Variation of G with latitude
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VARIATION OF G WITH LATITUDEgp =983.2 Gal ge = 978.0 GalRadius gp
rad-gerad = 6.6 Gal
Excess mass gpem-ge
em = -4.8 GalRotation gp
rot-gerot = 3,4 Gal
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VARIATION OF G WITH LATITUDE
In the long-term, the earth behaves like a fluid Reference ellipsoid (approximation to geoid)
The scalar potential verifies the Laplace equation and the solution can be expanded on Legendre polynomials
degree 0 degree 2 rotation
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VARIATION OF G WITH LATITUDE
We then get for the normal gravity (uniform density) due to position only
We are interested in the gravity anomaly gobs -gn
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VARIATION OF G WITH LATITUDE
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FREE AIR CORRECTIONAt the Earth’s surface
z is the elevation from sea level. The gravity anomaly is then(brings gravity to sea level)
Units: mGal, m
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BOUGUER CORRECTION
We also need to correct for the mass between sea level andelevation z. Bouguer assumes an infinite slab of density ρ andthickness z
Units: mGal, m, g/cm3
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BOUGUER CORRECTIONThe difficulty is to choose the right density ρ (average crustalrock has a density of 2.67
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TERRAIN CORRECTION
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TERRAIN CORRECTION (HAMMER)
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TERRAIN CORRECTION (LABORIOUS)
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OTHER CORRECTIONS AND PRECAUTIONS
• Isostatic correction: due to deeper sources regional trend correct or filter
• Tidal correction correct or drift if short time sales• Elevation• Latitude• Drift• Base
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GRAVITY OF SIMPLE SHAPES
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GRAVITY OF SIMPLE SHAPES
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GRAVITY OF SIMPLE SHAPESBedrock density 2.3, ore density 3.0, half cylinder at depth 100m radius for precision 0.1 mGal?
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GRAVITY OF SIMPLE SHAPES
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REGIONALS AND RESIDUALS
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REGIONALS AND RESIDUALS
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REGIONALS AND RESIDUALSGEOLOGICAL KNOWLEDGE ANDEXPERIENCE
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REGIONALS AND RESIDUALSTREND SURFACES
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REGIONALS AND RESIDUALSTREND SURFACES
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REGIONALS AND RESIDUALSTREND SURFACES
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REGIONALS AND RESIDUALSUPWARD CONTINUATION
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REGIONALS AND RESIDUALSSECOND DERIVATIVES
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REGIONALS AND RESIDUALSSECOND DERIVATIVES
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GRAVITY INTERPRETATIONHALF-MAXIMUM TECHNIQUE
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GRAVITY INTERPRETATIONHALF-MAXIMUM TECHNIQUE
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GRAVITY INTERPRETATION2ND DERIVATIVES TECHNIQUE
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GRAVITY INTERPRETATION2ND DERIVATIVES TECHNIQUE
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GRAVITY INTERPRETATION2ND DERIVATIVES TECHNIQUE
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DETECTION OF CAVITIES
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BEDROCK DEPTHS