Geoscience Reference
In-Depth Information
stage, then the receiving antenna only was moved 0.1 m until the antenna separation became 3.5 m.
At this point, both antennas were moved 0.25 m per trace, keeping a constant antenna separation
of 3.5 m.
To determine the soil moisture content, two different radar events are used. The ground-coupled
air wave travels directly between the transmitter and the receiver. This event is the first arrival and
has the EM velocity of air. The direct ground wave travels along the surface. The EM velocity from
this event corresponds to the EM velocity of the ground. We use the travel times of these two phases
and the distance between the antennas to determine the EM velocity. The EM velocity character-
izes the material through which the energy propagates. For our purposes, soil consists of a matrix
and pore space. This pore space is either filled with water or air. Water has a slow EM velocity
(0.033 m/ns), and air has a fast EM velocity (0.3 m/ns). In most areas, the material does not change
in terms of composition or structure over the short time intervals used in these types of experiments.
Thus, changes in soil moisture at the prototype surface barrier cause changes in EM velocity. Slower
measured EM velocities indicate a higher amount of water in the pore space.
The method indirectly infers the dielectric constant of the material through which the energy
propagates. The dielectric constant or dielectric permittivity represents the ability of a material to
polarize or store energy through separation of bound charges. The dielectric constant (κ) can be
computed from the EM velocity (
v
):
κ=
c
v
(23.1)
where
c
is the EM velocity of light (0.3 m/ns).
Water has a high dielectric constant of about 80. Air has a dielectric constant of 1. Dry soil
materials and sediments have dielectric constants between 3 and 10. Clays and silts may have a
dielectric constant as high as about 30 to 40. The large dielectric constant difference between water
and air enables mapping of changes in water content across a survey.
Soil moisture content can be derived from the EM velocity of the soil using Equation (23.1) to
convert first to dielectric constant. Mixing laws based on the amounts of the constituent materials
present are used to convert dielectric constant to soil moisture content (Knoll et al., 1995). The
dielectric constants can also be converted to soil moisture content using established petrophysical
relationships such as Topp's equation (Topp et al., 1980):
2
3
θ
=− +
0 053
.
.
0292
κ
−
0 00055
.
κ
+
0 000004 3
.
.
κ
(23.2)
where θ is the water content.
We analyzed the GPR profiles using a method that identifies changes in the arrival time of a
known radar event and then converts this time to EM velocity (Berktold et al., 1998; Du and Rum-
mel, 1994). This method is not widely applied in GPR surveys, but offers great potential to provide
spatially densely sampled EM velocity measurements that can be converted to the desired param-
eters, such as soil moisture content.
23.3 dAtA And AnAlySIS
We conducted 40 m long profiles to observe spatial changes in the GPR character across the proto-
type surface barrier. We also visited the prototype surface barrier site four times, March, May, and
September 2001, and January 2002, to determine the change in soil moisture during the different
seasons of the year. At Hanford, winters are cool and wet accounting for most of the precipitation,
and the summers are warm and dry. This testing methodology allowed us to observe changes in EM
energy travel times that we could relate to changes in soil moisture content.
