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VARIATION OF G WITH LATITUDE g p =983.2 Gal g e = 978.0 Gal Radius g p rad -g e rad = 6.6 Gal Excess mass g p em -g e em = -4.8 Gal Rotation g p rot -g e rot = 3,4 Gal
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
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
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
VARIATION OF G WITH LATITUDE
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
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
BOUGUER CORRECTIONThe difficulty is to choose the right density ρ (average crustalrock has a density of 2.67
TERRAIN CORRECTION
TERRAIN CORRECTION (HAMMER)
TERRAIN CORRECTION (LABORIOUS)
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
GRAVITY OF SIMPLE SHAPES
GRAVITY OF SIMPLE SHAPES
GRAVITY OF SIMPLE SHAPESBedrock density 2.3, ore density 3.0, half cylinder at depth 100m radius for precision 0.1 mGal?
GRAVITY OF SIMPLE SHAPES
REGIONALS AND RESIDUALS
REGIONALS AND RESIDUALS
REGIONALS AND RESIDUALSGEOLOGICAL KNOWLEDGE ANDEXPERIENCE
REGIONALS AND RESIDUALSTREND SURFACES
REGIONALS AND RESIDUALSTREND SURFACES
REGIONALS AND RESIDUALSTREND SURFACES
REGIONALS AND RESIDUALSUPWARD CONTINUATION
REGIONALS AND RESIDUALSSECOND DERIVATIVES
REGIONALS AND RESIDUALSSECOND DERIVATIVES
GRAVITY INTERPRETATIONHALF-MAXIMUM TECHNIQUE
GRAVITY INTERPRETATIONHALF-MAXIMUM TECHNIQUE
GRAVITY INTERPRETATION2ND DERIVATIVES TECHNIQUE
GRAVITY INTERPRETATION2ND DERIVATIVES TECHNIQUE
GRAVITY INTERPRETATION2ND DERIVATIVES TECHNIQUE
DETECTION OF CAVITIES
BEDROCK DEPTHS
