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2 The sampling dilema - Where to measure using proximal soil sensors?

2 The sampling dilema - Where to measure using proximal soil sensors?

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Proximal Soil Sensing for Measurements in Space and Time



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2. Proximal Soil Sensing Techniques

Presently, proximal soil sensors can measure the soil’s ability to accumulate and conduct electrical charge, to absorb, reflect, and/or emit EM

energy, to release ions, and to resist mechanical distortion. Using energies

in the EM spectrum as the framework, below we describe currently available technologies for PSS.



2.1. γ-rays

In the EM spectrum, γ-rays occur at quantum energies between 1 MeV

and 124 keV and have frequencies of 1020À1024 Hz and short wavelengths

of less than 10212 m (Fig. 1). They contain a very large amount of energy

and are the most penetrating radiation from natural or man-made sources.

2.1.1. γ-ray spectrometers

A γ-ray spectrometer is an instrument that measures the distribution of the

intensity of γ radiation versus the energy of each photon. Most soil γ-ray

spectrometers use scintillators with either thallium-doped sodium iodide

or thallium-doped cesium iodide crystals, although other materials are also

available (International Atomic Energy Agency (IAEA), 2003). When

these are hit by the ionizing radiation, they fluoresce and a photomultiplier

tube is used to measure the light from the crystal. The photomultiplier

tube is attached to an electronic amplifier, which quantifies the signal.

Active γ-ray sensors use a radioactive source (e.g., 137Cs) to emit photons

of energy that can then be detected using a γ-ray spectrometer. The theory

of operation for measuring soil properties such as water, or bulk density

using active γ-rays, indicates that when these are emitted and pass through

the soil, photons are transmitted following the BeerÀLambert law (Wang

et al., 1975). The attenuation of the signal is determined by the thickness

of the material, its density, and its mass attenuation coefficient.

Passive γ-ray sensors (Fig. 3A) measure the energy of photons emitted

from naturally occurring radioactive isotopes of the element from which

they originate. While many naturally occurring elements have radioactive

isotopes, only potassium (40K) and the decay series of uranium (238U

and 235U and their daughters) and thorium (232Th and its daughters) have

long half-lives, are abundant in the environment, and produce γ-rays of

sufficient energy and intensity to be measured. The result is a γ-ray energy

spectrum (Fig. 3B). Gamma-ray sensors have been more commonly used

from remote sensing platforms (Minty, 1997); however, the techniques

are also used to measure soil properties proximally (Viscarra Rossel et al.,

2007; Wong et al., 2009). The advantage of proximal γ-ray sensing over



252



(A)



R.A. Viscarra Rossel et al.



(B) 70



Counts (s–1)



60

50

40

30

K

U

1.46 1.76



20



Th

2.61



10

0

0



0.5



1



1.5

2

Energy (MeV)



2.5



3



Figure 3 (A) A proximal passive γ radiometric sensor mounted on a multisensor platform and (B) a γ-ray spectrum showing the energies of the potassium (K),

uranium (U), and thorium (Th) bands.



remote sensing is that it is more directly related to soil materials and less

prone to the effects of surface cover and geometry. Soil mineralogy, particle

size, and the effects of attenuating materials such as water and density

control the γ-ray signal. Soil parent material, the intensity of weathering,

and the geometry of near-surface soil layers are therefore also important.

2.1.2. Neutron scattering methods

Neutron scattering may be categorized into elastic and inelastic techniques. The most common soil sensor that uses elastic neutron scattering

is the neutron probe for measuring soil water (Gardner and Kirkham,

1952). Neutrons emitted from a radioactive source into the soil are slowed by elastic collision (i.e., the emitted neutrons have the same energy

as those that are injected) with the nuclei of atoms of low atomic weight,

such as hydrogen. Hydrogen can slow fast neutrons more effectively than

any other element present in soil and the density of the resulting cloud of

slow neutrons is a function of the amount of water in the soil.

Schrader and Stinner (1961) proposed inelastic neutron scattering

(INS) as a technique for elemental analysis of surfaces. Wielopolski et al.

