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Esci 203, Earthquakes & Earth Structure Geomagnetism

Esci 203, Earthquakes & Earth Structure Geomagnetism. John Townend EQC Fellow in Seismic Studies john.townend@vuw.ac.nz. Geophysics in the field. Overview. Importance for plate tectonics Some underlying physics Magnetic induction and the magnetic susceptibilities of common geomaterials

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Esci 203, Earthquakes & Earth Structure Geomagnetism

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  1. Esci 203, Earthquakes & Earth StructureGeomagnetism John Townend EQC Fellow in Seismic Studies john.townend@vuw.ac.nz

  2. Geophysics in the field

  3. Overview • Importance for plate tectonics • Some underlying physics • Magnetic induction and the magnetic susceptibilities of common geomaterials • The Earth’s magnetic field • Magnetic surveying and magnetic anomalies

  4. Magnetism and plate tectonics Figures courtesy of USGS

  5. Magnetic forces

  6. Magnetic field strength Magnetic field strength measured in tesla (T) • The average strength of the Earth’s magnetic field is ~510–5T, or 50,000 gamma (1 gamma  1 nT = 10–9T) cf.

  7. Magnetic induction • The magnetisation (M) induced in a body by an ambient magnetic field is determined by the physical properties of the body, and the field strength: where H is the prevailing magnetic field strength and k is the “magnetic susceptibility” • In exploration geophysics, we’re interested in the spatial distribution of k (cf. density) M = kH

  8. Magnetic susceptibilities • The susceptibility of a rock is controlled by the proportions of different minerals, and each mineral’s atomic structure

  9. Magnetic field geometry north declination east inclination down The local magnetic field orientation is measured in terms of declination (D, measured from north) and the inclination (I, positive downwards from the horizontal)

  10. The Earth’s magnetic field • The main field • Hypothesised to be caused by electrical currents in the outer core • The external field • Generally small component, produced by ionosphere/solar wind interactions • The crustal field • Magnetism induced by main field, plus remanent magnetism

  11. Main field The first order approximation to the main field is that of a simple bar magnet aligned almost parallel to the spin axis. This is referred to as a dipole field. The figure on the left is an oversimplification — see, e.g., Fig. 3.3 of Fowler

  12. Sources of the main field • Dipolar magnetic source (90%) • First described in 1600 by W. Gilbert • If this were the only source, the magnetic and geographic poles and equators would be the same • Non-dipolar source (10%) • Complex magnetic field produced by magnetohydrodynamic processes in the outer core

  13. The Earth’s main magnetic field • The main magnetic field is not due solely to a dipole aligned with the Earth’s rotation axis

  14. Temporal variations of the Earth’ magnetic field • Secular variations • Timescale of years • Diurnal variations (~20–30 nT) • Daily variations; need to be corrected for • Magnetic storms (~1000 nT) • Ionospheric activity, related to sun-spots • Paleomagnetic reversals • Abrupt (?) changes on Ma-timescales

  15. Solar wind and geomagnetic storms Solar wind and aurora australis 2118 UTC, 9 May 2004 www.sec.noaa.gov/SWN/ Geomagnetic storm,12 Aug. 2003

  16. Paleomagnetism • By measuring the magnetic inclination (I) of, say, a lava flow, we can determine the paleomagnetic latitude (l) at which it was erupted. 2 tan l = tan I • Given the magnetic declination (D) as well, and the current geographic position of the lava flow (lX, fX), we can determine the paleomagnetic pole’s latitude (lP) and longitude (fP) lP, fP = f (lX,fX, l, D) See, e.g., Mussett and Khan §10.1 or Fowler §3.1.3 for details

  17. Fmain Fanomalous Ftotal Magnetic surveying • The proton precession magnetometer measures the magnitude of the total magnetic field (Ftotal) • Watch out for AC power sources, belt buckles, knives, cars, fences, iron bars,…!

  18. Aeromagnetic surveying Used particularly for mineral exploration and regional geologic studies

  19. Magnetic map of greaterNew Zealand • Image courtesy of Rupert Sutherland, GNS. • See Sutherland (1996, Tectonophysics)

  20. Anomalies depend on the subsurface susceptibility distribution, and the local main field orientation 90°Nmag 45°Nmag 0°Nmag Local magnetic anomalies

  21. Shallow crustal magnetic anomalies

  22. Shallow crustal magnetic anomalies

  23. Magnetic surveying Aeromagnetic survey Ground survey Modelled magnetic and density structure

  24. Reading material • Mussett and Khan • Chapter 10: Sections 10.1, 10.2, 10.5 • Chapter 11: Sections 11.1–11.3 • Fowler (2005) • Sections 3.1 and 3.2 • Skip p. 48–51 if the maths is too daunting • http://galitzin.mines.edu/INTROGP • An introduction to exploration magnetics

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