Introduction - Geophysics for the Mineral Exploration Geoscientist - page 4

general, airborne measurements made from a low- ying
aircraft are more cost-effective than ground measurements
for surveys covering a large area or comprising a large
number of readings. The chief advantages of airborne
surveying relative to ground surveying are the greater
speed of data acquisition and the completeness of the
survey coverage.
As exploration progresses and focuses on smaller areas,
there is a general reduction in both the extent of geophys-
ical surveys and the distances between the individual
readings in a survey. Airborne surveys are usually part of
the reconnaissance phase, which is often the initial phase
of exploration, although some modern airborne systems
offer higher resolution by surveying very close to the
ground and may find application in the later stages of
exploration. Ground and drillhole surveys, on the other
hand, offer the highest resolution of the subsurface. They
are mostly used for further investigation of areas targeted
from the reconnaissance work for their higher prospectiv-
ity, i.e. they are used at the smaller prospect scale.
Methods that can be implemented from the air include
magnetics, known as aeromagnetics; gravity, sometimes
referred to as aerogravity or as currently implemented
for mineral exploration as airborne gravity gradiometry;
radiometrics; and electromagnetics, usually referred to as
airborne electromagnetics (AEM). All the geophysical
methods can be implemented downhole, i.e. in a drillhole.
Downhole surveys are a compact implementation of
conventional surface surveying techniques. There are two
quite distinct modes of making downhole measurements:
downhole logging and downhole surveying.
Downhole logging is where the in situ physical proper-
ties of the rocks penetrated by a drillhole are measured to
produce a continuous record of the measured parameter.
Downhole logs are commonly used for making strati-
graphic correlations between drillholes in the sedimentary
sequences that host coal seams and iron formations.
Measurements of several physical parameters, producing a
suite of logs, allow the physical characterisation of the local
geology, which is useful for the analysis of other geophysical
data and also to help plan future surveys, e.g. Mwenifumbo
et al.( 2004 ) . Despite the valuable information obtainable,
multiparameter logging is not ubiquitous in mineral explor-
ation. However, its use is increasing along with integrated
interpretation of multiple geophysical datasets.
Downhole surveying is designed to investigate the larger
region surrounding the drillhole, with physical property
variations obtained indirectly, and to indicate the direction

and even the shape of targets. That is, downhole electrical
conductivity logging measures the conductivity of the rocks
that form the drillhole walls, whereas a downhole electro-
magnetic survey detects conductivity variations, perhaps
owing to mineralisation, in the volume surrounding the
drillhole. Downhole geophysical surveys increase the radius
of investigation of the drillhole, increase the depth of inves-
tigation and provide greater resolution of buried targets.
Geophysical surveys are sometimes conducted in
open-pit and underground mines; measurements are made
in vertical shafts and/or along (inclined) drives, usually to
detect and delineate ore horizons. There exists a rather
small literature describing underground applications of
geophysics, e.g. Fallon et al.( 1997 ), Fullagar and Fallon
( 1997 ), and McDowell et al.( 2007 ) , despite many success-
ful surveys having been completed. Application and imple-
mentation of geophysics underground tend to be unique to
a particular situation, and survey design requires a fair
degree of ingenuity to adapt the arrangement of transmit-
ter and receiver to the con nes of the underground
environment. They are usually highly focused towards
determining a speci c characteristic of a small volume of
ground in the immediate surrounds. Electrical and mech-
anical interference from mine infrastructure limits the
sensitivity of surveys, which require a high level of plan-
ning and coordination with mining activities. Also, data
from in-mine surveys require particular skills to interpret
the more complex three-dimensional (3D) nature of the
responses obtained: for example, the response may eman-
ate from overhead, or the survey could pass through the
target. The generally unique nature of underground geo-
physical surveys and our desire to emphasise the principles
and common practices of geophysics in mineral explor-
ation restrict us from describing this most interesting
application of geophysics, other than to mention, where
appropriate, the possibilities of using a particular geophys-
ical method underground.

1.2.2 Geophysical methods and mineral deposits

The physical properties of the geological environment
most commonly measured in mining geophysics are
density, magnetism, radioactivity and electrical properties.
Elastic (seismic) properties are not commonly exploited. In
general, density, magnetism and radioactivity are used to
map the geology, the latter when the nature of the surface
materials is important. The limited use of electrical prop-
erties is due to their non-availability from an airborne