Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 328

but it may occur with these con gurations in highly iron-
rich soils when very strong primary fields are used. Elevat-
ing the array above the soil reduces the effect signi cantly,
so it is less likely to be observed in airborne TDEM data.

include graphite deposits and supergene manganese min-
eralisation (see Section 5.9.5.1 ) . EM surveys may be used
to identify potentially mineralised environments such as
kimberlite pipes. Examples of downhole and airborne EM
responses are described in Sections 5.8.3 and 5.9.5 , respect-
ively. Here we describe some examples of ground surveys.

5.7.6.6 Anisotropy effects
Electrical anisotropy (see Section 5.3.1.4 ) affects all
electrical and EM measurements. In electromagnetism,
anisotropy affects the orientation of the induced eddy
current system (see Section 5.7.1.4 ). Its effect may be seen
where, say, the dip of a conductive rock formation is
different in part of the survey area causing its foliation to
change orientation and orientation of the induced eddy
current system. A change in conductivity due to anisotropy
would then be expected.

5.7.7.1 Massive sulphide deposits
Figure 5.89 shows ground TDEM responses from four
massive sulphide deposits. These show anomalous
responses due to the respective target conductors, as well
as a variety of other secondary responses.
Data from the Ernest Henry IOCG deposit located near
Cloncurry, Queensland, Australia, are shown in
( Fig. 5.89a ). The geophysical characteristics of the deposit
are described by Webb and Rowston ( 1995 ) and Asten
( 2000 ). Copper

5.7.6.7 Inversion artefacts
The 1D assumption underlying LEI and CDI (see Section
5.7.4.3 ) can lead to seriously inaccurate models of the geo-
electric section when the electrical properties of the ground
are signi cantly heterogeneous and 3D in nature. The
inversion may model the data very well even though the
conductivity distribution in the subsurface is not 1D (Ellis,
1998 ) .
Inversion results are reliable where the assumption of
one-dimensionality holds, i.e. where the ground is approxi-
mated by a flat-lying layered conductivity structure.
Otherwise the parasection may contain conductivity zones
and apparent targets which are artefacts of the inversion
process. Where there are lateral changes in conductivity,
non-existent steeply dipping conductive zones appear
(see Fig. 5.83b ) . These may be the edges of flat-lying
conductors, vertical features such as dyke-like conductors
or steeply dipping conductivity contrasts such as occur
near faults. When steeply dipping conductors are present
they are usually detected but not properly de

gold mineralisation occurs in the matrix
of brecciated volcanics with an associated zone of super-
gene mineralisation occurring beneath about 45 m of con-
ductive sediments. A moving-loop survey was conducted
using coincident square loops (analogous to the in-loop
con guration described in Moving-loop mode in Section
5.7.3.2 ) 100 m in size, and dB/dt data were acquired. It
shows a weak and localised response (A) due to the super-
gene mineralisation. The anomaly is characteristic of a
shallow dipping plate (see Fig. 5.84b ) . The main body of
mineralisation is too poorly conductive to be detected
beneath the conductive overburden, the response of which
(B) extends to at least channel 11 along the whole profile.
The subtle peak in the measured response to the left of the
main anomaly (C) may be due to the zone of deeper
weathering. The geological section shown is located 350
m from the geophysical profile shown.
Trilogy ( Fig. 5.89b ) is a polymetallic deposit comprising
semi-massive sulphide and stringer mineralisation in silici-

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ned in terms fied carbonaceous phyllite/slate (Sampson and Bourne,

2001 ) in the Proterozoic Mount Barren Basin near
Ravensthorpe, Western Australia. The dB/dt data from a
moving in-loop survey using an 80 m square transmitter
loop show a broad response (A) to late times which dimin-
ishes slowly to the right of the section indicating the dip
direction of the conductive mineralisation. The weak late-
time responses to the left, (B) and (C), are probably related
to shallow mineralisation. The response (D), increasing in
width with delay time and disappearing by the latest times,
may be related to the depression in the weathered zone.
The responses of the conductive weathered zone, and
possibly including that of the host phyllite (the half-space

of shape or depth (Wolfgram et al., 2003 ) .

5.7.7 Examples of EM data from mineral
deposits

As would be expected, EM surveys are primarily used to
directly target mineralisation which would be expected
to be conductive: most commonly massive base metal
sulphides (SEDEX, VMS; e.g. Bishop and Lewis, 1992 ),
magmatic nickel deposits, e.g. King ( 2007 ) , and also
graphitic shear zones associated with unconformity-style
uranium mineralisation (Powell et al., 2007 ). Other targets