The Anatomy of a Mine - Mining and the Environment: From Ore to Metal - page 176

Electrical Methods
Mineral deposits and geological structures display a wide variety of electrical properties,
including electrical conductivity, and capacity to hold an electric charge. Electrical surveys
are normally conducted along surveyed grid lines and require electrodes, usually in the
form of either short metal stakes driven into the ground or porous ceramic pots i lled with
copper sulphate, to be placed in shallow holes dug with a spade. Both stakes and pots are
withdrawn on completion of a survey.
In the resistivity method, a current is introduced into the ground by two contact elec-
trodes and potential differences are measured between two or more other electrodes.
Electrodes are removed on completion of the survey. Telluric methods measure variations
in electric i elds caused by such diverse mechanisms as electrical storms and solar activity,
and are generally used to dei ne large-scale variations in conductivity of the Earth's crust.
The forced current electrode methods take advantage of the fact that a forced electric cur-
rent circulated by two electrodes results in voltages measured at two others. The electrodes
are usually arranged in i xed linear patterns and are systematically moved along survey
lines. The l ow of the induced current is affected by the properties of the rocks, includ-
ing mineral deposits that occur along the grid line. There are two basic electrode meth-
ods: (1) Induced Polarization (IP), the production of a double layer of charge at a mineral
interface; or production of changes in double-layer density of charge, brought about by
the application of an electric or magnetic i eld; and (2) Spontaneous Polarization (SP), the
electrochemical reactions of certain ore bodies causing spontaneous electrical potentials.

Forced Current Coil Methods
Collectively known as electromagnetic or EM methods, these depend upon electric current
being transmitted through a coil or loop of wire laid upon the ground. Rocks or mineral
deposits that are electrical conductors deform the resultant magnetic i eld. If the area to
be explored is large or remote, reconnaissance stage EM surveying may be achieved by
remote sensing. This technique is known as airborne EM surveying.

Seismic Surveys
These exploration techniques utilize the variation in the rate of propagation of shock
waves in different media. Seismic methods rely on studying the ways in which sound
or equivalent wave forms produced on the surface travel through the underlying rock.
Different rock formations and geological structures affect these energy waves in specii c
ways and, by studying the results obtained, it is possible to predict the nature of the con-
cealed geology.
There are two basic types of seismic surveying - refraction and rel ection. The refrac-
tion method is used to study ground conditions, such as depth of weathering or faulting,
within 50 metres of the surface. Therefore, it is extensively used in quarrying and con-
struction foundation studies. It is not generally used in exploration for mineral deposits.
The rel ection method is normally used for deep penetration and the understanding of
geological stratigraphy and structure.
The energy source is critical in seismic surveying. It determines the depth of penetration.
A vibrator or explosives are usually used in rel ection surveys. Vibrator systems are non-dam-
aging to the surface, but explosives, which are inserted in shallow shot holes, can affect small
areas of four to ten square metres, which require rehabilitation on completion of the survey.
Seismic methods are commonly used in exploration for oil, gas, and coal but are less
common in exploration for metalliferous deposits in geologically complex mineralized
areas. Large three-dimensional seismic exploration is a major activity with potentially sig-
nii cant environmental and social impacts.

Seismic methods rely on studying
the ways in which sound or
equivalent wave forms produced
on the surface travel through the
underlying rock.