Geoscience Reference
In-Depth Information
Throughout the text we emphasise the key aspects of
mineral exploration geophysics, in particular those aspects
that affect the interpretation of geophysical data. These
include petrophysics, the foundational science of geophys-
ics; numerical processing of the data; the creation and
interpretation of raster imagery; problems presented by
deeply weathered environments; geophysical characteris-
tics of geologically complex basement terrains; and the
inability to remove noise completely from the measure-
ments. We introduce the term
Target-scale
Prospect-scal e
Regional-scale
x 10000
Seismic
IP/resistivity
x 1000
CSAMT
Large-loop
EM
Aerogravity
IP/resistivity
(gradient array)
x 100
Helicopter AEM
Magnetics
Fixed-wing
AEM
, where
to fully understand the geophysical signal (the information
of interest) and the noise (the interference producing
uncertainty in the signal) requires information about the
subsurface, but the purpose of the geophysical survey is to
acquire this very information. We emphasise the need to
understand this fundamental aspect of geophysics when
working with geophysical data.
There have been many developments in geophysics in
recent years. We have deliberately avoided presenting older
techniques and practices not used widely today, leaving
descriptions of these to earlier texts.
The text is structured around the main geophysical
methods with each described in its own separate chapter.
General aspects of the nature of geophysical data, their
acquisition, processing, display and interpretation,
common to all methods, are described first in a general
chapter, Chapter 2 . Essential, and generally applicable,
details of vectors and waves are described in the online
Appendices 1 and 2 , respectively. The other chapters are
designed to be largely self-contained, but with extensive
cross-referencing to other chapters, in particular to
Chapter 2 . We have responded to the widespread comple-
mentary use of gravity and magnetics by describing them
in a single combined chapter, Chapter 3 . Geophysical
methods less commonly used by the mining industry are
described in online Appendices 3 to 6 . Appendix 7 lists
sources of information about mineral exploration geophys-
ics, especially case histories. The principles described are
demonstrated by examples of geophysical data and case
studies from a wide variety of mineral deposit types from
around the world. All deposits referred to are listed in
Table 1.2 and their locations shown on Fig. 1.4 .
At the conclusion of each chapter we provide a short list
of appropriate resource material for further reading on the
topic. The references cited throughout the text emphasise
those we believe suit the requirements of the exploration
geoscientist.
'
geophysical paradox
'
Aerogravity gradiometry
Gravity
x 10
Fixed-wing
aeromagnetics/radiometrics
Reconnaissance
gravity/aeromagnetics
Gravity/aeromagnetics
(public domain)
1
10 -1
10 0
10 1
Area (km 2 )
Figure 1.3 Approximate relative (a) areas and (b) line lengths
sampled by geophysical surveys costing the equivalent of a single
300 m deep diamond drillhole. The area of the drillhole is shown
for comparison. AEM - airborne electromagnetics, CSAMT -
controlled source audio-frequency magnetotellurics, GPS - global
positioning system, IP
time domain
electromagnetics. Redrawn with additions, with permission,
from Fritz ( 2000 ).
-
induced polarisation, TDEM
-
estimates are equated to the estimated total cost of a
single 300 m drillhole, including logging, assaying, reme-
diation etc. The drillhole provides reliable geological
information to a certain depth, but only from a very
small area. Drilling on a grid pattern at 25 m intervals
over an area of 1 km 2 would cost a few tens of millions
of dollars, but would only sample 3 ppm of the volume.
Geophysical methods provide information from vastly
greater areas and volumes, albeit in a form that is not
necessarily geologically explicit and will not necessarily
directly identify mineralisation. Despite this, appropri-
ately designed geophysical surveys and appropriately
chosen data analysis are highly effective for optimally
targeting expensive drillholes.
1.3 About this topic
Our focus is an explanation of the principles and modern
practice of geophysics in the search for mineral deposits.
The explanations are presented from a perspective relevant
to a mining industry geologist.
 
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