors (Meissner,
  1986)

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• Low conductivities unfractured crystalline
  rocks
• Generally moisture (1), chemical composition (2)
  and temperature (3 and
• 4) determine the conductivity in laboratory
  measurements.
• Field measurements show shielding or disturbing
  effects of sedimentary
• structures, edge effects and crustal influences.
• Moho does not represent a generally occuring
  electromagnetic
• discontinuity.

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Regional and local conductivity anomalies

• Methods used in the field investigation
  (Meissner, 1986)
• Method 1 4 electrode resistivity-depth probing
  of the direct-current conduction method
• Wide variety of direct-current techniques
• Resistivity-depth profiles for horizontally
  stratified media to map the lateral
  inhomogeneties
• The following Method 2 and 3 use the natural
  variations and pulsations (high frequency) of
  magnetic field of the Earth originating in the
  ionosphere
• Observed electric (Ex, Ey, Ez) and magnetic (Hx,
  Hy, Hz) components on the Earths surface
  External origin other generated by induction

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• Method 2 The magnetotelluric method
• Effective impedance
• Apparent resistivity
• ?a apparent resistivity
• ?o permability of free space

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• Method 3 The magnetic array or magnetic
  gradient method or Wiese vector method
• ?a(w) ? ?a(z)

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• Geomagnetic induction arrows (Wiese, 1965)
• Hva HN b HE
• If we combine the components of various signals
  to the same phase
• Hv(to)aw HN(to) bw HE(to)
• aw, bw determine the geomagnetic induction arrow
  in relation to the north direction.
• tan?wbw/aw
• For the time of the maximum of the vertical
  component, HV is chosen.

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• Method 4 The electromagnetic or controlled
  source magnetic induction method is widespread
  use by the mineral exploration industry.
• Controlled source alternating currents (coil or
  long wire)
• For crustal studies
• Diameter of loop1.5 km
• Recording (3-comp coils)100 km
• Frequency 0.1 - 400 Hz
• Apparent resistivity versus distance ? ? (z)
  models

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• (Meissner, 1986)

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• (Meissner, 1986)

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• (Meissner, 1986)

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• (Schön, 1998)

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Radial distribution of electrical conductivity

• For the conductivity down to 1500 km depth, use
  the short-period variations of the Earths
  magnetic field (less than a second several
  years)
• T lt 1 year Outside solid Earth induced
  currents flowing in the crust and mantle
• Diurnal variations interaction of conducting
  ionospheric layers of upper atmosphere with the
  main magnetic field solar wind and magnetic
  storms
• Others
• T27 days tides caused by the Moons orbital
  motion
• T 1 year
• T11 years sunspot cycle

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• The depth of penetration of an osciallation of
  period (T) above a uniform half-space of
  conductivity ? is
• Period Penetration Depth
• Tshort Shallow depth
• Tlong Deeper
• T1 s 1 year ? ? 1000-1500 km depth
• ? 0-300 km depth (regional)

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• Steps of calculation
• Data Vertical horizontal components of
  magnetic field (Diurnal and other variations T gt
  a few hours 300 1000 km)
• Method
• - Fourier analysis
• Spherical harmonic variations 24, 12, 8 and 6
  hour
• Spherical harmonic analysis were carried out on
  the world wide spread of data for each time of
  period to separate the internal and external
  parts of variation
• - Amplitude ratio and phase differences between
  external and internal parts of each period of
  variation can be determined.
• Use them to estimate conductivity distribution as
  a function of radius down to the depth of maximum
  penetration
• For the calculation of conductivity in the lower
  mantle, use secular variations of the Earths
  magnetic field.

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• (Bott, 1982)

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• Silicate minerals like fayalite are insulators at
  room temperature.
• Fayalite Fe2SiO4
• T 770 K ? Fe2SiO4 ? Spinel phase transition
  (Semi conductors)
• Impurity semi-conduction electronic
  semi-conduction ionic semi-conduction
• Olivine (Fayalite)
• Temperature
• 900 K Impurity semi-conduction ? dominant
• crust topmost mantle
• 1000 K ? Electronic semi-conduction ? dominant
• transition zone and lower mantle
• 1400 K ? Ionic conductivity at low pressure ?
  dominant
• upper mantle

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• (Bott, 1982)

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• Radial distribution of ? ? (Sm-1)
• Ocean water logged sediments 4 10-3 - 1
• Crystalline rocks of lithosphere 10-3
• impurity semi conduction
• Local anomalous regions
• Stable continental regions 10-2 at 80 km
• electronic semi conduction ? at deeper part
• at higher temperature
• Oceans 10-1 within small melt
  fraction asthenosphere
• 300-700 km ? ? ?
• temperature effect on
• electronic and phase transition
• 700 km 1
• Bottom of mantle 100

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REFERENCES

• Bott, M.H.P., 1982, The Interior of the Earth
  its structure, constitution and
• evolution, Elsevier, p 175-184 (ITU Mustafa Inan
  Library, QE 28.2.B68).
• Meissner, R., 1986, The Continental Crust, A
  Geopysical Approach,
• Academic Press, p 85-91.
• Schön, J.H., 1998, Handbook of Geophysical
  Exploration, Seismic
• Exploration, V 18 Physical Properties of rocks
  Fundamentals and Principles
• of Petrophysics, Pergamon press, p 379-457 (ITU
  Mustafa Inan Library,
• 431.6 P5 S34 1998).
• Telford, W.M., Geldart, L.P., Sheriff, R. E.,
  1990, Applied Geophysics,
• p 302-306 (ITU, Mustafa Inan Library).