Geoscience Reference
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possibility of disequilibrium, since the differing chemical
and physical properties of elements in the decay series may
lead to their separation. A detailed description of all these
aspects of the distribution of the radioelements is given in
lithotypes and rock units, and then to determine the radio-
metric properties of each region, such as the mean and
standard deviation of each elemental concentration. These
data are then used to calculate a residual image, being the
deviation of each pixel within a unit from the average value
for each unit. This may highlight subtle intra-unit vari-
ations associated with alteration or changes in lithology.
See Jayawardhana and Sheard (
1997
) and Dickson et al.
(
1996
) for examples of this approach.
Digital elevation data are useful for identifying areas of
erosion and sedimentation, where dissected cover is likely
and as a guide to where bedrock responses are most likely.
Draping radiometric data on the terrain often helps con-
siderably in understanding the responses. Wilford et al.
(
1997
) present some good examples of this kind of display
and discuss how they assist with interpretation.
4.7.1
Interpretation procedure
Analysis of radiometric data is less complex than for other
types of geophysical data, although the process is often
hindered by high noise levels caused by the effects of cover,
and deficiencies in the acquisition and reduction of the
survey data. In principle, areas with anomalous concen-
trations of one or more of the radioelements may be
identified as being potentially indicative of mineralising
environments. Geological interpretation of the data, on
the other hand, can be complex. Geological maps may be
created (radiolithic mapping) by assigning regions with
similar radioelement concentrations to pseudo-geological
units. However, like virtually every physical/chemical
property relevant to geophysical surveying, the different
rock types show a range of K, U and Th concentrations,
and unequivocal identi
cation of speci
c lithotypes is not
possible. As shown in
Figs. 4.12
to
4.14
, many rock types
are low in all three radioelements; and others, such as
granite and shale, may have similar concentrations of K,
U and Th. Often there are subtle changes in radioelement
content re
ecting lithological subdivisions which are not
evident in geological inspections. It is quite common, for
example, to map from the radiometric data zoning and
multiple intrusive phases in large granitic intrusions.
The influence of surficial cover and weathering pro-
cesses on the radiometric response means it is strongly
advisable to make an integrated interpretation using other
complementary datasets. An example of this kind of inter-
pretation for mineral exploration is provided by Dickson
et al.(
1996
). The obvious datasets to combine with radio-
metrics are multispectral remote sensing and aerial photo-
graphs because they also are only sensitive to surface
material. Multispectral data can be used to create images
showing the distribution of clays and iron oxides, both of
which in
uence the distribution of K, U and Th. Images
showing the distribution of vegetation may indicate where
the radiometric response may be subdued.
An integrated interpretation can be part of a more
sophisticated strategy designed to identify anomalous
regions. All available data, including elemental ratios and
other multichannel remote sensing data, are used to map
4.7.2
Interpretation pitfalls
Man-made disturbances of the ground surface often pro-
duce anomalously
radiometric responses.
Some examples include: the subsurface rocks exposed by
quarrying and open-pit mining, tailings from mines and
processing plants, transported gravels used for roads and
railway lines, phosphate-based fertilisers and atomic fall-
out. Consulting aerial photographs during the interpret-
ation should allow most of these types of features to be
recognised. Also important are variations in soil moisture,
surface water and snow, all of which may present more of a
problem since they are ephemeral.
'
high
'
and
'
low
'
4.7.2.1
Disequilibrium
The possibility, and indeed likelihood, of disequilibrium
(see
Sections 4.2.2
and
4.6.1
)
is a primary consideration in
the interpretation of radioelement concentrations obtained
from radiometric data. When disequilibrium occurs, the
inferred amount of eU, or less commonly eTh, will be
incorrect. The estimated concentration may be too high
or too low, depending on whether the location is one where
the mobile component of the relevant decay series has been
removed or has accumulated. It is possible for there to be
only low
-emissions from a uranium deposit, making it
dif
cult to detect with a
γ
-ray detector. Also, through the
solubility of Ra and mobility of Rn, it is possible to produce
214
Bi in locations remote from its parent
238
U source to
produce a uranium radiometric response in a non-
uranium bearing location. Note that the extent of the
resulting migration must be larger than the footprint of
γ
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