Field observations of shallow freeze and thaw processes using high‐frequency ground‐penetrating radar

We have used reflection profiles and common-midpoint (CMP) soundings with 900 MHz ground-penetrating radar (GPR) to monitor freezing and thawing processes during winter seasonal periods at two separate sites located in Ontario, Canada. GPR responds to the large contrast in dielectric permittivity between liquid water and ice. The profiles reveal the long-term development of a very shallow (<0·5 m) soil frost zone overlying unfrozen wet substratum. During the course of the winter season, long-term travel time analysis yielded physical properties of the frozen and unfrozen layers as well as the spatial distribution of the base of the soil frost zone. Short-term shallow thawing events overlying frozen substratum formed a dispersive waveguide for both the CMP and reflection profile surveys. Inversion of the dispersive wavefields for the CMP data yielded physical property estimates for the thawed and frozen soils and thawed layer thickness. We have shown that GPR can be used to monitor very shallow freezing and thawing events by responding to changes in the relative dielectric permittivity of the soil water phase (e.g. liquid water vs ice). The non-invasive collection of such data permits interpretation of dynamic temporal and spatial freeze–thaw events, which are important for characterizing a range of hydrological processes. Copyright © 2010 John Wiley & Sons, Ltd.