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contained the mineralised veins were mapped as linear
resistive zones (100s to 1000s of ohm-m) against a back-
ground with a resistivity of about 50
m. The resistivity
data, along with aeromagnetics and geological data, were
used to site the drillhole that discovered the Cindy deposit
under 5
Ω
N42 orebody
0
200
5.0
-
15 m of cover, and subsequently the Nancy and
Vera deposits.
Metres
3.0
Survey
traverses
5.6.7
Interpretation pitfalls
All the electrical parameters measured in resistivity/IP
surveys are subject to a variety of spurious responses from
a number of sources. The most common include conduct-
ive overburden, electromagnetic fields associated with the
wires connecting the electrodes, undulating topography,
electrical anisotropy of the subsurface material itself and
cultural interference. Their responses distort genuine target
anomalies and can masquerade as target anomalies. They
can be serious interpretation pitfalls unless procedures are
adopted to identify and account for them in the interpret-
ation of survey data. Man-made interference and cultural
anomalies are common to all electrical and EM measure-
ments and are discussed in
Section 5.4.2
.
10
5.0
2.0
Figure 5.56
Contours of chargeability (ms) from gradient array
resistivity/IP survey of the Pine Point MVT deposits (potential
dipole length
¼
61 m). Redrawn, with permission, from Lajoie and
self-potential or electromagnetic responses (Lajoie and
resistivity/IP survey de
ned chargeability anomalies of
about 7 times the background response associated with the
N42 orebody, although the resistivity responses were vari-
able.
Figure 5.56
shows the chargeability map produced
from the results of a subsequent gradient-array survey with
a potential dipole length of 61 m. In addition to the response
from the N42 orebody, distinct responses from three previ-
ously unknown orebodies to the south were defined.
5.6.7.1
Overburden effects
The electrical properties of the near-surface (see
Section
measurements made with electrodes on the surface. Elec-
tric current
to flow via the path of least resistance,
so where the overburden is highly conductive the transmit-
ted current will concentrate in the more conductive layer
with less entering the underlying higher-resistivity mater-
ial. Increasing the thickness of the surface layer and
increasing its conductivity causes a greater proportion of
the current to flow in it, with even less penetrating the
underlying material.
Telluric currents (see Atmospheric noise in
Section
'
prefers
'
Example - Pajingo epithermal gold deposits
Hoschke and Sexton (
2005
) describe various electrical
and EM surveys during exploration of the Pajingo
epithermal system, near Charters Towers, Queensland,
Australia. Low sulphidation epithermal alteration and
veining occur in intermediate volcanic rocks over an area
of about 150 km
2
. Conductive cover sediments cover
about 80% of the area. Gold occurs within quartz veins
0.5 to 3 m wide. Alteration is variable but tends to be
silicic near the mineralisation and dominated by clay
minerals further away.
Gradient array resistivity surveys were undertaken to
map the basement geology. Data were acquired as a series
of square blocks with side of 600 or 800 m and current
electrodes 1200 m or 1600 m apart. Line spacing was 40 m
and the potential dipole was 20 m in length.
Figure 5.57
shows a subset of the data. The zones of silicification which
flat-lying conductive overburden
and are a common source of noise that often obliterates
the weaker polarisation currents. Larger current dipoles are
required to penetrate the overburden; and larger current
and longer polarisation times in the time domain, to say
4 seconds, help increase IP signal levels. Conductive over-
burden also signi
cantly increases EM-coupling effects
(see
Section 5.6.7.2
).
Dipole
-
dipole pseudosections of modelled responses of
strated by the figure, the limited penetration of current
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