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27 Cards in this Set
- Front
- Back
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What are the 7 corrections that must be made to raw (measured) gravity data? |
1) Instrument Drift 2) Earth Tide 3) Latitude 4) Eotvos 5) Free-air correction 6) Bouguer correction 7) Terrain Correction |
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What is the instrument drift correction? |
Correct for instrument variations that take place independent of location. |
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Is drift added or subtracted from the rest of the data? |
subtracted |
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What is the Earth tide correction? |
Correct for variations in gravity arising from the attraction of the moon and sun. |
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Is Earth tide added or subtracted from the rest of the data? |
subtracted |
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What is the latitude correction? |
Correct for the oblate shape of the Earth that results from rotation. |
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Do we add or subtract the latitude correction? |
subtract, but need to survey a wide enough area to use it |
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What is the Eötvös correction? |
Correct for measurements taken on a moving platform. |
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Do we subtract or add the Eotvos correction? |
add |
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What is the free-air correction? |
Correct for a difference in elevation between your station and the reference datum. |
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Do we add or sub free-air correction? |
add |
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What is the Bouguer correction? |
Correct for "excess mass" between your station and the reference datum. |
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Do we add or sub the Bouguer correction? |
subtract |
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What is the terrain correction? |
Correct for the attraction of mass/terrain at locations not directly beneath your station. |
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Do we add or sub the terrain correction? |
add |
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What are the two anomalies we’ve discussed? |
1) Free air anomaly
2) Bouguer anomaly (both simple and complete Bouguer |
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What is the free air anomaly? |
= gravity measured – drift correction – latitude correction + Eotvos correction + free air correction |
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What is the simple bouguer anomaly? |
= gravity measured – drift correction – latitude correction + Eotvos correction + free-air correction – bouguer correction |
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What is the complete bouguer anomaly? |
= gravity measured – drift correction –latitude correction + Eotvos correction + free air correction – bouguer correction + terrain correction |
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What are the mass contributes to each anomaly? |
Free-air anomaly = contributions from mass below and above reference datum.
Bouguer anomaly = contributions from mass only below reference datum. |
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What are the approximate size of each of the corrections (in mGal) compared to the size of some of the anomalies we looked at in class? |
free-air anomaly can be +-300mGal, bouguer anomaly can be +100 to -600mGal |
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If you measure gravity on the ground floor of Lapham and on the 5th floor, gravity will increase as you go up. True or False? |
False |
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A gravity meter can measure the variation in gravity caused by moving up in elevation by a single meter. (In other words, the variation caused by moving 1 m further away from the center of the Earth.) True or False? |
True |
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Compared to a shallow sphere, the anomaly produced by a deeper sphere will be narrower and higher in amplitude. True or False? |
False |
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Because the densities of common rock types overlap, gravity (or density derived from gravity) is not typically useful in identifying a particular lithology. True or False? |
True |
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Your text reports densities in units of mega-grams per meter cubed (Mg/m3). 1 Mg/m3 equals |
1000 kg/m^3 |
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The attractive gravitational force between two bodies is proportional to: |
1) the product of the masses of the two bodies
2) the inverse of the distance between the two bodies squared
3) the universal gravitational constant, G |
