Geology Reference
In-Depth Information
B
A
+
+
+
+
S
L
-
N
-
-
-
Fig. 7.2
Schematic representation of an element of material in
which elementary dipoles align in the direction of an external
field
B
to produce an overall induced magnetization.
Fig. 7.1
The magnetic flux surrounding a bar magnet.
zation
or
magnetic polarization
, and results from the align-
ment of elementary dipoles (see below) within the
material in the direction of the field. As a result of this
alignment the material has magnetic poles distributed
over its surface which correspond to the ends of the
dipoles (Fig. 7.2). The intensity of induced magnetiza-
tion
J
i
of a material is defined as the dipole moment per
unit volume of material:
oped within it, so that
B
is expressed in Vs m
-2
(Weber
(Wb) m
-2
). The unit of the Wb m
-2
is designated the
tesla
(T). Permeability is consequently expressed in
Wb A
-1
m
-1
or Henry (H) m
-1
. The c.g.s. unit of
magnetic field strength is the
gauss
(G), numerically
equivalent to 10
-4
T.
The tesla is too large a unit in which to express the
small magnetic anomalies caused by rocks, and a subunit,
the
nanotesla
(nT), is employed (1 nT = 10
-9
T). The
c.g.s. system employs the numerically equivalent
gamma
(
g
), equal to 10
-5
G.
Common magnets exhibit a pair of poles and are
therefore referred to as dipoles. The
magnetic moment M
of a dipole with poles of strength
m
a distance
l
apart is
given by
M
LA
(7.5)
J
=
i
where
M
is the magnetic moment of a sample of length
L
and cross-sectional area
A
.
J
i
is consequently expressed
in A m
-1
. In the c.g.s. system intensity of magnetization
is expressed in emu cm
-3
(emu = electromagnetic unit),
where 1 emu cm
-3
= 1000 A m
-1
.
The induced intensity of magnetization is propor-
tional to the strength of the magnetizing force
H
of the
inducing field:
Mml
=
(7.4)
The magnetic moment of a current-carrying coil is pro-
portional to the number of turns in the coil, its cross-
sectional area and the magnitude of the current, so that
magnetic moment is expressed in A m
2
.
When a material is placed in a magnetic field it may
acquire a magnetization in the direction of the field
which is lost when the material is removed from the
field.This phenomenon is referred to as
induced magneti-
(7.6)
J
=
kH
i
where
k
is the
magnetic susceptibility
of the material. Since
J
i
and
H
are both measured in A m
-1
, susceptibility is di-
mensionless in the SI system. In the c.g.s. system suscep-
tibility is similarly dimensionless, but a consequence of
