Geology Reference
In-Depth Information
(a)
PFE = 100
×
(
ρ
d.c
-
ρ
h.f.
)/
ρ
h.f.
= 100
×
V
p
/(
V
o
-V
p
)
V
p
ρ
d.c
=
2
π
K
(
V
o
/
I
)
M = V
p
/ V
o
V
o
V
o
-V
p
Voltage observed at
voltage electrodes
ρ
h.f.
=
2
π
K
[(
V
o
-V
p
)/|]
V
p
(b)
Current,
I
, introduced at current electrodes
Figure 7.3
(a) Ground responses to equal-amplitude square-wave and spike
impulse currents, for an array with a geometric factor
K
(see Section 6.1.1).
The voltage
V
p
(discussed in the text) is seldom more than a small percentage
of the steady-state measured voltage V
o
. If the current is terminated shortly
after being introduced, as in any half-cycle of a high-frequency alternating
square wave, the measured voltage barely rises above
V
o
−
V
p
. The 'steady
state' and impulse currents introduced at the current electrodes are shown
in (b).
telluric and SP noise, 'DC' measurements are taken with current reversed
at intervals of the order of a few seconds, while the 'high' frequencies are
usually kept below 10 Hz to minimise electromagnetic induction.
7.2.5 Metal factors
A PFE can be divided by the DC resistivity to give a quantity which,
multiplied by 1000, 2000 or 2000π, produces a number of a convenient
size known as the
metal factor.
Metal factors emphasise rock volumes that
are both polarisable and conductive and may therefore be assumed to have
a significant sulphide (or graphite) content. Although this may be useful
when searching for massive sulphides, low resistivity is irrelevant and can
be actually misleading in exploration for disseminated deposits. As usual
when factors that should be considered separately are combined, the result
is confusion, not clarification.
7.2.6 Phase
The square-wave current of Figure 7.3 can be resolved by Fourier anal-
ysis into sinusoidal components of different amplitudes and frequencies
