Geoscience Reference
In-Depth Information
Magnetic permeability is analogous to electrical conductiv-
ity (see
Section 5.2.1.2
) and, put simply, accounts for how
easily a magnetic
field can exist within a material. For non-
magnetic (most) minerals
referring to magnetism acquired subsequently. Prolonged
exposure to the Earth
is magnetic
field can produce a sec-
ondary magnetism known as a viscous remanent magnet-
ism (VRM). This type of magnetisation accounts for many
remanent magnetisms being found parallel to the present-
day Earth
'
μ ≈ μ
0
.The ratio
μ=μ
0
is known
as the relative permeability (
μ
r
) and is approximately equal
to one for non-magnetic materials (Zhdanov and Keller,
The higher a material
'
s
field.
The ratio of the strengths of the remanent magnetism
(J
remanent
) and induced magnetism (J
induced
) is known as
the Königsberger ratio (Q):
'
s susceptibility, and/or the
stronger the external field, the stronger will be the magnet-
ism induced in the body. The induced field is parallel to the
field that caused it. However, this description so far neg-
lects the effects of self-demagnetisation and susceptibility
anisotropy (see
Sections 3.2.3.6
and
3.2.3.7
)
.
J
remanent
J
induced
¼
ð
:
Þ
Q
3
11
Because Q is a ratio it has no units. When it is greater than
1, remanent magnetism is dominant, and vice versa.
Although the Königsberger ratio gives an indication as to
whether induced or remanent magnetism is dominant, the
directions of each component also signi
cantly in
uence
the resultant overall magnetism. When the induced and
remanent magnetisms have similar directions, their effects
will be mainly additive creating a stronger overall magnet-
ism; and they will be subtractive when they have opposite
directions, so the resultant magnetism is less.
3.2.3.4
Remanent magnetism
For some materials, the external field may cause irrevers-
ible changes to the material
s magnetic properties (see
Ferromagnetism
in
Section 3.2.3.5
)
, so when the external
field is removed the material retains a permanent or rem-
anent magnetism. The strength and orientation of reman-
ent magnetism is related to the external magnetic
field. at
the time of its formation and is also affected by the mag-
netic mineral content of the rock, and by factors such as
magnetic grain size and microstructure (see
Section
ism is not truly permanent as it does change very slowly
with the long-term variations in the Earth
'
3.2.3.5
Types of magnetism
The magnetic properties of a material are determined by
the electron spins and their orbital motions in the atoms,
the concentration of magnetic atoms or ions, the inter-
action between the atoms, and the molecular lattice struc-
ture. For most atoms and ions the magnetic effects of these
cancel so that the atom or ion is non-magnetic. In many
other atoms they do not cancel, so overall, the atoms have a
magnetic dipole forming the material
'
is magnetic
field;
but it may be assumed to be permanent for our purposes.
There are numerous processes occurring in the natural
environment that affect remanent magnetism. At different
times during its existence a rock may be completely or
partially remagnetised, and all, or part, of an existing
remanent magnetism may be destroyed. Consequently,
several phases of remanent magnetism may co-exist in a
rock. The overall remanent magnetism is the combined
effect, i.e. their vector sum (see
Appendix 1
)
, of the various
permanent magnetisms and is called the natural remanent
magnetism (NRM).
Remanent magnetism is parallel, or very nearly so, to the
Earth
'
s intrinsic or spon-
taneous magnetisation
the magnetisation in the absence
of an external magnetic field. The reaction of the atomic
structure of matter to an external field can be classified into
three distinct types of magnetism: diamagnetism, para-
magnetism and ferromagnetism, which are reflected in
the material
-
is magnetic susceptibility. Ferromagnetism is
about a million times stronger than diamagnetism and
paramagnetism.
'
is magnetic
field at the time the magnetism was
created. Since the Earth
'
s
field changes in polarity and
direction with both time and location (see
Section 3.5.1
),
and since a rock unit is very likely to be subsequently
rotated by tectonic processes such as faulting and contin-
ental drift, only very recently acquired remanent
magnetisms are likely to be parallel to the present-day
Earth
'
Diamagnetism and paramagnetism
Materials in which the atomic electron spins align so that
their magnetic dipoles oppose an external magnetic
eld
have a characteristic weak negative susceptibility and are
diamagnetic. Geological materials with these characteris-
tics can be considered non-magnetic in geophysical
surveying. Materials in which the electron spins align so
s field. Remanent magnetism acquired at the time
of a rock
'
'
is formation is called primary, with secondary
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