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3.1. SOIL PHYSICS 131
3.1.10 Monitoring Field Tracer Experiment with Ground Penetrating Radar
and Time Domain Reflectometry
Participating scientist Carolin Ulbrich, Ute Wollschläger, Kurt Roth
Abstract We explored the feasibility of Ground Penetrating Radar to non-destructively monitor
solute movement in natural soils at the field scale. In a weekly radar time series the displacement
of a CaCl2-tracer was monitored. The test site was instrumented with Time Domain Reflectometry
probes in several depths to measure soil water content and electric conductivity.
Background On its way down from the soil surface
to aquifers, water passes through soil that
acts as a filter. The transport and decomposition
of fertilizers and contaminants determines the
quality of groundwater.
The interaction of this complex system cannot
be calculated exactly. So appropriate restrictions
have to be chosen and abstract models developed
in order to get a quantitative description of a
few important aspects which can be used for estimations.
To assess the applicability of a certain
model one has to verify its predictions with experiments.
At small scales (O(1 m)) there exist numerous validation
methods, but most of them cannot be
implemented at larger scales (O(100 m)) because
they are destructive or too time-consuming.
The aim of this work was to test Ground Penetrating
Radar (GPR) as a new technique to
non-destructively monitor subsurface solute transport
processes. It was compared with Time Domain
Reflectometry (TDR) measurements and
with data from soil sampling. Model predictions
of solute transport on this scale were applied.
Methods and results At the Grenzhof Test
Site, a conservative tracer fluid that absorbs the
radar wave was spread on a streak that crosses
GPR transects vertically. The transport of the
absorbing fluid was monitored weekly with a GPR
antenna along these transects. At the end of the
experiment a trench was excavated and sampled
along the streak. The samples were analyzed with
traditional methods to verify the radar data set.
In addition, a TDR time series was acquired in
the same area in three depths. The comparison to
the excavation samples showed that TDR can be
used to monitor tracer concentration at the instru-
Figure 3.10: GPR radargram. Amplitudes of
single radar pulses are plotted versus their travel
times at the distances where they have been
acquired. The recorded signals before ≈ 15 ns
originate from air- and ground-waves that travel
through the air and along the soil surface, respectively.
The salt tracer was applied between
5.8 m and 7.8 m. It absorbs the radar signal.
mented depth in a semi-quantitative approach.
The absorption effect of the highly concentrated
salt tracer was clearly visible in the GPR surveys.
Figure 3.10 shows a radargram acquired
one month after the application of the tracer. Reflections
at layer borders, visible in the GPR surveys
that were taken before the application of the
tracer, disappeared when the signal was absorbed
by the tracer pulse. The reflections could be detected
again when the tracer pulse passed the reflector.
Obviously, the spatial resolution and accuracy is
much better in traditional sampling experiments,
but the effort there is enormous and the destructive
manner impedes large scale surveys. TDR
provides point measurements with good accuracy
and excellent temporal resolution. GPR holds the
promise to large scale experiments with good temporal
resolution but coarse spatial resolution. A
significant further effort is required to make it
quantitative, however.
Outlook/Future work A solute transport experiment
is planned to be performed on the Grenzhof
Test Site. At a scale of about 300 m tracer
will be applied on small areas and the displacement
will be monitored with GPR and TDR. The
tracer distribution will be tracked in addition by
permanent electric resistivity measurements. This
will help to achieve a better understanding of
transport processes and to further develop the
methods of GPR and TDR monitoring of tracer
movement.
Main publication Ulbrich, Carolin, Diplomarbeit,
Institut für Umweltphysik, Universität
Heidelberg, October 2005