Geoscience Reference
In-Depth Information
Ew Line at 16 m
We st
East
0.25
0.25
0.20
0.20
0.15
0.15
0.10
0.10
0
10
20
Distance (m)
30
40
fIGURe 23.8
Soil moisture changes over time along the profile. As in Figure 23.7, the thick lines are the
five-point smoothed average of the thin lines. The March data are displayed in light gray, May is gray, Sep-
tember is dark gray, and January is black.
the winter months. The amplitudes from the GPR are influenced by many variable factors, including
the battery strength and coupling between the antenna and the ground. By normalizing the ground
wave amplitude with the air wave amplitude, we can remove coupling effects and other instrument
fluctuations in the ground wave amplitudes between the surveys. Again, the changes in amplitude
are due to changes in the moisture conditions. Our preliminary analysis indicates that a correlation
exists between high amplitude and wetter soil.
23.4 ConClUSIonS
GPR has great potential to observe temporal and spatial changes in the soil moisture content of
engineered barriers. Changes in the GPR character were easily observed over the course of this
experiment. The changes correspond to EM velocity changes that indicate the soil moisture var-
ies in an expected way. Amplitude analysis also indicates the soil moisture changes similar to the
results from the EM velocity analysis. Because the EM velocity changes are large between air and
water, GPR soil moisture estimates will probably be reliable. Further work is needed to calibrate
the EM velocity-soil moisture content relation to specific sites. We also need to test the accuracy of
quantitative estimates of the soil moisture using this method. The method can be made more conve-
nient by developing equipment for fast data acquisition and interpretation.
GPR proved successful at imaging changes in soil moisture along the profile and over the year.
The spatial changes in soil moisture from May to September may indicate greater evapotranspira-
tion on the west side of the prototype surface barrier. The seasonal GPR changes correlate with the
seasonal changes in soil moisture storage in the prototype barrier.
Using GPR to determine soil moisture content has many advantages over traditional methods:
(1) the cost and speed of data acquisition is relatively inexpensive; (2) the large spatial sampling
density provides greater coverage; and (3) the method is nonintrusive. GPR is a promising technique
to determine the three-dimensional distribution of the soil moisture content in the subsurface.
23.5 ACknoWledGMentS
The U.S. Department of Energy EM-50 Technical Task Plan RL21SS20, “The Application of Electromag-
netic Techniques to Cover Performance Monitoring,” funded this work. Battelle operates the Pacific North-
west National Laboratory for the U.S. Department of Energy under Contract DE-AC05-76RL01830.
