Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 110

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magnetic field associated with variations on time scales of
fractions of a second to months. This is due entirely to
sources external to the Earth, and is known as the external
field. The variable external field is a hindrance, because
temporal variations that occur during the course of a
magnetic survey must be compensated so that the mag-
netic measurements solely re ect the spatial variations in
the field.
The geomagnetic field at a location is defined by its
intensity, or strength, (F) and its direction, described by
its dip or inclination (Inc) and declination (Dec)
( Fig. 3.20b ), all of which change within and over the Earth.
Locations where the field lines are parallel to the Earth

There is also an unstable component of the Earth ' locations around the globe. This is known as the Inter-

national Geomagnetic Reference Field (IGRF). It gives the
direction and strength of the field at any location, and
attempts to predict temporal changes in these for the 5-
year epoch ahead, which is known as the provisional field.
It is updated every 5 years to account for observed (actual)
magnetic activity. In magnetic surveying, the IGRF repre-
sents a smooth long-wavelength variation superimposed
on the shorter-wavelength crustal features of interest. The
2004 edition of the IGRF is shown in Fig. 3.21 . Note how
the magnetic poles are not diametrically opposite each
other. The importance of the IGRF for magnetic surveying
is that it allows the strength and orientation of the geo-
magnetic field at the time and location of a magnetic
survey to be determined, which is important for both
enhancing and interpreting the data; see Sections 3.7.2
and 3.2.4 .

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surface, i.e. where the inclination is zero degrees, define the
magnetic equator. It occurs in the vicinity of the geographic
equator but there are regions where the two deviate signifi-

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cantly, by up to almost 15° of latitude. North of the mag-
netic equator the field points downward into the Earth and
inclination is reckoned positive (referred to as northern
magnetic latitude). South of the magnetic equator the eld
points upwards and out of the Earth and inclination is
negative (referred to as southern magnetic latitude). The
magnetic poles are by de nition those locations where the
field is vertical, i.e. perpendicular to the Earth

3.5.1.1 Temporal variations in the geomagnetic field
The longer-period variation of the Earth s magnetic field,

of greater than about a year, is known as the secular
variation. It is thought to be primarily due to changes in
the electric currents producing the internal field. There are
also variations in the inclination and declination, and the
magnetic north pole is also drifting westwards. However,
these very small changes are of little signi cance in mag-
netic prospecting. Secular variations in the strength of the
field have an effect when merging data from magnetic
surveys conducted over periods of years or decades, typical
of national survey programmes.
Palaeomagnetic studies indicate that the geomagnetic
field has reversed a number of times (Collinson, 1983 ).
The present direction of the field is known as the normal
direction, established from its previously reverse direction
about 780,000 years ago. Intervals of consistent polarity
range from about 50,000 years to about 5 million years.
The pole reversals have an obvious impact on the reman-
ent magnetisation of crustal rocks, which strongly in u-
ences their magnetic responses.
There are several short-period variations of the Earth

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is surface.
Inclination is +90° at the magnetic north pole and

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90° at
the magnetic south pole. Note that these do not coincide
with the geomagnetic or the geographic poles. Declination
describes the eld

s direction in the horizontal plane, i.e.
direction of the horizontal component of the field
( Fig. 3.20b ), and is measured positive clockwise with
respect to geographic north, i.e. it is the difference between
true north and magnetic north. A compass needle aligns
itself in this direction, which is known as the magnetic
meridian.
The geomagnetic

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field reaches its maximum strength at
the magnetic poles, about 60,000 nT at the magnetic north
pole and 67,000 nT at the magnetic south pole. Its min-
imum strength, about 25,000 nT, occurs in Brazil and the
southern Atlantic Ocean. This is important because it
means that a source

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field that affect magnetic surveying; they are a form of
environmental noise (see Section 2.4.1 ), which we now
describe.

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is induced magnetism is less where
the field is weaker (see Section 3.2.3.3 ). Magnetic anomal-
ies in South America and Southern Africa have about half
the amplitude of the same sources in, say, Australia, where
the field inclination is similar ( Figs. 3.21a and b ) .
The relatively stable component of the geomagnetic eld
is described by a mathematical model developed from
observations over many years at a large number of

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Diurnal variation
Variations of the Earth

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s magnetic field of less than about a
year are due to changes in the external field, which are
related to sources external to the Earth. These are chiefly