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3 Ultraviolet, visible, and infrared reflectance spectroscopy

3 Ultraviolet, visible, and infrared reflectance spectroscopy

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Proximal Soil Sensing for Measurements in Space and Time






Near infrared

Mid infrared


Log 1/R

Iron oxides



Iron oxides

Clay minerals





Clay minerals




Iron oxides

Organic matter















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


R.A. Viscarra Rossel et al.

fewer reports of portable, mid-IR systems for PSS ( Jahn and Upadhyaya,

2010; Reeves et al., 2010).

2.4. Laser-induced breakdown spectroscopy

LIBS is made possible because of lasers. The technology uses an optically

focused short-pulsed laser to heat the surface of the soil sample to the

point of volatilization and ablation. This results in the generation of a hightemperature plasma on the surface of the sample. It is important to note

that the plasma forms over a limited area so that only a very small amount

of sample is measured during each event. As it cools, the excited atomic,

ionic, and molecular fragments produced in the plasma emit radiation

characteristic of the elemental composition within the volatilized material.

A spectrometer capable of resolving spectra in the range 200À900 nm is

used to detect the emitted radiation. A sample spectrum is shown in Fig. 5.

LIBS has been used for elemental analysis in geochemical exploration

(Mosier-Boss et al., 2002), for the analysis of soil carbon (Cremers et al.,

2001) and other elements (Hilbk-Kortenbruck et al., 2001). Fiber optic

technology has made it possible to develop portable (Harmon et al., 2005)

and mobile LIBS systems (Bousquet et al., 2008).

2.5. Microwaves

The quantum energy of microwaves ranges between B12.4 and 12.4 μeV,

near frequencies of 3 3 1011À3 3 109 Hz, and wavelengths of 1 3 1023À

5 3 1025 m (Fig. 1). Microwave sensors are typically used for remote sensing

Figure 5 Typical soil laser-induced breakdown spectroscopy (LIBS) spectrum.

Proximal Soil Sensing for Measurements in Space and Time


( Jackson et al., 1984), but sensors have been constructed for measuring soil

water proximally (Whalley, 1991). They measure either changes in the

emissivity of the soil or changes in microwave attenuation caused by changes

in water content. This dependence of the soil’s emissivity on its water

content is due to the large contrast between the dielectric properties of free

water (k0 5 80), dry soil (k0 5 2À5 depending on its bulk density), and air

(k0 5 1). The large dielectric constant for water results from the alignment of

the electric dipole moment of the water molecule in response to an applied

field. As the water content of a soil increases, its dielectric constant and

attenuation increase, and changes in soil emissivity are produced. Therefore,

microwave sensors measure the thermal radiation emitted by the soil, which

is generated within the volume of the soil and is dependent on the water

content (i.e., dielectric properties) and temperature of the soil.

2.6. Radio waves

Radio waves occur in the EM spectrum at frequencies less than

3 3 109 Hz with wavelengths greater than 1 3 1023 μ and energies less

than 12.4 μeV (Fig. 1).

2.6.1. Time- and frequency-domain reflectometry and capacitance

Time-domain reflectometry (TDR), frequency-domain reflectometry

(FDR), and capacitance sensors use the dielectric properties of soils, that is,

their permittivity, to measure water content. TDR instruments consist of

a transmission line (TL) and a waveguide made up of two or three parallel

metal rods that are inserted into the soil. The instrument produces a series of

precisely timed electrical pulses with frequencies of 2 3 107À3 3 109 Hz,

which travel along the TLs and waveguide. These frequencies provide a

response that is less dependent on soil-specific properties like texture, salt

content, and temperature. Impedance along the waveguide varies with

the dielectric constant of the bulk soil. The soil bulk dielectric constant is

determined by measuring the time it takes for the electromagnetic pulse to

propagate along the TL and waveguide surrounded by the soil. Since

the propagation velocity is a function of the soil bulk dielectric constant, the

latter is proportional to the square of the transit time out and back along

the TL and waveguide. Because the dielectric constant of soil depends on

the amount of water present, soil volumetric water content can be inferred

from the reflected measurements. Noborio (2001) provides a good overview

of the use of TDR for the measurement of soil water content and electrical


FDR and capacitance probes consist of two or more capacitors (rods,

plates, or rings) that are inserted into the soil. Plates are usually annuli

arranged concentrically to facilitate borehole measurements (Dean et al.,

1987). These capacitors use the soil as a dielectric and hence depend on the


R.A. Viscarra Rossel et al.

soil water content. When the capacitor is connected to an oscillator to form

an electric circuit, changes in soil water can be detected by changes in the

circuit’s operating frequency. In FDR, the oscillator frequency is controlled

within a certain range to determine the resonant frequency at which the

amplitude is greatest, which is a measure of the soil water content.

