Geoscience Reference
In-Depth Information
discriminating various sulphide and oxide minerals in a
number of deposit types. It takes considerable time to
make spectral measurements and as a consequence the
method remains chie y in the realms of research and is
not commonly used in geophysical prospecting.
In
spacing and their relative positions, and array location
relative to the target. As would be expected, that part of
the subsurface closest to the array exerts the greatest
in uence on the measured parameters. It follows that by
positioning the array in different locations information
about different parts of the survey area is obtained.
'
'
frequency domain surveys, current is
transmitted as a very low-frequency alternating square
wave, in the range 0.1 to 3 Hz, but usually 0.1 Hz. Meas-
urements are made at two frequencies, the higher usually
three times the lower frequency, and the apparent resistiv-
ity of the ground is calculated for both ( Fig. 5.37b ) . The
resistivity at the lower frequency is taken as the apparent
resistivity of the ground. The difference in resistivity at the
two frequencies, relative to the resistivity at the higher
frequency, is used to calculate the induced polarisation
parameter known as the percentage frequency effect (PFE)
given by:
conventional
5.6.4.1 Depth penetration
The distance between the current electrodes controls the
influence that features at depth have on the measurement.
Figure 5.38a is a cross-section showing the distribution of
current flowing through a half-space; cf. Fig. 5.34f . The
numbers on the lines show the percentage of the total
current flowing above the line. The 50% line has a max-
imum depth equal to half the current dipole length (X AB /2)
and almost 90%
flows above a depth equal to three times
the dipole length.
When presenting and interpreting the data, a pseudo-
depth for a given reading can be assigned, which is a
function of the array geometry and the electrode separ-
ations
Þ¼ ρ low ρ high
ρ high
PFE
ð
%
100
ð
5
:
19
Þ
where
ρ high are the resistivities at the lower and
the higher frequencies, respectively.
A related IP parameter is the metal factor (MF). This is
the PFE normalised (divided) by the resistivity measured at
the lower frequency. It is intended to remove variations in
PFE related to the host rock resistivity and to highlight
zones of anomalous
ρ low and
(see Electrode arrays
in Section 5.6.5 ) . By
'
expanding
'
an array (increasing the electrode separations
X AB
a)
I
. In conductive terrains,
however, the parameter is dominated by the effects of the
low resistivity of the host rock, resulting in spurious MF
anomalies in regions where there is no signi cant increase
in polarisation. For this reason MF is not normally used.
There is also a delay or phase shift (see Appendix 2 )
between the transmitted sine wave current and the meas-
ured voltage at each frequency that can also be used as an
indication of electrical polarisation ( Fig. 5.37c ) . The phase
shifts at two frequencies can be combined into a single
parameter known as the relative phase shift (RPS), which is
relatively immune to EM-coupling effects (see Section
5.6.7.2 ) , and given by:
'
metal content
'
_
+
A
B
10
X AB /2
50
90
90
60
70
80
80
b)
Normalised current density (%)
0 0 0 0 0 0 0 0 0 0 0
0
100
25
200
50
RPS
¼ ϕ low ϕ high
ð
5
:
20
Þ
300
400
500
100
200
Dipole
length -
X AB (m)
where
ϕ high are the phase shifts at the lower and the
higher frequencies, respectively, and have units of degrees.
ϕ low and
300
Figure 5.38 The half-space current distribution in the axial section of
a surface dipole. (a) A current dipole of length X AB . The numbers
on the lines show the percentage of current
5.6.4 Measurement of resistivity/IP
flowing above the line.
Fundamental to the acquisition of electrical data are two
basic variables: array geometry, both in terms of electrode
(b) Plot of percentage current that
flows above a given depth.
Based on a diagram in Robinson and Coruh ( 1988 ).
 
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