The Anatomy of a Mine - Mining and the Environment: From Ore to Metal - page 175

been developed with mixed success. Certain plant species can take up metals from the soil
through their root systems and concentrate them in various parts of the plants, such as the
leaves or bark. Geo-botanical surveys based on direct sampling of plants or ground litter have
been undertaken with some success around the world, notably in the Copper Belt of Africa.

Geo-physical Exploration

Mineral deposits usually possess physical properties that are different from those of the
surrounding host rocks. Mining geologists use scientii c techniques called geophysical
methods to measure these variations in the physical properties of rocks (such as density,
magnetism, electrical conductivity, natural radioactivity, or heat capacity) as a guide to
the possible locations of mineral deposits. Geophysical methods offer a means of seeing
into the Earth itself, something few other exploration techniques can do. They are a very
important component of most modern exploration programmes.
Geophysical observations are not new. However, the systematic measurement of geo-
physical properties as a means of discovering deposits did not commence until the late
1940s (Kearey et al . 2002). Technology development was rapid and actual geophysical sur-
veying commenced in the 1950s and continues to be an integral part of today's exploration
activity. When conducted from the air, geophysical exploration is called remote sensing.
It may be undertaken from i xed-wing aircraft or helicopters normally l ying 60 to 200
metres above the surface or from satellites up to 1,000 kilometres above the Earth. Ground
geophysical surveys are expensive and are generally only undertaken over relatively small
areas of established interest.

Geophysical methods offer
means of seeing into the Earth
itself, something few other
exploration techniques can do.

Gravimetric and Magnetic Methods
The Earth acts as a giant magnet, generating a i eld about itself that inl uences or cap-
tures other objects that are either magnetic or may be magnetized, particularly objects (or
respectively, ore deposits) containing iron. Magnetometers, that allow magnetic surveys,
are simple but highly sensitive. Magnetic surveys may be undertaken from the air or on
the ground. Survey data are processed in the ofi ce and presented as a magnetic map.
There are three common gravimetric and magnetic methods.
●

Electromagnetic methods are electrical exploration methods. They determine the mag-
netic i eld that is associated with an electrical current induced through the ground.

●

Gravity methods map the force of gravity at different locations with a gravimeter deter-
mining differences in specii c gravity of rock masses and, through this, the distribution
of masses of different specii c gravity. Often ore minerals have a higher density than the
surrounding rocks. By measuring variations in the Earth's gravity i eld over an area, an
indication of the nature of the underlying geology and the likely presence of deposits can
be obtained. Surveys are often undertaken over large areas, and provide regional infor-
mation on the nature of rocks often at considerable depth. As the gravity survey instru-
ments have no surface impact and no special access development is required, this type of
surveying has no environmental impact on the area being surveyed.

●

Magnetic methods are commonly used in geophysical prospecting to map variations in
the magnetic i eld of the Earth attributable to changes in structure or magnetic suscep-
tibility in certain near-surface rocks. Most magnetic prospecting is done with airborne
instruments. Detailed magnetic surveys designed for closer studies of the geology may be
undertaken on the ground. Ground surveys normally require a grid of pegged lines over
relatively small areas. Stringent conditions apply to any land clearing for such grid lines.