GRAVITY SURVEY (i) -Introduction-

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GRAVITY SURVEY (i)-Introduction-
June, 2004
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Gravity Survey

• Measurements of the gravitational field at a
  series of different locations over an area of
  interest. The objective in exploration work is to
  associate variations with differences in the
  distribution of densities and hence rock types.
  Occasionally the whole gravitational field is
  measured or derivatives of the gravitational
  field, but usually the difference between the
  gravity field at two points is measured
• (From R. E. Sheriff Encyclopedic
  Dictionary of Exploration Geophysics, published
  by the Society of Exploration Geophysics )

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• In gravity prospecting, geophysicists measure
  minute variations in the force of gravity from
  rocks up to a few miles beneath the earth's
  surface.
• Different types of rocks have different
  densities, and the denser rocks have the greater
  gravitational attraction.

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• Theory of gravity
• Use two of Newtons laws
• Universal law of gravitation
• Where G6.67 10-11 N m2/kg2 (m3/kg.s2)
• Second law of motion F mg
• We can combine them to obtain the gravitational
  acceleration at the surface of the earth

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• Gravity and potentials
• g is a vector field
• where r1 is the unit vector pointing toward the
  center of the Earth
• Gravitational potential
• U is a scalar field which makes it easier to work
  with g is a vector field
• Definition The gravitational potential, U, due
  to a point mass m, at a distance r from m, is the
  work done by the gravitational force in moving a
  unit mass from infinity to to a position r from
  m.

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• Relating g to U
• U is a scalar field which makes it easier to work
  with
• Potentials are additive
• Gravity is a conservative force
• And gravitational acceleration can be easily
• determined from the potential
• It follows that
• For smaller scale problems we usually deal with
  g,
• and sum the vertical component of g

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• Units for g
• SI unit for g m/s2
• 1 cm/s2 1 Gal (for Galileo) 0.01 m/s2
• milliGal or mGal 10-3 Gal typical unit for
  field studies
• some text book uses the gravity unit (g.u.)
• 1 g.u. 0.1 mGal
• Normal value of g at the surface of the Earth
• gE 9.8 m/s2 980 cm/s2 980 Gal 980,000
  mGal 9800 g.u.

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Gravity Meter A gravimeter is any instrument
designed to measure spatial variations in
gravitational acceleration
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