that occurs underneath soils, where weather and biotic influence are marginal.
Using this term, regolith is characterised by the disintegration of parent rock and
by weathering processes that change the gamma-ray signal, otherwise the regolith
and parent rock could not be distinguished from each other. If the mentioned
processes change the signal found in regolith, the same should happen in soils
upon which in principle the same processes act. We learned earlier that the three
standard elements measured by gamma-ray spectrometry (K, Th and U) show
different behaviours in the environment. K is easily liberated by chemical
weathering and re-translocated in the landscape, whereas Th is less mobile and
tends to occur dispersed, and U tends to locally accumulate and is conditionally
redox-sensitive. Therefore, in theory a differentiation in the gamma-ray signal of
each should be found, as soil forming processes have a different effect on at least
two of the three measured elements. Also, dilution or accumulation of all three
elements tends to occur, changing the overall signal when compared to the parent
rock. In the following section, we review some important soil forming processes
and check their potential influence on the gamma-ray signal.
Organic matter accumulation adds additional material to the soil, and this
accumulation normally shows a depth gradient, but no grain-size effect. Organic
matter can absorb K, but not in the sense of a specific absorption; therefore, it can be
expected that organic matter accumulation will dilute the overall gamma-ray signal.
Whether this dilution is detectable depends on the degree of accumulation; for
example, low organic matter concentrations (i.e., <2 mass %), as found in many
terrestrial arable soils, show hardly any influence, whereas soils dominated by
organic matter (i.e., in bogs and H-horizons), or showing thick organic layers on
top (O-horizons), significantly attenuate the parent rock signal.
The effect of clay illuviation potentially depends on the type of clay minerals
present. The translocation of high activity clays, which normally contain a substantial concentration of K due to their origin from micas, most probably leads to a
decreasing K signal in the topsoil (A and E horizon) and an increasing K signal in
the enriched horizon (Bt). The effect of the formation of concretions depends on
whether a residual or absolute accumulation occurs. A residual accumulation is
characterised by lower K, but higher Th and U concentrations, due to the fact that K
is more easily leached out of the soil profile under acid conditions. In the case of
accumulation due to lateral sub-surface flows, the effect is assumed to depend on
the precipitated minerals. Carbonate, anhydrite or silica accretions potentially lead
to an attenuation of the overall gamma-ray signal, whereas Fe-oxide accretion
probably leads to increased U concentrations if the source region is characterised
by a higher share of U-containing minerals. In the case of potassium salt accumulation, K can be assumed to be selectively enriched. These short theoretical
reflections show that soil forming processes can change the global gamma-ray
signal, as well as the element specific signals found. Consequently, if the influence
of a process on element concentrations is sufficiently strong (greater than the
accuracy of the measurement) and understood, gamma-ray spectrometry can
support soil type and/or property mapping.
38
K. Stahr et al.
Using this term, regolith is characterised by the disintegration of parent rock and
by weathering processes that change the gamma-ray signal, otherwise the regolith
and parent rock could not be distinguished from each other. If the mentioned
processes change the signal found in regolith, the same should happen in soils
upon which in principle the same processes act. We learned earlier that the three
standard elements measured by gamma-ray spectrometry (K, Th and U) show
different behaviours in the environment. K is easily liberated by chemical
weathering and re-translocated in the landscape, whereas Th is less mobile and
tends to occur dispersed, and U tends to locally accumulate and is conditionally
redox-sensitive. Therefore, in theory a differentiation in the gamma-ray signal of
each should be found, as soil forming processes have a different effect on at least
two of the three measured elements. Also, dilution or accumulation of all three
elements tends to occur, changing the overall signal when compared to the parent
rock. In the following section, we review some important soil forming processes
and check their potential influence on the gamma-ray signal.
Organic matter accumulation adds additional material to the soil, and this
accumulation normally shows a depth gradient, but no grain-size effect. Organic
matter can absorb K, but not in the sense of a specific absorption; therefore, it can be
expected that organic matter accumulation will dilute the overall gamma-ray signal.
Whether this dilution is detectable depends on the degree of accumulation; for
example, low organic matter concentrations (i.e., <2 mass %), as found in many
terrestrial arable soils, show hardly any influence, whereas soils dominated by
organic matter (i.e., in bogs and H-horizons), or showing thick organic layers on
top (O-horizons), significantly attenuate the parent rock signal.
The effect of clay illuviation potentially depends on the type of clay minerals
present. The translocation of high activity clays, which normally contain a substantial concentration of K due to their origin from micas, most probably leads to a
decreasing K signal in the topsoil (A and E horizon) and an increasing K signal in
the enriched horizon (Bt). The effect of the formation of concretions depends on
whether a residual or absolute accumulation occurs. A residual accumulation is
characterised by lower K, but higher Th and U concentrations, due to the fact that K
is more easily leached out of the soil profile under acid conditions. In the case of
accumulation due to lateral sub-surface flows, the effect is assumed to depend on
the precipitated minerals. Carbonate, anhydrite or silica accretions potentially lead
to an attenuation of the overall gamma-ray signal, whereas Fe-oxide accretion
probably leads to increased U concentrations if the source region is characterised
by a higher share of U-containing minerals. In the case of potassium salt accumulation, K can be assumed to be selectively enriched. These short theoretical
reflections show that soil forming processes can change the global gamma-ray
signal, as well as the element specific signals found. Consequently, if the influence
of a process on element concentrations is sufficiently strong (greater than the
accuracy of the measurement) and understood, gamma-ray spectrometry can
support soil type and/or property mapping.
38
K. Stahr et al.