integrated via a multiplication with the potential cation exchange capacity of the
clay fraction, then the different depth trends and the differences in magnitude of the
K measurement between high and low activity RSGs could be qualitatively
reconstructed (Fig. 2.2b). For a better quantitative assessment, the bulk mineral
composition, including the share of K bearing feldspars, would be needed.
In conclusion, clay illuviation and chemical weathering influence the gammaray signals produced, especially by K. Portable gamma-ray spectrometers can be
used in order to distinguish low and high activity clay profiles, if analysed reference
profiles are available.
This means at the same time that gamma-ray spectrometry can semiquantitatively measure the cation exchange capacity (CEC), which is used as a
diagnostic criterion for the differentiation of clay illuviation type RSGs in the
World Reference Base for Soil Resources (WRB). By adding a field pH-meter –
for approximating the base saturation – as a second important diagnostic criterion,
all illuviation type RSGs (Luvisol, Alisol, Lixisol, Acrisol) can be classified in
the field.
2.2.6 Gamma-Ray Signals at the Landscape Scale
The multiple possibilities that gamma-ray spectrometers offer, from its handheld to
airborne versions, make this technology suitable for mapping at several different
scales. Along dirt roads, cars can be used, while rugged terrain can be accessed
using helicopters and larger areas mapped from aircraft. However, the question
arises: what can be mapped? The literature looks at soil properties, soil related
processes and soil mapping. For example, Anderson-Mayes (1997) and Dent (2007)
reported on salinity mapping, Pracilio et al. (2006) approached texture, and Beckett
(2007) worked on soil porosity and density. Dickson et al. (1996) studied erosion,
Gunn et al. (1997) investigated land use and degradation using this method, and
finally Cook et al. (1996), Bierwirth and Brodie (2005) and Wilford and Minty
(2007) worked on soil type mapping. All these studies detected a more or less good
correlation between the subject studied and radiometric measurements. However,
most also treated this phenomenon statistically for prediction purposes, without
making an effort to understand why there was a correlation, i.e., to understand the
radiometric response to material properties in a mechanistic sense, plus they used
exclusively surface measurements. However, gamma-ray data can only be fully
utilized if the factors influencing the radiometric response are understood. Using an
inverse argument, we can state that all the properties which can be predicted with
the help of radiometric data also have an influence on the gamma-ray signal. Their
quantitative impact and cross-over effects need to be determined in more detail in
the future. In the following sections, we want to concentrate on the experiences
gained with soil type mapping at the landscape and regional scales in the upland
areas of northern Thailand.
44
K. Stahr et al.
clay fraction, then the different depth trends and the differences in magnitude of the
K measurement between high and low activity RSGs could be qualitatively
reconstructed (Fig. 2.2b). For a better quantitative assessment, the bulk mineral
composition, including the share of K bearing feldspars, would be needed.
In conclusion, clay illuviation and chemical weathering influence the gammaray signals produced, especially by K. Portable gamma-ray spectrometers can be
used in order to distinguish low and high activity clay profiles, if analysed reference
profiles are available.
This means at the same time that gamma-ray spectrometry can semiquantitatively measure the cation exchange capacity (CEC), which is used as a
diagnostic criterion for the differentiation of clay illuviation type RSGs in the
World Reference Base for Soil Resources (WRB). By adding a field pH-meter –
for approximating the base saturation – as a second important diagnostic criterion,
all illuviation type RSGs (Luvisol, Alisol, Lixisol, Acrisol) can be classified in
the field.
2.2.6 Gamma-Ray Signals at the Landscape Scale
The multiple possibilities that gamma-ray spectrometers offer, from its handheld to
airborne versions, make this technology suitable for mapping at several different
scales. Along dirt roads, cars can be used, while rugged terrain can be accessed
using helicopters and larger areas mapped from aircraft. However, the question
arises: what can be mapped? The literature looks at soil properties, soil related
processes and soil mapping. For example, Anderson-Mayes (1997) and Dent (2007)
reported on salinity mapping, Pracilio et al. (2006) approached texture, and Beckett
(2007) worked on soil porosity and density. Dickson et al. (1996) studied erosion,
Gunn et al. (1997) investigated land use and degradation using this method, and
finally Cook et al. (1996), Bierwirth and Brodie (2005) and Wilford and Minty
(2007) worked on soil type mapping. All these studies detected a more or less good
correlation between the subject studied and radiometric measurements. However,
most also treated this phenomenon statistically for prediction purposes, without
making an effort to understand why there was a correlation, i.e., to understand the
radiometric response to material properties in a mechanistic sense, plus they used
exclusively surface measurements. However, gamma-ray data can only be fully
utilized if the factors influencing the radiometric response are understood. Using an
inverse argument, we can state that all the properties which can be predicted with
the help of radiometric data also have an influence on the gamma-ray signal. Their
quantitative impact and cross-over effects need to be determined in more detail in
the future. In the following sections, we want to concentrate on the experiences
gained with soil type mapping at the landscape and regional scales in the upland
areas of northern Thailand.
44
K. Stahr et al.