MAGNETIC METHOD - Field Geophysics - page 66

Figure 3.1 Combination of magnetic dipoles to form an extended magnet.
The positive and negative poles at the ends of adjacent dipoles cancel
each other out. The pole strength of the magnet is the same as that of the
constituent dipoles, but its magnetic moment is equal to its length multiplied
by that pole strength.
The susceptibility , k , is small for most natural materials, and may be either
negative (diamagnetism) or positive (paramagnetism). The fields produced
by such materials are usually considered too small to affect survey magne-
tometers. Modern high-sensitivity magnetometers are creating exceptions
to this rule, but most useful magnetic anomalies are still due to the small
number of ferro- or ferri-magnetic substances in which the molecular mag-
nets are held parallel by intermolecular exchange forces. Below the Curie
temperature , these forces are strong enough to overcome the effects of ther-
mal agitation. Magnetite, pyrrhotite and maghemite, all of which have Curie
temperatures of about 600 ◦ C, are the only important naturally occurring
strongly magnetic minerals and, of the three, magnetite is by far the most
common. Hematite, the most abundant iron mineral, normally has a very
small susceptibility, and many iron ore deposits do not produce significant
magnetic anomalies.
3.1.3 Susceptibilities of rocks and minerals
The susceptibility of a rock usually depends on its magnetite content.
Sediments and acid igneous rocks have small susceptibilities whereas
basalts, dolerites, gabbros and serpentinites are usually strongly magnetic.
Weathering generally reduces susceptibility because magnetite is oxidised
to hematite, but some laterites are magnetic because of the presence of
maghemite and remanently magnetised hematite. The susceptibilities, in
rationalised SI units, of some common rocks and minerals are given in
Table 1.2. Negative values, indicating diamagnetism, are observed only
with very pure materials, since diamagnetic moments are nearly always
swamped by any paramagnetism present.
The magnetic properties of highly magnetic rocks tend to be extremely
variable and their magnetisation is not strictly proportional to the applied
field. Quoted susceptibilities are for Earth-average field strengths.