In capacitance, a measure of the soil’s permittivity is determined by

measuring the charge time of the capacitor in the soil. There are three

basic parts to a capacitance sensor (Dally et al., 1993): the target plate, the

air space, and the sensor head. The target plate accumulates the voltage

that will eventually dissipate across the air space. The air space is the gap

between the target plate and the sensor head. This space can be filled

with soil to increase or decrease the voltage being accumulated on the

target plate. The sensor head measures the voltage accumulated by the

target plate and dissipated through the soil. Whalley et al. (1992) developed one such sensor, which was also found to be sensitive to fluctuations

in soil bulk density. Liu et al. (1996) and later Andrade-Sanchez et al.

(2007) and Adamchuk et al. (2009) also evaluated a dielectric-based moisture sensor under dynamic conditions by incorporating it into a nylon

block attached to an instrumented tine (Fig. 6). A series of studies demonstrated that salinity, texture, and temperature also affected measurements. Soil-specific calibrations are recommended because the operating

frequency of these devices is generally below 1 3 108 Hz. At these frequencies, the bulk permittivity of the soil may change and measurements

are more affected by texture, salinity, and bulk density.

Figure 6

A capacitance soil water content sensor prototype.

Proximal Soil Sensing for Measurements in Space and Time


2.6.2. Nuclear magnetic resonance

Nuclear magnetic resonance (NMR) is based on the interaction between

nuclear magnetic moments and applied static and radiofrequency magnetic

fields (Matzkanin and Paetzotd, 1982). Paetzold et al. (1985) developed a

tractor-mounted NMR instrument and used the technique to measure soil

water content. The instrument detected and measured the NMR signal

from the hydrogen in water. By adjusting the strength of the radiofrequency and static magnetic fields, the researchers were able to measure soil

water content at depths of 38, 51, and 63 mm. Their findings suggested

that the NMR signal is a linear function of volumetric water content

and is not affected by clay mineralogy, soil organic matter, or texture.

Furthermore, the technique can distinguish between water that is bound

by clay particles and not available to plants, and that which is available for

plant use.

Magnetic resonance sounding (MRS) uses the NMR principle that is

used in medical brain scanning (i.e., MRI or magnetic resonance imaging)

to measure subsurface free water and hydraulic properties (Lubczynski and

Roy, 2003). It is also known as surface NMR and can be used to measure

water content and porosity to depths up to 1500 m.

2.6.3. Ground-penetrating radar

GPR uses the transmission and reflection of high-frequency (106À109 Hz)

electromagnetic waves in the soil. They have transmitter and receiver

antennas that can be moved across the soil surface (Fig. 7). Much like with

TDR sensors, the primary control on the transmission and reflection of the

electromagnetic energy is the dielectric constant. Because of the large contrast between the dielectric constants of water, air, and minerals, GPR can

be used to measure variations in soil water content (Lambot et al., 2004).

Unlike TDR, however, GPR measurements are non invasive and the

sensors can measure soil water content of relatively large volumes of soil.

The resolution of GPR images can be varied through the use of different

antennae frequencies. Typically, higher frequencies increase the resolution

at the expense of depth of penetration. Daniels et al. (1988) describe the

fundamental principles of GPR. Knight (2001) provides an overview of

GPR in environmental applications and Huisman et al. (2003) review its

use for soil water determinations. The penetration depth of GPR measurements is affected by the electrical conductivity of the soil. Good

penetration depth of up to about 15 m can be achieved in dry sandy soils

or massive dry materials such as granite and limestone. As the conductivity

increases, penetration depth decreases because the electromagnetic energy

is more quickly dissipated into heat, causing a loss in signal strength at

depth. In highly conductive soils, such as those with large amounts of clay,

water and/or salt, penetration depth can be only a few centimeters. Slowly


R.A. Viscarra Rossel et al.

Figure 7

A ground-penetrating radar (GPR) system.

changing water contents are also difficult to detect with GPR, and water

profiling is generally not possible with most types of instruments. More

abrupt changes, such as wetting fronts, are easier to detect and this use of

GPR is more appropriately applied in irrigated regions.

2.6.4. Electromagnetic induction

EMI is a highly adaptable noninvasive technique that measures the apparent

bulk electrical conductivity of soil (ECa). The instruments commonly have

a transmitter and a receiver. Using a varying magnetic field of relatively low

frequency (kHz), the technique induces currents in the ground in a way

that ensures their amplitude is linearly related to the conductivity of the

soil. The magnitude of these currents is determined by measuring the magnetic field that they generate. McNeill (1980) provides a good account.

