Something Geophysical

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Something Geophysical

• Kori Newman
• Orals Review Session
• 20 January 2006

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Types of Geophysics

• Seismology (refraction and reflection)
• Magnetics
• Gravity
• Heat flow

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Body waves
P wave velocity v(l2µ)/r
S wave velocity v(µ/r) vs vp/v3
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Seismic Phases
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Snells Law sin q1/ n1 sin q2/n2 No matter wha
t layers you compare, this is always a constant,
p, the ray parameter. As you go to a higher veloc
ity material, the ray will get bent closer to the
horizontal. This is how we get rays that curve
through the earth When q90, then you get a criti
cal reflection. It travels along the boundary at
the faster velocity, and somehow comes back up
again (Huygens principle allows for wavelets to
be formed from any point on the propagating wave
front).
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Evidence of a Solid Inner Core

• S wave shadow zone from 103-180, shows that
  outer core is a fluid (S waves cant travel
  through fluids). Late P wave arrivals at 140
  originally lead to the belief of a homogenous low
  velocity core. Magnetic field also an indicator
  of a fluid core.
• Inge Lehmann, 1936, proposed the existence of a
  solid inner core.
  
• Faint arrivals in the P wave shadow zone
  (105-142) explained by a reflection off an
  inner core

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Surface Waves
http//www.geo.mtu.edu/UPSeis/waves.html
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T phase
http//www.ldeo.columbia.edu/res/pi/tphase/
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Tolstoy and Bohnenstiehl, 2005
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Marine Reflection Profiling

• Images subsurface
• Depth of section related to frequency and volume
  of sound source
  
• Strength of reflectors depends on impedance
  contrast (velocity and density)
  
• Travel time can be converted to depth

http//www.ldeo.columbia.edu/menke/lithosphere/so
nar/sonar.html
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Magnetics

• Field is 30,000 nT at equator, 60,000 nT at
  poles
  
• Inclined geocentric dipole model accounts for
  90 of the Earths surface magnetic field
  
• North pole is magnetic south (field lines come
  out the geographic south pole and into the
  north)
  

• Field believed to be produced by the turbulent
  liquid outer core
  
• Field flips 1-5 times per million years
• We are believed to be in the middle of a reversal
  because the geomagnetic field is weakening

http//www.psc.edu/science/Glatzmaier/field_big.gi
f
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I is inclination D is declination
tan I 2 tan l
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Polar Wander
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Self Exciting Disk Dynamo

• Mechanism where a conductor rotating in a
  magnetic field can produce a current that then
  produces the magnetic field
  
• Not a perpetual motion machine. In this model
  the disk must be manually spun. In the Earth
  its thought that the solidification of the outer
  core powers the dynamo.

Butler, 1992 (http//www.geo.arizona.edu/Paleomag/
book/)
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Rock Magnetism

• Magnetite (Fe3O4) and hematite (Fe2O3) are the
  major magnetic minerals
  
• Remnant magnetism can be thermal, detrital or
  chemical
  
• Preserved inclinations show the latitude at which
  the rock was formed (tan I 2 tan l)

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Gravity

• FGmm/r 2 Universal Law of gravitation --
  Falls off with 1/r 2
  
• Range of earths field 978-983 Gals (greatest at
  the poles why? see next slide)
  
• Geoid equipotential to sea level.
• Two methods of measuring
• Absolute pendulum, simple free fall
• Relative measurements springs
• Correct for latitude, elevation, and mass to use
  what we do know to find out more information
  about what we dont know.

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Earths not perfectly round
Shape Oblate spheroid (that mean the earth
bulges at the equator, and is shorter at the
poles) Why? Balance of forces centrifugal due
spin and gravity inward -- shape change so that a
plumb bob will point perpendicular to the Earths
surface Less oblate during ice age due to unloadi
ng of equatorial regions from the accumulation of
ice at poles
Equatorial radius 6378.16 km Polar
radius 6356.77 km Flattening of the earth (Re
-Rp)/Re1/298.247
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Satellite Seafloor Mapping

• Increased gravity over topographic features
• The ocean bulges over these features
• Smith Sandwell map in this slide, but first
  done by Bill Haxby

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Isostasy

• Any given column should be the same density
• Loading an area will make it sink

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Crustal Thickness
Mountain belts have deep crustal roots
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Thermal Isostasy

• Cooling as material moves away from ridges
  results in thermal subsidence
  
• Ridge axis height falls off as 1/v(age)

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Heat Flow

• Heat flux out of the Earth
• Total surface heat flow is 4.431033 W (mean
  surface heat flow is 87 mW/m2
  
• High heat flow at volcanic areas
• High heat flow in stable continental crust
  usually due to radioactivity
  
• Near surface thermal gradient is 25 C/km

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