We have examined the applicability of f-k analysis to the GPR direct wave measurements for water content. When the vadose zone condition is dry over wet, we obtained a f-k spectra where most of energy is bounded by the air and dry layer velocity. When wet soil overlies dry soil, the f-k spectra shows a contrasting response where most of energy travels with the velocity bounded by two layer velocities. In this case, most of energy is trapped and guided within the first layer and exhibits velocity dispersion. By inverting the dispersion curve, we can determine the two layer’s dielectric constants as well as layer thickness. In these experiments, f-k analysis provided a good means to examine the vadose zone condition in both scenarios.