Geoscience Reference
In-Depth Information
approximately from the velocity. Water content is then usually derived from the dielectric constant
using the popular empirical Topp equation (Topp et al., 1980) or mixture formulae (i.e., Roth et al.,
1990) which describe the relationships between dielectric and hydraulic parameters. Using this pro-
cedure, Hubbard et al. (1997), Parkin et al. (2000), Greaves et al. (1996), and Sénéchal et al. (2000a)
investigated the use of radar data for estimating subsurface water content.
Another method of estimating water content distributions from GPR data is to look for a vari-
able that statistically or geostatistically describes this distribution. In this case, the analysis of
radar data is used for characterizing the heterogeneity of the subsurface. To calculate any kind
of distribution from radar data, the recorded radar traces are analyzed using different attributes.
This method is analogous to seismic trace analyses. Chen and Sidney (1997) gave an overview of
seismic attributes that are specific measurements of geometric, kinematic, dynamic, or statistical
features derived from the recorded data. Using this procedure for electromagnetic applications,
Sénéchal et al. (2000b) gave a complex interpretation of a three-dimensional GPR data set using
attributes calculated from amplitude analysis of reflected radar waves. They got an understand-
ing of the lateral continuities and discontinuities of the reflectors, the geometry of structures, and
their dynamic characteristics. Knight et al. (1997) used the amplitude values recorded in the radar
traces for a geostatistical analysis of the GPR data. The spatial variation in dielectric properties in
the subsurface was closely related to the spatial variation in grain size. The geostatistical analysis
captured information about the spatial distribution of the dominant sedimentological features. Rea
and Knight (1998) found agreement using geostatistical analysis of a digitized photograph and the
radar data (amplitudes). The GPR data imaged the spatial distribution of lithologies and could be
used to quantify the correlation structure of the sedimentary unit. They also observed agreement
between the spatial variation in dielectric properties in the subsurface and the spatial variation of
the grain size, both indicating the heterogeneity of the subsurface. The authors hypothesized that
information extracted from the GPR data can be used to describe spatial variability of hydraulic
properties. Also Tercier et al. (2000) found that geostatistical analysis of GPR data gave an effective
way of quantifying the correlation structure of the two-dimensional GPR image. Charlton (2000)
used GPR techniques for spatially distributed measurements of volumetric soil moisture. He found
significant relationships between maximum amplitude and moisture content, indicating the poten-
tial of GPR for a quantitative assessment of soil moisture at different depths.
This study summarizes and completes—by showing new data—two earlier experimental works
on GPR application in well-defined systems (Schmalz et al., 2002, 2003; Stoffregen et al., 2002).
At first, we focused on the general suitability of the GPR technique to depict temporal soil water
content changes by analyzing the electromagnetic wave propagation. A more detailed analysis of
the GPR signal was performed in a second step in order to get more insight into the spatial heteroge-
neity of the soil moisture distribution. We selected certain attributes of the individual radar traces,
analyzed them statistically, and compared the computations to soil water content distributions as
derived from hydraulic simulation studies. It is important to note that the GPR technique can be
used to obtain different kinds of information. At first, the velocity analysis of the magnetic wave can
be applied to obtain absolute soil water content information as an integral over the soil depth which
is penetrated by the waves. At second, certain attributes of the GPR signal can be analyzed in order
to obtain depth-resolved information of the underground.
25.2
MAteRIAl And MethodS
25.2.1 gPR t
e c h n i q u e
The propagation velocity of GPR electromagnetic waves is determined by the dielectric constant or
permittivity of the medium to be investigated, which on its part is a function of the water content.
The dielectric constant ε for water is 80, for various soil and geological materials 5 to 15, and 1 for
