Geoscience Reference
In-Depth Information
best done in association with a digital terrain model, so
topography may be used instead of total count.
lithological units and are useful for characterising different
lithotypes. They are also useful for highlighting zones of
preferential radioelement enrichment and alteration.
Caution must be exercised when working with channel
ratios, as a high ratio value is obtained when the level of the
radioelement in the denominator (lower term) is anomal-
ously low. This can be a spurious indicator of concentra-
tions of the radioelement occupying the numerator (upper
term); amplifying (squaring) the numerator helps to min-
imise this effect.
The various ratio groups can be displayed as ternary
images, e.g. (K/eTh, eTh/eU, K/eU) etc. Moreover, a par-
ticular radioelement can be combined with its respective
ratios to form a composite ternary image of that radioele-
ment, for example (K, K/eTh, K/eU), (eTh, eTh/K, eTh/eU)
and (eU, eU/eTh, eU/K), and ampli
4.5.3
Channel ratios
Concentrations of K, eU and eTh tend to be correlated in
most rock types (see
Section 4.6
). Anomalous areas occur
where this
situation breaks down owing to enrich-
ment and/or depletion of one or more of the three radio-
elements, which can often be identi
ed in the elemental
channel data. The individual channels show the absolute
concentrations of the elements, whereas their ratios, i.e.
eTh/K, eU/K, eU/eTh and their reciprocals, show their
relative concentrations. Using amplified channel values
increases the influence that variations in those channels
have on the ratio and reduces the effects of variations in
the denominator term. They are particularly effective for
channels having low count rates with respect to the other
channels, typically eU and eTh. The ratios eU
2
/eTh and
eTh
2
/K often provide higher resolution than their corres-
ponding elemental ratios.
Another strategy is to combine various elements into a
single term. The ratios K/(eU + eTh), eU/(K + eTh) and
eTh/(K + eU) show the variation of one element with
respect to the other two; and (eTh
'
normal
'
ed values can be
substituted where appropriate. These images emphasise
the abundance of the principal element relative to its ratios
with the other two elements. Areas where the principal
element has high concentration will appear white when the
other two elements both have lower concentrations, and
will appear red where the concentrations of both increase.
4.5.4
Multivariant methods
eU)/K can be used
to target areas where both U and Th vary with respect to K.
Ampli
ed values can be substituted where appropriate.
Note that the elements forming the various types of
ratios should be speci
ed in the same measurement units,
i.e. as counts per second or as concentrations in ppm
(multiply K% concentration by 10,000 to obtain ppm).
Channel ratios can be highly variable and large amplitudes
can be attenuated by displaying the logarithm or the
inverse tangent of the ratio.
Channel amplification and channel ratios are very
important enhancements for radiometric data because they
reduce the effects of the amount of exposure and soil
water-content etc. For example, the absolute level of radi-
ation from a particular rock outcrop will be reduced in
areas where it is weathered deeper or subject to surface
moisture, but the elemental ratios can be expected to
remain (relatively) unaffected, indicating the same litho-
type. They are also less sensitive to errors in the height
correction and to non-planar source geometry etc. Ratios
of elemental concentrations provide higher resolution of
variations in elemental concentrations than the elemental
channels
Multivariant techniques can be applied to assist integration
and interpretation of radiometric and complementary
datasets. These include factor, cluster and principal-
component analysis (PCA) (Davis,
1986
). Their common
goal is to reduce the complexity of interpretation by iden-
tifying fewer, or particularly significant, parameters that
most effectively represent variations in the various data-
sets. A discussion of these quite complex statistical
methods is beyond our scope. Some examples are
4.6
Radioelements in the geological
environment
Radiometric responses ultimately depend on the presence
of mineral species that contain one or more of the radio-
elements K, U and Th. These may be primary constituents
of the mineral or may occur in trace quantities as impur-
ities in the crystal lattice.
Potassium, which is by far the most abundant of the
three radioactive elements in the geological environment,
themselves,
so they correlate better with
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