Geoscience Reference
In-Depth Information
where
c
is again the EM velocity in air,
x
is antenna separation (3.5 m),
t
ground
is the arrival time
of the ground wave, and
t
air
is the arrival time of the air wave.
The dielectric constant estimate represents the dielectric constant in the material sampled by the
radar wave. Through experiments, Berktold et al. (1998) determined that the ground wave samples
below the surface to a depth between one-half to one wavelength. The wavelength (λ) is computed
from the frequency (
f
) and the velocity of the phase by
λ=
v
f
(23.4)
In the GPR data, the antenna frequency is 100 MHz and the EM velocity is about 0.12 m/ns. Thus,
the dielectric constant is for the material to a depth of about 0.6 to 1.2 m.
Figure 23.7 shows the changes in EM velocity along the GPR profile over the course of the
experiment. To more easily see the trends in the values, we applied a five-point running average to
smooth the values. In March, the EM velocity is about 0.09 to 0.095 m/ns on the west side of the
profile, then increases to about 0.105 m/ns on the east side. The EM velocity increases significantly
in May to EM velocities of about 0.12 m/ns, then remains about the same or slightly slower in Sep-
tember. The increase in EM velocity from west to east is not observed in the May data but appears in
the September data. This change in trend from May to September may suggest greater evapotrans-
piration on the west side of the prototype surface barrier. In January, the EM velocity decreased to
about 0.09 to 0.095 m/ns along the profile.
Figure 23.8 shows the soil moisture content estimates derived from Topp's equation (Equation
(23.2)). Again, we applied a five-point running average to smooth the values. The graph shows recip-
rocal trends from Figure 23.7; high soil moisture content in March and January, and low moisture
content in May and September. From the plot, we infer that the soil dries out from March to May.
The soil moisture content remains about the same through September indicating a lack of precipita-
tion. By January, the soil moisture has increased to the highest values observed. Between January
and March, the soil has again started the cyclical drying process.
We also briefly investigated the effects of soil moisture on amplitude. Du and Rummel (1994)
note that the amplitude of the ground wave increases as κ relative to that of the air wave. Thus,
the ground wave is better observed in wet soils compared to dry soils. Reviewing Figure 23.5 and
Figure 23.6, the ground wave amplitudes weaken in the drier spring and summer months relative to
Ew Line at 16 m
We st
East
0.14
0.14
0.12
0.12
0.10
0.10
0.08
0.08
0
10
20
Distance (m)
30
40
fIGURe 23.7
Velocity changes along the profile from March to January. The thick lines are the five-point
smoothed average of the thin lines. The March data are displayed in light gray, May is gray, September is dark
gray, and January is black.