(2008) proposed it for the measurement of soil carbon and other elements. INS relies on the detection of γ-rays that are emitted following

the capture and reemission of fast neutrons as the sample is bombarded

with neutrons from a pulsed neutron generator. The emitted γ-rays are

characteristic of the excited nuclide and the γ-rays intensity is directly

related to the elemental content of the sample. The detectors used are the

same as those used in γ-ray spectroscopy (see above).

Zreda et al. (2008) described a passive technique where the neutron

source is derived from the naturally occurring neutrons generated through



Proximal Soil Sensing for Measurements in Space and Time



253



cosmic-ray interaction with the atmosphere. The sensor’s detector counts

neutrons that are back scattered out of the soil and these can be correlated

to soil water content. Since the method relies on a natural source, low

counts are obtained and significant integration is required. As such, it is

suited to stationary measurements (e.g., for monitoring changes in soil

water).



2.2. X-rays

X-rays have quantum energies between 124 keV and 124 eV and occur in

the EM spectrum at a frequency range of 1017À1020 Hz and wavelengths

of about 10212À1029 m (Fig. 1). They contain a large amount of energy

and have been classified as either “hard” (shorter wavelengths) or “soft”

(longer wavelengths).

2.2.1. X-ray fluorescence

XRF is used to measure elements in soil samples. The technique

relies on the fluorescence at specific energies of atoms that are excited

when irradiated with X-rays. Detection of the specific fluorescent

photons enables the qualitative and quantitative analysis of the elements

in a sample. XRF spectroscopy has been used in the laboratory for

many years. Kalnicky and Singhvi (2001) provide a comprehensive

overview of XRF for environmental analysis. Portable, handheld XRF

technology has gained acceptance as an analytical approach in the

environmental community, particularly for rapid measurement of metal

contaminants.

2.2.2. X-ray diffractometry

X-ray diffraction (XRD) is a nondestructive technique used to acquire

detailed information on the mineral composition of the soil. Moore and

Reynolds (1997) provide details of the laboratory technique. The use of

portable XRD systems and research prototypes are starting to appear in

the literature because many of the strict hardware requirements, such as

reproducible alignment of the X-ray detectors and long acquisition times,

are being overcome (Gianoncelli et al., 2008). Sarrazin et al. (2005)

describe the development and testing of a portable combined XRD/

XRF system that can be used in the field. The system was constructed

for remote planetary exploration, but has also been used as a field tool

for geological research. Gianoncelli et al. (2008) also describe the development of a portable XRD/XRF system for simultaneous elemental

analysis and phase identification of inorganic materials.



254



R.A. Viscarra Rossel et al.



2.3. Ultraviolet, visible, and infrared reflectance

spectroscopy

Diffuse reflectance spectroscopy has been used in soil science research

since the 1950s and 1960s (Bowers and Hanks, 1965; Brooks, 1952).

However, it is only in around the past 20 years, most likely coinciding

with the establishment of chemometrics and multivariate statistical techniques in analytical chemistry, that its usefulness and importance in soil

science has been realized. Interest in using reflectance spectroscopy to

measure soil properties is widespread because the techniques are rapid, relatively inexpensive, require minimal sample preparation, are nondestructive,

require no hazardous chemicals, and several soil properties can be measured

from a single scan (Viscarra Rossel et al., 2006a).

Ultraviolet radiation possesses quantum energies of 124À2.1 eV,

has a frequency range of about 1015À1017 Hz, and wavelengths of around

1029À1027 m (Fig. 1). There is little in the literature on the use of ultraviolet radiation for PSS, and often the technique is combined with visible

or infrared spectroscopy (Islam et al., 2003). Visible light has energies

between 2.1 and 1.65 eV, frequency in the range 4 3 1014À7 3 1014 Hz,

and wavelengths of 7 3 1027À4 3 1027 m (Fig. 1). Absorptions of ultraviolet and visible radiation occur under high energies due to the excitation of outer electrons. Absorption of energy by an atom or molecule

involves the promotion of electrons from their ground state to an excited

state. Absorptions in organic molecules are restricted to certain functional

groups (chromophores) that contain valance electrons of low excitation

energy. Many inorganic species, such as iron oxides in soil, show charge

transfer absorptions (also called charge transfer complexes) (Schwertmann

and Taylor, 1989). For a complex to demonstrate charge transfer behavior,

one of its components must be able to donate electrons and another must

be able to accept them. Thus, absorptions involve transfer of an electron

from the donor to an orbital associated with the acceptor. A soil spectrum

in the visible range is shown in Fig. 4A.

