Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 295

comparatively cheap and in appropriate situations can
provide useful results.

between current pole-electrode (B) and the poten-

tial electrode (M)). IP parameters can also be measured.
In order to con rm anomalous responses related to the
conductive body, the whole survey procedure can be
repeated with the buried current electrode located outside
the conductor. For example, for the case of an electrode in
a drillhole, it may be raised above the body by say, 10 m.
The potential distribution about the pole-electrode is
mapped and used to confirm that anomalous features
observed with the electrode embedded in the conductor
are related to the conductive body and not due to the
background geology. This background response can be
removed from the original results to enhance the reso-
lution of anomalous features, a form of regional

' roving '

5.6.9.1 AP survey practice
The survey equipment required is the same as that for
resistivity/IP surveying (see Section 5.6.4 ) . Details of the
electrodes are described in Section 5.4.1. To conduct an AP
survey, an electrical potential from the transmitter used for
resistivity/IP work is applied to a pair of current electrodes.
The electrode configuration is the pole pole array (see

Section 5.6.5 ) with the current pole-electrode located
(buried) in the conductive target body ( Fig. 5.63 ) . It is
the means by which the potential is applied to the target
conductor, so that the conductor acts as a giant buried
current electrode. The other current electrode is located
remote from the survey area so that it is effectively at
in

-

- residual

separation (see Section 2.9.2 ) .
Measurements are normally made on an equidimen-
sional grid, or along survey traverses perpendicular to the
expected strike of the conductor, and a map of the surface
potential produced. Station spacing is typically 5 to 50 m
with survey lines spaced of 25 to 100 m apart. Potential
measurements can also be made down adjacent, uncased
and water- lled, drillholes; typically at intervals 5 to 10 m,
or closer to increase resolution in the vicinity of rapid
changes in conductor geometry.

uence on the electric
field (the shape of the equipotential surfaces) in the vicinity
of the conductor.
Mapping the electric field about the conductive body
will reveal its presence and its shape (see Fig. 5.35 ). This
is done by making potential measurements with a pair of
non-polarising electrodes, one of which is established as a
' xed

nity where it has minimum in

dis-
tance, from the body. Measurements are made with the
other

' reference electrode and located at an ' in nite '

5.6.9.2 Display and interpretation of AP data
Maps and cross-sections of the measured potential are
normally presented as contours instead of images, because
contours generally provide better resolution of subtle gra-
dients than images. The amplitudes and shapes of the
contours, and the gradients between them (the spacing of
the contours), are interpreted in terms of conductor geom-
etry (see Section 2.10.2 ). AP data can be interpreted using
computer models and can be quite reliably interpreted
using qualitative methods. As mentioned previously, the
main application of the AP method is mapping of con-
ductive bodies at prospect scale, based on observed vari-
ations in electrical potential. An increase in potential
indicates the presence of an electrically connected con-
ductor, and gradients in the field re ect conductor geom-
etry and electrical contacts.
A common application of the AP method is to assess
the signi cance of a conductor intersected by a drillhole. If
the intersection is part of a small body at great depth, the
equipotential surfaces will be spherical and centred about
the current electrode. Contours of the potential field on
any plane, such as the ground surface, will be circular and
centred about the projection of the current electrode

' '

potential electrode moved systematically
over the survey area (including down drillholes), which is
usually centred on the energised conductor. The potential
difference is measured relative to the distant electrode and
the measurements normalised for variations in the current
transmitted into the body (the voltage is divided by the
current transmitted during the measurement), so the
results are expressed in units of V/A. Sometimes the data
are transformed to apparent resistivity using the geometric
correction factor given in Table 5.2 (X BM is the distance

roving

To distant
electrode

To distant
electrode

I V

Current electrode

?

?

Conductive zone

Figure 5.63 Schematic illustration of the applied potential method for
surface and downhole measurements. ' s