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Potential Fields Methods Potential Fields

Potential Fields Methods Potential Fields A potential field is a field in which the magnitude and direction (vector) of the measurement depends on the location of the measuring instrument.

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Potential Fields Methods Potential Fields

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  1. Potential Fields Methods Potential Fields A potential field isa field in which the magnitude and direction (vector) of the measurement depends on the location of the measuring instrument. The two geophysical potential field methods we will looks at are magnetism and gravity. Magnetic methods are based on the interactions of earth materials and the earth’s magnetic field. More on this later. We will now discuss gravity...

  2. Gravity Newton’s Law of Gravitation (describes the force between two bodies) F = G (m1m2) / r2 Where: F = force of attraction between the two objects (in Newtons, N, kg m / s2) G = Universal Gravitational Constant (6.67x10-11 N m2/kg2) m1,m2 = the masses of the bodies r = the distance between the centers of mass of the two objects

  3. Gravity For Earth’s gravity field: g = G M / R2 Where: g = gravitational acceleration observed at the Earth’s surface (in units of m/s2, or Gal) G = Universal Gravitational Constant (6.67x10-11 N m2/kg2) M = the mass of the Earth R = the distance from the observation point to the Earth’s center of mass Note: • g does NOT depend on the mass of the object attracted to earth • g follows an inverse square law, meaning that as your distance from the Earth’s center of mass doubles, the value of g is reduced by 1/22 or 1/4, etc.

  4. Gravity For Earth’s gravity field: g varies from 978 Gal at the equator to 983 Gal at the poles (because of the equatorial bulge and centripital force) The gravity anomalies we will be looking at are small relative to the mass of the Earth (obviously), so we tend to look at gravity in units of milligals (mGal) so: 1 mGal = 10-3 Gal = 10-5m/s2 Or the value of g at the equator is 978000 mGal

  5. Gravity For Earth’s gravity field: To make measurements of gravity useful, the data must be reduced to remove effects of the earth and just deal with anomalies. The measurement of g at any specific location is a function of (1) latitude, (2) elevation, and (3) the mass distribution around the observation point.

  6. Gravity For Earth’s gravity field: To make measurements of gravity useful, the data must be reduced to remove effects of the earth and just deal with anomalies. Step #1 : Remove latitude effect glat.cor = gobs - gt Where (all in mGal) glat.cor = the latitude-corrected gravity value gobs = your observed (measured) gravity reading gt = the theoretical gravity for the latitude of the observation point

  7. Gravity For Earth’s gravity field: To make measurements of gravity useful, the data must be reduced to remove effects of the earth and just deal with anomalies. ... gt = ge (1+0.005278895sin2f+0.000023462sin4f) where gt = the theoretical gravity for the latitude of the observation point ge = theoretical gravity at the equator (978031.85 mGal) f = latitude of the observation point (degrees)

  8. Gravity For Earth’s gravity field: To make measurements of gravity useful, the data must be reduced to remove effects of the earth and just deal with anomalies. Step #2 Make the free air correction (FAC), which accounts for the local change in gravity due to elevation. gFAC = glat.cor + FAC where FAC = elevation x (0.308 mGal/m)

  9. Gravity For Earth’s gravity field: To make measurements of gravity useful, the data must be reduced to remove effects of the earth and just deal with anomalies. Step #3 Correct for the mass surrounding the observation point both vertically (simple Bouguer correction) and horizontally (complete Bouguer correction).

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