Radiometric method - Geophysics for the Mineral Exploration Geoscientist - page 207

in U, 37% in Th and 14% in the K channel (B. Minty, pers
comm).
There is no doubt that these reduction algorithms sig-
ni cantly increase the resolution of survey data. The
improvements exhibited in the corrected survey data, as
judged from the clarity of geology-related patterns and
textures, are often dramatic. Good examples are presented
by Minty ( 1998 ) and Dickson and Taylor ( 2000 ).

uranium-channel -rays. It is constantly present in the

atmosphere because its daughter products attach them-
selves to airborne aerosols and dust particles. Its distribu-
tion is not uniform; it is affected by such factors as wind,
moisture and temperature, resulting in signi cant vari-
ations in concentration with location (especially height)
and time. Furthermore, increasing barometric pressure
forces Rn into the pores of the soil, from which it is then
released into the atmosphere during periods of lower pres-
sure. The problem can be severe, with Rn sometimes
accounting for a significant portion of the counts in the
U channel.
One method of removing atmospheric Rn requires the
use of an upward-looking radiation detector shielded from
the terrestrial radiation. Calibration surveys are conducted
over water on days having different amounts of 222 Rn in
the air. The ratio of counts measured by the upward-
looking detector to those measured by the main
downward-looking detector in each energy window pro-
vides channel correction factors that are applied to the
downward-looking measurements. The main disadvantage
of this approach is the additional space and weight associ-
ated with the shielded upward-looking detector, and that
Rn is not necessarily uniformly distributed within the air.
Spectral signature correction methods are based on the
differential attenuation of

γ

4.4.3 Background radiation

Gamma-radiation from the aircraft and from space is
background radioactivity that contributes to the measure-
ments. The former is constant and due to radioactive
elements in the materials from which the aircraft and
recording equipment are made. Cosmic radiation has a
fairly constant energy spectrum everywhere, although its
amplitude reduces with decreasing altitude.
Removal of the cosmic background is based on the fact
that any radiation greater than 3.0 MeV must be of cosmic
origin, since terrestrial

-rays have lower energies (see
Fig. 4.6 ) . Airborne spectrometers include a cosmic-energy
channel to measure the cosmic radiation, usually in the
range 3 to 6 MeV ( Table 4.1 ) . The cosmic contribution in
any energy window is proportional to the radioactivity in
the cosmic window and, because its energy spectrum is
fairly constant, its contribution can be determined. Correc-
tion parameters for the cosmic background are determined
by acquiring survey data at a number of survey heights
(typically 3000 to 4000 m) over a large body of water, such
as a lake or the ocean, where ground radiation is minimal
owing to the water

γ

γ

-rays of different energies. As
described in Section 4.2.3 , lower-energy

γ

-rays experience
greater attenuation than higher-energy rays. This means
that a decay series spectrum changes as the distance
between source and detector changes, with the greatest
spectral modification occurring in the lower energy part
of the spectrum. The relative amplitudes of the various
photopeaks will be different for a source close to the
detector, such as atmospheric Rn, and for more distant
sources in the ground. Spectral signature methods compare
peaks in the observed spectra and estimate the relative
contributions from terrestrial and atmospheric sources.
Those used are the 214 Bi peaks at 0.609 and 1.765 MeV,
and the 214 Pb peaks at 0.295 and 0.352 MeV (see Fig. 4.6b ) .
Detailed descriptions of these methods and examples of
their effectiveness in removing Rn-related noise are pro-
vided by Minty ( 1992 ) and Jurza et al.( 2005 ).
Note that methods based on the 214 Bi 0.609 MeV
photopeak are only effective in areas devoid of 137 Cs (see
Section 4.2.5.1 ) , because its single photopeak at 0.662 MeV
contaminates the 0.609 MeV emissions from U and Rn.
Consequently, these methods are likely to be ineffective in
much of the northern hemisphere.

-rays (see Section
4.2.3 ) . For all energy windows, there is a linear relationship
between the intensity of the radiation and survey height,
with the aircraft contribution appearing as a constant
background. The cosmic contribution in each channel of
normal survey data can be calculated from the slope of the
linearity, for that channel, and the counts measured in the
cosmic channel. This is combined with the constant air-
craft response for the energy window and removed as a
single background correction.

' s ability to absorb γ

4.4.4 Atmospheric radon

One of the most dif cult aspects of radiometric data reduc-
tion is compensating the effects of radon gas ( 222 Rn) and
its daughters in the atmosphere. Radon-222 occurs above
214 Bi in the 238 U decay series ( Fig. 4.5c ) so it is a source of