Geology Reference
In-Depth Information
7
Magnetic surveying
m
pm
mm
r
7.1 Introduction
012
2
F
=
(7.1)
4
R
The aim of a magnetic survey is to investigate subsurface
geology on the basis of anomalies in the Earth's mag-
netic field resulting from the magnetic properties of the
underlying rocks. Although most rock-forming miner-
als are effectively non-magnetic, certain rock types
contain sufficient magnetic minerals to produce signifi-
cant magnetic anomalies. Similarly, man-made ferrous
objects also generate magnetic anomalies. Magnetic sur-
veying thus has a broad range of applications, from small-
scale engineering or archaeological surveys to detect
buried metallic objects, to large-scale surveys carried out
to investigate regional geological structure.
Magnetic surveys can be performed on land, at sea
and in the air. Consequently, the technique is widely
employed, and the speed of operation of airborne
surveys makes the method very attractive in the
search for types of ore deposit that contain magnetic
minerals.
where
m
0
and
m
R
are constants corresponding to the
mag-
netic permeability of vacuum
and the
relative magnetic
permeability
of the medium separating the poles (see
later). The force is attractive if the poles are of different
sign and repulsive if they are of like sign.
The
magnetic field B
due to a pole of strength
m
at a dis-
tance
r
from the pole is defined as the force exerted on a
unit positive pole at that point
m
pm
m
0
(7.2)
B
=
2
4
r
R
Magnetic fields can be defined in terms of
magnetic poten-
tials
in a similar manner to gravitational fields. For a
single pole of strength
m
, the magnetic potential
V
at a
distance
r
from the pole is given by
m
pm
m
0
V
=
(7.3)
4
r
R
The magnetic field component in any direction is then
given by the partial derivative of the potential in that
direction.
In the SI system of units, magnetic parameters are de-
fined in terms of the flow of electrical current (see e.g.
Reilly 1972). If a current is passed through a coil consist-
ing of several turns of wire, a
magnetic flux
flows through
and around the coil annulus which arises from a
magne-
tizing force H
.The magnitude of
H
is proportional to the
number of turns in the coil and the strength of the cur-
rent, and inversely proportional to the length of the wire,
so that
H
is expressed in A m
-1
. The density of the mag-
netic flux, measured over an area perpendicular to the
direction of flow, is known as the
magnetic induction
or
magnetic field B
of the coil.
B
is proportional to
H
and the
constant of proportionality
m
is known as the
magnetic
permeability
. Lenz's law of induction relates the rate of
change of magnetic flux in a circuit to the voltage devel-
7.2 Basic concepts
Within the vicinity of a bar magnet a magnetic flux is
developed which flows from one end of the magnet
to the other (Fig. 7.1).This flux can be mapped from the
directions assumed by a small compass needle suspended
within it. The points within the magnet where the
flux converges are known as the poles of the magnet.
A freely-suspended bar magnet similarly aligns in
the flux of the Earth's magnetic field. The pole of
the magnet which tends to point in the direction of
the Earth's north pole is called the north-seeking or
positive pole, and this is balanced by a south-seeking
or negative pole of identical strength at the opposite end
of the magnet.
The force
F
between two magnetic poles of strengths
m
1
and
m
2
separated by a distance
r
is given by
