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134 CHAPTER 3. TERRESTRIAL SYSTEMS
3.1.13 Simulation and Observation of an Evanescent Wave in Ground Penetrating
Radar Applications
Participating scientist Holger Gerhards, Benedikt Oswald
Abstract A radiating dipole near a single boundary layer was simulated to observe air and ground
wave occurring in Ground Penetrating Radar applications. A semi-analytical approach to solve
Maxwell’s equations in frequency domain with Green’s functions was used. The evanescent characteristic
of the ground wave in air was demonstrated theoretically and experimentally.
x
air
soil
(4)
(3)
(1)
antenna
z
(2)
Background Ground Penetrating Radar
(GPR) is a fast and nondestructive electromagnetic
method to do near subsurface explorations
to estimate soil water content. Reflections are
measured, which occur at boundaries with a dielectric
contrast. Because the relative dielectric
permittivity (εw ≈ 81) of water is much higher
than the soil matrix (εs ≈ 3) in the used frequency
range between 0.1 to 1.0 GHz and assuming that
high water content changes coincides with soil
layers, temporal changes of water content can be
easily detected analyzing travel times.
As shown in Fig. 3.13 for a two layer (air/soil)
model a direct wave from transmitter to receiver
in air and another in the soil can be observed. The
measured ground wave in the air has an evanescent
character, which means, that its amplitude
decays exponentially with the height of the receiving
antenna over the soil. The knowledge about
this phenomena can help to separate the interfering
air and ground waves or to obtain information
about the dielectric properties of the soil surface.
Funding DFG RO 1080 / 10-1
Methods and results A radiating dipole
within a two layer medium was modeled using a
semi-analytical approach based on a spectral decomposition
of the Green’s functions in frequency
domain. The obtained electric field components
were transformed into time domain to be able to
compare the simulation with field measurements.
Figure 3.13: Propagation modes at a single
boundary layer; (1) and (2) spherical wave like
propagation modes in the according material,
(3) ground wave coupling in air (evanescent
wave), (4) air wave coupling in soil (lateral/head
wave)
The exponential decay and its frequency dependence
can be shown by analyzing the amplitudes
of the ground wave wavelet and its shape with
increasing height above the soil.
Analogous to the simulation experiments were
arranged, where the evanescent behavior of the
ground wave was verified and therefore validates
the semi-analytical approach. Furthermore, it was
shown, how far the ground wave interferes with
the air wave.
To obtain insight on the parameters that determine
the attenuation, an equation was derived
from the plane wave approach used in optics. The
dependency on frequency and dielectric contrast
was obtained, which can be qualitatively shown
with the simulated and measured time signals.
Outlook/Future work A frequency analysis
must be applied to check the derived frequency
dependence for experimental data. If air and
ground waves interfere with each other, a quantitative
separation must be developed, such as using
a wavelet analysis. Furthermore, the semianalytical
approach will be extended to simulated
multilayered models, which may help to understand
the depth-dependency of the ground wave
when a near subsurface water content gradient exists.
Main publication Gerhards, Holger, Diplomarbeit,
Universität Jena, 2004