2.2.3 Parent Rock Signals in the Northern Thai Highlands
Gamma-ray spectrometry has predominantly been used for rock and mineral
prospection. Also for its application with soils, we first need information on the
parent rock (background) signal, in order to understand the influence of soil
formation on the measured signal. Table 2.1 presents measured values from the
study area in comparison to data from Australia, as published by Dickson and Scott
(1997). We note that our study data were measured in the natural environment,
whereas the Australian data were derived from laboratory measurements.
In northern Thailand, the highest gamma-ray readings occurred with granites
and were associated partly with metamorphized rocks (such as gneiss and
migmatite). This concurred with the findings of Dickson and Scott (1997: 188),
who concluded that “. . .there is a trend for increasing radioelement content with
increasing Si content, i.e., felsic rocks have a higher radioelement content than
ultrabasic and mafic rocks. . .”, as underlined by the latite readings from Thailand.
The concentration in clastic sediments seemed to be related to the dominant grainsize class; sandstones and related rocks showed, on average, lower concentrations
than siltstones and claystones. Extremely low K values were recorded for limestone, which is by definition (>75 mass % carbonate) poor in K bearing silicates.
Measurements from Thailand did not always match with the ranges found in
Australia, a fact which may be attributed to regional differences in terms of
geochemical composition. Here, it must be stated that globally reported data are
still relatively scarce (due to political, economic and security reasons), and that the
grouping of data according to rock types is therefore arbitrary.
In conclusion, more standardised data on gamma-ray rock signatures are
required. The differences in rock types lead to the assumption that also soils that
strongly depend on rock type (i.e., azonal soils like Leptosols, Regosols) or
secondary enrichments (Calcisols, Gypsisols) can potentially be distinguished via
gamma-ray spectrometry.
2.2.4 Influence of Soil Forming Processes on the Gamma-Ray
Signal: Case Study in Bor Krai Village, Thailand
The Bor Krai case study represents the most intensive village level study of soils in
the northern Thai highlands. The reason is the existing petrographic diversity in
terms of limestone, marls, claystone and latite (a basic magmatite), plus the
presence of hydrothermal formations rich in aluminium-hydroxides. Table 2.1
shows the gamma-ray signals for soils in contrast to those of the supposed parent
materials. The dominant parent material according to the geological map is Permian
limestone. Limestone is rich in carbonates, but poor in silicates and other minerals,
the latter of which can lead to a strong gamma-ray signal. As a consequence, the
limestone signals here were weak, especially for K and Th. However, eU
2 Beyond the Horizons: Challenges and Prospects for Soil Science and Soil. . .
39
Gamma-ray spectrometry has predominantly been used for rock and mineral
prospection. Also for its application with soils, we first need information on the
parent rock (background) signal, in order to understand the influence of soil
formation on the measured signal. Table 2.1 presents measured values from the
study area in comparison to data from Australia, as published by Dickson and Scott
(1997). We note that our study data were measured in the natural environment,
whereas the Australian data were derived from laboratory measurements.
In northern Thailand, the highest gamma-ray readings occurred with granites
and were associated partly with metamorphized rocks (such as gneiss and
migmatite). This concurred with the findings of Dickson and Scott (1997: 188),
who concluded that “. . .there is a trend for increasing radioelement content with
increasing Si content, i.e., felsic rocks have a higher radioelement content than
ultrabasic and mafic rocks. . .”, as underlined by the latite readings from Thailand.
The concentration in clastic sediments seemed to be related to the dominant grainsize class; sandstones and related rocks showed, on average, lower concentrations
than siltstones and claystones. Extremely low K values were recorded for limestone, which is by definition (>75 mass % carbonate) poor in K bearing silicates.
Measurements from Thailand did not always match with the ranges found in
Australia, a fact which may be attributed to regional differences in terms of
geochemical composition. Here, it must be stated that globally reported data are
still relatively scarce (due to political, economic and security reasons), and that the
grouping of data according to rock types is therefore arbitrary.
In conclusion, more standardised data on gamma-ray rock signatures are
required. The differences in rock types lead to the assumption that also soils that
strongly depend on rock type (i.e., azonal soils like Leptosols, Regosols) or
secondary enrichments (Calcisols, Gypsisols) can potentially be distinguished via
gamma-ray spectrometry.
2.2.4 Influence of Soil Forming Processes on the Gamma-Ray
Signal: Case Study in Bor Krai Village, Thailand
The Bor Krai case study represents the most intensive village level study of soils in
the northern Thai highlands. The reason is the existing petrographic diversity in
terms of limestone, marls, claystone and latite (a basic magmatite), plus the
presence of hydrothermal formations rich in aluminium-hydroxides. Table 2.1
shows the gamma-ray signals for soils in contrast to those of the supposed parent
materials. The dominant parent material according to the geological map is Permian
limestone. Limestone is rich in carbonates, but poor in silicates and other minerals,
the latter of which can lead to a strong gamma-ray signal. As a consequence, the
limestone signals here were weak, especially for K and Th. However, eU
2 Beyond the Horizons: Challenges and Prospects for Soil Science and Soil. . .
39