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