MAGNETIC METHOD - Field Geophysics - page 67

3.1.4 Remanence
Ferro- and ferri-magnetic materials may have permanent as well as induced
magnetic moments, so that their magnetisations are not necessarily in the
direction of the Earth's field. The Konigsberger ratio , of the permanent mag-
netisation to the magnetisation that would be induced in an Earth-standard
field of 50 000 nT, is generally large in highly magnetic rocks and small in
weakly magnetic ones, but is occasionally extraordinarily high ( > 10 000)
in hematite, despite the low susceptibility. Magnetic anomalies due en-
tirely to remanent magnetisation are therefore very occasionally produced
by hematite ores.
3.2 The Magnetic Field of the Earth
The magnetic fields of geological bodies are superimposed on a background
of the Earth's main field. Variations in magnitude and direction of this field
influence both the magnitudes and the shapes of local anomalies.
The terms North and South that are commonly used to describe magnetic
polarity are replaced in geophysics by positive and negative . The direction
of a magnetic field is conventionally defined as the direction in which a unit
positive pole would move, and geophysicists give little thought to whether
it is the North or South magnetic pole that is positive.
3.2.1 The main field of the Earth
The Earth's main magnetic field originates in electric currents circulating
in the liquid outer core, but can be largely modelled by a dipole source
at the Earth's centre. Distortions in the dipole field extending over regions
thousands of kilometres across can be thought of as caused by a relatively
small number of subsidiary dipoles at the core-mantle boundary.
The variations with latitude of the magnitude and direction of an ideal
dipole field aligned along the Earth's spin axis are shown in Figure 3.2.
Near the Equator the dip angles change almost twice as fast as the latitude
angles. Neither the magnetic equator, which links points of zero magnetic
dip on the Earth's surface, nor the magnetic poles coincide exactly with
their geographic equivalents (Figure 3.3), and the main dipole would have
to be inclined at about 11 ◦ to the spin axis to explain the Earth's actual field.
The North Magnetic Pole has, in the last ten years, moved from northern
Canada into the Arctic Ocean, and the South Magnetic Pole is currently in
the Southern Ocean at about 65 ◦ S 138 ◦ E. Differences between the directions
of true and magnetic north are known as declinations, presumably because
a compass needle ought to point north but declines to do so.
Dip angles estimated from the global map (Figure 3.3) can be used to
obtain rough estimates of magnetic latitudes and hence (using Figure 3.2) of
regional gradients. This approach is useful in determining whether a regional