Geoscience Reference
In-Depth Information
are distinctly different. In the first part of this chapter
we describe the nature of density and magnetism and
the properties of the associated gravity and magnetic
fields. The acquisition and reduction of gravity and
magnetic data are then described separately, since these
are dissimilar. This is followed by a detailed description
of aspects common to both methods: namely the pro-
cessing, display and interpretation of the data.
discussing interpretation we emphasize the geological
causes of variations in rock density and magnetism and
their relationship to changes in the gravity and mag-
netic fields. Finally, we present some case studies chosen
to demonstrate the application of the gravity and mag-
netic methods in exploration targeting and geological
mapping, and also to illustrate some of the processing
and interpretation methods described.
In
3.2
Gravity and magnetic fields
important characteristic of potential fields for geophysics
because sources of gravity and magnetic fields do not exist
in isolation; so the ability to determine the combined effect
of a group of several adjacent sources is very important. In
the case of the Earth
Gravity and magnetism involve the interaction of objects at
a distance through the respective fields surrounding the
objects. A gravity
field is caused by an object
'
s mass, a
s gravity
field, the equipotential sur-
faces approximate the shape of the Earth. The geoid (see
Section 3.4.4
)
is the equipotential surface coinciding with
mean sea level and everywhere de
nes the horizontal.
An object
'
magnetic
field by its magnetism. It is common to refer to
the object as the source of these
fields. Gravity and mag-
netic
fields are both types of potential
field (
fields that
require work to be expended to move masses or magnetic
objects within them). Whether a body is affected by a
potential
field depends on the body
s gravitational potential energy changes when
it is moved along a path that passes through an equipoten-
tial surface, so the potential difference between it and
surrounding masses changes. Moving an object from the
ground to a higher location opposes the action of gravity
and involves crossing equipotential surfaces and increasing
'
s characteristics and
the type of
field. For example, gravity
fields affect objects
having mass, i.e. everything, whereas magnetic
fields only
affect objects that are magnetic.
The potential about an object is represented by a series of
equipotential surfaces, where every point on a surface has
the same potential energy. Potential is a scalar quantity (see
Section 2.2.2
)
and decreases with distance from the source.
For example, the gravitational potential of a homogeneous
spherical object is described by surrounding spherical equi-
by imaginary field lines directed towards the centre of mass
and which intersect the equipotential surfaces perpendicu-
larly, and so form a radial pattern. The
'
a)
Equipotential surface
Gravitational
field line
CoM
field lines represent
the direction of motion of an object in
uenced only by the
field. Potential
fields are vector quantities, i.e. they have
both magnitude and direction (see
Appendix 1
)
. The
strength of the
field, also called its magnitude or intensity,
decreases with distance from the source. The sources of
gravity
fields are polar or monopoles, i.e. they have one
polarity, and always attract, unlike electrical and magnetic
fields, which have poles of opposite polarity that can either
attract or repel each other (see
Fig. 5.2
).
When multiple sources are present the fields of adjacent
bodies interact and their equipotential surfaces merge. The
field at any point is the vector sum of the fields (see
Appendix 1
)
associated with each body. This is an
b)
Attractive force (F)
m
1
m
2
CoM
CoM
r
Figure 3.2
Gravitational attraction. (a) The gravitational
equipotential surfaces and gravity
field of a spherical mass. (b) The
gravitational attraction between two masses (m
1
and m
2
). CoM is the
centre of mass.
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