Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 89

be remotely detected through its effect on the gravity field. It
should be noted that variations in the vertical component of
gravity measured beside or below the source, e.g. downhole
or with underground measurements, will be different;
explanations of these are beyond our scope.
More information about the shape and location of the
source of a gravity anomaly can be obtained if, instead of
measuring just gravity, tensor measurements are made (see
Section 2.2.3 ). Three-component gradient or full-tensor
gradiometers measure the gravity gradient in three perpen-
dicular directions. Data of this kind provide more infor-
mation about the source than gravity alone. However, and
as shown in Fig. 3.4 , the individual gradients are not easily
correlated with the overall shape of the source, but collect-
ively they are very useful for quantitative interpretation
using inverse modelling methods (see Section 2.11.2.1 ).

a)

Gzz

Gxx

Gxy

3.2.3 Magnetism and magnetic fields

In describing the principles of magnetism and magnetic
fields, we will use the term

Gyy

when referring to
the properties of a body that is magnetic and the term
'

' magnetism '

to describe the process of acquiring mag-
netism. In the literature it is common to use magnetisation
and magnetism as equivalent terms, which can be unneces-
sarily confusing.

magnetisation '

Gxz

3.2.3.1 Magnets and magnetic dipoles
An important characteristic of magnetism is that it can be
intrinsic, i.e. a material can inherently possess magnetism,
or the magnetism can be induced through the material s

being affected by an external magnetic field, such as the
Earth

'

Gyz

s magnetic field. These two types of magnetism are
called permanent or remanent magnetism and induced
magnetism, respectively. Some materials exhibit both kinds
of magnetism (at the same time); for some their magnetism
is predominantly or solely of one type; and some other
materials are incapable of becoming magnetic. The overall
magnetism of a body is the vector sum (see Appendix 1 ) of
its induced (J Induced ) and remanent (J Remanent ) magnetisms.
The ubiquitous bar magnet is a convenient vehicle for
describing the principles of magnetism. Figure 3.5a shows
such a magnet and its external magnetic field. Magnetism
can be described in terms of the magnetic pole, the funda-
mental element of magnetism. This is the basis of the older
cgs system of measuring magnetism. Magnetic poles have
either positive or negative polarity, referred to as north and
south poles, respectively. Unlike poles attract and like poles

'

b)

Source

Depth

Figure 3.4 The gradients (a) of the gravity field which form the

gravity tensor measured on a horizontal surface above a dense
spherical source (b).