The near-infrared (NIR) portion of the EM spectrum has a frequency range

of 1.2 3 1014À4 3 1014 Hz, with wavelengths of 7 3 1027À2.5 3 1026 m,

while the mid-IR has a frequency range of 3 3 1012À1.2 3 1014 Hz

and wavelengths of 2.5 3 1026À2.5 3 1025 m (Fig. 1). Energies in the

NIR range between 1.65 eV and 124 meV, while those in the mid-IR

range from 124 to 12.4 meV. The farÀIR has an energy range of

12.4À1.24 meV, a frequency range of 3 3 1012À3 3 1011 Hz, and wavelengths between 2.5 3 1025 and 5 3 1025 m (Fig. 1); however, there are

no publications on the use of farÀIR for PSS. Conversely, there is a vast

amount of literature on the use of visÀNIR and mid-IR for soil analysis

(Stenberg et al., 2010) and, increasingly, on the use of these techniques

for PSS (Ben-Dor et al., 2008; Christy, 2008; Reeves et al., 2010; Viscarra



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Proximal Soil Sensing for Measurements in Space and Time



(A)



(B)



(C)



2.5

Visible



Near infrared



Mid infrared



2



Log 1/R



Iron oxides



1.5



Water



Iron oxides



Clay minerals



Carbon



Quartz



Colour



1



Clay minerals

Carbon

Carbonate



0.5



Iron oxides



Organic matter

Carbonates

Water



0

400



500



600



700



1200



1700



2200



2500



7500



12,500



17,500



Wavelength (nm)



Figure 4 Typical soil spectrum in the (A) visible (vis), (B) near-infrared (NIR),

and (C) mid-infrared (mid-IR) portions of the EM spectrum.



Rossel et al., 2009; Waiser et al., 2007). Infrared radiation does not have

enough energy to induce electronic transitions as with ultraviolet and

visible light. Their absorptions are restricted to compounds with smaller

energy differences in the possible vibrational states. For a molecule to absorb

infrared energy, the vibrations within a molecule must cause a net charge

in the dipole moment of the molecule. The alternating electrical field of

the radiation interacts with fluctuations in the dipole moment of the molecule. If the frequency of the radiation matches the vibrational frequency

of the molecule, then radiation will be absorbed, causing a change in the

amplitude of the molecular vibration. The positions of the molecules are not

fixed and are subject to different stretching or bending vibrations.

The mid-IR contains more information on soil mineral and organic

composition than the visÀNIR, and its multivariate calibrations are generally

more robust (Viscarra Rossel et al., 2006a). The reason is that the fundamental molecular vibrations of soil components occur in the mid-IR, while only

their overtones and combinations are detected in the NIR (Stenberg et al.,

2010). Hence, soil NIR spectra display fewer and much broader absorption

features compared to mid-IR spectra (Fig. 4B and C, respectively).

The adaptation of visÀNIR spectrometers for PSS has been ongoing

for the past two decades, with the first field prototype mobile systems

developed by Shonk et al. (1991) and Sudduth and Hummel (1993).

Since then, other prototype mobile systems have been developed by

Shibusawa et al. (2001), Mouazen et al. (2005), Stenberg et al. (2007), and

Christy (2008), who described a commercially available mobile visÀNIR

system. Alternatively, stationary PSS of visÀNIR reflectance has been

implemented using portable instruments (Ben-Dor et al., 2008; Kusumo

et al., 2011; Viscarra Rossel et al., 2009; Waiser et al., 2007). There are



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