Geology Reference
In-Depth Information
ρ
a
90
340
140
126
106
800
200
485
365
332
252
954
208
84
193
165
211
306
248
384
350
320
342
333
105
108
287
320
94
285
248
550
323
353
368
144
116
535
159
106
296
312
550
350
PFE
0.8
2
4.9
3.8
4.8
4.5
3.8
3.6
2.4
2.6
1
1.1
2.4
4.6
3.9
4
6.5
6.7
4.5
3.4
2.6
2.2
1.8
1.8
5
3.2
5.8
8.5
6.3
3.5
3.6
2.8
2.4
1.3
2.4
2.9
2.8
4.7
6.5
10.2
5.3
4.1
3.2
3.4
MF
2.2
14
38
36
53
5.6
19
7.5
7.9
3.6
6.6
1.2
12
55
20
24
31
18
8.8
7.4
6.9
3.8
22
5.4
47
30
13
18
91
22
14
6.6
6.7
6.4
20
24
8.7
40
96
18
13
5.9
9.7
0/B
Economic
sulphides >5%
Barren
porphyry
Pyrite
minor chalco <3%
Fig. 8.32
Pseudosections of apparent
resistivity (
r
a
), percentage frequency effect
(PFE) and metal factor parameter (MF) for
a double-dipole IP traverse across a zone of
massive sulphides whose shape is known
from subsequent test drilling. Current and
potential electrode spacing
a
was 100 feet
(30.5 m). Frequencies used for the IP
measurements were 0.31 and 5.0 Hz. (After
Fountain 1972.)
0
100 ft
a
na
a
Δ
V
Plotting point
economic importance, for example water-filled shear
zones and graphite-bearing sediments can both generate
strong IP effects. Field operations are slow and the
method is consequently far more expensive than most
other ground geophysical techniques, survey costs being
comparable with those of a gravity investigation.
in locating low-grade ore deposits such as disseminated
sulphides (e.g. Langore
et al
. 1989).These have a strong IP
effect but are non-conducting and therefore are not read-
ily detectable by the electromagnetic methods discussed
in Chapter 9. IP is by far the most effective geophysical
method that can be used in the search for such targets.
Figure 8.34 shows the chargeability profile for a time-
domain IP survey using a pole-dipole array across the
Gortdrum copper-silver ore body in Ireland. Although
the deposit is of low grade, containing less than 2%
conducting minerals, the chargeability anomaly is well
defined and centred over the ore body. In contrast, the
8.3.6 Applications of induced
polarization surveying
In spite of its drawbacks, the IP method is extensively
used in base metal exploration as it has a high success rate
