Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 291

Low High

Resistivity Polarisation

High Low

Pseudosections Earth models

0
1
2
3
4
5

Conductive
overburden

n =1

n =2
n =3
n =4

n =5
n =6
n =7
n =8

Conductive overburden

Thickness change

0
1
2
3
4
5

n =1

n =2
n =3
n =4

n =5
n =6
n =7
n =8

Conductive overburden - thickness change

Extent of highly conductive zone

0
1
2
3
4
5

n =1

n =2
n =3
n =4

Conductive
zone

n =5
n =6
n =7
n =8

Conductive overburden - more conductive zone

Projection of source body

0
1
2
3
4
5

n =1

n =2
n =3
n =4

n =5
n =6
n =7
n =8

Source
body

Conductive overburden - in contact with underlying source body

Projection of source body

0
1
2
3
4
5

n =1

n =2
n =3
n =4

Source
body

n =5
n =6
n =7
n =8

Conductive overburden - not in contact with underlying source body

Figure 5.58 The computed dipole dipole array resistivity responses of some simple 2D conductivity models with conductive overburden.

Horizontal scale is one dipole length per division.

-

Figure 5.59 shows how EM-coupling appears in IP phase
pseudosection data. These data are from the Goongewa
(formerly Twelve Mile Bore) MVT-type Pb

depth. The decoupled data resolve the ' pants-legs ' anomaly

due to the polarisable mineralisation.
All electrode arrays are prone to EM-coupling, to some
degree. The Schlumberger and gradient arrays, with their
long cable runs to the distant current electrodes, are highly
susceptible to EM-coupling. Strategies for minimising the
effect include: minimising the dipole lengths, minimising

Zn deposit,
Canning Basin, Western Australia (Scott et al., 1994 ) . Since
the effects are greater for larger dipole separations, the raw
phase shows a consistent increase with dipole separation
(n) and, therefore, an apparent increase in IP effect with

-