where c is again the EM velocity in air, x is antenna separation (3.5 m), tground is the arrival time of the ground wave, and tair 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 (X) is computed from the frequency (f) and the velocity of the phase by

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 September. 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 evapotranspiration 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 reciprocal 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 precipitation. 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 Jk 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 c

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