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# Gravity method

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### Gravity method

1. 1. Gravity Method By Dr. Amin E. Khalil
2. 2. 2 21 r MM GF
3. 3. gMF 2 21 2 r MM GgM 2 r M Gg
4. 4. R = radius of the earth Earth is considered as sphere and hence radius is constant.
5. 5. Earth is not sphere and hence radius is not constant. Hence correction must be applied
6. 6. Because many factors affect the value of g. As an example: •Earth flattening. •Surface relief. • anomalous masses inside the earth
7. 7. What is geoid? It is equipotential surface where the g is always constant. The equipotential surface is usually taken as the sea level. However, due to some heterogeneity in the earth Geoid may differ slightly from Sea level.
8. 8. Reduction is carried out to correct for: •Latitude •Elevation •Excess mass •Terrain •Drift •Isostacy •Earth’s rotation
9. 9. The earth is flattened, i.e the radius at the equator is longer than at the poles. gn = 978031.85 (1.0 + 0.005278895 sin2(lat) + 0.000023462 sin4(lat)) (mGal)
10. 10. geoid
11. 11. gb = gobs - gn + 0.3086h - 0.04193r h (mGal)
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