Geology Reference
In-Depth Information
Fig. 9.19
Contour map of real component anomalies over part of the Skellefteå orefield, northern Sweden, obtained using an airborne
system with vertical coplanar coils. Mean ground clearance 30 m, operating frequency 3.5 kHz. Contours in ppm of the primary field.
(After Parasnis 1973.)
frequency, ranging from 10
-5
Hz up to the audio range,
and overlap the frequency range utilized in the AFMAG
method (Section 9.4.3).
nique capable of penetrating to the depths of interest to
the oil industry.Although variable in both their direction
and intensity, telluric currents cause a mean potential
gradient at the Earth's surface of about 10 mV km
-1
.
Telluric currents are used in prospecting by measuring
the potential differences they cause between points at the
surface. Obviously no current electrodes are required
and potential differences are monitored using non-
polarizing electrodes or plates made of a chemically
inert substance such as lead. Electrode spacing is typical-
ly 300-600 m in oil exploration and 30 m or less in
9.11.2 Surveying with telluric currents
Telluric currents flow within the Earth in large circular
patterns that stay fixed with respect to the Sun. They
normally flow in sheets parallel to the surface and extend
to depths of several kilometres in the low frequencies.
The telluric method is, in fact, the only electrical tech-
