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In-Depth Information
measurement of naturally occurring EM
fields is less
common (see online
Appendix 4
f
for a description of these
methods). Surveys are conducted from the air (airborne
electromagnetics; AEM), on the ground surface, and in
drillholes (downhole electromagnetics; DHEM). Although
primarily used as an exploration tool, there is increasing
use of EM in-mine, the intention being to map an orebody
accurately prior to, and during, mining to reduce the
amount of delineation drilling required. The basic aim of
EM surveying is to map spatial variations in electrical
conductivity with the data presented in the form of
pseudo-maps, cross-sections or volumes showing the con-
ductivity variations in a continuous form. The data may
also be used to infer the location of a
(a)
D48
D47
CE
D11
0
25
Metres
D45
CE
Graphitic intersection
(b)
(c)
Normalised
potential
(V/A)
Normalised
potential
(V/A)
, i.e. a discrete
zone of conductivity approximated by some simple shape.
New survey and interpretation systems are continually
evolving, and EM theory related to geophysical surveying is
continually being developed. The result is a complex and
diverse science. Making and understanding EM measure-
ments in areas of conductive overburden (see
Section 5.3.4
)
is especially challenging. Future developments are aimed at
improving signal detection and resolution of more subtle
conductivity contrasts, better depth penetration, especially
beneath conductive overburden, and reducing the cost of
EM surveying. Considerable work is ongoing in the math-
ematically complex area of improving data interpretation
tools, which includes the development of associated soft-
ware for building more accurate and complex 3D conduct-
ivity models of the ground from ever-increasing data
volumes. These developments allow better integration of
geological information and a greater role for the geologist
in the analysis of the data than is the case at present.
There exists a plethora of EM systems, instrumentation
and interpretation techniques, past and present. Confus-
ingly, survey results are dependent upon many variables, in
particular the speci
c EM system used. We have attempted
to elucidate the principles and practice of this complex area
of exploration geophysics, with emphasis on currently
available systems and survey practice. The reader is
older EM methods, including contemporaneous survey
procedures and interpretation techniques.
'
target
'
0.3
0.4
0.5
0
.3
0.4
0.5
0.
6
10
10
Electrically
continuous
20
20
Electrically
continuous
30
30
Electrically
continuous
40
40
D47 (source in D48)
50
50
Depth
(m)
D11 (source in D45)
60
Depth
(m)
Figure 5.67
Downhole applied potential logs from the Victoria
graphite deposit. (a) Drill section and stratigraphy as interpreted
from the drillhole intersections. Locations of the buried current
electrode labelled as CE. (b) Pro
le of potential in drillhole D11 with
the current electrode in the upper of two high-grade intersections in
drillhole D45. (c) Profile of potential in drillhole D47 with the
current electrode in the upper of two intersections in drillhole D48.
See text for details. Based on diagrams in Mwenifumbo (
1997
).
other two intersections, indicating they are electrically isol-
ated. Compare these data with
Fig. 5.65a
and
b
.
Continuity between the various intersections was estab-
lished by the AP survey, leading to the interpretation
shown in
Fig. 5.67a
.
The resulting interpretation was dif-
ferent, in important aspects, from the contemporaneous
geological model and was crucial to the decision to begin
mining in the area between drillholes D11 and D48.
5.7
Electromagnetic methods
5.7.1
Principles of electromagnetic surveying
Electromagnetic (EM) surveys as used by the minerals
industry are chie
y a type of active geophysical method,
i.e. they use artificially created electromagnetic fields. The
The EM method is based on the principle of
electromagnetic induction described in
Section 5.2.2
.
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