Geology Reference
In-Depth Information
tion of charged particles in coupled convective cells
within the outer, fluid, part of the Earth's core. The ex-
change of dominance between such cells is believed to
produce the periodic changes in polarity of the geomag-
netic field revealed by palaeomagnetic studies. The
circulation patterns within the core are not fixed and
change slowly with time. This is reflected in a slow,
progressive, temporal change in all the geomagnetic
elements known as
secular variation
. Such variation is
predictable and a well-known example is the gradual
rotation of the north magnetic pole around the geo-
graphic pole.
Magnetic effects of external origin cause the geo-
magnetic field to vary on a daily basis to produce
diurnal variations
. Under normal conditions (Q or quiet
days) the diurnal variation is smooth and regular and
has an amplitude of about 20-80 nT, being at a maxi-
mum in polar regions. Such variation results from
the magnetic field induced by the flow of charged parti-
cles within the ionosphere towards the magnetic poles,
as both the circulation patterns and diurnal variations
vary in sympathy with the tidal effects of the Sun and
Moon.
Some days (D or disturbed days) are distinguished by
far less regular diurnal variations and involve large, short-
term disturbances in the geomagnetic field, with ampli-
tudes of up to 1000 nT, known as
magnetic storms
. Such
days are usually associated with intense solar activity and
result from the arrival in the ionosphere of charged solar
particles. Magnetic surveying should be discontinued
during such storms because of the impossibility of
correcting the data collected for the rapid and high-
amplitude changes in the magnetic field.
Magnetic
North pole
Fig. 7.7
The variation of the inclination of the total magnetic
field with latitude based on a simple dipole approximation of the
geomagnetic field. (After Sharma 1976.)
residual field can then be approximated by the effects of a
second, smaller, dipole.The process can be continued by
fitting dipoles of ever decreasing moment until the
observed geomagnetic field is simulated to any required
degree of accuracy. The effects of each fictitious dipole
contribute to a function known as a harmonic and the
technique of successive approximations of the observed
field is known as spherical harmonic analysis - the
equivalent of Fourier analysis in spherical polar coordi-
nates. The method has been used to compute the for-
mula of the International Geomagnetic Reference Field
(IGRF) which defines the theoretical undisturbed mag-
netic field at any point on the Earth's surface. In mag-
netic surveying, the IGRF is used to remove from the
magnetic data those magnetic variations attributable to
this theoretical field. The formula is considerably more
complex than the equivalent Gravity Formula used for
latitude correction (see Section 6.8.2) as a large number
of harmonics is employed (Barraclough & Malin 1971,
Peddie 1983).
The geomagnetic field cannot in fact result from per-
manent magnetism in the Earth's deep interior. The re-
quired dipolar magnetic moments are far greater than is
considered realistic and the prevailing high temperatures
are far in excess of the Curie temperature of any known
magnetic material.The cause of the geomagnetic field is
attributed to a dynamo action produced by the circula-
7.5 Magnetic anomalies
All magnetic anomalies caused by rocks are superim-
posed on the geomagnetic field in the same way that
gravity anomalies are superimposed on the Earth's
gravitational field. The magnetic case is more complex,
however, as the geomagnetic field varies not only in am-
plitude, but also in direction, whereas the gravitational
field is everywhere, by definition, vertical.
Describing the normal geomagnetic field by a vector
diagram (Fig. 7.8(a)), the geomagnetic elements are
related
BHZ
2
=+
2
2
(7.7)
