Geology Reference
In-Depth Information
True
North
Basic
igneous
Magnetic
North
200
H
D
I
East
100
B
Acid
igneous
Z
Metamorphic
Sandstone
Shale
Limestone
0
0-22
0-133
0-118
0-463
0-519
4-773
Range
Fig. 7.6
The geomagnetic elements.
Fig. 7.5
Histogram showing mean values and ranges in
susceptibility of common rock types. (After Dobrin & Savit 1988).
the interpretation of the resulting anomalies. The geo-
magnetic field is geometrically more complex than the
gravity field of the Earth and exhibits irregular variation
in both orientation and magnitude with latitude, longi-
tude and time.
At any point on the Earth's surface a freely suspended
magnetic needle will assume a position in space in the di-
rection of the ambient geomagnetic field.This will gen-
erally be at an angle to both the vertical and geographic
north. In order to describe the magnetic field vector, use
is made of descriptors known as the geomagnetic ele-
ments (Fig. 7.6). The
total field vector B
has a vertical
component
Z
and a horizontal component
H
in the di-
rection of magnetic north.The dip of
B
is the
inclination
I
of the field and the horizontal angle between geo-
graphic and magnetic north is the
declination D
.
B
varies in strength from about 25 000 nT in equatorial
regions to about 70 000 nT at the poles.
In the northern hemisphere the magnetic field gener-
ally dips downward towards the north and becomes
vertical at the north magnetic pole (Fig. 7.7). In the
southern hemisphere the dip is generally upwards to-
wards the north. The line of zero inclination approxi-
mates the geographic equator, and is known as the
magnetic equator.
About 90% of the Earth's field can be represented by
the field of a theoretical magnetic dipole at the centre of
the Earth inclined at about 11.5° to the axis of rotation.
The magnetic moment of this fictitious
geocentric dipole
can be calculated from the observed field. If this dipole
field is subtracted from the observed magnetic field, the
In general the magnetite content and, hence, the sus-
ceptibility of rocks is extremely variable and there can be
considerable overlap between different lithologies. It
is not usually possible to identify with certainty the
causative lithology of any anomaly from magnetic
information alone. However, sedimentary rocks are
effectively non-magnetic unless they contain a signifi-
cant amount of magnetite in the heavy mineral fraction.
Where magnetic anomalies are observed over sediment-
covered areas the anomalies are generally caused by an
underlying igneous or metamorphic basement, or by
intrusions into the sediments.
Common causes of magnetic anomalies include
dykes, faulted, folded or truncated sills and lava flows,
massive basic intrusions, metamorphic basement rocks
and magnetite ore bodies. Magnetic anomalies range in
amplitude from a few tens of nT over deep metamorphic
basement to several hundred nT over basic intrusions
and may reach an amplitude of several thousand nT over
magnetite ores.
7.4 The geomagnetic field
Magnetic anomalies caused by rocks are localized effects
superimposed on the normal magnetic field of the Earth
(geomagnetic field). Consequently, knowledge of the
behaviour of the geomagnetic field is necessary both in
the reduction of magnetic data to a suitable datum and in
