Geology Reference
In-Depth Information
3
MAGNETIC METHOD
Compasses and dip-needles were used in the Middle Ages to find magnetite
ores in Sweden, making the magnetic method the oldest of all applied
geophysical techniques. It is still one of the most widely used, even though
significant magnetic effects are produced by only a very small number of
minerals.
Magnetic field strengths are now usually measured in
nanotesla
(nT). The
term '
gamma
', originally defining a unit equal to 10
−
5
gauss and numerically
equal to the nT, is still occasionally used
3.1 Magnetic Properties
Although governed by the same fundamental equations, magnetic and grav-
ity surveys are very different. The magnetic properties of adjacent rock
masses may differ by several orders of magnitude rather than a small per-
centage (see Table 1.2).
3.1.1 Poles, dipoles and magnetisation
An isolated magnetic pole would, if it existed, produce a field obeying the
inverse-square law. In reality, the fundamental magnetic source is the dipole
(see Section 1.2.5) but, since a line of dipoles end-to-end produces the same
effect as positive and negative poles isolated at opposite ends of the line (Fig-
ure 3.1), the pole concept is often useful.
A dipole placed in a magnetic field tends to rotate, and so is said to have
a
magnetic moment.
The moment of the simple magnet of Figure 3.1, which
is effectively a positive pole,
m
, at a distance 2
L
from a negative pole with
strength
−
m
, is equal to 2
Lm
. The magnetisation of a solid body is defined
by its magnetic moment per unit volume and is a vector, having direction as
well as magnitude.
3.1.2 Susceptibility
A body placed in a magnetic field acquires a magnetisation,
M
, that, if small,
is proportional to the field:
M
=
kH
