11
Advanced Ground-Penetrating Radar for Soil Moisture Retrieval
soil moisture variability, which is generally resulting in a poor agreement with smallsupport scale calibrating measurements (e.g., Ceballos et al. 2005). The difference in
support scales between large-scale remote sensing methods and small-scale invasive
sensors may indeed reach several orders of magnitude, therefore making these two
methods hardly comparable.
Proximal soil moisture sensing methods that are groundbased but noninvasive
may bridge the scale gap that remains in soil moisture sensing techniques, making
possible the characterization of soil moisture at an intermediate scale between remote
sensing and invasive sensors. Proximal soil moisture sensing includes ground penetrating radar (GPR), electromagnetic induction sensors (e.g., Martinez et al. 2010),
and ground-based radiometers (e.g., Jonard et al. 2011). Among these options, the
GPR method for the determination of soil moisture was the most used and applied.
2.2 SOIL MOISTURE SENSING BY GROUNDPENETRATING RADAR
GPR is based on the propagation of a radar electromagnetic wave (typically in the
range of 10–2000 MHz) into the ground. Wave propagation is governed by soil electromagnetic properties, that is, the dielectric permittivity ε, the electrical conductivity σ, and the magnetic permeability μ. For nonmagnetic soils as prevalent in the
environment, μ is equal to the free-space magnetic permeability μ 0 and does not
impact on the electromagnetic wave propagation. As the dielectric permittivity of
water (ε w ≈ 80) is much larger than the one of the soil particles (ε s ≈ 5) and air (ε a =
1), GPR wave propagation velocity in the soil is principally determined by its water
content. GPR can image the soil with a high spatial resolution and up to a depth of
several meters, depending on the frequency range of the electromagnetic waves. A
review about recent development of GPR can be found in the work of Slob et al.
(2010). In the areas of vadose zone hydrology and water resources management,
GPR has been used to identify soil stratigraphy (Davis and Annan 1989; Grandjean
et al. 2006), to locate water tables (Doolittle et al. 2006), to trace wetting front movement (Saintenoy et al. 2008), to identify soil hydraulic parameters (Binley et al.
2002; Cassiani and Binley 2005; Kowalsky et al. 2005; Jadoon et al. 2008; Lambot
et al. 2009), to assess soil salinity (al Hagrey and Müller 2000), and to monitor contaminants (Cassidy 2007).
For soil moisture sensing, an excellent review of GPR applications was given by
Huisman et al. (2003), where several methodologies of soil moisture determination
using GPR wave propagation velocity or surface reflection were distinguished:
1. Determination of the wave propagation time to a known reflecting interface
using a single-offset surface GPR (Grote et al. 2003; Lunt et al. 2005; van
Overmeeren et al. 1997; Weiler et al. 1998)
2. Detection of the velocity-dependent reflecting hyperbola of a buried object
using a single-offset surface GPR along a transect (Windsor et al. 2005)
3. Determination of the wave propagation velocity using multioffset surface GPR measurements above a reflecting layer (i.e., common midpoint
method, Jacob and Hermance 2004)
Advanced Ground-Penetrating Radar for Soil Moisture Retrieval
soil moisture variability, which is generally resulting in a poor agreement with smallsupport scale calibrating measurements (e.g., Ceballos et al. 2005). The difference in
support scales between large-scale remote sensing methods and small-scale invasive
sensors may indeed reach several orders of magnitude, therefore making these two
methods hardly comparable.
Proximal soil moisture sensing methods that are groundbased but noninvasive
may bridge the scale gap that remains in soil moisture sensing techniques, making
possible the characterization of soil moisture at an intermediate scale between remote
sensing and invasive sensors. Proximal soil moisture sensing includes ground penetrating radar (GPR), electromagnetic induction sensors (e.g., Martinez et al. 2010),
and ground-based radiometers (e.g., Jonard et al. 2011). Among these options, the
GPR method for the determination of soil moisture was the most used and applied.
2.2 SOIL MOISTURE SENSING BY GROUNDPENETRATING RADAR
GPR is based on the propagation of a radar electromagnetic wave (typically in the
range of 10–2000 MHz) into the ground. Wave propagation is governed by soil electromagnetic properties, that is, the dielectric permittivity ε, the electrical conductivity σ, and the magnetic permeability μ. For nonmagnetic soils as prevalent in the
environment, μ is equal to the free-space magnetic permeability μ 0 and does not
impact on the electromagnetic wave propagation. As the dielectric permittivity of
water (ε w ≈ 80) is much larger than the one of the soil particles (ε s ≈ 5) and air (ε a =
1), GPR wave propagation velocity in the soil is principally determined by its water
content. GPR can image the soil with a high spatial resolution and up to a depth of
several meters, depending on the frequency range of the electromagnetic waves. A
review about recent development of GPR can be found in the work of Slob et al.
(2010). In the areas of vadose zone hydrology and water resources management,
GPR has been used to identify soil stratigraphy (Davis and Annan 1989; Grandjean
et al. 2006), to locate water tables (Doolittle et al. 2006), to trace wetting front movement (Saintenoy et al. 2008), to identify soil hydraulic parameters (Binley et al.
2002; Cassiani and Binley 2005; Kowalsky et al. 2005; Jadoon et al. 2008; Lambot
et al. 2009), to assess soil salinity (al Hagrey and Müller 2000), and to monitor contaminants (Cassidy 2007).
For soil moisture sensing, an excellent review of GPR applications was given by
Huisman et al. (2003), where several methodologies of soil moisture determination
using GPR wave propagation velocity or surface reflection were distinguished:
1. Determination of the wave propagation time to a known reflecting interface
using a single-offset surface GPR (Grote et al. 2003; Lunt et al. 2005; van
Overmeeren et al. 1997; Weiler et al. 1998)
2. Detection of the velocity-dependent reflecting hyperbola of a buried object
using a single-offset surface GPR along a transect (Windsor et al. 2005)
3. Determination of the wave propagation velocity using multioffset surface GPR measurements above a reflecting layer (i.e., common midpoint
method, Jacob and Hermance 2004)