Effects of the transition zone above a water table on the reflection of GPR waves

We simulate two levels of water table ( at 72 and 48 cm depth) by injecting water in a sand box that also contains several buried objects. GPR profiles acquired with a 1200 MHz antenna at the top of the sand box do not show any clear reflections from the water table. This is because of the existence of a 'transition zone' in which the velocity is a continuously decreasing function of depth. The reflection coefficient in this case decreases with increasing frequency and even vanishes for a cut-off frequency f(0) which itself increases with decreasing transition zone thickness. By modeling in the frequency domain, we explain the absence of the high-frequency GPR reflections from the top of the saturated zone. When the wavelength is small ( high frequency) compared to the thickness of the transition layer, the reflection coefficient is negligible and hence no reflections from the water table will be observed.