Introduction - Geophysics for the Mineral Exploration Geoscientist - page 2

a)

Response
from mineralisation

Response
from contact

Location

A A'

b)

Receiver
(airborne survey)

Receiver
(ground survey)

A B A'

Receiver
(downhole
survey)

Geophysical respons e

Mineralisation

B

Response
from
mineralisation

Natural energy

B'

Response
from
contact

B'

Figure 1.1 Geophysical surveying schematically
illustrated detecting mineralisation and mapping a
contact between different rock types. Instruments
(receivers) make measurements of a physical parameter
at a series of locations on or above the surface (A

c)

Transmitter

A B A'

A 0 )or -

B 0 ). The data are plotted as a function of
location or depth down the drillhole (a). (b) Passive
geophysical surveying where a natural source of energy is
used and only a receiver is required. (c) Active
geophysical surveying where an arti

-

downhole (B

Mineralisation

Artificial energy

cial source of energy
(transmitter) and a receiver are both required.

Energy originating
from mineralisation

B'

to be contradictory. Any contradiction is resolved by the
'

1.1 Physical versus chemical
characterisation of the geological
environment

basis of investigating the
geology. For example, porosity and pore contents are
commonly important in

chemical ' versus ' physical '

uences on physical properties,
but are not a factor in the various schemes used by
geologists to assign a lithological name, these schemes
being based on mineralogical content and to a lesser extent
the distribution of the minerals.
Some geophysical methods can measure the actual
physical property of the subsurface, but all methods are
sensitive to physical property contrasts or relative
changes in properties, i.e. the juxtaposition of rocks with
different physical properties. It is the changes in physical
properties that are detected and mapped. This relativist
geophysics approach is another fundamental aspect that
differs from the absolutist geological approach. For
example, one way of geologically classifying igneous rocks

The geophysical view of the geological environment
focuses on variations in the physical properties within
some volume of rock. This is in direct contrast with the
geological view, which is primarily of variations in the bulk
chemistry of the geology. The bulk chemistry is inferred
from visual and chemical assessment of the proportions of
different silicate and carbonate minerals at locations where
the geology happens to be exposed, or has been drilled.
These two fundamentally different approaches to assessing
the geological environment mean that a particular area of
geology may appear homogeneous to a geologist but may
be geophysically heterogeneous, and vice versa. The two
perspectives are complementary, but they may also appear