Geology Reference
In-Depth Information
the transport of radon generated at depth (Abdoh &
Pilkington 1989).This technique is advantageous when
there is no great difference in rock properties across
the fault that could be detected by other geophysical
methods.
emissions, the information obtained relates only to the
top metre or so of the ground.
The interpretation of radiometric data is mainly qual-
itative, although characteristic curves are available for
certain elementary shapes which provide the parameter:
(surface area) ¥ (source intensity).
10.5 Field surveys
As previously stated, Geiger counter investigations are
limited to ground surveys. Count rates are noted and
their significance assessed with respect to background
effects resulting from the potassium content of the local
rocks, nuclear fallout and cosmic radiation.An apprecia-
ble anomaly would usually be in excess of three times the
background count rate.
Scintillation counters may also be used in ground sur-
veys and are usually sited on rock exposures.The ground
surface should be relatively flat so that radioactive emis-
sions originate from the half-space below the instru-
ment. If this condition does not obtain, a lead collimator
can be used to ensure that radioactive emissions do not
arrive from elevated areas flanking the instrument.
Most radiometric surveying is carried out from the
air, employing larger scintillation sensors than in ground
instruments, with a consequent increase in measure-
ment sensitivity. Instruments are interfaced with strip
recorders and position fixing is by means of the methods
discussed in Section 7.8. Radiometric measurements are
normally taken in conjunction with magnetic and elec-
tromagnetic readings, so providing additional datasets at
minimal extra cost. In surveying for relatively small de-
posits the slow speed of helicopters is often advantageous
and provides greater discrimination and amplitude of
response. Flight altitude is usually less than 100 m and,
because of the weak penetrative powers of radioactive
10.6 Example of radiometric surveying
Figure 10.3 shows a ground magnetic and gamma-
ray profile across a zone of uranium mineralization in
Labrador. This was obtained from contour maps of a
small area identified from a regional airborne survey.
There are strong coincident magnetic and radiometric
anomalies, the source of which was investigated by two
boreholes. The anomalies arise from magnetite and
pitchblende, located immediately beneath the anomaly
maxima, in an argillaceous and quartzitic host. Pitch-
blende is a variety of massive, botryoidal or colloform
uraninite.
Further reading
Durrance, E.M. (1986)
Radioactivity in Geology.
Ellis Horwood,
Chichester.
Milsom, J. (1989)
Field Geophysics.
Open University Press, Milton
Keynes.
Telford, W.M. (1982) Radon mapping in the search for uranium.
In
: Fitch, A.A. (ed.),
Developments in Geophysical Exploration
Methods.
Applied Science, London, 155-94.
Telford, W.M., Geldart, L.P. & Sheriff, R.E. (1990)
Applied Geo-
physics
, 2nd edn. Cambridge University Press, Cambridge.
Wollenberg, H.A. (1977) Radiometric methods.
In
: Morse, J.G.
(ed.),
Nuclear Methods in Mineral Exploration and Production.
Elsevier, Amsterdam, 5-36.
