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

Abstract

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 f0 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.