Introduction - Geophysics for the Mineral Exploration Geoscientist - page 3

is according to their silica content, with absolute values
used to de ne categories such as felsic, intermediate,
ma c etc. The geophysical approach is equivalent to being
able to tell that one rock contains, say, 20% more silica
than another, without knowing whether one or both are
ma c, felsic etc.
The link between the geological and geophysical
perspectives of the Earth is petrophysics

resistivity method. An extension to this is the induced
polarisation (IP) method which measures the ability of rocks
to store electric charge. Electrical properties can also be
investigated by using electric currents created and measured
through the phenomenon of electromagnetic induction.
These are the electromagnetic (EM) methods, and whilst
electrical conductivity remains an important factor, different
implementations of the technique can cause other electrical
properties of the rocks to influence the measurements.
The physical-property-based categorisation described
above is complemented by a two-fold classification of the
geophysical methods into either passive or active methods
( Fig. 1.1b and c ).
Passive methods use natural sources of energy, of which
the Earth

-

the study of
the physical properties of rocks and minerals, which is
the foundation of the interpretation of geophysical data.
Petrophysics is a subject that we emphasise strongly
throughout this topic, although it is a subject in which
some important aspects are not fully understood and more
research is urgently required.

fields are two examples, to
investigate the ground. The geophysical measurement is
made with some form of instrument, known as a detector,
sensor or receiver. The receiver measures the response of the
local geology to the natural energy. The passive geophysical
methods are the gravity, magnetic, radiometric and SP
methods, plus a form of electromagnetic surveying known
as magnetotellurics (described in online Appendix 4 ) .
Active geophysical methods involve the deliberate
introduction of some form of energy into the ground, for
example seismic waves, electric currents, electromagnetic
waves etc. Again, the ground

' s gravity and magnetic

1.2 Geophysical methods in exploration
and mining

Geophysical methods are used in mineral exploration for
geological mapping and to identify geological environ-
ments favourable for mineralisation, i.e. to directly detect,
or target, the mineralised environment. During exploit-
ation of mineral resources, geophysics is used both in
delineating and evaluating the ore itself, and in the engin-
eering-led process of accessing and extracting the ore.
There are five main classes of geophysical methods,
distinguished according to the physical properties of the
geology to which they respond. The gravity and magnetic
methods detect differences in density and magnetism,
respectively, by measuring variations in the Earth

s response to the introduced
energy is measured with some form of detector. The need
to supplement the detector with a source of this energy,
often called the transmitter, means that the active methods
are more complicated and expensive to work with. How-
ever, they do have the advantage that the transmission of
the energy into the ground can be controlled to produce
responses that provide particular information about the
subsurface, and to focus on the response from some region
(usually depth) of particular interest. Note that, confus-
ingly, the cause of a geophysical response in the subsurface
is also commonly called a source

'

s gravity
and magnetic fields. The radiometric method detects
variations in natural radioactivity, from which the radio-
element content of the rocks can be estimated. The seismic
method detects variations in the elastic properties of the
rocks, manifest as variations in the behaviour of seismic
waves passing through them. Seismic surveys are highly
effective for investigating layered stratigraphy, so they are
the mainstay of the petroleum industry but are compara-
tively rarely used by the minerals industry.
The electrical methods, based on the electrical properties
of rocks and minerals, are the most diverse of the ve
classes. Electrical conductivity, or its reciprocal resistivity,
can be obtained by measuring differences in electrical
potentials in the rocks. When the potentials arise from
natural processes the technique is known as the spontan-
eous potential or self-potential (SP) method. When they
are associated with artificially generated electric currents
passing through the rocks, the technique is known as the

'

- a term and context we

use extensively throughout the text.

1.2.1 Airborne, ground and in-ground surveys

Geophysical surveying involves making a series of meas-
urements over an area of interest with survey parameters
appropriate to the scale of the geological features being
investigated. Usually, a single survey instrument is used to
traverse the area, either on the ground, in the air or within
a drillhole ( Fig. 1.1 ). Surveys from space or on water are
also possible but are uncommon in the mining industry. In