EMI has been used extensively in mapping soils since De Jong et al. (1979)

first reported it. It has been particularly useful for mapping saline soils

(Rhoades, 1993) and for precision agriculture (Corwin and Lesch, 2003).

EMI coupled with a global positioning system (GPS) provides a rapid

soil-mapping tool and, until now, may be the most commonly used proximal soil sensor (Fig. 8). Because most soil and rock minerals are very

good insulators, the electrical conductivity sensed by an EMI unit is electrolytic and it takes place through the poreÀwater system. The following

factors are therefore important: shape, size, and connectivity of the

pore system; water content; concentration of dissolved electrolytes in the

soil water; temperature and phase of the pore water; and amount and

composition of colloids (Rhoades et al., 1989).

Proximal Soil Sensing for Measurements in Space and Time

Figure 8


An on-the-go soil electromagnetic induction (EM-38) system.

While clay content, electrical conductivity of the soil solution, and

water content are often recognized as the controlling factors that must be

accounted for when calibrating EMI measurements (Williams and Baker,

1982; Williams and Hoey, 1987), it is not that simple. It is the pore

system and its contents rather than the clay content per se that should be

considered. Soils with significant clay content usually have a pore geometry dominated by finer-sized pores. In comparison to a sandy soil, greater

proportions of these pores are filled and connected at comparable water

contents, and this gives rise to their larger electrical conductivity. The

bulk density of the soil should also be considered because it determines

total porosity. Clay soils in most cropping areas usually have a substantial

CEC, and cations in solution are in equilibrium with the charged clay

surface—these cations also contribute to the electrolyte concentration.

Finally, colloids—particularly those associated with organic matter—may

also contribute to the measured conductivity.

Effective measurement depth is a function of coil spacing and, under

some conditions, frequency. For commercial EMI sensors, the depth of

measurement can range from B0.37 m to more than 60 m. There are a

number of operational issues with EMI sensors, including temperature

effects (Robinson et al., 2004; Sudduth et al., 2001) and spurious signals

due to nearby metal objects (Lamb et al., 2005).

2.7. Magnetic, gravimetric, and seismic sensors

2.7.1. Magnetics

Magnetic sensors, or magnetometers, measure variations in the strength

of the earth’s magnetic field and the data reflect the spatial distribution of


R.A. Viscarra Rossel et al.

magnetization in the ground. Magnetization of naturally occurring

materials and rocks is determined by the quantity of magnetic minerals

and by the strength and direction of the permanent magnetization carried

by those minerals (Hansen et al., 2005). Typically, magnetics has been

used for the detection of geological bodies; however, there is increasing

use of the technique for near-surface applications. For example, for

mapping field drainage for hydrologic modeling (Rogers et al., 2005); to

better understand soil genesis and formation (Mathe and Leveque, 2003);

to detect anthropogenic pollution on topsoils; through their associations

with iron oxides (Schibler et al., 2002); and for rapid identification and

mapping of soil heavy metal contamination ( Jordanovaa et al., 2008).

2.7.2. Gravity

Gravity data can be collected using gravimeters (or gravitometers) and

provide information on the local gravitational field. There are two types

of gravimeter: relative and absolute. A relative gravimeter measures relative differences in the vertical component of the earth’s gravitational field

based on variations in the extension of an internal spring in the gravimeter.

The technique has typically been used to determine the subsurface configuration of structural basins, aquifer thickness, and geological composition.

An absolute gravimeter measures the acceleration of free fall of a control

mass. Absolute gravimetry can be used to measure mass water balances at

regional or local scales (Nabighian et al., 2005).

2.7.3. Seismology

Seismic reflection methods are sensitive to the speed of propagation of

various kinds of elastic waves. The elastic properties and mass density of the

medium in which the waves travel control the velocity of the waves and can

be used to infer properties of the earth’s subsurface. Reflection seismology

is used in exploration for hydrocarbons, coal, ores, minerals, and geothermal

energy. It is also used for basic research into the nature and origin of rocks

that make up the Earth’s crust (McCarthy and Thompson, 1988). It can

be used in near-surface applications for engineering, groundwater and environmental surveying (Harry et al., 2005). A method similar to reflection

seismology, which uses electromagnetic instead of elastic waves, is GPR.

2.8. Contact electrodes

This section refers to techniques for measuring electrical properties of

soils, such as their resistivity and dielectric, using direct injection of current into the soil using electrodes.

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