Geology Reference
In-Depth Information
2
GRAVITY METHOD
Differences in rock density produce small changes in the Earth's gravity field
that can be measured using portable instruments known as gravity meters or
gravimeters.
2.1 Physical Basis of the Gravity Method
The attraction between two point masses
m
1
and
m
2
is given by Newton's
Law:
F
=
Gm
1
.
m
2
/
r
2
The gravitational constant,
G
, has a value of 6.67
×
10
−
11
Nm
2
kg
−
2
. Gravity
fields, measured in the SI system in newtons per kilogram, are equivalent to
accelerations, for which the numerically identical and more widely used unit
is the m s
−
2
. This is inconveniently large for geophysical work and is divided
by a million to produce the more practical 'gravity unit' (µNkg
−
1
, µms
−
2
or 'g.u.'). In principle, the g.u. should long ago have replaced the c.g.s.
'milliGal' (mGal), which is equal to 10 g.u., but the older unit obstinately
refuses to die. Because it is used in virtually all equipment manuals and
most publications, it is also used in this chapter.
2.1.1 Gravity field of the Earth
The Earth's gravity field is approximately equal to that of a sphere with the
same average radius and total mass, but increases slightly (by about 0.5%, or
5000 mGal) from the Equator to the poles. The rate of change with latitude
is zero at the poles and Equator, and reaches a maximum of about 0.8 mGal
per kilometre north or south at 45
◦
latitude (Figure 2.1). The relationship,
in mGal, between 'normal' sea-level gravity and latitude (
λ
) is described by
the 1967
International Gravity Formula (IGF67)
:
g
norm
=
978 031
.
85
+
5162
.
927 sin
2
λ
+
22
.
95 sin
4
λ
The theoretical sea-level gravity at the Equator is thus 978 031.85 mGal.
This formula replaced an earlier, 1930, version with slightly different
