Geoscience Reference
In-Depth Information
topography on dipole
dipole pseudosec-
from shallow bodies.
There are no means of accurately removing terrain
effects from resistivity/IP data, the practical approach to
interpretation being to include topographic variations in
the computed model. Of course, it is perfectly plausible for
an economic target to be responsible for a change in
topography, owing to greater or decreased resistance to
weathering, the anomalous electrical responses drawing
attention to the area.
-
dipole and pole
-
5.6.8.1
Example of resistivity/IP logging - Uley
graphite deposit
At Uley in South Australia, mineralisation occurs in a
graphite schist within a succession of schist and gneiss,
beneath a cover of laterite and calcrete. Graphite ore zones
are up to 12 m thick and are stratiform and strata-bound.
The host rocks are strongly weathered. Numerous elec-
trical and EM surveys have been made in the area (Barrett
and Dentith,
2003
).
Figure 5.61
shows the downhole logs of various electrical
properties from a drillhole through the graphite mineral-
isation. A correlation between IP anomalies, SP highs and
carbon assays is evident. The data indicate that a charge-
able source has been intersected at a depth of about 40 m.
This is consistent with the results from modelling of
surface IP data. The logs also show that there is little
relationship between mineralisation and resistivity; this is
inconsistent with IP and subsequent EM surveys and is
probably due to the differing sample volumes measured
in surface
5.6.7.4
Anisotropy effects
Electrical anisotropy affects all electrical and EM measure-
ments and is discussed in
Section 5.3.1.4
.
In resistivity/IP
surveying for mineral targets, the electrode array is usually
oriented in a
fixed direction for the full survey area, usually
perpendicular to strike. It is only anomalous variations in
the measured parameters along the survey direction that
are of interest for detecting targets, so anisotropy is usually
of little concern in
field surveys. However, its effect will be
seen where, say, the dip of a rock formation is different in
part of the survey area causing its foliation to change
orientation relative to the electric
field of the survey array.
Changes in resistivity and polarisation due to anisotropy
would then be expected.
Anisotropy is also likely to be noticed when comparing
the absolute values of measured parameters from different
arrays where the principal direction of current flow in each
could be different: for example chiefly horizontal flow for
measurements using the Wenner, Schlumberger and gradi-
ent arrays, and significantly vertical flow for the dipole
surveys
compared with the downhole
measurements.
C
(%)
SP
(mV)
Apparent
resistivity
( m)
Chargeability
(ms)
0
30
-100
100
300
0
800
1600
0
400
800
0
20
40
-
dipole array. Comparison of the measured electrical proper-
ties is difficult, as it also is when comparing EM conductivity
data with electrical resistivity data owing to the principally
different directions of current
60
flow in each method.
80
5.6.8
Resistivity/IP logging
100
Resistivity/IP and resistance logging in hard-rock terrains
is mainly undertaken to de
ne the electrical properties of a
known succession and to aid in the interpretation of elec-
trical and EM surveys. In soft-rock terrains the aim is more
often to establish stratigraphic correlations and in some
cases infer subtle changes in physical characteristics indica-
tive of facies changes. Resistivity/IP and single-point resist-
ance logging for these purposes are demonstrated in the
following two examples.
120
140
Figure 5.61
Assay data and downhole logs from the Uley graphite
deposit. The resistivity/IP data were acquired using the dipole
-
dipole
array (dipole length
0.75 m). Redrawn, with permission, from
Barrett and Dentith (
2003
).
ΒΌ
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