C025 – Local Gravity Correction

Description

This calculates the value of gravity based on latitude and elevation above sea level.

References

The Geodetic Reference System 1967

Woollard G.P. 1979 “The new gravity system – changes in international gravity base values and anomaly values” Geophysics, 44 1352 – 1366

IAG Developed Geodetic Reference System 1980, leading to World Geodetic Reference System 1984 (WGS84)

Kelton calculation reference C025

FLOCALC calculation reference F025

Options

Formula

• IGF 1967

Calculation is performed in accordance with the Geodetic Reference System 1967

• Wollard 1979

Calculation is performed in accordance with the formulae given in Woollard G.P. 1979 “The new gravity system – changes in international gravity base values and anomaly values” Geophysics, 44 1352 – 1366.

• IAC 1980/ WGS 1984

Calculation is performed in accordance with the IAG Developed Geodetic Reference System 1980, leading to World Geodetic Reference System 1984 (WGS84)

Location

• Offshore

Use this option for calculating gravity for an offshore location

• Onshore

This gives the option of making an additional correction for the density of the rock base

Calculation

All calculations are performed in SI units unless otherwise stated, the formulae use the geographic latitude in radians.

Calculation of gravity at sea level

IGF 1967

Wollard 1979

IAC 1980/ WGS 1984

 Where λ = Geographic latitude in radians

Elevation Correction

The effect of elevation allows for the ‘free-air’ gradient of gravity given by Newtons Law of Gravity, and Bouguer slab effect resulting from the layer of rock between sea-level datum and the site.

 Where ρr = Density of the rock e = Elevation above sea level

Height Correction

A small effect for height above ground level of the site (i.e. bench height) involving the free air gradient alone

 Where h = Height above ground level

Bouguer Gravity Anomaly B

The Bouguer gravity anomaly depends upon the distribution of the underlying rocks anc can only be determined by measurement.

Local Gravity

The value of local gravity can be calculated by

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