Geology Reference
In-Depth Information
(a)
+20
R
θ
S
S
P
P
Surface
0
θ
P
θ
S
-20
R
Current
concentration
Ore
P
S
R
= tilt
= primary
= secondary
= resultant
(b)
90
θ
60
S
Surface
S
θ
θ
θ
S
R
R
P
P
P
Current
concentration
R
Fig. 9.4
Tilt-angle profiles resulting from
(a) vertical and (b) horizontal transmitter
loops. (After Parasnis 1973.)
Ore
Modern instruments have three coils with their
axes at right angles. They can thus detect the signal
whatever its direction, and find the null orientation
electronically and automatically. Some instruments
will measure signals from two or more transmitters si-
multaneously. In this case transmitters are chosen whose
signals arrive in the survey area at very different
azimuths.
The VLF method has the advantages that the field
equipment is small and light, being conveniently oper-
ated by one person, and that there is no need to install
a transmitter. However, for a particular survey area,
there may be no suitable transmitter providing a mag-
netic vector across the geological strike. A further disad-
vantage is that the depth of penetration is somewhat less
than that attainable by tilt-angle methods using a local
transmitter. The VLF method can be used in airborne
EM surveying.
Outcrop of
massive sulphide
0
100m
N
50
0
50
Tilt
Transmitter
Fig. 9.5
Example of tilt-angle survey using a vertical loop
transmitter. (After Parasnis 1973.)
