Geology Reference
In-Depth Information
and mineral concentration, IP measurements are usually
made at frequencies at, or below, 10 Hz to remain in the
non-inductive regions.
Two measurements are commonly made.The
percent-
age frequency effect
(PFE) is defined as
8.3.4 Field operations
IP equipment is similar to resistivity apparatus but uses a
current about 10 times that of a resistivity spread; it is also
rather more bulky and elaborate.Theoretically, any stan-
dard electrode spread may be employed but in practice
the double-dipole, pole-dipole and Schlumberger con-
figurations (Fig. 8.33) are the most effective. Electrode
spacings may vary from 3 to 300 m with the larger
spacings used in reconnaissance surveys. To reduce the
labour of moving current electrodes and generator,
several pairs of current electrodes may be used, all con-
nected via a switching device to the generator.Traverses
are made over the area of interest plotting the IP reading
at the mid-point of the electrode array (marked by
crosses in Fig. 8.33).
Noise in an IP survey can result from several phenom-
ena. Telluric currents cause similar anomalous effects to
those encountered in resistivity measurements. Noise
also results from the general IP effect of barren rocks
caused by membrane polarization. Noise generated by
the measuring equipment results from electromagnetic
coupling between adjacent wires. Such effects are com-
mon when alternating current is used since currents can
be induced to flow in adjacent conductors. Conse-
quently, cables should be at least 10 m apart and if they
must cross they should do so at right angles to minimize
electromagnetic induction effects.
-
(
rr
r
)
01
.
10
PFE =
100
(8.23)
10
where
r
0.1
and
r
10
are apparent resistivities at measuring
frequencies of 0.1 and 10 Hz. The
metal factor
(MF) is
defined as
-
(
rr
rr
)
5
01 10
01 10
.
MF =¥
2
(8.24)
p
.
This factor normalizes the PFE with respect to the lower
frequency resistivity and consequently removes, to a
certain extent, the variation of the IP effect with the
effective resistivity of the host rock.
A common method of presenting IP measurements
is the
pseudosection
, in which readings are plotted so as to
reflect the depth of penetration. Figure 8.31 illustrates
how a pseudosection is constructed for the double-
dipole array geometry illustrated in Fig. 8.33. Measured
values are plotted at the intersections of lines sloping at
45° from the centres of the potential and current elec-
trode pairs.Values are thus plotted at depths which reflect
the increasing depth of penetration as the length of the di-
pole separation increases.The values are then contoured.
VES resistivity data can also be presented in this way with
the plotted depth proportional to the current electrode
separation. Pseudosections give only a crude representa-
tion of the IP response distribution at depth: for example,
the apparent dip of the anomalous body is not always the
same as the true dip. An example of this method of
presentation is shown in Fig. 8.32.
8.3.5 Interpretation of induced
polarization data
Quantitative interpretation is considerably more com-
plex than for the resistivity method.The IP response has
been computed analytically for simple features such as
spheres, ellipsoids, dykes, vertical contacts and horizon-
tal layers, enabling indirect interpretation (numerical
modelling) techniques to be used.
Laboratory modelling can also be employed in indi-
rect interpretation to simulate an observed IP anomaly.
For example, apparent resistivities may be measured for
various shapes and resistivities of a gelatine-copper
sulphate body immersed in water.
Much IP interpretation is, however, only qualitative.
Simple parameters of the anomalies, such as sharpness,
symmetry, amplitude and spatial distribution may be
used to estimate the location, lateral extent, dip and
depth of the anomalous zone.
The IP method suffers from the same disadvantages
as resistivity surveying (see Section 8.2.9). Further,
the sources of significant IP anomalies are often not of
n
= 1
n
= 2
n
= 3
n
= 4
Δ
V
Δ
V
Δ
V
Δ
V
n
= 1
n
= 2
n
= 3
n
= 4
Fig. 8.31
The presentation of double-dipole IP results on a
pseudosection.
n
represents the relative spacing between the
current and potential electrode pairs.
