V

Distance (m)

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.

Ew Line at 16 m

West East

Ew Line at 16 m

West East

Distance (m)

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, September is dark gray, and January is black.

Distance (m)

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, September 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 varies 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 convenient 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 evapotranspiration 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.

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