Radiometric method - Geophysics for the Mineral Exploration Geoscientist - page 194

e.g. heavy-mineral sands. Renewed interest in the
method has also been driven by the requirement to
map the materials making up the near-surface. In this
context, the radiometric data are usually interpreted in
combination with topographic and satellite-borne
remote-sensing data. Some applications include rego-
lith mapping for mineral exploration and the mapping
of soil types for environmental studies.
The radiometric method has several characteristics
that make it unique amongst the geophysical methods.
Firstly, the measured radioactivity originates from only
the top few centimetres of the Earth's crust so, unlike
other geophysical methods, radiometrics has only a very
limited ability to see into the subsurface. Secondly,
because it is possible to identify the elemental source
of the radiation from the energy of the

There exists a large literature describing the radiomet-
ricmethod, althoughmuch of it is focused on applications
in the glaciated terrains of the northern hemisphere.
Significant developments in the past 10 years, particularly
in data processing and reduction methods, and progress
in understanding the behaviour of the radioelements in
arid terrains such as Australia, mean that much of the
published material has now been superseded.
The process of converting measured radioactivity to
elemental concentrations of the ground involves a
multi-stage reduction of the survey data to remove
responses of non-geological origin. It is based largely
on measurements made specifically for this

purpose. Much of this reduction process is empirical,
and understanding its basis and limitations requires a
reasonable knowledge of the characteristics of radio-
activity. This, therefore, is the starting point for our
description of the radiometric method, with emphasis
on aspects relevant to geophysical surveying. The inter-
pretation of radiometric data and the computed elem-
ental maps requires an understanding of the behaviour
of radioactive elements in the geological environment,
which is emphasised in the latter part of the chapter.

calibration

-rays emitted,
radiometric data are used to map variations in the
chemical rather than the physical characteristics of the
survey area. Interpretation of radiometric data straddles
the boundary between geochemistry and geophysics.
This may explain why the methodologies for interpret-
ing radiometrics are less well-developed than those of
other geophysical methods.

γ

4.2 Radioactivity

isotope is called the parent and the post-decay isotope is
the daughter product. The daughter product may itself be
radioactive, as may its daughters. In this case a decay
series forms, continuing until a stable (non-radioactive)
daughter isotope is created. Some radioisotopes have
more than one mode of decay with a proportion of the
radioisotope decaying through one mode and the
remainder in another. This is known as branched decay,
and the relative proportions of the different types of
decay are always the same.

An atom that contains equal numbers of protons and
electrons will be electrically neutral. The number of
protons or electrons in the atom is the element

is atomic
number (Z), which defines its chemical properties and its
place in the periodic table of the elements. The number of
neutrons in the nucleus of the atom is the atom

'

s neutron
number (N), and the total number of protons and neutrons
(Z + N) in the nucleus is the atom

'

s mass number (A).
Atoms of the same element having different neutron
numbers are called isotopes and are identi ed by their
mass number. For example, uranium (U) contains 92
protons, but can have 142, 143 or 146 neutrons, forming
the isotopes 234 U, 235 U and 238 U respectively.
Radioactive materials, also called radioisotopes,are
unstable and spontaneously emit radiation as part of
their transformation into a more stable, non-radioactive,
state. The radiation occurs in three forms, known as
alpha- (

'

4.2.1 Radioactive decay

Alpha-decay involves emission of a helium ( 4 He) nucleus,
β β - ) or positron (

+ ),

-decay the emission of an electron ( β

and γ -decay the emission of a photon. Alpha- and β -decays

γ

are the emission of particles whereas -decay is the

emission of high-frequency electromagnetic radiation (see
Fig. 5.1 ). The particles originate from within atomic nuclei
and, since their emission causes a change in atomic
number, a different element is created.

) radiation, with the
emission of the different kinds of radiation referred to as
α

α ), beta- ( β )andgamma- ( γ

-, β -and γ -decay, respectively. The original (pre-decay)