12
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
4. Determination of the surface ground-wave velocity using multioffset and
single-offset surface GPR (Huisman et al. 2002; Galagedara et al. 2003,
2005a,b; Grote et al. 2003, 2010)
5. Determination of the two-dimensional (2-D) spatial distribution of water
by transmission tomography using borehole GPR (Binley et al. 2001;
Alumbaugh et al. 2002; Looms et al. 2008)
6. Determination of the surface reflection coefficient using off-ground, airlaunched GPR (Chanzy et al. 1996; Serbin and Or 2003, 2005)
Although these techniques are well established, they still suffer from major limitations originating from the strongly simplifying assumptions on which they rely
with respect to electromagnetic wave propagation phenomena. As a result, a bias
is introduced in the estimates due to the adequacy of a limited GPR model, and
moreover, only a part of the information contained in the radar data is used, generally the propagation time. In addition, these techniques are not appropriate in a realtime mapping context, as usually, several cumbersome measurements are needed at
a given location. According to Huisman et al. (2003), the main limitation of GPR
methods may be the use of uncertain petrophysical relationships relating soil dielectric permittivity, which is directly retrieved from the GPR data, to soil moisture.
Recently, some authors have proposed innovative soil moisture retrieval techniques using the same GPR sensors. In that respect, Benedetto (2010) used a
Rayleigh scattering-based method for directly determining the soil moisture, without the need of calibrating the GPR system or the petrophysical relationship. Oden
et al. (2008) determined soil surface electromagnetic properties from early-time
GPR wavelet analysis. Lastly, van der Kruk (2006) and van der Kruk et al. (2007)
developed an inversion method of dispersed waveforms trapped in a surface waveguide (i.e., when the soil is layered by freezing, thawing, or a wetting front) for
retrieving its dielectric permittivity and thickness. Inversion of GPR data coupled
with an accurate electromagnetic model for wave propagation in GPR systems,
including GPR antenna modeling, may therefore increase the retrieval capabilities
from GPR data.
2.3 FULL-WAVEFORM INVERSION OF GPR DATA
A full-waveform electromagnetic model for the particular case of zero-offset, offground GPR was developed by Lambot et al. (2004a), where a single GPR antenna
plays simultaneously the role of an emitter an and a receiver and is situated at some
distance above the soil. The model includes propagation effects within the antenna
and antenna–soil interactions, while this is usually not accounted for using common GPR methods, and an exact solution of three-dimensional (3-D) Maxwell’s
equations for wave propagation in multilayered media is considered, instead of the
commonly used one-dimensional approach. Ultrawideband frequency-dependent
GPR waveforms propagated to the soil are generated using a vector network analyzer (VNA). The main advantage of the VNA technology over traditional GPR systems is that the measured quantities constitute international standards and are well
defined physically with proper calibration of the system. Soil electrical properties
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
4. Determination of the surface ground-wave velocity using multioffset and
single-offset surface GPR (Huisman et al. 2002; Galagedara et al. 2003,
2005a,b; Grote et al. 2003, 2010)
5. Determination of the two-dimensional (2-D) spatial distribution of water
by transmission tomography using borehole GPR (Binley et al. 2001;
Alumbaugh et al. 2002; Looms et al. 2008)
6. Determination of the surface reflection coefficient using off-ground, airlaunched GPR (Chanzy et al. 1996; Serbin and Or 2003, 2005)
Although these techniques are well established, they still suffer from major limitations originating from the strongly simplifying assumptions on which they rely
with respect to electromagnetic wave propagation phenomena. As a result, a bias
is introduced in the estimates due to the adequacy of a limited GPR model, and
moreover, only a part of the information contained in the radar data is used, generally the propagation time. In addition, these techniques are not appropriate in a realtime mapping context, as usually, several cumbersome measurements are needed at
a given location. According to Huisman et al. (2003), the main limitation of GPR
methods may be the use of uncertain petrophysical relationships relating soil dielectric permittivity, which is directly retrieved from the GPR data, to soil moisture.
Recently, some authors have proposed innovative soil moisture retrieval techniques using the same GPR sensors. In that respect, Benedetto (2010) used a
Rayleigh scattering-based method for directly determining the soil moisture, without the need of calibrating the GPR system or the petrophysical relationship. Oden
et al. (2008) determined soil surface electromagnetic properties from early-time
GPR wavelet analysis. Lastly, van der Kruk (2006) and van der Kruk et al. (2007)
developed an inversion method of dispersed waveforms trapped in a surface waveguide (i.e., when the soil is layered by freezing, thawing, or a wetting front) for
retrieving its dielectric permittivity and thickness. Inversion of GPR data coupled
with an accurate electromagnetic model for wave propagation in GPR systems,
including GPR antenna modeling, may therefore increase the retrieval capabilities
from GPR data.
2.3 FULL-WAVEFORM INVERSION OF GPR DATA
A full-waveform electromagnetic model for the particular case of zero-offset, offground GPR was developed by Lambot et al. (2004a), where a single GPR antenna
plays simultaneously the role of an emitter an and a receiver and is situated at some
distance above the soil. The model includes propagation effects within the antenna
and antenna–soil interactions, while this is usually not accounted for using common GPR methods, and an exact solution of three-dimensional (3-D) Maxwell’s
equations for wave propagation in multilayered media is considered, instead of the
commonly used one-dimensional approach. Ultrawideband frequency-dependent
GPR waveforms propagated to the soil are generated using a vector network analyzer (VNA). The main advantage of the VNA technology over traditional GPR systems is that the measured quantities constitute international standards and are well
defined physically with proper calibration of the system. Soil electrical properties